Chapter 1: Environments and Their Affects on Corrosion Processes – Corrosion

1.0                Environments and Their Affects on Corrosion Processes

The environment plays a major role in the selection of materials for corrosion resistance. Environmental factors that may influence corrosion include the environment’s composition, pH level, humidity, wind or water currents, and temperature. These factors exist in the following types of environments, atmospheric, fresh water, saltwater, and soil, which will be discussed in this section. Additional micro-environments, such as specific acid susceptibilities of metals   and microorganisms will be covered.

1.1       Atmospheric Environments

Atmospheric corrosion can vary widely depending on contaminants present, humidity and rainfall, wind and temperature.   Extensive atmospheric testing programs have been conducted    to compute corrosion rates of metals and to characterize metals’  susceptibilities  to  various  forms of corrosion. A typical atmospheric testing rack  is  shown  in  Figure  1.  Such  studies have lead to the broad categorization of environments into rural, urban, industrial, marine, and combinations of them. A general characterization of the four main types is listed in Table 1.

Deviations of corrosion rates within the four categories  have  lead  to  a  further  subcategorization based upon weather and climate. These additional factors  influencing  corrosion   are   temperature,   humidity   and   rainfall.   The   relative   corrosion   rates   for  these environments are found in Table 1.

Figure 1            Atmospheric Corrosion Test Rack

Table 1 Types of Atmospheric Environments

Table 2 General Corrosion Rates for Different Atmospheric Environments

1.1.1 Atmospheric Contaminants

The primary sources of atmospheric contaminants come from chlorides in marine locations, and industrial and automobile pollutants. These contaminants deposit onto metal surfaces where they react primarily with oxygen, water, and free electrons producing metal compounds which have varying degrees of solubility; producing varying increased corrosion rates over non-corrosive environments.

The presence of chloride salts in the atmosphere significantly increased the corrosion rates of most metals. In the case of ferrous metals, chloride anions combine with ferrous cations to produce iron chloride. Iron chloride is more soluble than the ferrous hydroxide produced in a benign environment, leading to an increased corrosion rate. Other metals such as copper and  zinc produce metal chlorides which are less soluble than ferrous chlorides. These metals therefore experience increased corrosion rates, but not to the extent of ferrous metals. It is notable that deicing salts used on roadways in winter months produce corrosivity in those environments similar to marine atmospheric environments.

Sulfur dioxide (SO₂) and nitrous oxides (NO_x) are found in industrial and urban environments from the burning of fossil fuels. Sulfur dioxide deposited on metal surfaces will react with oxygen and free electrons from the metal surface, producing sulfate ions, as expressed in Equation 1. The sulfate ions lead to the formation of metal sulfates, which in turn react with water to complete the corrosion process, Equation 2.

As seen in Equation 2, sulfate ions are again produed in the case of ferrous metals, producing a self-contained corrosion process once sulfur dioxide is present. This process may not occur as readily with other metals nor are most of the metal sulfates produced as soluble as iron sulfate. The presence of nitrous oxides may also increase corrosion rates of metals in a like manner, although they do not deposit on metals as readily as sulfur dioxide.

There are a few additional atmospheric contaminants that are less abundant or may be found in special industrial environments. Hydrogen sulfide is extremely corrosive to most metals. This compound is readily found in oil-refining and petroleum industries. Hydrogen chloride and chlorine gas have been found to produce higher corrosion rates than chloride salt environments. Ammonia, sulfur trioxide, and smoke particles will also  increase  atmospheric  corrosion  of most metals. The typical concentrations of these major contaminants are found in Table 3.

Table 3 Typical Concentrations of Several Atmospheric Contaminants

1.1.2 Humidity and Rainfall

Humidity is also a major factor in determining the corrosion rate of metals, as moisture provides the electrolyte, which is required for corrosion reactions to take place. In general, the corrosion rate increases as humidity increases. The critical level of relative humidity in order for serious corrosion to occur in the absence of other electrolytes is usually taken to be 60%.³ This critical level of relative humidity may vary depending on the impurities present in the atmosphere. Rainfall can increase or decrease corrosion processes. In areas where stagnant water may accumulate, a localized corrosion cell will most likely be the result. However, rain may also wash corrosive deposits off from metal surfaces, decreasing corrosivity.

1.1.3 Wind

The wind plays a role in the direction and distance atmospheric contaminants are dispersed. The corrosivity of atmospheric environments and therefore general corrosion rates of metals, are related to their distance and proximity to coastal waters and industrial plants.

1.1.4 Temperature

Temperature can have a significant effect on the corrosion of metals, with increased rates of corrosion as temperature is increased. Temperature may also affect the form of attack that the corrosion takes on; for example, changing the temperature may change the corrosion mechanism from uniform to pitting. It can also evaporate condensed moisture on metallic surfaces leaving behind corrosive contaminants. High temperatures can  produce  a  form  of  corrosion  where gas becomes the electrolyte as opposed to a liquid medium.

1.1.5 Atmospheric Corrosivity Algorithms

There has been some corrosion algorithms  developed  to  compute  corrosivity  values  for  given environmental conditions. A couple methods are  described  here  which  include  the Pacer Lime Program sponsored by the USAF, and ISO Standard  9223.  These  methods however, use average values  to  compute  their  corrosivity  indices  and  only  provide  a general characterization of atmospheric corrosivity for various environments.

An atmospheric corrosivity severity classification  system  was  developed  by  Summit  and  Fink under the Pacer Lime  Program  to  provide  management  information  for  the  maintenance of aircraft  ⁴  Measurements  of  environmental  conditions  were  made  at numerous USAF bases to compute a corrosivity algorithm. Environmental  conditions  considered as part of the algorithm are distance to coastal waters, SO₂ content, total suspended particles, humidity, and rainfall. A severity index was created and used to schedule the  frequency of various preventive maintenance tasks on aircraft. The algorithm developed to schedule the frequency of washing aircraft, is shown in Figure 2.

Figure 2            Corrosion Severity Algorithm for Planning an Aircraft Washing Schedule

The ISO 9223 Standard uses the time of wetness and the deposition rates of sulfur dioxide and chlorides to compute an atmospheric corrosivity index.⁶ The time of wetness is in units of hours per year, and consists of the time when the relative humidity is >80% and the temperature is >0ºC. The three conditions are divided into five ranges of values used to produce five corrosion categories represented in Table 4.

Table 4 ISO 9223 Corrosion Categories/Rates after One Year Exposure

1.1.6 Managing Atmospheric Corrosion

General methods to minimize the effects of atmospheric corrosion include the following:

• Proper selection of material for the type of environment and corrosive contaminants present.
• Proper component/system design to limit contaminants and water build-up.
• Use organic and/or metallic coatings and sealants wherever
• Vapor phase corrosion inhibitors may be used in microenvironments, such as the inside of boilers.

1.2       Water Environments

Factors contributing to the corrosivity of water environments include the composition, pH level, temperature, water velocity, and biological organisms. Water environments are divided into natural or fresh water, and seawater type environments. Fresh waters are used extensively in cooling systems, boiler feed waters, processing of materials and products, washing and drinking waters.

1.2.1 Water Compositions

The composition of water can be quite different, dependent upon the materials and contaminants picked up from the atmosphere during rainfall, the surrounding soil, and man-made pollutants dumped or spilled into waterways. The compounds most responsible for general water corrosivity are dissolved gases and salts. There may additionally be dissolved compounds present in specific areas from pollutants.

The primary dissolved gases affecting corrosion in waters are oxygen and sulfurous gases. Oxygen is by far the biggest concern, as it directly relates to higher corrosion rates for many metals. The concentration of oxygen is greatest at water surfaces and in the presence of algae. Sulfur dioxide and hydrogen sulfide significantly increase corrosivity and are found in waters as a result of pollutants and/or microorganisms. Sulfate reducing bacteria converts sulfates to sulfides. Nitrogen, like in atmospheric environments, is less abundant, but will increase  corrosion rates of metals where present.

The ions from dissolved salts mostly responsible for increased water corrosivity are chlorides   and sulfates. These ions react with metal cations to produce corrosion reactions. A run down of the  most  common  constituents  and  ions  present  in  seawater  are  listed  in Table 5. Cations present may reduce corrosion by reacting with available anions. The measure of calcium and magnesium solid precipitates determines the hardness of water. Table 6 represents the general characteristics of some natural waters. A measure of the water’s electrical resistivity gives a general indication of corrosivity.

Table 5 Typical Contents and Ions Found in Seawater

Table 6 Typical Natural Water Analyses

A = very soft lake water : B = moderately soft surface water : C = slightly hard river water : D = moderately hard river water

E = hard borehole water : F = slightly hard borehole water with bicarbonate ions : G = very hard groundwater

1.2.2 pH Level

The pH level of both natural and seawaters is usually within 4.5 to 8.5. Copper is one metal in which the corrosion rate increases in acidic water. Copper from the corrosion process will then deposit on other materials present producing a greenish stain. The deposition of copper onto aluminum or galvanized metals sets up pitting corrosion.

1.2.3 Temperature

Higher temperatures normally produce increased corrosion rates in water, like other environments. Increased temperatures do decrease oxygen solubility in water. However, warm temperatures will also increase biological growth which can increase oxygen content. And like  in any environment, higher temperatures generally speed up corrosion reactions. The temperature, as well as oxygen content and salinity as a function of ocean depth are depicted in Figure 3.

Figure 3            Some Corrosivity Factors as a Function of Depth in the Pacific Ocean (West of Port Hueneme, California)

1.2.4 Water Velocity and Agitation

The water velocity and agitation may increase or decrease corrosion rates, dependent upon the particular metal. The relative degree of attack on some marine metals is shown in Figure Most metals have a critical velocity, beyond which significant corrosion occurs.

Figure 4            Localized Attack (Crevice and Pitting) of Some Metals in Flowing Seawater

1.2.5 Biological Organisms

All biological organisms, either animals or plants, alter the composition of surrounding water, which may result in increased or decreased corrosion rates. Some organisms merely provide a protective layer, limiting oxygen from reaching the metal’s surface. Others increase the oxygen or sulfide content, increasing the corrosive attack on  metals.  Microorganisms  will  be  discussed further  in  Section  2.9.6.  Figure  5  shows  some  corrosivity  factors  and  their  effect on the uniform corrosion rate of carbon steel in the Pacific Ocean.

Figure 5            Factors Affecting the Corrosion of Carbon Steel in the Pacific Ocean

1.2.6 Managing Underwater Corrosion

Methods to limit uniform underwater corrosion include:

• Proper selection and design of
• Specialized underwater paints should be used where feasible (to include formulations for the deterrence of
• Cathodic protection should be used when
• Preventive maintenance to remove microorganisms and other marine animals from surfaces.

1.3    Soil Environments

The corrosivity of soils is affected by water, the degree of aeration, pH level, temperature, salt content, and biological activity. The soil’s particle size plays a role in that smaller particle sizes will hold water, and have less oxygen content; while the opposite exists for large particle sizes. Loose soil will also have greater oxygen content. The oxygen content is highest near or at the soil’s surface and decreases with increasing depth. Soils normally have pH levels in the range 5  to 8. At these levels, the corrosion rate is not significantly affected. However, acidic soils will increase the rate of attack on most metals, and will affect other factors such as microbiological activity. Salt content affects soils in the same manner as atmospheric and water environments, with the major corrosive species once again being chloride and sulfate ions. Sulfate reducing bacteria is the most detrimental microorganism to metal corrosion. Like the water environment, a measure of  the  soil’s  electrical  resistivity  is  a  general  indication  of  its  corrosiveness.  Lower resistivity equates to higher corrosivity.  Table  7  lists  soil  resistivity  ratings  based  upon resistivity. Mappings of soil resistivities are used to determine where to lay underground piping. It is beneficial to route pipelines through soil with like resistivities and ideally, high resistivities. Care must be taken when laying pipelines in the vicinity of other structures, such     as railways, to minimize their exposure to stray currents.

Table 7 Soil Corrosivity Ratings

1.3.1 Managing Corrosion in Soil Methods to limit corrosion in soils include:

• Proper selection and design of materials
• Metals should be coated where possible. Bituminous coal tar and asphalt dip coatings are effective for under ground piping. Imbedding pipes in concrete has also been used to limit corrosion.
• Cathodic protection should be used where

Pipes in the vicinity of electric railroad tracks or other similar equipment must be electrically insulated. One method is to coat the piping with hot asphalt followed by a concrete encasement. Organic dielectric coatings have also been developed for this purpose.

Chapter 2: Forms of Corrosion – Corrosion

There are eight major forms of corrosion accounting for the vast majority of corrosion problems observed, along with several lesser seen forms occurring in specific environments. Uniform or general corrosion proceeds independent of the material’s microstructure and component design. It is highly dependent upon the environmental conditions and the material’s composition, generally occurring at a slow rate. All the remaining forms of corrosion are localized, dependent upon the environments, the components and systems designs, and/or the microstructure of the materials. These forms typically produce higher corrosion rates than uniform corrosion, and in some cases can be quite rapid. Each of the various forms of corrosion should be evaluated for a material and environment when designing new systems. The following sections cover all the forms of corrosion observed with some materials’ susceptibilities and general protection schemes for the various forms.

2.0       Uniform Corrosion

Uniform corrosion is a generalized corrosive attack that occurs over a large area on the surface of a material. It is only dependent upon the material’s composition and the environment.  The  result is a thinning of the material until failure occurs. Uniform corrosion rates are fairly predictable, following an exponential relationship as follows.

The decrease in corrosion rate with time is a direct result of an oxide scale layer forming on the metal’s surface, which then deters further corrosion from occurring. There are extreme cases however, where the corrosivity of the environment is  severe  and  prevents  an  oxide  layer  from forming.   In this case, the corrosion rate will be constant with time.   Figure 6 depicts     this relationship for uniform corrosion. Equation 3 may be used to predict the long-term corrosion damage from short-term tests. There are some problems with  this  prediction  however.  Environments usually change over time so that corrosion rates will deviate from   those predicted by the equation. Also, the development of additional forms of corrosion will likely accelerate attack in localized areas.

Figure 6           Uniform Corrosion Rates

Uniform corrosion is measured in weight loss or thickness loss and is converted from one to the other using the equation.

2.1.1 Metal Susceptibilities to Uniform Corrosion

Magnesium  and  low  alloy  ferrous  alloys  are  by  far  the  most  susceptible  metals  to uniform corrosion as shown in Figure 7. Additional metal classes, not addressed in the figure, normally have negligible uniform atmospheric corrosion rates. For the susceptible metals, increased alloying with specific elements can increase uniform corrosion resistance. Alloying  for general corrosion   resistance   of   the   different   metal    classes    is    covered    in    Section 4.0, Corrosion Characteristics and Properties of Metals. Alloying should also consider the environmental   composition   and   degree   of   corrosivity.   The   relative    uniform    corrosion susceptibilities of a steel piling exposed to marine environments are shown in Figure  8.

Figure 7            Atmospheric Corrosion Rates of Various Metals

Figure 8            Relative Uniform Corrosion Rates of a Steel Piling in Marine Environments

2.1.2 Managing Uniform Corrosion

The selection of materials for uniform corrosion resistance should simply take into consideration the susceptibility of the metal to the type of environment that will be encountered. Organic or metallic coatings should be used wherever feasible. When coatings are not used, surface treatments that artificially produce the metal oxide layer prior to exposure will result in a more uniform oxide layer and the thickness may be controlled. There are also surface treatments  where additional elements are incorporated for corrosion resistance, such as chromium. Also, vapor phase inhibitors may be used in such applications as boilers to combat corrosive elements and adjust the pH level of the environment.

2.1       Galvanic Corrosion

Galvanic corrosion occurs when two metals having different electrical potentials (dissimilar metals) are electrically connected, either through physically touching each other or through an electrically conducting medium, such as an electrolyte. Systems meeting these requirements essentially form an electrochemical cell which will conduct electricity. The induced electrical current can then attract electrons away from one of the metals, which thus acts as the anode in the electrochemical cell.  This usually results in acceleration of the rate of corrosion of the anode.  The opposing metal, the cathode, will consequently receive a boost in its resistance to corrosion, since it can supply any imminent corrosion reactions with electrons from an external source. Galvanic corrosion is usually observed to be greatest near the surface where the two metals are    in contact. Figure  9  shows  galvanic  corrosion  on  a  metal  component  near  a  dissimilar  metal fastener.

Figure 9            Galvanic Corrosion between a Stainless Steel Screw and Aluminum

In general, corrosion is the result of an electrochemical reaction that occurs between an anode and a cathode. In the case of uniform corrosion, the metal being corroded acts as both the anode and the cathode in the reaction, where localized areas on the surface of the metal have slightly different electrical potentials. However, galvanic corrosion occurs between two dissimilar metals. The metal with a lower potential relative to the other metal acts as the anode, while the metal with a higher potential acts as the cathode. The corrosion reaction/corrosion current (flow of electrical current) is driven by an electrical potential  gradient.  Some  typical  electrical  potentials  for some common  metallic  elements  are  shown  in  Table  8.  (Note  –  these  potentials  were  taken in standard conditions,  but  actual  potentials  vary  in  metals  and  alloys,  especially  under various environmental conditions.)

Table 8 Electromotive Series of Metals

2.2.1 Factors Influencing Galvanic Corrosion

There are a number of driving forces that influence the occurrence of galvanic corrosion and the rate at which it occurs. Among these influencing factors are the difference in the electrical potentials of the coupled metals, the relative area, and the system geometry. Other driving forces that factor into promoting or preventing galvanic corrosion include the polarization (the shift in electrode potential during electrolysis) of the metals, the electrical resistance and electrical current of the system, the type, pH, and concentration of the electrolyte, and the degree of aeration or motion of the electrolyte.

2.2.1.1 Potential Difference

The main driving force for galvanic corrosion is the electrical potential difference between the two dissimilar metals; thus, typically the bigger the difference, the more rapid the rate of galvanic corrosion. Galvanic corrosion occurs mainly at the contact area of the two metals and dissipates with distance from the junction. A fundamental expression that shows the electrical potential of a galvanic system is given in Equation 5.

There are numerous resources from which the standard electrode potentials of specific metals and alloys can be obtained. The rate of galvanic corrosion in specific environments, however, should not be determined based on the standard electrode potentials of metals. These standard potentials are determined as the potential of a metal in equilibrium with a specific concentration of the electrolyte.⁹⁶ Furthermore, a galvanic system is dynamic and the reactions are dependent on a number of other factors, including electrolyte concentration, temperature, and pH, as well as oxygen content and fluid motion.

It is not always necessary, however, to have two distinct metals in order to create a galvanic couple. There are instances where galvanic corrosion occurs within the same metal. This can happen when the metal has both an active and passive state, for example, one part is covered with an oxide film and hence passivated, while another part of the metal is exposed to the atmosphere. This condition would create a potential difference causing the unpassivated area of the metal to galvanically corrode.

2.2.1.2 Relative Area

The size of the metal components in the galvanic system also influences the rate and degree of corrosion. For example, a system with a relatively large cathode (less reactive metal) and a relatively small anode (more reactive metal) will corrode via galvanic corrosion to a greater extent than will a system with electrodes of equal size. Furthermore, a system with a relatively large anode compared to a small cathode will not typically exhibit galvanic corrosion on the anode to a significant extent. In general, corrosion of the anode is proportional to the relative area of the cathode. That is, the induced electrical current increases proportionally with an increase in cathodic area relative to the area of the anode. The opposite is generally true as well: current decreases proportionally with a decrease in relative cathodic area.

2.2.1.3 Geometry

Component geometry is another factor affecting the flow of current, which consequently influences the rate of galvanic corrosion. Current does not easily travel around corners, for instance.

2.2.1.4 Electrolyte and Environment

The rate of galvanic corrosion is partially dependent on the concentration, oxygen content and motion of the electrolyte, as well as the temperature of the environment. For instance, higher temperatures typically cause an increase in the rate of galvanic corrosion, while higher concentrations of the electrolyte will result in a decrease in the corrosion rate.96 The pH of the electrolyte solution may also affect the occurrence of galvanic corrosion in a dissimilar metal system. For example, a metal that is the cathode in a neutral or basic electrolyte may become the anode if the electrolyte becomes acidic.96 A higher oxygen content in the electrolyte also typically results in an increase in the rate of galvanic corrosion. Electrolyte motion can also increase the rate of corrosion, since it may remove some of the oxidized metal from the anode surface, allowing for further oxidation of the metal.

2.2.2 Material Selection

In most cases, galvanic corrosion can be easily avoided if proper attention is given to the selection of materials during design of a system. It is often beneficial for performance and operational reasons for a system to utilize more than one type of metal, but this may introduce a potential galvanic corrosion problem. Therefore, sufficient consideration should be given to material selection with regard to the electrical potential differences of the metals.

2.2.2.1 Galvanic Series

The potential difference of two metals is qualitatively determined by their relative placement    on the Galvanic Series, shown in Table 9.  Some metals are listed more than once.  This is   either because they exhibit different  galvanic  properties  when  given  different  heat  treatments,  or  because they can be in two different states.   The metal is in an active state    when the metal surface has direct interaction with the environment, and the metal  is  in  a passive state when a noble film has formed on the surface.

This table can be helpful in estimating the likelihood of corrosion of a specified bimetallic system by gauging the distance between the two metals on the galvanic series. To state it  simply, avoid using metals that are far apart on the galvanic series. The chart is not useful, however, in predicting the degree or rate of corrosion, since there are several other factors that influence the magnitude of corrosion in a given bimetallic system.

The metal that is higher on the Galvanic Series chart is less reactive and thus acts as the cathode, while the metal appearing lower in the series is more reactive and acts as the anode in the electrochemical cell. For example, if copper was to be electrically coupled with tin  and immersed in seawater, then copper would be the anode and would corrode more readily than tin, which would act as the cathode. In environments other than seawater, the metal with the least resistance to corrosion in the surroundings acts as the anode and is then more readily corroded than the other, more noble metal.

2.2.2.2 Other Material Selection Charts

There have been a number of charts and tables created in  order  to  aid  in  the  material  selection process and eliminate the potential for galvanic corrosion. Table 10 lists specific metal and alloy compatibilities  with  other  specific  metals  and  alloys  in  seawater  with  respect     to  galvanic  corrosion.   This table shows whether a certain combination of metals or alloys       is compatible,  unfavorable or uncertain.  Note that the stainless steels listed in the table are all  in the same state (active or passive). Table 11, on the other hand, lists metal and alloy compatibilities with  respect  to  galvanic  corrosion  in  environments  other  than  seawater, such as marine and industrial atmospheres.

Table 9 Galvanic Series in Seawater

Table 10           GalvanicCorrosion Compatibilities of Metals and Alloys in Seawater

Table 11           Galvanic Corrosion Compatibilities of Metals and Alloys in Marine and Industrial Environments

Note – Joined metals presented in this table are of equal area.
PH – Precipitation Hardening is a specific type of heat treatment/aging

2.2.3 Managing Galvanic Corrosion

If proper design, material selection, implementation, and maintenance steps are followed, it is relatively simple to avoid the occurrence of galvanic corrosion in a new system. MIL-STD-889 (active) is a DOD standard on dissimilar metals. The purpose of this standard is to define and classify dissimilar metals and establish requirements for protecting coupled dissimilar metals in all military equipment parts, components and assemblies.⁹⁶ To  further  aid  in  properly  avoiding this form of corrosion, Table 12 provides a brief list of guidelines to  minimize  galvanic corrosion. Some of these are explained in more detail in the sections to follow.

2.2.3.1 Area Effects

Taking into account the relative areas of galvanically coupled metallic systems can minimize galvanic corrosion. The size of the cathodic metal in the bimetallic system should not be significantly larger than the size of the anodic metal, since this would cause a greater degree of corrosion of the anodic member. Instead, the anodic metal should have an equal or larger area. For example, the more noble metal should be used for rivets, bolts and other fasteners, thus making the area of the anode much greater than that of the cathodic component.

2.2.3.2 Cathodic Protection

Galvanic corrosion can be intentionally induced in order to protect a more important metallic component. This method of protection involves using a highly active metal, one that is lower on the galvanic series, to be sacrificially corroded. This sacrificial anode protects the more important, cathodic metal from corrosion. Magnesium and zinc are commonly used as sacrificial anodes. Sacrificial anodes are often replaced in-service as they are consumed through galvanic corrosion, as intended.

2.2.3.3 Insulate Dissimilar Metals

Electrically resistive, non-metallic materials can be used to insulate two dissimilar metals. This in effect, breaks the electrical connection or at least increases the electrical resistivity resulting in a reduction, if not elimination, of the potential for galvanic corrosion.

2.2.3.4 Coatings

Metallic coatings are commonly used to protect bimetallic systems against galvanic corrosion. These coatings can provide protection by acting as barriers to corrosion or by readily accepting corrosion, thereby saving the important metal component from being corroded. For example, zinc is often used as a coating for steel, and since it is not very corrosion resistant, it will corrode preferentially to protect the steel. Thus, the zinc coating acts as a sacrificial anode.

Noble metal coatings are typically used as barrier coatings, since they are relatively unreactive. These coatings can isolate the important metal from the surrounding environment; however, pores, defects, or damaged areas in these barrier coatings are areas still susceptible to being galvanically corroded. Furthermore, the areas under these discontinuities (also known as holidays) in the coating system are likely to be targeted for severe localized corrosion. In addition, if the anodic metal in the galvanically coupled system is coated with a barrier coating without coating the cathodic member as well, it can have severe negative effects due to the reduced anodic area. Moreover, if the anode is coated, while the cathode is not, the former cathode may become anodic to the former anode.

2.2.3.5 Crevices

Threaded joints with dissimilar metals that are far apart on the galvanic series should be avoided. It is recommended that crevices be sealed either by welding or brazing to protect against galvanic corrosion.

Chapter 2.3: Crevice Corrosion – Corrosion

2.3       Crevice Corrosion

Crevice corrosion occurs as a result of water or other liquid entrapment in localized areas dependent upon component/system design. These designs include primarily sharp angles, fasteners, joints, washers and gaskets. Crevice corrosion can also occur under debris build up on surfaces, sometimes referred to as “poultice corrosion.” Poultice corrosion can be quite severe, due to an increasing acidity in the crevice area.

2.3.1 Crevice Corrosion Mechanism

The combination of low oxygen content in the crevice area compared to the surroundings, sets   up an anodic imbalance creating a highly corrosive microenvironment, as depicted in Figure 10.  Crevice corrosion is of particular concern in aircraft lap joints.   In severe cases, the build up of corrosion products in the lap joint can cause separation of the two metals, known as pillowing.

Figure 10          Crevice Corrosion Process in Steel

2.3.2 Crevice Geometry

The crevice gap, depth, and the surface ratios of materials can all affect the degree of crevice corrosion. Tighter gaps have been known to increase the rate of crevice corrosion of stainless steels in chloride environments. This has been attributed to the reduced volume of electrolyte that becomes acidified resulting in a higher rate of attack. The larger crevice depth and greater surface area of metals will generally increase the rate of crevice corrosion.

2.3.3 Metals Susceptible to Crevice Corrosion

In general, materials that are passive have a greater susceptibility to crevice corrosion. These include aluminum alloys and particularly stainless steels. Titanium alloys normally have good resistance to crevice corrosion. However, they may become susceptible in elevated temperature, acidic environments containing chlorides. In seawater environments, copper alloys can experience crevice corrosion that occurs on the outside of the crevice.

2.3.4 Managing Crevice Corrosion

New components and systems should be designed to minimize areas where crevice corrosion may occur. Welded joints are preferable to fastened joints. Where crevices are unavoidable, metals with a greater resistance to crevice corrosion in the intended environment should be selected. Avoid the use of hydrophilic materials in fastening systems and gaskets. Crevice areas should be sealed to prevent the ingress of water.   Also, a regular cleaning schedule should be implemented to remove any debris build up. Figure  11  illustrates  several  methods  that  may be implemented to decrease crevice corrosion.

Figure 11          Methods to Mitigate Crevice Corrosion

2.3       Pitting Corrosion

Pitting corrosion, also simply known as pitting, is an extremely localized form of corrosion that occurs when a corrosive medium attacks a metal at specific points causing small holes or pits to form. This usually happens when a protective coating or oxide film is perforated, due to mechanical damage or chemical degradation. Pitting can be one of the most dangerous forms of corrosion because it is difficult to anticipate and prevent, relatively difficult to detect, occurs very rapidly, and penetrates a metal without causing it to lose a significant amount of weight. Failure of a metal due to the effects of pitting corrosion can thus occur very suddenly. Pitting  can have side effects too, for example, cracks may initiate at the edge of a pit due to an increase in the local stress. In addition, pits can coalesce underneath the surface, which can weaken the material considerably. Figure 12 shows  the  result  of  pitting  of  an  aluminum  railing  that  was located near an ocean.

Figure 12          Pitting Corrosion of an Aluminum Railing near the Atlantic Ocean

2.4.1 Pitting Mechanism

Pitting often begins at a specific area of a passivated metal where there is a break in the passivation layer, which then acts as the anodic area, while the rest of the metal acts as the cathodic area. With a potential difference between the anode and cathode, extremely localized corrosion initiates, and since the surrounding area is passivated, the corrosion remains localized and causes pits to form in the metal. Moreover, since the anodic area is significantly smaller  than the cathodic area, corrosion continues at a rapid pace.

A further danger of pitting is that corrosion in pits becomes self-sustaining by an autocatalytic process. Such a process involves the progression of pit growth by the dissolution of the metal near the bottom of the pit. It is thought that the environment is very acidic near the bottom of the pit, thus propagating the dissolution of the metal. The dissolution reaction, where electrons associated with a metal-metal bond are dispelled and a metal ion breaks away from the bulk material, works in conjunction with a cathodic reaction near the surface adjacent to the pit. The cathodic reaction supplies excess electrons to facilitate a reduction reaction by forming hydroxide ions from water molecules and diatomic oxygen. To maintain neutrality, anions (negative ions) from the electrolyte migrate into the pit where there is an excess of positive charge, and associate with the metal ions. Subsequently, this species is dissociated in water to form a metal hydroxide and an acid, which results in a reduction in the pH near the bottom of the pit. This means that there is an excess of positively charged hydrogen ions and anions, which stimulate and propagate further dissolution of the metal near the bottom of the pit. These reactions are shown in Equation 6, Equation 7, and Equation 8.

Pitting corrosion is also very difficult to measure and predict, as there are usually numerous pits of varying depths and diameters, which do not form consistently under specified conditions. The holes that form from corrosive attack, however, tend to be greater in depth than in diameter. These pits typically form on the top-surface of a metal and proceed to deepen in the same direction as gravity. Thus, they do not usually form on surface planes that are parallel to the direction of gravity, but rather on those that are perpendicular to gravity. Moreover, pits do not tend to proceed away from the direction of gravity. Basically, they do not form on the bottom surface of a metal and proceed away from the direction of gravity. Initiation of the holes is a gradual and fairly long process, but once they are formed, the rate of growth of the pit increases significantly. Pitting usually occurs in static or low velocity fluid systems, since pitting  corrosion will tend to decrease as fluid velocity increases. Pitting is often difficult to measure since the metal usually experiences minimal weight loss during the corrosion process. Also, pits can be filled in with corrosion products.

2.4.2 Metals Susceptible to Pitting Corrosion

Stainless steels tend to be the most susceptible to pitting corrosion among metals and alloys. For example, stainless steels tend to form deep pits in seawater, and environments containing higher concentrations of chlorine or bromine solutions. Polishing the surface of stainless steels can increase the resistance to pitting corrosion compared to etching or grinding the surface. Alloying can have a significant impact on the pitting resistance of stainless steels. The effects of some of the alloying elements of stainless steels on the corresponding resistance to pitting are provided in Table 13.

Table 13           Effects of Alloying Elements on Pitting Resistance of Stainless Steel Alloys

Conventional steel has a greater resistance to pitting corrosion than stainless steels, but is still susceptible, especially when unprotected.  Aluminum in an environment containing chlorides  and aluminum brass in contaminated or polluted water are usually susceptible to pitting. Titanium is strongly resistant to pitting corrosion. The relative  pitting  resistance  of  some metals is shown in Figure 13.

Figure 13          Relative Pitting Resistance of Some Metals

2.4.3 Managing Pitting Corrosion

Proper material selection is very effective in preventing the occurrence of pitting corrosion.  Field testing, though, is often necessary to determine whether the chosen material is suitable for the proposed environment. Another option for protecting against pitting is to mitigate aggressive environments and environmental components (e.g. chloride ions, low pH, etc.). Inhibitors may sometimes stop pitting corrosion completely. Further efforts during design of the system can aid in preventing pitting corrosion, for example, by eliminating stagnant solutions or by the inclusion of cathodic protection.

2.5       Intergranular Corrosion

Intergranular corrosion attacks the interior of metals along grain boundaries. It is associated with impurities which tend to deposit at grain boundaries and/or a difference in phase precipitated at grain boundaries. Heating of some metals can cause a “sensitization” or an increase in the level  of inhomogeniety at grain boundaries. Therefore, some heat treatments and weldments can result in a propensity for intergranular corrosion. Susceptible materials may also become sensitized if used in operation at a high enough temperature environment to cause such changes in internal crystallographic structure.

2.5.1 Metals Susceptible to Intergranular Corrosion

Intergranular corrosion can occur in many alloys. The most predominant susceptibilities have been observed in stainless steels and some aluminum and nickel-based alloys. Stainless steels, especially ferritic stainless steels, have been found to become sensitized, particularly after welding. Welding causes the precipitation chromium carbide phases at grain boundaries in the heat affected zone (HAZ).   This in turn results in intergranular corrosion within the HAZ of    the stainless steels. Aluminum alloys also suffer  intergranular  attack  as  a  result  of  precipitates at grain boundaries that are more active. Alloys that  fall  into  this  type  of  corrosion include 5083, 7030, 2024, and 7075. Exfoliation corrosion is considered a type of intergranular corrosion in materials that have  been  mechanically  worked  to  produce  elongated grains in one direction. This form of corrosion has been experienced in certain aluminum alloys. High nickel alloys can be susceptible  by  precipitation  of  intermetallic  phases at grain boundaries.  The intergranular corrosion process is however more complicated   in nickel-alloys than in stainless steels or aluminum alloys.

2.5.2 Managing Intergranular Corrosion

Methods to limit intergranular corrosion include:

• Keep impurity levels to a minimum
• Proper selection of heat treatments to reduce precipitation at grain
• Specifically for stainless steels, reduce the carbon content, and add stabilizing elements (Ti, Nb, Ta) which preferentially form more stable carbides than chromium

2.6       Selective Leaching (Dealloying Corrosion)

Dealloying, also called selective leaching, is a rare form of corrosion where one element is targeted and consequently extracted from a metal alloy, leaving behind an altered structure. The most common form of selective leaching is dezincification, where zinc is extracted from brass alloys or other alloys containing significant zinc content. Left behind are structures that have experienced little or no dimensional change, but whose parent material is weakened, porous and brittle. Dealloying is a dangerous form of corrosion because it reduces a strong, ductile metal to one that is weak, brittle and subsequently susceptible to failure. Since there is little change in the metal’s dimensions dealloying may go undetected, and failure can occur suddenly. Moreover,  the porous structure is open to the penetration of liquids and gases deep into the metal, which can result in further degradation. Selective leaching often occurs in acidic environments.

2.6.1 Dezincification

There are essentially two forms of dezincification: uniform and localized. Uniform dezincification occurs when zinc is leached from a broad area of the brass surface, whereas, the localized form, also known as plug-type dezincification, penetrates deeply into the brass. In the localized form, the metal in the surrounding area is not significantly corroded by dezincification.

The widely accepted mechanism of dezincification involves the dissolution of brass where the zinc remains suspended in the corrosive solution while the copper is plated back on to the brass. Although dezincification can occur in the absence of oxygen, its presence accelerates the corrosion rate. Copper-zinc alloys with greater than 15% zinc are  susceptible  to  dezincification. Figure 14 shows a photograph of corrosion by dezincification.

Figure 14          Dezincification of Brass Containing a High Zinc Content

2.6.2 Susceptible Metals

Although brass with a relatively high zinc content is the most common alloy to experience the selective leaching form of corrosion, other metals and alloys, as shown in Table 14, are susceptible to this form of corrosion.

2.6.3 Managing Selective Leaching

Reducing the aggressive nature of the atmosphere by removing oxygen and avoiding stagnant solutions/debris buildup can prevent dezincification. Cathodic protection can also be used for prevention. However, the best alternative, economically, may be to use a more resistant material such as red brass, which only contains 15% Zn. Adding tin to brass also provides an improvement in the resistance to dezincification. Additionally, inhibiting elements, such as arsenic, antimony and phosphorous can be added in small amounts to the metal to provide further improvement. Avoiding the use of a copper metal containing a significant amount of zinc altogether may be necessary in systems exposed to severe dezincification environments.

Table 14           Combinations of Alloys and Environments Subject to Dealloying and Elements Preferentially Removed

2.7       Erosion Corrosion

Erosion corrosion is a form of attack resulting from the interaction of an electrolytic solution in motion relative to a metal surface. It has typically been thought of as involving small solid particles dispersed within a liquid stream. The fluid motion causes wear and abrasion, increasing rates of corrosion over uniform (non-motion) corrosion under the same conditions. Erosion corrosion is evident in pipelines, cooling systems, valves, boiler systems, propellers, impellers, as well as numerous other components. Specialized types of erosion corrosion occur as a result of impingement and cavitation. Impingement refers to a directional change of the solution whereby  a greater force is exhibited on a surface such as the outside curve of an elbow joint. Cavitation is the phenomenon of collapsing vapor bubbles which can cause surface damage if they repeatedly hit one particular location on a metal.

2.7.1 Factors Affecting Erosion Corrosion

All the factors that influence the resistance of material to erosion corrosion and their exact relationship are difficult to define. One property that factors in is hardness. In general harder materials resist erosion corrosion better, but there are some exceptions. Surface smoothness, fluid velocity, fluid density, angle of impact, and the general corrosion resistance of the material to the environment are other properties that factor in. Equation 9 predicts the erosion rate of metals using some of these factors. However, this prediction is only for erosion and does not include the added effects of corrosion. Erosion in a corrosive environment would be expected to occur at a higher rate.

2.7.2 Managing Erosion Corrosion

There are some design techniques that can be used to limit erosion corrosion as listed below with a couple methods depicted in Figure 15.

Avoid turbulent flow.

Add deflector plates where flow impinges on a wall. Add plates to protect welded areas from the fluid stream.

Put piping of concentrate additions vertically into the center of a vessel.

Figure 15 Techniques to Combat Erosion Corrosion

2.7       Stress-Corrosion Cracking

Stress corrosion is an environmentally induced cracking phenomenon that sometimes occurs when a metal is subjected to a tensile stress and a corrosive environment simultaneously. This is not to be confused with similar phenomena such as hydrogen embrittlement, in which the metal is embrittled by hydrogen, often resulting in the formation of cracks. Moreover, SCC is not defined as the cause of cracking that occurs when the surface of the metal is corroded resulting in the creation of a nucleating point for a crack.  Rather, it is a synergistic effort of a corrosive  agent and a modest, static stress. Another form of corrosion similar to SCC, although with a subtle difference, is corrosion fatigue, and is discussed in Section 2.9.1. The key difference is that SCC occurs with a static stress, while corrosion fatigue requires a dynamic or cyclic stress.

Stress corrosion cracking (SCC) is a process that takes place within the material, where the cracks propagate through the internal structure, usually leaving the surface unharmed. Furthermore, there are two main forms of SCC, intergranular and transgranular. For the intergranular form, the cracking progresses mostly along grain boundaries, whereas, in transgranular SCC, the cracking does not strictly adhere to the grain boundaries, instead it can penetrate grains. Most cracks tend to propagate in a direction that is perpendicular to the direction of applied stress. Aside from an applied mechanical stress, a residual, thermal, or welding stress along with the appropriate corrosive agent may also be sufficient to promote SCC. Pitting corrosion, especially in notch-sensitive metals, has been found to be one cause for the initiation of SCC.

SCC is a dangerous form of corrosion because it can be difficult to detect, and it can occur at stress levels which fall within the range that the metal is designed to handle. Furthermore, the mechanism of SCC is not well understood. There are a number of proposed mechanisms that attempt to explain  the  phenomenon  of  SCC,  but  none  have  done  so  with  complete  success. Figure 16 shows pictures of the two types of stress corrosion cracking.

Figure 16          Pictures of Stress Corrosion Cracking, (a) Intergranular, (b) Transgranular

2.8.1 Environmental Influence on SCC

Stress corrosion cracking is dependent on the environment based on a number of factors including temperature, solution, metallic structure and composition, and stress.¹² However, not all environments are equally potent to all metals; that is, specific metals are susceptible to specific chemical species, and some alloys are susceptible to SCC in one environment while others are more resistant.

Increasing the temperature of a system often works to accelerate the rate of SCC. The presence of chlorides or oxygen in the environment can also significantly influence the occurrence and rate of SCC. SCC is a concern in alloys that produce a surface film in certain environments,  since the film may protect the alloy from other forms of  corrosion,  but  not  SCC.  Some specific environments that can cause SCC of certain metals are listed in Table 15.

Table 15           Environments that May Cause Stress Corrosion of Metals

2.8.2 Managing Stress Corrosion Cracking

There are several methods that may be used to minimize the risk of SCC. Some of these methods include:

• Choose a material that is resistant to SCC.
• Employ  proper  design  features  for  the  anticipated  forms of corrosion. Corrosion pits may produce crack initiation sites.
• Minimize stresses including thermal stresses. Environment modifications (pH, oxygen content).
• Use surface treatments (shot peening, laser treatments) which increase the surface resistance to SCC.
• Any barrier coatings will deter SCC as long as it remains intact.
• Reduce exposure of end grains (i.e. end grains can act as initiation sites for cracking because of preferential corrosion and/or a local stress concentration).

Chapter 2.9: Other Forms of Corrosion – Corrosion

2.9       Other Forms of Corrosion

Not all types of corrosion can be easily classified as one of the eight major forms of corrosion described in the preceding sections.  Therefore, some of the less common or more unique forms  of corrosion are described in the following sections. These forms of corrosion may, in some instances, be considered as a subgroup of one of the eight major forms.

2.9.1 Corrosion Fatigue

Corrosion fatigue is a decrease in fatigue strength due to the effects of corrosion. Corrosion fatigue cracking differs from SCC and hydrogen induced cracking in that the applied stresses are cyclic rather than static. Fatigue cracking is often characterized by “beach marks” or striation patterns which are perpendicular to the crack propagation direction, as shown in Figure 17. Both the stress required for crack initiation and propagation can be lower in corrosive environments. Factors influencing corrosion fatigue include material strength, fracture toughness, and environmental conditions. There are two primary material properties used to assess fatigue, the number of cycles to failure for an applied stress level or the crack growth per cycle for a stress intensity factor.

Figure 17          Characteristic Fatigue Striation Pattern

2.9.1.1 Factors Affecting Corrosion Fatigue

The selection of materials for increased fracture toughness involves a trade-off with strength. Increased strength normally reduces fracture toughness and vice versa. One method to enhance fracture toughness while maintaining strength is reducing the metal’s average grain size. Additionally, highly polished surfaces resist crack initiation better as do lower temperatures. There are a couple of surface treatments that may be used to induce residual compressive stresses, thereby increasing fatigue strength. They include shot peening, laser shock peening,  and recently, low plasticity burnishing. The metals’ particular susceptibility to environmental conditions, as always, is a factor.

2.9.1.2 Stress-Life (S-N) Data

One type of reported fatigue data is stress-life or S-N curves, which plot the stress amplitude versus the number of cycles to failure. This follows the empirical relationship

Figure 18          S-N Data for 7075-T6 in Air and NaCl Solution

2.9.1.3 Fatigue Crack Growth Data

Information on fatigue can also be found in the form of crack growth plots. The relation in this case is

There are three types of fatigue crack growth behavior as  depicted in Figure 19.   Type A    exists for materials affected by the corrosive environment for crack initiation and crack growth. Type B behavior exists for materials where no environmental effect exists below the stress intensity  threshold fro SCC.   Type C is a combination of types A and B.   Aluminum alloys     in seawater follow type A behavior as can be seen in Figure 20.

Figure 19          Types of Fatigue Crack Growth Rates

2.9.1.4 Managing Corrosion Fatigue

Methods to deter corrosion fatigue include the following:

• Employ designs which minimize stresses to the components
• Choose heat treatments that reduce residual stresses
• Use surface treatments that enhance corrosion fatigue resistance such as shot peening or laser treatments
• Use barrier coatings or corrosion preventive compounds to block corrosive species from the metal.

Figure 20          Crack Growth rates for 7075-T6

2.9.2 Fretting Corrosion

Fretting corrosion occurs where two metals are in contact and there is a relatively small motion between the materials. It can be thought of as the combination of wear and a corrosive environment. This process usually presents itself in material interfaces not designed to be in motion with respect to each other. Typical applications that have produced fretting corrosion are motor shafts and electrical contacts. In the case of motor shafts, machinery vibration causes fretting and usually results in decreased fatigue life, known as fretting fatigue. Proper alignment of the rotating shafts is critical to reducing fretting fatigue failures. A second form of fretting corrosion appears in electrical contacts where thermal expansion and contraction cycles result in degradation of the contacting materials. Electrical contacts are most often coated with a noble metal, which are resistant to fretting corrosion. Cyclic motion, however, can cause wear and failure of the coating leaving the base metal vulnerable to fretting corrosion and other forms of attack. Once the base metal is exposed, the formation of highly resistive oxides occurs resulting in intermittent or open electrical circuits. Fretting corrosion is often undetected due to the nature of its existence in hidden material interfaces. The best way to mitigate fretting corrosion is to be knowledgeable of the typical material combinations and applications where it occurs and the methods used to combat it. Factors contributing to fretting corrosion include contact conditions, environmental conditions, and materials properties.¹⁸  These  factors  all  interact  to  produce  fretting  corrosion  or  fretting fatigue, as represented in Figure 21.

Figure 21          Contributing Factors to Fretting Corrosion

2.9.2.1 Metals’ Susceptibilities to Fretting Corrosion

The susceptibilities of some material combinations to fretting are listed in Table 16.

2.9.2.2 Managing Fretting Corrosion

Methods used to reduce fretting corrosion include the following:

• Soft metal against hard metal contacts
• Roughen surface to reduce slippage
• Increase load to reduce relative motion
• Low viscosity fluids in combination with phosphate treated surfaces
• Increase surface hardness of contacting metals
• Use one metal with a low coefficient of friction
• Use corrosion preventive compounds on electrical contacts

Table 16           Resistance to Fretting Corrosion of Various Material Couples under Dry Conditions

2.9.3 Hydrogen Damage

There are a number of different forms of hydrogen damage to metallic materials, resulting from the combined factors of hydrogen and residual or tensile stresses. Hydrogen damage can result  in cracking, embrittlement, loss of ductility, blistering and flaking, and also microperforation.

2.9.3.1Hydrogen Induced Cracking

Hydrogen induced cracking (HIC) refers to the cracking of a ductile alloy when under constant stress and where hydrogen gas is present. Hydrogen is absorbed into areas of high triaxial stress producing the observed damage.

2.9.3.2 Hydrogen Embrittlement

Hydrogen embrittlement is the brittle fracture of a ductile alloy during plastic deformation in a hydrogen gas containing environment.

2.9.3.3 Loss of Tensile Ductility

The loss of tensile ductility occurs with metals exposed to hydrogen which results in a significant reduction in elongation and reduction in area. It is most often observed in low strength alloys  and has been witnessed in steels, stainless steels, aluminum alloys, nickel alloys, and titanium alloys.

2.9.3.4 High Temperature Hydrogen Attack

High pressure hydrogen will attack carbon and low-alloy steels at high temperatures. The hydrogen will diffuse into the metal and react with carbon resulting in the formation of methane. This in turn results in decarburization of the alloy and possibly cracks formation.

2.9.3.5 Blistering

Blistering occurs primarily in low strength metals. It is a result of atomic hydrogen diffusion  into defect areas of the alloy. The monotonic atoms combine into gas molecules in voids within the metal. Then, the high pressure of H₂ entrapped within the metal causes the material to blister or rupture. This form of attack has been observed in low strength steels exposed to H₂S or when cleaned in pickling baths.

2.9.3.6 Shatter Cracks, Flaking, and Fish Eyes

These forms of hydrogen damage are similar to blistering and are seen primarily during processing. Hydrogen is more soluble at the melting temperatures of metals allowing it to enter defect areas. The decreased solubility of hydrogen when cooled then produces the damage features.

2.9.3.7 Microperforation

Microperforation has been seen in steels in a high pressure hydrogen and room temperature environment. The hydrogen produces fissures in steel alloys such that gases and liquids can permeate the material.

2.9.3.8 Degradation in Flow Properties

An increase in creep rates occurs in iron alloys and steels under ambient conditions in hydrogen environments, and in several alloys at elevated temperatures.

2.9.3.9 Hydride Formation

The precipitation of metal hydride phases in magnesium, tantalum, niobium, vanadium, uranium, zirconium, titanium, and their alloys, in the presence of hydrogen produces a degradation of mechanical properties and cracking.

2.9.3.10 Metals’ Susceptible to Hydrogen Damage

Table 17 lists susceptible metals to the various types of hydrogen attack..

Table 17           Metals’ Susceptibilities to Hydrogen Damage

2.9.3.11 Managing Hydrogen Damage Methods to deter hydrogen damage are to:

• Limit hydrogen introduced into the metal during
• Limit hydrogen in the operating
• Structural designs to reduce stresses (below threshold for subcritical crack growth in a given environment)
• Use barrier coatings
• Use low hydrogen welding rods

2.9.4 High Temperature Corrosion

High temperature corrosion is an attack on a metal at elevated temperatures in a gaseous environment rather than in a liquid. The most prominent high temperature corrosion reaction is oxidation, although sulfidation and carburization may also occur. Most metals exposed to a high temperature oxidative environment will produce an oxide scale layer which protects the metal from further corrosion. It uniformly covers the entire surface.  Ionic transport through the scale  is the rate controlling process. The corrosion rate will normally decrease after the scale is produced following a parabolic relationship with time. In severe corrosive environments where a protective scale cannot form, the corrosion rate will follow a more linear path.

Sulfidation occurs when the concentration of sulfur gas is high enough such a sulfide layer forms. Sulfides are less stable and grow much faster than oxides.  As a result, sulfides react  more readily with metals and penetrate deeper into the metal. They are replaced with the more stable oxides as reactions continue to occur. It is preferred in such environments to have a protective oxide scale first produced, which then protects the metal against subsequent sulfidation.

Hot corrosion is a term describing the high temperature attack of gas turbine engine components in the path of hot gases. It is a sulfidation process involving the formation of condensed salts containing sodium sulfate and/or potassium sulfate. Increasing the chromium content in the metal alloys improves the corrosion resistance but also results in decreased strength.

Carburization is a rare form of high temperature corrosion where carbon atoms are absorbed into a metals’ surface. It only occurs in environments with a very low oxygen partial pressure. Austenitic stainless steels are susceptible under such conditions due to the high solubility of carbon in austenite. Alloying studies to reduce carburization have shown that silicon, niobium, tungsten, titanium, and the rare earth metals increase resistance. Elements which increase damage include lead, molybdenum, boron, cobalt, and zirconium.

2.9.4.1 Metals’ Susceptible to High Temperature Corrosion

Although high temperature corrosion testing, especially on superalloy materials for gas turbine applications, has been conducted, no qualitative relationship has been determined. Materials are selected for corrosion resistance dependent upon their comparative rates of attack from tests and from field experience.

2.9.4.2 Managing High Temperature Corrosion

Methods to reduce high temperature corrosion include:

• Proper metal
• Change in operating
• Structural designs to limit
• High temperature barrier coatings (ceramics)

2.9.5 Exfoliation

Exfoliation corrosion is considered a form of intergranular corrosion that attacks metals which have been mechanically deformed, primarily by extrusion or rolling, producing elongated grains directionally aligned. Most often, the attack is initiated at exposed endgrains, as has been the case with aircraft skins around fasteners see Figure 22.  This form of corrosion is most evident  in some of the aluminum alloys  and  is  shown  in  Figure  23.  Metals  susceptible  to  exfoliation corrosion are aggressively attacked in environments corrosive to that  particular metal.   As an example, AA 2024-T4 is known to perform well in urban type environments,     but is severely attacked in marine environments.

Figure 22          Exfoliation Corrosion Initiated at Endgrains

Figure 23          Exfoliation of an Aluminum Alloy in a Marine Environment

2.9.5.1 Managing Exfoliation Corrosion

As with intergranular corrosion, the proper selection of alloy and heat treatment to avoid precipitation at grain boundaries is the primary method to combat exfoliation. Reducing the area of endgrain surfaces will limit the initiation of attack, as well as the use of barrier coatings.

2.9.6 Microbiological Corrosion

MIC is actually not a form of corrosion, but rather is a process that can influence and even initiate corrosion. It can accelerate most forms of corrosion; including uniform corrosion, pitting corrosion, crevice corrosion, galvanic corrosion, intergranular corrosion, dealloying, and stress corrosion cracking. In fact, if unfamiliar with MIC, some corrosion problems may be misdiagnosed as conventional chloride induced corrosion. One prominent indicator of MIC is a higher rate of attack than one would normally expect. MIC can affect numerous systems, and  can be found virtually anyplace where aqueous environments exist. It is not exclusive to water-based systems, occurring in fuel and lubrication systems as well. Table 18 lists applications where MIC has been found to be prominent while Figure 24 shows one such location.

Table 18           Systems with Persistent MIC Problems

Figure 24          Interior of a Ballast Tank on a Navy Ship

2.9.6.1 Types of Microorganisms

The types of microorganisms with species attributable to MIC include algae, fungi, and bacteria.²³ Algae produce oxygen in the presence of light (photosynthesis) and consume oxygen in darkness. They can be found in most any aquatic environment ranging from freshwater to concentrated salt water. The availability of oxygen has been found to be a major factor in corrosion of metals in saltwater environments. Algae flourish in temperatures of 32 – 104ºF and pH levels of 5.5 – 9.0. Fungi consist of mycelium structures which are an outgrowth of a single cell or spore. Mycelia are immobile, but can grow to reach macroscopic dimensions. Fungi are most often found in soils, although some species are capable of living in water environments. They metabolize organic matter, producing organic acids.

Bacteria are generally classified by their affinity to oxygen. Aerobic species require oxygen to metabolize while anaerobic species require a lack of oxygen to do the same. Facultative bacteria can grow in either environment, although they prefer aerobic conditions. Microaerophilic bacteria require low concentrations of oxygen. Oddly enough, aerobic and anaerobic organisms have often been found to co-exist in the same location. This is because aerobic species deplete the immediate surroundings of oxygen creating an ideal environment for anaerobes. Bacteria are further  classified  by  shape   into   spherical   (bacillus),   rod   (coccus),   comma   (vibrio),   and filamentous (myces) species. Figure  25 is an example of rod-shaped bacteria observed  using transmission electron microscopy.

Figure 25          Rod-Shaped Pseudomonas Bacteria

Microorganisms in the planktonic state refer to those organisms floating freely in the aqueous environment or in air. They can resist harsh environments including acids, alcohols, and disinfectants, drying, freezing, and boiling.²⁵ Some spores have the ability to last hundreds of years and then germinate once favorable conditions exist. Microorganisms in the sessile state are those that have attached themselves to a surface and have developed a protective membrane, collectively called a biofilm. Microorganisms have the ability to reproduce quickly; some doubling in as little as 18 minutes. When left untreated, they can rapidly colonize in stagnant aqueous environments, potentially introducing a highly active corrosion cell.

2.9.6.2 Microorganisms that Accelerate Corrosion

Once a microorganism forms a biofilm on a material’s surface, a microenvironment is created that is dramatically different from the bulk surroundings. Changes in pH, dissolved oxygen, and organic and inorganic compounds in the microenvironment can lead to electrochemical reactions which increase corrosion rates. Microorganisms may also produce hydrogen which can promote hydrogen damage in metals. Most microorganisms form an extracellular membrane which protects the organism from toxic chemicals and allows nutrients to filter through.²⁵ Biofilms are resistant to many chemicals by virtue of their protective membrane and ability to breakdown numerous compounds. They are significantly more resistant to biocides (chemicals used to kill microorganisms) than planktonic organisms. Some bacteria even metabolize corrosion  inhibitors, such as aliphatic amines and nitrites, decreasing the inhibitor’s ability to control corrosion. Microorganisms’ metabolic reactions attributable to metallic corrosion  involve  sulfide production, acid production, ammonia production, metal deposition, and metal oxidation and reduction. Several groups of microorganisms have been attributed to MIC, and are listed below.²⁶ Following these recognized forms; Table 19 lists some specific microorganisms within these categories, along with their characteristics.

Sulfate Reducing Bacteria (SRB)

Sulfate reducing bacteria are anaerobic microorganisms that have been found to be involved with numerous MIC problems affecting a variety of systems and alloys. They can survive in an aerobic environment for a period of time until finding a compatible environment. SRB chemically reduce sulfates to sulfides, producing compounds such as hydrogen sulfide (H₂S), or iron sulfide (Fe₂S) in the case of ferrous metals. The most common strains exist in the temperature range of 25 – 35ºC, although there are some that can function well at temperatures of 60ºC. They can be detected through the presence of black precipitates in the liquid media or deposited on surfaces, as well as a characteristic hydrogen sulfide smell.

Sulfur/Sulfide Oxidizing Bacteria (SOB)

Sulfide oxidizing bacteria are an aerobic species which oxidize sulfide or elemental sulfur into sulfates. Some species oxidize sulfur into sulfuric acid leading to a highly acidic micro-environment. The high acidity has been associated with the degradation of coating materials in a number of applications. They are primarily found in mineral deposits and are common in wastewater systems. SRB is often found in conjunction with SOB.

Iron/Manganese Oxidizing Bacteria

Iron and manganese oxidizing bacteria have been found in conjunction with MIC, and are typically located over corrosion pits on steels.   Some species are known to accumulate iron       or  manganese  compounds  resulting  from  the  oxidation  process.  The  higher  concentration of manganese in biofilms  has  been  attributed  to  corrosion  of  ferrous  alloys  including  pitting of stainless steels in treated water systems. Iron tubercles have also been observed to  form as a result of the oxidation process (Figure 26).

Table 19           Common Microorganisms Found in Conjunction with MIC

Figure 26          Tubercles as a Result of MIC (courtesy of Metallurgical Technologies, Inc.)

Slime Forming Bacteria

Slime forming bacteria are aerobic organisms which develop polysaccharide “slime” on the exterior of cells. The slime controls permeation of nutrients to the cells and may breakdown various substances, including biocides. Slime formers have been responsible for the decreased performance of heat exchangers as well as clogging of fuel lines and filters. They can prevent oxygen from reaching the underlying metal surface, creating an environment suitable for anaerobic organisms.

Organic Acid Producing Bacteria

Some anaerobic organisms also produce organic acids. These bacteria are more apt to be found in closed systems including gas transmission lines and sometimes closed water systems.

Acid Producing Fungi

Some fungi produce organic acids which attack iron and aluminum alloys. Like slime formers, they can create environments suitable for anaerobic species. These organisms have been attributed to the widespread corrosion problems observed in aluminum fuel tanks in aircraft.

2.9.6.3 Metals Affected by MIC

Since MIC is a mechanism that accelerates corrosion, it should be expected to occur more often in metal alloys with susceptibilities to the various forms of corrosion, and in environments conducive to biological activity. Metals used in the applications listed in Table 1, and thus exposed to microbial activity and the potential of MIC, include mild steels, stainless steels, copper alloys, nickel alloys and titanium alloys. In general, mild steels can exhibit everything from uniform corrosion to environmentally assisted cracking, while the remaining alloys usually only show localized forms. Mild steels, stainless steels, aluminum, copper, and nickel alloys all have shown effects of MIC, while titanium alloys have been found to be virtually resistant to MIC under ambient conditions.

Mild Steels

MIC problems have been widely documented in piping systems, storage tanks, cooling towers, and aquatic structures. Mild steels are widely used in these applications due to their low cost,  but are one of the most readily corroded metals. Mild steels are normally coated for corrosion protection, while cathodic protection may also be used for select applications. Galvanization (zinc coating) is commonly used to protect steel in atmospheric environments. Bituminous coal tar and asphalt dip coatings are often used on the exterior of buried pipelines and tanks, while polymeric coatings are used for atmospheric and water environments. However, biofilms tend to form at flaws in the coating surfaces. Furthermore, acid producing microorganisms have been found to dissolve zinc and some polymeric coatings. Numerous cases have also been documented where microorganisms caused debonding of coatings from the underlying metal. Underneath the delaminated coating in turn, creates an ideal environment for further microbial growth.

Poor quality water systems and components with areas that accumulate stagnant water/debris are prone to MIC. In some extreme cases, untreated water left stagnant within mild steel piping has caused uniform corrosion throughout the low lying areas. This has been seen to occur in underground pipes that have been left unused for periods of time. Many power plant piping failures have been found to be the result of introducing untreated water into a system. SRB has been the primary culprit in such cases. A change to a more corrosion resistant material is not always the answer when it comes to solving MIC problems. For example, an upgrade from carbon steel to stainless steel in a nuclear power plant caused a change in MIC problems that in some instances were even more severe. SRB has also been found in conjunction with underdeposit corrosion occurring in cooling towers. Wet soils containing clay have played a major role in the occurrence of underground MIC problems. Under such conditions, the exterior of underground piping and storage tanks have experienced coating delamination and corrosion as a result of biofilm growth.

Stainless Steels

Stainless steels have suffered MIC problems in the same type of scenarios as mild steels – primarily in locations where water accumulates. There are two notable problems that have surfaced with stainless steel MIC. One is an accelerated corrosion rate, primarily through pitting or crevice corrosion that occurs in low lying areas, joints, and corner locations. Stainless steel tanks and piping systems are sometimes hydrotested subsequent to manufacture and prior to field use. Several cases of severe MIC have been documented whereby hydrotesting using well water was performed, and the product was then stored for a period of time before being placed into service. The tanks and piping were not adequately dried, nor was a biocide used to deter biofilm growth. In one particular case, a 304 stainless steel pipeline for freshwater service, failed 15 months after being hydrotested.³⁰ A second MIC problem discovered with stainless steels occurs adjacent to weldments. Microorganisms readily attack areas around welds due to the inhomogeneous nature of the region. In one case, perforation occurred in a 0.2 inch diameter 316L stainless steel pipe adjacent to a welded seam after four months in service under intermittent flow conditions.³¹ Stainless steels containing 6% molybdenum or greater, have been found to be virtually resistant to MIC.

Aluminum Alloys

The major applications where MIC has attacked aluminum alloys have been in fuel storage tanks and aircraft fuel tanks. MIC problems exist in the low-lying areas of tanks and at water–fuel interfaces. Contaminants in fuels, such as surfactants and water soluble salts, have largely contributed to the formation of biofilms in these systems. Fungi and bacteria have been found to be the main culprits. Cladosporium resinae, a fungus, has widely been attributed to corrosion of aircraft fuel tanks. Its presence decreases the pH to around 3-4, which can attack the protective coatings and underlying metal. Pseudomonas aeruginosa and Candida species are also likely to be found in conjunction with MIC of aluminum fuel tanks.

Additionally, heavy fungal growth on interior surfaces of helicopters has occurred subsequent to depot maintenance and prior to returned field use.³² Fungal growth had been reported in passenger areas of the H-53 helicopter and was therefore slated for cleaning during refurbishment. Fungi could be found on virtually all interior surfaces of the helicopter. The surfaces were cleaned with 100% isopropanol, treated with a biocide, followed by application of a corrosion preventive compound. The procedure removed most of the microorganisms present and was effective at killing spores. However, some biofilms remained, which rapidly reproduced before the aircraft was even returned to service.

Copper Alloys

Copper alloys find use in seawater piping systems and heat exchangers, which are susceptible to MIC. Microbial products that can be harmful to copper alloys include CO₂, H₂S, NH₃, organic and inorganic acids, and sulfides. MIC observed in copper alloys includes pitting corrosion, dealloying and stress-corrosion cracking. Higher alloying content in copper usually results in a lower corrosion resistance. Although MIC has been found in both, more problems have been documented with 70/30 than with 90/10 Cu/Ni alloys. MIC has also been documented in Admiralty brass (Cu-30Zn-1Sn), aluminum brass (Cu-20Zn-2Al), and aluminum bronze (Cu- 7Al-2.5Fe). Ammonia and sulfides have gained considerable attention as compounds that are corrosive to copper alloys. Admiralty brass tubes have been found to suffer stress-corrosion cracking in the presence of ammonia. Seawater that is high in sulfide content, has caused pitting and stress-corrosion cracking in copper alloys. SRB has also been known to attack copper alloys causing dealloying of nickel or zinc in some cases.

Nickel Alloys

Nickel alloys are used in high velocity water environments including evaporators, heat exchangers, pumps, valves, and turbines blades, as they generally have a higher resistance to erosive wear than copper alloys.  However, some nickel alloys are susceptible to pitting and crevice attack under stagnant water conditions, so that downtime and unused periods can lead to potential MIC problems. Monel 400 (66.5Ni-31.5Cu-1.25Fe) has been found to be susceptible to underdeposit MIC. Pitting corrosion, intergranular corrosion, and dealloying of nickel have all been observed with this alloy in the presence of SRB. Ni-Cr alloys have been found to be virtually resistant to MIC.

2.9.6.4 Monitoring/Detection Methods

Early detection of potential MIC is crucial to the prevention of equipment failure and extensive maintenance. The most common detection methods involve sampling bulk liquids from within the system and monitoring physical, chemical, and biological characteristics. The goal is to identify favorable conditions for biofilm formation and growth, so that the internal environment may be adjusted as appropriate. Visual inspections of accessible areas should also be performed on a routine basis. Additional methods that may be utilized include coupon monitoring, electrochemical sensor and biosensor techniques. Monitoring equipment is available for measuring a number of properties of the bulk system. A common practice has been to monitor temperature, pH, conductivity, and total dissolved solids directly from the operating system, while taking samples for portable or laboratory testing methods to evaluate dissolved gases, bacteria counts, and for bacteria identification.²¹ Bacteria counting, via cultured growth, may be helpful, but strict conditions must be followed to produce meaningful results. The most important factor in bacterial counts is observing changes in trends rather than in actual numbers. Consistency is crucial where deviations in sample location, temperature, growing media, growth time, and even changes in technicians can affect results. A strict schedule must also be maintained. Changes in bacteria counts are used to adjust biocide usage and may also be indicative of biofilm growth in the case of differences in counts across     a system. Bacteria cultures can  also  be  used  to  identify  specific  species  present  (Figure  27). Direct bacteria counts can be performed using a microscope to inspect bacteria which have been placed onto a slide and may also  be  stained  for  viewing,  as  shown  in  Figure  28.  Visual  inspections should be performed on exposed surfaces where algae and fungal growth   can occur, and on surfaces exposed during maintenance procedures.  The presence of SRB can  be detected by observing black particles in the liquid media and/or deposited on surfaces, a  result of iron sulfide and/or copper sulfide formation, or a distinct hydrogen sulfide odor.³³ Fluorescent dyes can be used to enhance visual  detection,  as  biofilms  absorb  some  of  the  dye whereby an ultraviolet light is then used to expose the microorganisms.

Figure 27          Bacteria Culture

Figure 28          Inspection of Bacteria on a Stained Microscope Slide

Coupons have been found quite useful in detecting MIC, especially when used in conjunction with additional monitoring techniques. Coupons are small metal samples placed within the system and periodically extracted to measure corrosion rates through weight loss and possibly to collect microorganisms from biofilms on the coupon for identification. Proper placement of the coupons within the system plays a key role in MIC monitoring and detection. Coupons should  be placed in locations where MIC is likely to occur. Electrochemical sensing techniques, such as electrical impedance spectroscopy and electrochemical noise, are other means of detecting MIC. Electrochemical sensors detect characteristics of corrosion reactions, such as changes in electrical conductivity. As with coupons, strategic placement of the sensors in the systems is crucial to detecting MIC.

One type of sensor designed specifically for biofilm detection uses a probe that attracts microbial growth.³⁶ Utilizing experience of the electrochemical conditions under which biofilms occur, probes have been developed that replicate these preferred conditions. The sensor then alerts operators when biofilm activity is present. Sensors should ideally be placed in areas where biofilm growth is more likely. Another method that may be used specifically to detect microorganisms in water systems is the use of fluorogenic bioreporters.³⁷ These are compounds (dyes) that change their fluorescence upon interaction with microorganisms. Activity is determined by the ratio of fluorescence of the reacted dye, extracted from the system or measured in-service, to the unreacted dye. The ratio increases with biological activity and can be used to effectively regulate the use of biocides. This method however, does not distinguish between planktonic and sessile organisms. Thus, problems could be growing in the system without being detected.

2.9.6.5 Mitigation Methods

Clearly, the best way to prevent MIC is to prevent the growth of biofilms altogether. Once a biofilm has formed, it is more resistant to biocides, and can rapidly grow if not completely removed. The emphasis is placed on cleanliness and incorporating established corrosion prevention and control techniques for the various metal alloys and forms of corrosion. Monitoring and detection of microorganisms will effectively guide preventive maintenance procedures.

Cleanliness of systems involves monitoring the quality of water, fuel, or lubricants present in the system. This includes water content in fuel and lubrication systems. Water should be monitored and removed when the content becomes too high. All fluids should be monitored for solid particles and filtered to prevent particle contamination. Contaminants increase the likelihood of biofilms through their use as nutrients.  Bacterial counts and biosensing help to adjust the level  of biocides introduced to the system. Biocides are widely used and are effective at killing planktonic microorganisms. The cost of biocides is significant however, and along with their toxicity, effective management of biocide use can reduce costs and damaging effects on the environment. Cleanliness also includes scheduled cleaning of exterior components where any debris accumulation has occurred. Non-abrasive cleaning methods are preferred as to  not damage coatings. Inspection/cleaning should also be performed on normally inaccessible components that are exposed during maintenance/repair procedures. Designing systems that minimize MIC prone areas and providing accessibility for maintenance as appropriate helps to promote system cleanliness. This involves eliminating stagnant and low-flow areas, minimizing crevices and welds, incorporating filtration, drains, and access ports for treatments, monitoring/sampling, and cleaning.

Established corrosion prevention and control methods that are employed to protect metals from the various forms of corrosion will also help mitigate MIC. This includes designing systems to minimize stagnant water conditions, proper base material and coating selection, possible  cathodic protection, sealing crevices and around fasteners, using gaskets to minimize galvanic corrosion, proper heat treatments, and post weld treatments. For underground structures, providing ample drainage by backfilling with gravel or sand will help prevent MIC. In some cases, a change to an alternate material such as PVC piping has greatly reduced underground pipeline corrosion problems.  Coatings can be formulated with biocides, though such coatings  are not generally used on the interior of systems. Smooth surface finishes with minimized  defects are preferred. Research into alternative coatings that may deter MIC has shown polydimethylsiloxane coated 4340 steel to have favorable results.³⁸ In laboratory tests, the silicone compounds significantly reduced MIC of the steel in a 0.6M NaCl solution over a two year period.

2.9.7 Liquid and Solid Metal Embrittlement

Liquid metal embrittlement (LME) is a brittle fracture of a normally ductile metal when in contact with a liquid metal and stressed in tension. There is no change in the yield behavior of the metal; however, fracture can occur well below the metal’s yield strength. The stress required for crack propagation is lower than that for crack initiation. As a result, the crack initiation and propagation are seen instantaneously with a complete fracture of the metal. Fracture surfaces are usually completely covered with the liquid metal. The movement of the liquid metal into the crack is attributed to the rapid crack propagation through the material. In some cases, solid metals at temperatures in the vicinity of their melting points have also been shown to cause embrittlement. This phenomenon has  been  termed  solid  metal  induced  embrittlement (SMIE). Table 20 lists LME interactions of various metals.

Table 20           Liquid Metal Embrittlement Observed in Various Metals

Liquid metal embrittlement has been seen in processing environments and a handful of operational applications. Plating some metals, such as cadmium plated titanium or steel can produce embrittlement. Zircalloy tubes in nuclear reactors have been known to be embrittled by liquid and solid cadmium. Lithium causes LME in lithium metal cooled reactors.

2.9.8 Molten Salt Corrosion

Molten salt corrosion is the degradation of metal containers by molten or fused salts. There are two general mechanisms attributed to this form of corrosion. The most common is the oxidation of the metal much like in aqueous environments. The second is the dissolution of the metal.  To  a lesser extent, all forms of aqueous corrosion have been observed in fused salts.

2.9.8.1 Effects of the More Common Molten Salts on Metals

These effects are summarized in Table

2.9.8.2 Managing Molten Salt Corrosion

Methods to reduce molten salt corrosion include:

• Use materials that form a passive nonsoluble films
• Minimize the entry of oxidizing species
• Lower the temperatures

Table 21           Effects of the More Common Molten Salts on Metals

2.9.9 Filiform Corrosion

Filiform corrosion is an attack of a metal substrate material underneath a polymeric film. The corrosion initiation is generally due to a defect in the coating. Corrosive elements to the metal substrate deposit in the defect area causing corrosion of the metal as well as bulging and cracking of the coating. The corrosion tends  to  spread  one-dimensionally  in  a  random  manner  creating patterns resembling a worm path or tentacles emanating form a point, see Figure 29. There is an associated “head” where the corrosion is preceding and the “tail” where the  corrosion originated. Filiform corrosion has been observed on steel cans, aluminum foils and painted aluminum alloys, as well as other lacquered metals. It normally occurs  in  high  humidity (> 65% RH) although it may result in lower humidity environments of severe corrosivity. The width  of  the  corrosion  paths are in the range of 0.05 – 3 mm depending on  the coating material and the corrosivity of the environment.

Figure 29          Filiform Corrosion

2.9.9.1 Managing Filiform Corrosion

Methods that reduce filiform corrosion include the following:

• Use less active metal substrates
• Reduce humidity
• Use zinc primers on steel
• Use multiple coat/paint systems

Other corrosion protection methods can be used, and are discussed in terms of combating corrosion in general in Section 3.0.

2.9.10 Stray-Current Corrosion

Stray-current corrosion is an attack of a metal due to the formation of an electric current through that metal which is unintended. This corrosion form is independent of environmental conditions. A direct current is more damaging than alternating currents. In alternating currents, damage will decrease as frequency increases. A major source of stray-currents is underground power lines. Damage to active-passive metals such as aluminum alloys and stainless steels is greater than that for active metals.

2.9.10.1 Managing Stray-Current Corrosion

The best way to combat stray-current corrosion is to prevent the current using insulation techniques. Coatings will not protect the metal and may even accelerate the attack if a flaw in  the coating exists. If the current cannot be prevented, methods to deter corrosion include the following:

• Grounding the Stray Current
• Sacrificial Anodes
• Insulation

2.9.11 Grooving Corrosion in Carbon Steel

Grooving corrosion in carbon steels is a specialized form of corrosion that exists for electric resistance welded piping subsequently exposed to aggressive waters.⁴⁰ This welding process produces a redistribution of sulfides along the weld line. The result is a preferred attack in the weld area producing grooves in the material. Post weld heat treatments appear to influence grooving corrosion with temperatures of around 750ºC producing a higher susceptibility and higher temperatures, on the order of 1000ºC, decreasing susceptibility.

Chapter 3: Corrosion Characteristics and Properties of Metals – Corrosion

As discussed in the previous sections, the extent and form of corrosion occurring on a metal is predominantly dictated by the environmental conditions, and thus, the interaction of the metal with the surrounding environment. Some metals are inherently resistant to the effects of corrosion, while others are inherently susceptible. This section discusses the nature of the more common types of metals and alloys in terms of their corrosion characteristics and properties.  This is not a comprehensive evaluation of metals and alloys, however; these are general observations, and are not intended to provide complete guidance in materials selection. Instead, an investigation of literature should precede the selection of a material.

The corrosion characteristics and properties of metals come from field experience and extensive testing in natural, simulated, and accelerated environments. The results of testing are used to rate materials and determine what alloying and heat treatments are beneficial to corrosion resistance. Field experiences, as well as test results, are used to document susceptibilities of materials under specific conditions. The following section uses information from both testing and field experience to facilitate the selection of metals for varying applications. The section is organized by the relative usage of the metal classes with the most widely used ferrous metals covered first.

3.0       Steels

Steels can be largely grouped into three categories with respect to corrosion resistance.⁹ Carbon steels contain up to approximately 2% total alloying content with the primary additions of carbon, manganese, phosphorus and sulfur. The second group is the low alloy steels (or sometimes referred to as mild alloy steels) containing roughly 2 – 11% total alloying content. Corrosion resistance can be enhanced over carbon steels with additions of copper, nickel, chromium, silicon, and phosphorus. High corrosion resistant steels (stainless steels) can only be obtained with ³ 11% Cr along with varying amounts of other elements.

3.1.1 Alloying for Corrosion Resistance

The primary alloying elements to increase corrosion resistance of steels are copper, chromium, silicon, phosphorus, and nickel. Broad categories of steel materials based upon alloying content include the low alloy steels, weathering steels, and stainless steels.

3.1.1.1 Carbon and Low Alloy Steels

For carbon steels, copper additions of 0.01 to 0.05% have the greatest effect for increasing  general corrosion resistance, as seen in Figure 30. The relationship of the other elements on corrosion  resistance  is  displayed  in  Figure  31.    Small additions of chromium significantly increase tensile strength as well as increasing corrosion resistance leading to the high strength  low alloy (HSLA) steels. Weathering steels is a term describing low-alloy steels with small additions of chromium, nickel, and copper. They can provide good service without any coatings in   a   non-marine atmospheric  environment. Many inland bridge structures make use of weathering  steels.  Larger additions of chromium are required for a dramatic increase in corrosion resistance as previously mentioned.

Figure 30          Effects of Copper Addition on the Uniform Atmospheric Corrosion of Steel

Figure 31          Effects of Alloying Elements on the Uniform Industrial Atmospheric Corrosion of Steel.

3.1.1.2 Stainless Steels

Stainless steels contain 11 percent or more of chromium. The higher chromium content results  in the formation of a chromium oxide protective film, greatly increasing the oxidation resistance of the steel. Stainless steels are most often exposed to a passivating solution to improve formation of the protective film.⁶⁶ Corrosion resistance will generally increase with an increase in chromium content and decrease with an increase in carbon content. Stainless steels are excellent for oxidizing environments but are susceptible in halogen acids or halogen salt solutions. They are also susceptible to pitting in seawater.

3.1.1.3 Austenitic Stainless Steels

Austenitic stainless steels are the most commonly used class of stainless steels. They may be used in mild to severe corrosive environments, dependent upon alloying and are nonmagnetic compared with other steels.⁶⁶ They may be utilized in environments with temperatures reaching 600ºC and for low temperatures in the cryogenic range. Almost all austenitic stainless steels are modifications from the 18Cr – 8Ni (304) alloy. Difficulty in processing stainless steels limits increasing concentrations of chromium. The addition of nitrogen has been found to be an austenite phase stabilizer which allows higher additions of molybdenum, up to about 6%, increasing the material’s corrosion resistance in chloride environments. Other additions which improve  corrosion  resistance  to  specified   environments   include   high   chromium   alloys for high  temperature  service  and  high  nickel  alloys  for  inorganic  acids.  Table  22 represents  a summary of the austenitic stainless steel alloys and their modifications in regards   to corrosion resistance.

Table 22 Austenitic Stainless Steel Alloys

3.1.1.4 Ferritic Stainless Steels

Ferritic stainless steels generally do not match the corrosion resistance of the austenitic grades. They exhibit relatively high yield strength and a low ductility and are magnetic. The ferrites  have a low solubility for some elements such as carbon and nitrogen. The ferritic stainless steels will transition from ductile to brittle over a small temperature range, occurring above ambient temperature for increasing carbon and nitrogen content and more so with increased chromium content. Ferritic alloys have been developed using an argon-oxygen decarburization (AOD) process, significantly reducing carbon and nitrogen levels. Also, reactive elements such as titanium and niobium may be added to precipitate some of the carbon and nitrogen. Ferritic stainless steel alloys containing carbon and nitrogen are susceptible to intergranular corrosion by heat sensitization through heat treating, welding, or other thermal exposure. Newer alloys, such as 444, have lower carbon and nitrogen content, using AOD, allowing higher chromium and molybdenum content resulting in an alloy more amenable to welding and somewhat tougher, although still limited by a lack of toughness. Ferritic stainless steels do offer use in thermal transfer applications as a result of  their  high  resistance  to  SCC  in  chloride  environments. The 409 alloy was specifically developed for use in automotive exhaust components. Table 23 lists the ferritic alloys with their corrosion characteristics.

Table 23           Ferritic Stainless Steel Alloys

3.1.1.5 Martensitic Stainless Steels

Martensitic stainless steels have a much lower corrosion resistance than austenitic grades, and usually slightly lower than the ferritic grades. The martensitic stainless steels contain lower Cr and higher C concentrations compared with the other stainless steels. This structure results in a strong but brittle class of materials. They may be tempered to improve toughness, but to limited degree. Additions of nitrogen, nickel, and molybdenum at lower levels of carbon have been found to produce alloys with better toughness and corrosion resistance properties. The corrosion characteristics of the martensitic grades are summarized in Table 24.

Table 24           Martensitic Stainless Steel Alloys

3.1.1.6 Precipitation Hardening Stainless Steels

Precipitation-hardening (PH) stainless steels are Cr-Ni alloys that are hardened at moderately high temperatures, by adding elements such as copper and/or aluminum which form intermetallic precipitates. PH stainless steels may have austenitic, semi-austenitic, or martensitic structures. They must not be further exposed to elevated temperatures, once hardened, as the precipitates will  be  altered,  degrading  the  material’s  strength.  This  includes  welding   and environmental exposures. The corrosion aspects of the PH stainless steels are presented in Table 25.

Table 25           Precipitation-Hardened Stainless Steel Alloys

3.1.1.7 Duplex Stainless Steels

Duplex stainless steels are two phase materials containing roughly equal amounts of ferrite and austenite phases developed specifically as a high corrosion resistant material. They contain high levels of chromium (20 – 30%), Ni (5 – 10%), and low carbon content (< 0.03%). They may additionally contain molybdenum, nitrogen, tungsten, and copper as modifiers to increase corrosion resistance in specific environments. Duplex stainless steels offer strength about double that of austenitic stainless steels, with increased resistance to chloride induced SCC and pitting. They are typically used in temperatures ranging from -60 to 300ºC. There are four primary  alloys of duplex stainless steels used which are:

Duplex stainless steels have been used extensively in oil and gas production equipment having excellent resistance to the corrosive byproducts. They have also replaced other stainless steels that had corrosion problems in chemically corrosive environments and in heat transfer equipment due to their better resistance to SCC.

3.1.1.8 Iron-based Superalloys

Iron-based superalloys are also an extension of the stainless steels. They contain 20 – 30% chromium plus other alloying elements. They offer good corrosion resistance in a service temperature higher than the duplex stainless steels, but lower than nickel-based superalloys (up to about 815ºC). The cost of the iron-based superalloys is lower than nickel-based superalloys making them marketable in this service temperature range. Iron-based superalloys are used in structural components for furnaces, in steam and gas turbines, and in chemical processing equipment.

3.1.2 Resistance to Forms of Corrosion

The resistance of steels to corrosion varies greatly with alloying content playing a major role. Carbon and low alloy steels have the second highest uniform corrosion rates of any metals, while high alloyed stainless steels are generally only susceptible to localized corrosion. The following sections highlight the susceptibilities of steels to forms of corrosion.

3.1.2.1 Uniform Corrosion

Carbon and low alloy  steels  are  susceptible  to  uniform  atmospheric  corrosion  while stainless steels are considered resistant. Figure 32 summarizes  data  collected  on  various carbon and low alloy steels tested for uniform corrosion  in  a  natural  atmospheric  environment. The graphs clearly show the decline in corrosion rate over time,  with  the exception of the severe marine environment.

Figure 32          Uniform Corrosion of Steels in Various Atmospheric Environments

3.1.2.2 Pitting and Crevice Corrosion

Stainless steels are susceptible to pitting and crevice corrosion in marine environments, especially when fully immersed in saltwater. Stainless steels have been used on ships and can provide excellent service in marine atmospheric environments, as long as deposits are routinely washed off from surfaces. Accumulation of salt deposits will cause pitting and crevice corrosion to occur. All stainless steel alloys will exhibit pitting in low velocity seawater (less than five feet per second). Higher velocities prevent deposits and marine growth to occur so that pitting will not occur on exposed surfaces. Crevice corrosion has been found to occur, even at high velocities. The addition of molybdenum is beneficial for pitting and crevice corrosion resistance.

3.1.2.3 Stress Corrosion Cracking

Stress corrosion cracking of steels is largely a combination of their strength and environmental susceptibility. High strength steels are  susceptible  to  SCC  in  corrosive  environments.  Failures of stainless steels in marine atmospheres have often been a result of SCC. Table 26, Table 27, and Table 28 categorize the SCC susceptibilities of steels in marine atmospheric environments.

Table 26           Steels with a High Resistance to Stress Corrosion Cracking in Atmospheric Marine Environments

Table 27           Steels with a High Resistance to Stress Corrosion Cracking in Atmospheric Marine Environments if Used with Caution

Table 28           Steels with a Low Resistance to Stress Corrosion Cracking in Atmospheric Marine Environments

3.1.2.4 Intergranular Corrosion

Intergranular corrosion has been observed in some stainless steels primarily as a result of the precipitation of chromium carbides at grain boundaries. In austenitic stainless steels, chromium carbides are completed dissolved above temperatures of 1900ºF.⁴⁰ When slowly cooled from these temperatures, the formation of chromium carbides at grain boundaries can result. They  may also be formed by reheating austenitic stainless steels into the temperature range of 800 – 1200ºF. The formation of chromium carbide precipitates at grain boundaries in ferritic stainless steels occurs for temperatures above 1700ºF. The sensitized area in welded austenitic stainless steels occurs in the heat affected zone while in ferritic stainless steels, the sensitized area is likely to be in the fusion zone and the weld itself. Methods to reduce sensitivity of stainless steels to intergranular corrosion include limiting carbon content and the addition of titanium and/or niobium which preferentially form carbides.

3.1.2.5 Hydrogen Damage in Steels

There are several different mechanisms of hydrogen attack as discussed in Section 2.9.3. Steels are susceptible to all except metal hydride formation. High strength steels are the most susceptible, although even ductile steels have been known to suffer from hydrogen damage.

3.1.3    Corrosion Resistance in Chemical Environments

Acidic environments are involved in most of the severe corrosion problems encountered with steels, as with most metals, although alkaline environments can also be responsible for increased corrosion. The corrosion rate of steels in acids depends upon the composition and concentration of  acid,  as  well  as  temperature.⁶⁸          The corrosion rate of steels in hydrochloric acids will continuously   increase   with   increasing  acid  concentration.    In sulfuric  acids,  however, the corrosion rate increases until a level of concentration where  passivity  is  reached,  see Figure 33.  If  the  passive  film  is  damaged  by  mechanical  or  chemical  means,  the  corrosion rate will significantly increase in concentrated solutions.

Figure 33          Uniform Corrosion of Carbon Steel by Sulfuric Acid at Room Temperature

Nitric acid readily attacks carbon and low alloy steels. Austenitic stainless steels, as well as aluminum alloys, form strong adherent oxide films. This makes them the most applicable metals for use with nitric acid.

Sodium and potassium hydroxides have similar effects on steels. The uniform corrosion rates  are generally £ 2 mils/yr, for all concentration levels. The problem with exposure of low alloy steels to these materials is the susceptibility to SCC, sometimes referred to as caustic embrittlement. The  relation  of  temperature  and  sodium  hydroxide  concentration  to  observed cracking is shown in Figure 34.

3.1.4 Corrosion Protection of Steels

Corrosion protection of carbon and low-alloy steels is almost always required. There  a  numerous coatings, coating processes, and methods used to limit corrosion of these steels,   which include the following:

• Conversion coatings
• Surface modification
• Inhibitors
• Corrosion preventive compounds
• Metal claddings
• Hot-dip coating processes
• Continuous electrodeposition
• Electroplating
• Organic coatings (paints)
• Zinc-rich coatings
• Porcelain enameling
• Thermal spraying processes
• Vapor-deposited coatings
• Pack cementation coatings

Figure 34 Susceptibility of low alloy steels to SCC in NaOH

Chapter 3.2: Aluminum and Its Alloys – Corrosion

3.2       Aluminum and Its Alloys

In general, aluminum and its alloys are more resistant to corrosion than mild steel. They are known to have a very good resistance to corrosion in a variety of environments and chemical compounds, even though aluminum is a relatively reactive metal. Aluminum and its alloys also have a good resistance to various forms of corrosive attack. For the most part, the lower temperature corrosion resistance of aluminum is virtually equivalent to that of stainless steel, and it provides reasonable protection at elevated temperatures. Pure aluminum, however, tends to have a greater corrosion resistance than its alloys, and impurities in the aluminum only act to increase the metal’s susceptibility to corrosion. This is true especially for surface impurities;  clean surfaces are much more effective at resisting corrosion than are surfaces with deposits.

Aluminum’s excellent resistance to corrosion can usually be attributed to the rapid formation of an oxide film on the metal’s surface, which acts as a barrier to corrosive environments. For instance, the film inhibits corrosion very effectively in lower temperature, atmospheric and aqueous corrosive environments. An important aspect of the film is that it forms quickly in many environments, but can also be produced artificially by sending an electric current through the metal. This is called anodizing. The tough, virtually transparent, non-flaking, aluminum oxide film is capable quickly of repairing itself when it is scratched or abraded. Therefore, in order to defeat the protective film, continuous mechanical abrasion or chemical degradation in an oxygen deficient atmosphere is required. A further benefit is that the surface oxide film can be modified or thickened to enhance its corrosion protection.

3.2.1 Alloys and Alloying Elements

Although alloying other elements with aluminum can improve certain properties, it tends to have a negative effect on its corrosion resistance. Some elements, such as magnesium, however, can be alloyed in amounts of about <1% without significantly decreasing the corrosion resistance compared to pure aluminum. Common alloying elements include copper, magnesium, silicon, and zinc. Iron is not usually intentionally used as an alloying element, rather it is commonly a contaminant, and it is typically attributed as being the primary cause of pitting in aluminum alloys. Some of the general classes of aluminum alloys and their corrosion characteristics are described in the following sections. Although there are a number of specific aluminum alloys, only a few are discussed.

3.2.1.1 Aluminum (1000 Series)

The 1000 series of aluminum alloys has approximately 99% aluminum with the remaining percent consisting of other elements, which are considered impurities. Similar to  pure aluminum, this series of metals has excellent corrosion resistance to many environments, but with increasing impurity content the corrosion resistance decreases.

3.2.1.2 Copper (2000 Series)

The 2000 series of aluminum alloys contains copper as the principal alloying element. These are higher strength alloys and are consequently used mainly for structural applications, but they have a much lower corrosion resistance compared to other aluminum alloys. Therefore, alloys with little or no copper are used for applications where corrosion resistance is important. This series of alloys, in general, is prone to stress corrosion cracking and exfoliation, and typically copper alloying results in the occurrence of uniform, pitting, and intergranular forms of corrosion to a greater extent. For instance, copper additions greater than 0.15% decreases the resistance to pitting corrosion. Alloys containing copper are also more susceptible to corrosion in seawater and marine environments.

If alloys in this series are slightly overaged, their resistance to SCC is improved to the point where the alloys are no longer susceptible to this form of corrosion. Solution heat treatment and artificial aging of 2000-series aluminum alloys, however results in CuAl₂ precipitates at grain boundaries, which causes the alloy to be susceptible to intergranular corrosion.⁶⁹

Aluminum alloy 2020 in general is not suitable to be used for structural applications, but in the T651 condition it does exhibit an excellent resistance to SCC.^{43, 69} Aluminum alloys 2024-T851 and 2219-T851 are also highly resistant to SCC.

3.2.1.3 Manganese (3000 Series)

Manganese is the primary alloying element in the 3000 series of aluminum alloys. These alloys exhibit a very good resistance to corrosion, in general, and are particularly very resistant to SCC.

3.2.1.4 Silicon (4000 Series)

Silicon is the main alloying element in the 4000 series of aluminum alloys, but it has little effect on the corrosion resistance of aluminum. In particular, this series of alloys is characteristically very resistant to SCC.

3.2.1.5 Magnesium and Silicon (5000 and 6000 Series)

Magnesium is the main alloying element in the 5000 series of aluminum alloys, and it provides extra protection against aqueous corrosion. Magnesium can also serve to increase the resistance to corrosion in salt water and under alkaline conditions compared to unalloyed aluminum. It may, however, also help to advance SCC and intergranular corrosion, if it is present in the grain boundaries as an anodic magnesium aluminum phase. If the magnesium content exceeds the specified limit, it tends to precipitate another phase with aluminum, and consequently causes an increase in susceptibility to intergranular corrosion. Aluminum-magnesium alloys also have a tendency to be susceptible to exfoliation.

Aluminum alloys 5083, 5086, and 5456 in the H30-series of conditions should not be used for structural applications since, they are very susceptible to SCC. Aluminum alloy 5454-H34, on the other hand, has an excellent resistance to SCC. Furthermore, the H116 and H117 tempers for the 5000-series of aluminum alloys offer a good resistance to exfoliation.

The 6000 series of aluminum alloys contain magnesium and silicon as the primary alloying elements. These alloys are stronger while maintaining the same excellent resistance to aqueous corrosion as the 5000-series alloys. However, silicon in amounts greater than 0.1% reduces the resistance to pitting corrosion, and decreases the corrosion resistance in marine environments. Furthermore, excess silicon decreases the resistance to intergranular corrosion. Alloys  containing magnesium or magnesium and silicon tend to have the best resistance to corrosion in seawater and marine environments of any of the other aluminum alloys.

In general, similar to the 5000-series, the 6000 series alloys are susceptible to SCC. In particular these alloys with >3% magnesium can be very susceptible to SCC. Cold-worked aluminum- magnesium and aluminum-magnesium-silicon alloys containing <3% Mg, however, are very resistant to SCC.⁴³

3.2.1.6 Zinc (7000 Series)

Zinc is the primary alloying element in the 7000 series of aluminum alloys, and in general, as an alloying element it only has a small influence on the corrosion resistance of aluminum. These alloys, however, are characteristically much more susceptible to aqueous corrosion. A high zinc content may result in decreased resistance to intergranular corrosion, SCC, and exfoliation corrosion. In addition, zinc may decrease the resistance of aluminum to acidic environments, but may increase the resistance to alkaline environments.

Within the 7000 series of aluminum alloys, some alloys are especially susceptible to SCC and are therefore not suitable for structural applications. Overaging in the 7000 series of aluminum, however, tends to reduce their susceptibility to SCC. Aluminum alloys 7079 and 7178 are not suitable for structural applications.⁴³ The high strength aluminum alloy 7075 in the T6 condition is very susceptible to SCC and exfoliation, but in the T73 condition it has a greater resistance to SCC.⁶⁹ 7075 in the T7351 condition has an excellent resistance to SCC.⁶⁹ In general for the 7000-series aluminum alloys, the T76 tempers have a greater resistance to exfoliation than the T73 tempers.

3.2.1.7 Chromium

Chromium can be a beneficial alloying element because it typically provides improved corrosion resistance. For instance, Cr improves the corrosion resistance of Al-Mg and Al-Mg-Zn alloys when added in small amounts (0.1-0.3%). Furthermore, Cr increases SCC resistance in high- strength alloys, however, it does tend to increase the pitting potential in water for high purity aluminum.

3.2.1.8 Lithium

Lithium is a chemically active metal and may increase aluminum’s susceptibility to corrosion. For instance, it seems that lithium additions of <3% result in a slightly more anodic aluminum.⁷⁰ This indicates that additions of lithium, however, may only increase the susceptibility of aluminum to corrosion marginally. Moreover, studies have shown that the susceptibility of the aluminum lithium alloy to corrosion is largely dependent on the d phase, which is the AlLi phase. Increasing the amount of d phase present, for example, increases the alloy’s susceptibility to corrosion.⁷¹

Two of the more common aluminum lithium alloys are 2090 and 8090. 2090 is similar to 7075 aluminum in terms of resistance to SCC, and has a higher resistance to exfoliation corrosion than 7075.⁷⁰ 8090 aluminum with an altered surface structure (heat treatment T82551) has been shown to have a greater general resistance to corrosion than 2090 aluminum. Both 2090 and 8090 aluminum have been shown to be susceptible to pitting and intergranular corrosion.⁷¹  Alloy 2097 is another aluminum lithium alloy and has shown improved pitting corrosion resistance compared to an aluminum copper alloy (2124) and comparable general corrosion resistance.⁷²

3.2.1.9 Comparison of the Corrosion Resistance of Aluminum Alloys

A comparison of the relative corrosion resistance of the various groups of aluminum alloys    is provided in Table 29.

3.2.2 Resistance to Forms of Corrosive Attack

Although aluminum and its alloys have a good resistance to various forms of corrosive attack, they are still susceptible to some mechanisms including galvanic, pitting, SCC, intergranular, crevice corrosion, corrosion fatigue, and occasionally filiform corrosion. Susceptibility to other forms of corrosion is often dependent on alloy composition and heat treatment.

Table 29           Comparison of the General Corrosion Resistance for the Series of Aluminum Alloys

3.2.2.1 Galvanic

Aluminum, especially when joined with steel, is very susceptible to galvanic corrosion. In seawater, its place on the galvanic series is very low, and thus it is very anodic.  It will,  therefore, corrode preferentially when joined with a dissimilar metal that is higher up on the series. Graphite is very high on the galvanic series, and thus any contact with aluminum will adversely affect it in terms of corrosion. The significance of this is that graphite pencils used to make marks on aluminum may initiate corrosion by galvanic action.

3.2.2.2 Pitting

Pitting is one of the most common forms of corrosion found in aluminum and its  alloys. Chloride containing environments pose one of the biggest threats to aluminum in terms of corrosion, since pitting corrosion tends to occur in salt water and marine environments.  Seawater flowing at a high velocity relative to the aluminum is especially corrosive in the form of pitting since it will inhibit the protective oxide layer from automatically healing itself.

3.2.2.3 SCC

Additions of copper, magnesium and zinc in sufficient amounts can lead to SCC of aluminum alloys. SCC is dependent on the environment that the aluminum alloy is exposed to. For  instance, chloride, bromide and iodide environments are particularly dangerous to aluminum since SCC tends to occur in such environments. Aluminum alloys tend to resist SCC in hydrogen, argon and air with no moisture content. Marine environments, which are commonly encountered in applications of aluminum alloys, typically promote SCC of aluminum alloys. Increasing the pH in chloride environments, however, works to inhibit SCC in aluminum and its alloys.

Furthermore, SCC can also be dependent on the heat treatment and grain orientation.⁶⁹ For example, 7075-T6 (or 2024-T4) is most susceptible to SCC when the tensile stress is applied in the short transverse direction, is less susceptible to SCC when it is applied in the long transverse direction, and is least susceptible to SCC when it is applied in the longitudinal direction.⁶⁹ (This is only the case for thick specimens; thin aluminum sheets and castings are typically not affected by this.) Shot peening can be used to improve resistance of aluminum alloy structural forgings, machined plates and extrusions to SCC and corrosion fatigue. Some environments that are known induce or retard SCC in aluminum alloys are given in Table 30.   Table 31 provides      a relative comparison of various aluminum alloys and their resistance to SCC.

Table 30           Some Environments Known to Cause or Not to Cause Stress Corrosion Cracking in Aluminum Alloys

Table 31           Rating for Resistance to Stress Corrosion Cracking Aluminum Alloys in the Short Transverse Grain Direction (STGD)

3.2.2.4 Intergranular Corrosion

Inhomogeneities in the alloy structure are commonly the cause of intergranular corrosion in aluminum alloys. Furthermore, alloys with a high copper content tend to be susceptible to intergranular corrosion.

3.2.2.5 Crevice Corrosion and Exfoliation

Aluminum is also susceptible to crevice corrosion, and since it is often used in components where joining and fastening is required, crevices must be eliminated to avoid this particular form of corrosion. Exfoliation in aluminum is commonly a consequence of crevice or galvanic corrosion. Aluminum alloys that have elongated grain structures are susceptible to exfoliation.

3.2.3 Corrosion Resistance in Various Environments

Periodic cleansing (e.g. rain) of aluminum and aluminum alloys can extend their life significantly in terms of corrosion resistance. This is especially true if the metals are exposed to environments with high salt content or other air contaminants, such as in marine and industrial locations, respectively. Furthermore, aluminum should not be stored in humid environments unless coated with a corrosion inhibitor.

3.2.3.1 Water

Aluminum is strongly resistant to corrosion in normal atmospheric environments, fresh water environments, distilled water environments and other aqueous environments. Both water containing a significant amount of carbon dioxide and polluted water, however, can be more corrosive to aluminum.

3.2.3.2 Acid and Alkaline environments

Aluminum is resistant to neutral and acidic environments, in general, because of its ability to form an oxide film. However, aluminum is more prone to corrosion in alkaline environments. More specifically, aluminum has a general resistance to corrosion in environments with a pH in the range of 3 to 8.5. Under basic (alkaline) conditions the metal is attacked much more readily than the film. Thus, if the basic medium finds a hole in the oxide film, corrosion will occur usually in the form of pitting. Conversely, under acidic conditions the oxide film is attacked more readily than aluminum, thus, if corrosion occurs it will most likely be in the form of uniform corrosion. Corrosion inhibitors can expand the pH operating range of aluminum metals and alloys in alkaline environments up to approximately 11.5.

3.2.3.3 Soil

The corrosion resistance of aluminum and aluminum alloys in soil is dependent on the nature and conditions of the underground environment. In dry, sandy soil, aluminum and its alloys are sufficiently resistant to corrosion, but in wet, acidic or alkaline soils the metals are more susceptible to corrosion.

3.3       Copper and Its Alloys

Copper is a noble metal that has an inherent resistance to corrosion in a variety of environments. Despite its excellent corrosion resistance to a broad range of environments, copper corrodes rapidly in certain environments, unlike some other noble metals. Even so, it’s good corrosion resistance generally applies to atmospheric environments, industrial environments, freshwater environments, and seawater environments, as well as a number of acidic and alkaline conditions. Pure copper is especially resistant to the aforementioned environments.

Copper is a low-cost alternative to stainless steels and nickel-base alloys when selecting a material for a corrosion resistant application. Copper alloys provide good strength at lower temperatures with a good resistance to corrosion in a broad range of environments. Among other applications, copper is useful for architectural applications (e.g. roofing), freshwater handling systems and plumbing, seawater handling systems, chemical process equipment and heat exchangers, and electrical systems.

3.3.1 Alloys and Alloying Elements

There are three main types of copper alloys: copper-tin (bronze), copper-zinc (brass), and copper-nickel (cupro-nickels). Each of these main alloys may be alloyed with additional elements, which in some cases provides increased corrosion resistance and improved material properties. The following sections briefly describe the corrosion characteristics of some  common copper alloys.

3.3.1.1 Pure Copper and High-Copper alloys

Pure copper is accepted as having greater than 99% copper, while high-copper alloys have greater than 96% copper. Both pure copper and high-copper alloys have excellent resistance to corrosion, especially in seawater. They are also highly resistant to microbiological-influenced corrosion, as copper is toxic to microorganisms. They are, however, susceptible to erosion- corrosion.

3.3.1.2 Bronze

Alloying tin (Sn) with copper improves the resistance to corrosion in fresh water and seawater environments. Hence, bronze has an excellent corrosion resistance in fresh water and in contaminated water, as well as a very good resistance to corrosion in marine environments. Furthermore, alloys that contain approximately 8 to 10% tin have a good resistance to attack by impingement, which is a form of erosion corrosion. Bronze has moderate resistance to pitting corrosion. Moreover, the addition of tin to copper pushes copper more toward the cathodic end  of the galvanic series, further protecting it from galvanic corrosion.

Aluminum is added in 5-12% to Cu-Ni-Fe-Si-Sn systems to make aluminum bronze alloys, which show improvements in general corrosion resistance and exhibit excellent resistance to impingement attack (erosion corrosion) and high temperature corrosion. With an aluminum content of less than 8%, aluminum bronze alloys have an excellent resistance to pitting. Aluminum bronze alloys can be used in nonoxidizing mineral acids, organic acids, neutral saline solutions, alkalis, seawater, brackish water and fresh water without being significantly susceptible to corrosion.⁷³ They are not generally suitable, however, for use in nitric acid, metallic salts, humidified chlorinated hydrocarbons and ammonia.⁷³

Phosphorous is added to copper-tin alloys to provide enhanced resistance to nonoxidizing acids (except HCl) and flowing seawater. These phosphor bronze alloys also have superior resistance to SCC compared to brass. The addition of silicon can make bronze susceptible to pitting, as  well as embrittlement in the presence of high-pressure steam environments.⁷³

3.3.1.3Brass

Brass is a copper alloy with a significant zinc content. The content of zinc can be as great as about 40%, but corrosion by selective leaching (dezincification) can be significant when the content is more than 15%. The effect of zinc content on the susceptibility of brass to pitting and dezincification is shown in Figure 35. Copper alloys that have more than 85% copper are resistant to dezincification, but may also be more susceptible to corrosive attack  by impingement. Low concentrations of zinc in brass leads to a very good resistance to pitting. The addition of zinc to copper moves it further down the galvanic series toward the anodic end, and therefore, it is more susceptible to galvanic corrosion. High zinc content can also lead to a greater susceptibility to SCC. Brasses with 20-40% Zn, for example, are highly susceptible to SCC, while brass alloys with less than 15% Zn are highly resistant to SCC. For marine environments, brasses with a copper content between 65 and 85% are the most resistant to corrosion. Copper-zinc alloys have a good resistance to corrosion in fresh water environments. The one type of brass that is considered to have the best corrosion resistance in fresh water is red brass (85% Cu, 15% Zn).

Figure 35          The Effect of Zinc Content on the Corrosion of Brass in an Ammonium Chloride Environment

Alloying brass compounds with additional elements can enhance the corrosion resistance. The addition 1% Sn, for example, improves the resistance to dezincification in 70 Cu-30 Zn alloys; this alloy is called admiralty brass. (The addition of 0.75% Sn to 60 Cu-40Zn produces the alloy called Naval brass.) Alloying nickel with brass produces nickel-silver alloys, which have a good resistance to fresh water corrosion, are resistant to dezincification, and significantly improves corrosion resistance in salt water. The addition of Pb, Te, Be, Cr, or Mn to brass has no significant affect on its corrosion resistance.⁷³

Al addition (2%) added to 76 Cu-22 Zn produces aluminum brass, which has improved corrosion resistance. These alloys exhibit improved resistance to impingement attack in seawater flowing at high velocities, but are still susceptible to dezincification.⁷³ The addition of arsenic, phosphorous or antimony can be used to increase the resistance of aluminum brass, admiralty brass or naval brass to dezincification.⁷³ Arsenic added to aluminum brass in an amount of approximately 0.10%, for example, improves dezincification resistance.

3.3.1.4 Copper-Nickel

Copper-nickel has a resistance to fresh water, contaminated water, and marine environments that is similar to that of bronze. It is also more noble than pure copper on the galvanic series, and therefore, less susceptible to galvanic corrosion. Copper-nickel alloys with a composition  of 70% Cu and 30% Ni have the best resistance to corrosion in aqueous and acidic environments, in addition to having a very good resistance to SCC and impingement attacks. Copper-nickel alloys have a moderate resistance to pitting. Cu with 10% Ni also has a very good resistance to impingement attack and SCC. Copper-nickel alloys have a moderate resistance to pitting, although some specific alloys have an excellent resistance to pitting in seawater (e.g. alloys C70600 and C71500).⁷³ Copper-nickel alloys with additions of Fe are usually very resistant to SCC. Cu-18Ni-17Zn and Cu-18Ni-27Zn exhibit good corrosion resistance in freshwater and seawater, and a good resistance to dezincification.⁷³ Some copper-nickel alloys, however, are susceptible to crevice corrosion in seawater.

3.3.1.5 Other Alloying Elements

Copper-silicon alloys have a greater resistance to SCC than brass, in general. Copper-beryllium alloys are the only copper alloys that have shown a susceptibility to pitting in atmospheric environments. Additions of phosphorous in amounts greater than 0.04% can lead to serious  SCC. Additions of aluminum result in a more anodic metal compared to pure copper, in terms of galvanic corrosion.

3.3.2 Resistance to Forms of Corrosive Attack

Despite its excellent corrosion resistance in general, copper and its alloys are susceptible to several forms of corrosion. Copper is susceptible, to some extent, to uniform corrosion, galvanic corrosion, dealloying (selective leaching), pitting, SCC, erosion corrosion, fretting, intergranular corrosion, and corrosion fatigue. These forms of corrosion with respect to copper and its alloys are described in the following sections. (They are also described in more general terms in  Section 1.0.)

3.3.2.1 Uniform Corrosion

Copper and its alloys have a strong resistance to uniform corrosion under normal conditions, but over long periods of exposure they will exhibit to some extent this form of non-localized corrosion. Immersion in or uniform exposure to aerated or oxidizing acids or sulfur containing compounds, etc., will accelerate the process of uniform corrosion on copper and its alloys.

3.3.2.2 Galvanic Corrosion

Copper has a relatively noble (cathodic) position on the Galvanic Series compared to many structural metals and alloys, thus it will most likely not corrode preferentially when electrically coupled with one of them. When coupled with more noble metals, however, such as nickel, titanium, and some stainless steels, copper will corrode preferentially by galvanic action.

3.3.2.3 Dealloying (Dezincification)

When considering a copper alloy it is very important to realize its potential for dezincification, if it has a significant zinc content (>15% in Cu-Zn alloys). Dezincification is a process which leaches out the zinc leaving behind a less ductile, porous copper structure that is more susceptible to fracture. This often occurs in ambient waters or salt solutions. Dealloying also occurs in some copper-aluminum alloys, where the aluminum is selectively leached from the alloy. This has a greater occurrence in alloys with more than 8% aluminum.

3.3.2.4 Pitting

Most often, pitting does not occur to a significant extent in copper, that is, not enough to cause any considerable damage. If very thin copper or copper alloys are used, however, perforation is possible by pitting. Moreover, if copper is used in low flow velocity or stagnant seawater, it  does have a slightly higher propensity for pitting.

3.3.2.5 Crevice Corrosion

Copper and its alloys are generally resistant to crevice corrosion, although a few specific alloys may have a tendency to experience a form of crevice corrosion. Typically, copper alloys containing aluminum or chromium have a higher susceptibility to crevice corrosion.

3.3.2.6 Erosion Corrosion

Copper and its alloys are susceptible to erosion corrosion, which is also characterized as impingement attack. This is especially the case for copper when immersed in polluted water, contaminated water, seawater, or water containing sulfur compounds. Erosion corrosion from cavitation also has a tendency to occur on copper alloys.

3.3.2.7 SCC

Copper and its alloys are susceptible to stress corrosion cracking, especially in the presence of ammonia and ammonium compounds.  Stress corrosion cracking of copper alloys is presumed   to  be  integrally  related  to  dealloying.⁷³   Table   32   presents   some   copper   alloys   and their corresponding resistance to SCC.

Table 32           Resistance of Some Copper Alloys towards Stress-Corrosion Cracking

3.3.3 Corrosion Resistance in Various Environments

3.3.3.1 Atmospheric Environments

With the exception of instances where ammonia (NH₃), sulfur compounds (H₂SO₄) or certain other chemical agents are present, copper and its alloys generally exhibit an excellent resistance to corrosion in atmospheric environments, including clean (rural), polluted (industrial), marine and  tropical.  Copper  and  its  alloys  are  therefore  suitable  for  long-term  use  in   atmospheric  environments.  Table  33  provides  the  corrosion   rates   of   certain   copper alloys in various atmospheric environments.

Table 33           Uniform Corrosion Rates of Some Copper Alloys in Several Atmospheric Environments

3.3.3.2 Water Environments

In fresh water environments, copper tends to form a protective coating on the surface, and is typically very resistant to corrosion in such environments. The corrosion rate is slightly higher  in soft water or water having a significant amount of dissolved CO₂. Marine environments typically pose little threat to copper and most copper alloys, although at high flow velocities in seawater copper is very susceptible to erosion corrosion. Copper and its alloys are also very resistant to biofouling.

Copper is generally resistant to corrosion in steam environments. If there is a significant concentration of CO₂, oxygen or ammonia in the steam, however, the copper is more susceptible to corrosion.

3.3.3.3 Acids/Alkalines

Copper does not usually corrode in the presence of acids unless there are oxidizing agents (e.g. oxygen, HNO₃) available. For instance, copper and sulfuric acid do not react unless oxygen is present. Hence, it is susceptible to oxidizing acids, in addition to oxidizing heavy metal salts, sulfur, and ammonia. Exposure to environments containing ammonia can result in rapid and severe attack in the form of uniform corrosion or SCC. However copper is resistant to neutral solutions and solutions with a pH slightly on the alkaline side. The most threatening environments are ammonia, cyanide solutions, oxidizing salts and acids, or salts and acids         in oxidizing conditions. Table 34 provides the uniform corrosion rate of copper  in  three different acids.

Table 34           Corrosion Rate of Copper in Several Acids

3.3.3.4 Soil

Copper is generally very resistant to corrosion in soil, and copper-tin (bronze) alloys are especially  resistant  to  corrosion  in  soil.          The presence of organic compounds, ammonium compounds, sulfates, or cinder, however, adversely affects the corrosion resistance of copper. Figure 36 shows the rate of uniform corrosion for copper in four different types of soils over a long period of time.

Chapter 3.4: Nickel and Its Alloys – Corrosion

Nickel and nickel alloys are commonly used for applications subject to severe corrosion problems, since they exhibit excellent corrosion resistant properties in addition to having other desirable material properties. They are more expensive, however, than copper alloys and stainless steels. Nickel is a relatively noble metal, and does not readily corrode without the presence of an oxidizing agent. Under certain conditions nickel will form a passive film that protects the metal from corrosion.

Figure 36         Rate of Corrosion of Copper in Different Types of Soils⁷³

There is a wide range of environments that nickel and its alloys are resistant to. They typically show good resistance to corrosion in atmospheric environments, fresh water, distilled water, seawater and nonoxidizing acid environments. Nickel also has a very good resistance to  corrosion in alkaline environments and solutions, halogens, reducing salts, and other oxidizing halides.⁷⁴ They have good resistance to corrosion at lower and higher temperatures, and in neutral solutions or solutions with a pH slightly less than 7. Nickel has a strong resistance to high  stresses that may cause SCC. Nickel is susceptible to strong oxidizers (e.g. nitric acid, ammonia) and sulfuric environments at high temperature, however, which can cause general corrosion and possibly intergranular corrosion.

3.4.1 Alloys and Alloying Elements

Many of the superalloys are nickel-based or have a high nickel content, and have a good resistance to corrosion. Several of the key alloying elements and their impact on the corrosion resistance of nickel are reviewed in the following sections. In addition, some nickel-based alloys are considered as well.

3.4.1.1 Chromium

The addition of chromium enhances the resistance of nickel to high temperature corrosion. Chromium additions improve the resistance to oxidation at high temperatures and the resistance to oxidizing acids such as nitric and chromic acids. Furthermore, chromium improves the resistance of nickel to carburization and sulfidation at higher temperatures, but negatively impacts the resistance to corrosion in high temperature environments containing nitrogen or fluorine. Chromium forms a passive film on the nickel alloy in these types of environments. It also provides resistance to corrosion in liquid environments at lower temperatures, and to SCC, pitting and crevice corrosion. The maximum corrosion resistance is achieved with a chromium content of approximately 20%, and corrosion resistant superalloys usually contain 15-30% Cr.

3.4.1.2 Nickel-Chromium-Iron Alloys

Inconel 600 is a Ni-Cr-Fe alloy that is very resistant to corrosion in organic acids, caustic soda, and alkalis, but is only moderately resistant to corrosion in mineral acids. It is also resistant to atmospheric corrosion, high temperature corrosion, SCC, oxidation, carburization and nitridation.⁷⁴ Hastelloy G-30 has excellent resistance to corrosion in nitric acid, and  also resistant to sulfuric acid, phosphoric acid, fluorides, and oxidizing acids in general. Inconel 690 exhibits excellent resistance to oxidizing agents, sulfuric acid, and nitric acid. It is also very resistant to high temperature corrosion.

3.4.1.3 Nickel-Chromium-Molybdenum Alloys

Inconel 617 is a nickel-chromium-molybdenum alloy that exhibits excellent resistance to oxidation. Inconel 625 is resistant to pitting, crevice corrosion and oxidation at high temperatures, as well as to highly corrosive environments.⁷⁴ It also shows resistance to corrosion in halides, as well as to carburization, which can cause corrosive degradation of the material. Hastelloys C-276 and C-4 offer resistance to localized corrosion as well as SCC. Inconel 625  and Hastelloy C-276 can be resistant to hydrochloric acid even in the presence of oxidizing agents. Hastelloy C-22 provides superior resistance to oxidation, as well as excellent resistance to SCC and localized corrosion. Hastelloy C-2000 offers very good resistance to uniform corrosion in a wide range of environments, as well as very good resistance to SCC and localized corrosion.

3.4.1.4 Nickel-Chromium-Iron-Molybdenum Alloys

Incoloy 825 is a nickel-chromium-iron-molybdenum alloy that exhibits excellent resistance to sulfuric acid and phosphoric acid, moderate resistance to hydrochloric acid, and less resistance to corrosion in alkalis and halogens. Incoloy 825 is resistant to SCC, pitting and intergranular corrosion.⁷⁴ Hastelloy G and Hastelloy G-3 are suitable for service in sulfuric acid and phosphoric acid. Hastelloy G-30 is resistant to corrosion in phosphoric acid, sulfuric acid, nitric acid, fluorides, and oxidizing acids.⁷⁴ Hastelloy D-205 exhibits excellent corrosion resistance in sulfuric acid at high temperatures and to oxidizing agents. Most of the alloys in this group are very resistant to atmospheric corrosion.

3.4.1.5 Copper

Nickel-copper alloys have excellent resistance to corrosion in seawater, some acids, alkalis, and halides. Additions of copper typically improve nickel’s resistance to nonaerated, nonoxidizing acids. For example, additions of 30-40% Cu typically result in nickel having a good resistance to sulfuric acid and an excellent resistance to hydrofluoric acid. Copper is the main alloying element in Monel superalloys, which contain approximately 70% Ni and 30% Cu and have a good resistance to hydrofluoric acid. Copper can be added to Ni-Cr-Mo-Fe alloys to improve their resistance to hydrochloric, sulfuric and phosphoric acids.⁷⁴

3.4.1.6 Nickel-Copper Alloys

Nickel-copper alloys possess corrosion resistance similar to that of pure nickel, that is, they are resistant to corrosion in a broad range of environments. They are also similar to nickel in that they are susceptible to corrosion in oxidizing environments. Nickel-copper alloys have a good resistance to corrosion in sulfuric acid, seawater, and halogens.

Monel 400 is a nickel copper alloy with additional alloying elements, and is very resistant to seawater, sulfuric acid, alkalis, and halogen acids, including hydrofluoric acid as long as oxygen is not present in significant quantities. The resistance of Monel 400 to corrosion in low concentrations of nonoxidizing hydrochloric acid is very good even at higher temperatures (up to 200°C). It is much more susceptible to corrosion in hydrochloric acid containing oxidizing agents. Monel 400 is also very resistant to atmospheric corrosion and to corrosion in flowing seawater. Monel 400 exhibits very good resistance to erosion corrosion in seawater, but is susceptible to pitting and crevice corrosion in stagnant or low-flow velocity seawater. Monel K- 500 has corrosion characteristics similar to Monel 400.

3.4.1.7 Aluminum

Aluminum additions help to provide resistance to oxidation, sulfidation (which can cause corrosive degradation) and carburization at high temperatures, but may also make nickel more susceptible to high temperature corrosion in nitriding environments. With greater than 4% Al content, an oxidation inhibiting aluminum oxide film is capable of forming on the surface of the nickel alloy; however, it only occurs at high temperatures (>870°C). Once the film is formed it will protect against lower temperature oxidation too, but if it is abraded or removed, the alloy will no longer have the same oxidation resistance. Aluminum may result in a degradation of the hot corrosion resistance in superalloys, but it is also dependent on Cr content and the temperature of the environment.

3.4.1.8 Titanium

Additions of titanium to nickel are not typically used in nickel alloys intended for use in lower temperature applications. Titanium may provide some improvement in nickel’s resistance to hot corrosion, but it may also degrade the resistance to SCC, if carbon, oxygen or nitrogen is present. Titanium additions are used in superalloys with aluminum to improve the strength, and a high titanium to aluminum content ratio results in improved hot corrosion resistance.

3.4.1.9 Molybdenum

Additions of molybdenum improve the resistance of nickel to crevice corrosion, pitting corrosion in seawater, and to corrosion in nonoxidizing acids. Up to 28% Mo is used for nonoxidizing environments of hydrochloric, phosphoric, hydrofluoric and sulfuric acids. However, molybdenum also degrades nickel’s resistance to hot corrosion and to nitric acid, and decreases the resistance of nickel to oxidation at high temperatures. Furthermore, nickel-molybdenum alloys are susceptible to corrosion in oxidizing acid environments.

3.4.1.10 Nickel-Molybdenum Alloys

Nickel-molybdenum alloys exhibit excellent corrosion resistance in hydrochloric, sulfuric, and phosphoric acid environments. They are however, more susceptible to corrosion in oxidizing environments, and are especially susceptible to corrosion in nitric acid environments.

Hastelloy B and Hastelloy B-2 exhibit good resistance to corrosion in hydrofluoric acid environments at low temperatures. Welded components made from Hastelloy B may be susceptible to intergranular corrosion. Hastelloy B-2 is a nickel-molybdenum alloy that has excellent resistance to aluminum-chloride environments, while Hastelloy B-3 exhibits good resistance to SCC. Both of these alloys exhibit superior resistance to corrosion in hydrochloric acid compared to all of the nickel-based alloys, but they are more susceptible to this environment if oxidizing agents are present.

3.4.1.11 Tungsten

Tungsten additions improve the resistance of nickel to localized corrosion and corrosion in the presence of nonoxidizing acids. Tungsten can quickly increase the density of a nickel alloy, however, because of its relatively high atomic weight. When used with 13-16% Mo in amounts of 3-4%, tungsten provides extra local corrosion resistance. However, alloying tungsten with nickel superalloys can result in a poorer resistance to hot corrosion. Tungsten may negatively affect the resistance of nickel to oxidation at high temperatures.

3.4.1.12 Silicon

Nickel alloys containing silicon often have it in small amounts as a contaminant from a processing step during fabrication. Silicon additions however, are sometimes intentionally used (typically 9-11%) to provide hot corrosion resistance in concentrated sulfuric acid environments. Moreover, silicon additions improve the resistance of nickel to high temperature corrosion; specifically oxidation, nitridation, sulfidation and carburization.⁷⁴

3.4.1.13 Iron

Additions of iron can be used to reduce the cost of nickel, since nickel is more expensive than iron, but it does not offer much in terms of corrosion resistance. Iron additions, though, can improve nickel’s resistance to sulfuric acid, and may also improve the resistance to carburization at high temperatures.

3.4.1.14 Cobalt

Cobalt is not typically used in significant amounts since it has corrosion resistance characteristics very similar to nickel and is more expensive. It does however, provide improvement to high temperature carburization and sulfidation resistance in nickel alloys.⁷⁴

3.4.1.15 Other Alloying Elements

Yttrium, lanthanum, and other elements may also improve the corrosion resistance of nickel and its alloys. Yttrium generally improves the resistance to high temperature oxidation, sulfidation and carburization. Tantalum and niobium can improve the corrosion resistance at higher temperatures and the resistance of nickel alloys to intergranular corrosion. Niobium  may increase the resistance of nickel to carburization at high temperatures, but may also decrease the resistance of nickel to nitridation at high temperatures. Carbon can improve the resistance of nickel to nitridation and carburization at high temperatures, but decreases the resistance to high temperature oxidation. Manganese typically reduces the resistance of nickel to high temperature oxidation and nitridation.

3.4.2 Resistance to Forms of Corrosive Attack

In general, nickel and its alloys have an excellent resistance to corrosion. In certain conditions,  of course, they may be susceptible to some forms of attack. A few of the forms of corrosion are described below in terms of the resistance or susceptibility of nickel and its alloys.

3.4.2.1 Uniform Corrosion

Nickel and its alloys are very resistant to general corrosion in a wide range of environments. Since it is a relatively noble metal, alloys with a high nickel content typically exhibit a good resistance to uniform corrosion.

3.4.2.2 Galvanic Corrosion

Since nickel is a relatively noble metal and also a fairly cathodic in the Galvanic Series compared to most other metals, it is not very susceptible to galvanic corrosion. It may however, exhibit a degree of corrosion due to galvanic action, if it is coupled with a more noble metal.

3.4.2.3 Pitting and Crevice Corrosion

Nickel is somewhat susceptible to pitting and crevice corrosion in seawater and other environments. Typically,  surface  impurities  are  the  cause  of  pitting,  since  they  can  act  as a nucleating point for corrosion.  Crevice corrosion can occur, particularly in areas where there  is stagnant seawater.

3.4.2.4 Intergranular Corrosion

Nickel has a good resistance to intergranular corrosion, although in certain nickel alloys with inappropriate  heat  treatments  it  may  be  susceptible  to  this  particular  form  of  corrosion.   In general, a higher nickel content corresponds to a better resistance to intergranular corrosion.  In environments containing sulfur, nickel alloys have an increased susceptibility  to  intergranular corrosion.

3.4.2.5 Stress Corrosion Cracking

Alloys that contain mostly nickel with a small amount of iron tend to be susceptible to SCC. Nickel alloys seem to have a greater resistance to SCC compared to stainless steels.

3.4.2.6 High Temperature Corrosion

Nickel and its alloys may be susceptible to oxidation, carburization, nitridation, sulfidation and halogenation at high temperatures.⁷⁴ Certain alloying elements however, can provide augmented resistance to high temperature corrosion in environments where these contaminants are present.

3.4.3 Corrosion Resistance in Various Environments

3.4.3.1 Atmospheric Environments

Nickel and nickel alloys demonstrate very good resistance to corrosion in atmospheric environments, although after extended periods of exposure many alloys will develop a thin, adherent film, especially in industrial environments. Even so, nickel and  its  alloys  are  generally suitable for use in atmospheric environments, due to their strong corrosion resistance. Table 35 gives corrosion data on some nickel-based alloys after exposure to the atmosphere.

Table 35           Atmospheric Corrosion of Nickel-Base Alloys

3.4.3.2 Water Environments

Fresh water environments do not pose much of a threat to nickel and nickel alloys, since they generally exhibit a good corrosion resistance in these environments. Therefore, they are suitable for applications that require the exposure to or handling of fresh water environments.

The resistance of nickel and nickel-based alloys to corrosion in seawater is dependent on factors such as the velocity of flow of seawater. Some alloys may exhibit good corrosion resistance to flowing seawater, for example, but are susceptible to corrosion in stagnant or  low-flow  seawater. Table 36  shows  the  corrosion  resistance  of  several  nickel-based  alloys  exposed  to stagnant seawater.

Table 36          Corrosion Resistance of Several Nickel-Based Alloys Exposed to Stagnant Seawater.

3.4.3.3 Acids/Alkalis

Nickel and nickel-based alloys are generally resistant to corrosion in nonaerated and nonoxidizing acids. Since sulfuric acid is not considered to be an oxidizing acid up to a concentration of about 50-60 wt.%, for example, most nickel alloys are generally resistant to corrosion in this environment.⁷⁴ The corrosion rate of these alloys, though, typically increases with increasing sulfuric acid concentration. Nickel and its alloys are generally resistant to corrosion in acrylic acid and fatty acids.⁷⁴

The presence of oxidizing agents in acids or the aeration of acids can significantly increase the corrosion rate of the nickel alloy. For instance, nickel alloys are typically resistant to HCl in low concentrations, but the presence of Cu²⁺ or Fe³⁺, for example, may increase the corrosion rate of these alloys considerably. Some alloys, however, offer a better resistance in acids with oxidizing agents present, than others. Alloys containing chromium are more resistant to these types of acids, such as nitric and chromic acids, while molybdenum additions tend to degrade the resistance of nickel to these acid environments. The uniform corrosion rates of some nickel-based alloys in several acid environments are given in Table 37.

Table 37 Corrosion Rate of Several Nickel-Based Alloys in Various Acid Environments

Nickel is strongly resistant to corrosion in alkalis, but environmental contaminants can cause an increase in the corrosion rate. Nickel is not, however, resistant to ammonium hydroxide solutions.⁷⁴ The corrosion resistance of nickel alloys in alkalis tends to decrease with decreasing nickel content.

3.5       Titanium and Its Alloys

Titanium is an inherently reactive metal, but it performs very well against a wide range corrosive environments. It may be the best available metal for corrosion resistance, but it is also very expensive, thus it is not used for many applications. This inherent corrosion resistance can be primarily attributed to a continuous, self-healing, protective oxide film, which forms in the presence of oxygen or water vapor. The protective film helps resist corrosion in oxidizing environments.   In environments that do not contain an oxygen source, however, titanium is susceptible to corrosion.

3.5.1 Alloys and Alloying Elements

In general, additions of large amounts of an alloying element reduce the corrosion resistance of pure titanium. Small amounts of palladium, platinum, and rhodium, however, increase the resistance to corrosion, including corrosion in moderate concentrations of hydrochloric and sulfuric acids. Additions of approximately 30% molybdenum improve the resistance to hydrochloric acid.

Other typical alloying elements used in titanium alloys include aluminum, chromium, iron, manganese, molybdenum, tin, vanadium, and zirconium.  Aluminum additions above 6% causes  a significant degradation in the SCC resistance, while titanium aluminide intermetallics may   have increased resistance to oxidation and oxygen embrittlement.  Additions of approximately  2% nickel improves crevice corrosion resistance in hot brine environments, but reduces the resistance to hydrogen embrittlement and also degrades the formability of titanium. Table 38 shows the resistance or various titanium alloys to SCC in a hot-salt environment.

Table 38           Relative Resistance of Titanium Alloys to Hot-Salt Stress Corrosion

3.5.2 Resistance to Forms of Corrosive Attack

Titanium and its alloys typically exhibit an excellent resistance to corrosion. Titanium is generally resistant to oxidation, galvanic corrosion, SCC, corrosion fatigue, and erosion corrosion. A few of the forms of corrosion and their correlation to titanium are briefly discussed in the following sections.

3.5.2.1 Stress Corrosion Cracking

Titanium is susceptible to SCC in the presence of hot-salts or gaseous chloride ions and residual stresses. Severe SCC usually only occurs in the presence of hydrobromic acid or red fuming nitric acid at elevated temperatures; otherwise SCC is not much of a threat to titanium, which is also generally resistant to SCC in seawater, fresh waters and body fluids. Titanium has exhibited susceptibility to SCC in liquid and gaseous oxygen at cryogenic temperatures.

3.5.2.2 Pitting

The occurrence of pitting on titanium is rare, although it can result from iron adsorbed on the surface of titanium. Titanium resists pitting better than stainless steels and copper-nickels.

3.5.2.3 Other Forms

Titanium is susceptible to crevice corrosion, to liquid metal embrittlement in the presence of Cd and Ag, and is also susceptible to embrittlement as a result of the dissolution of hydrogen, oxygen, and nitrogen. Furthermore, titanium and its alloys have a high susceptibility to fretting  at interfaces with titanium or other metals, which can significantly reduce its fatigue life.⁴³ Titanium does, however, have a strong resistance to erosion corrosion and impingement attack, as well as a good resistance to corrosion fatigue.

3.5.3 Corrosion Resistance in Various Environments

Titanium has an excellent resistance to atmospheric corrosion in unpolluted, marine, and industrial environments. It is also highly resistant to corrosion in water, seawater, and chloride solutions. In a wide variety of other chemical environments its corrosion resistance is similar to or better than most other metals. Furthermore, outstanding corrosion resistance at lower temperatures is characteristic of titanium.

Corrosion in inorganic salts and acids and ammonia solutions is easily resisted by titanium. The corrosion resistance of titanium in seawater and body fluids is superior to all other structural metals, and is therefore often used in orthopedic implants. Titanium is also resistant to hypochlorites, chlorine solutions, molten sulfur, wet chlorine gas, H₂S gas up to 260°C, and carbon dioxide up to 260°C. It is susceptible to dry chlorine gas and ionizable fluoride compounds (e.g. sodium fluoride, hydrogen fluoride). Furthermore, molten sodium hydroxide and hot, strong alkali solutions are a couple of the few substances which can attack titanium severely.

Titanium is resistant to most oxidizing acids and organic acids, but is susceptible to reducing acids, strong sulfuric and hydrochloric acids, phosphoric acids, oxalic acids, and fuming nitric acids. The corrosive effects of fuming nitric acid and chlorine gas, however, can be mitigated by adding small amounts of water. Moreover, oxidizing inhibitors and heavy metal ions are effective in mitigating the corrosive attack of acids.

3.6         Magnesium and Its Alloys

Magnesium has the lowest density of the metals but it also has the highest susceptibility to corrosion effectively eliminating it from use in most applications. An oxide layer will form on magnesium; however the thin film layers that form are usually soluble in water and readily breakdown in the presence of ions such as chloride and bromide. Increasing temperatures will also accelerate degradation of the protective film leading to widespread corrosion of magnesium. Galvanic corrosion of magnesium is always a consideration since it is anodic to most metals. Coatings should always be used to protect magnesium alloys in structural applications. Use of  Mg alloys in moving components has led to rapid breakdown of the coating system leading to corrosion of the unprotected Mg alloy material. Therefore, magnesium alloys are almost entirely used in non-moving structural applications with proper protective methods.

3.6.1 Alloying for Corrosion Resistance

Alloying magnesium does not lead to any improvements in its corrosion resistance and in some cases can lead to a severe  degradation  in  corrosion  resistance.  The  effects  of  various  alloying elements on the uniform corrosion rates of magnesium are shown in Figure 37.

Figure 37         Effect of Alloying on the Uniform Corrosion Rate of Magnesium⁷⁵

Iron, with a concentration of ³ 0.017%, and also Ni, Co, and Cu significantly increase the corrosion rate of Mg. Aluminum improves the strength and hardness of Mg without seriously degrading its corrosion resistance. A content of 6% Al provides the best combination of strength and ductility. Although it may lead to an increase in corrosion rate, zinc is second to aluminum in its strengthening effect on Mg. It is used in combination with small amounts of other elements such as zirconium and the rare earths to produce a precipitation hardenable alloy. Zn also  counters the corrosion effects of Fe and Ni contaminants found in Mg alloys. Manganese has  been found to slightly increase resistance to salt water environments by interacting with iron and other heavy metal elements in the alloy. Mn has a low solubility in Mg and thus is used in small amounts, about 1.5% maximum and 0.3% with Al.  Some  common  magnesium  alloys  with their respective applications are listed in Table 39.

Table 39           Common Magnesium Alloys

3.6.2 Resistance to Forms of Corrosion

Magnesium and magnesium alloys can be highly susceptible to a number of corrosion forms including general corrosion. Issues unique to Mg alloys will be covered in this section.  Corrosion prevention and protection methods are almost always necessary when using magnesium materials.

3.6.2.1 Uniform Corrosion

Magnesium alloys have the highest uniform corrosion rates of any metal as shown previously in

Figure 22 in Section 1.1-Uniform Corrosion. An oxide protective layer will  form  on  magnesium once exposed, however this film is easily degraded by a number of environmental conditions and chemical compounds. Corrosion protection methods are almost  always  used with magnesium alloys. Magnesium alloys are not normally  used  in  moving  components where the coatings are easily damaged.

3.6.2.2 Galvanic Corrosion

All metals are cathodic to (more noble than) magnesium. Aluminum alloys are closest to magnesium in the galvanic series, although some aluminum alloys may still pose galvanic corrosion problems when in contact with magnesium alloys. Copper, nickel, and iron cause severe galvanic corrosion to magnesium, and thus aluminum alloys absent of these elements (5000 and 6000 Al series) are preferred for use when in contact with magnesium alloys. Aluminum alloys 5052, 5056, and 6061 have been found to have the least galvanic effect on Mg alloys in a marine atmospheric environment.⁷⁵

3.6.2.3 Stress Corrosion Cracking

The SCC susceptibility of magnesium is generally more severe in alloys containing Al and/or  Zn. The addition of aluminum, above 0.15 to 2.5%, creates the highest susceptibility in Mg alloys.⁷⁵ The susceptibility  increases  with  increasing  Al  content  to  a  peak  at  about  6%,  see  Figure 38.   The addition of zinc increases SCC susceptibility, but not to the extent as      does Al containing alloys. Mg alloys absent of both Al and Zn are the most resistant to SCC.

Figure 38         Stress versus Time to Failure for Mg-Al Alloys in a saltwater solution

3.6.3 Corrosion Resistance to Various Environments

Magnesium is susceptible to numerous environmental conditions. Only mild atmospheres and stagnant fresh water which do not break down the MgO surface layer on magnesium are acceptable for magnesium. Any agitation of the water environment or addition of salts into the environment will attack the protective coating and lead to corrosion of the magnesium.  Humidity levels of 30% can produce mild corrosion with severe corrosion occurring at a level of 80%. All acids with the exception of hydrofluoric acid and H₂CrO₄ readily attack magnesium. Magnesium does resist corrosion in the presence of dilute alkalis. Organic acids, such as fruit juices and carbonated drinks severely attack magnesium. Other organic compounds do not affect magnesium at room temperatures but may lead to corrosion at elevated temperatures.

3.6.4 Corrosion Protection of Magnesium Alloys

Special care must be given to the fabrication of magnesium joints due to the high susceptibility of this metal. When joining two magnesium parts, a chemical conversion coating should be  used, followed by one or more primer coats having alkali resistance such as an epoxy or vinyl resin. Fasteners for Mg-Mg joints include 5056 Al rivets, 6061 Al bolts, and cadmium or zinc plated steel bolts. For joining magnesium with a dissimilar metal, the surfaces must be insulated with an organic tape, sealing compound, or an alkali resistant paint. The joint should be fastened with cadmium plated steel bolts and nuts with a 5052 aluminum washer separating the steel and magnesium. Only 5056, 6061, and 6053 aluminum alloy bolts and screws can be used bare, to join magnesium. All other metal fasteners should be  coated  when  used  with  magnesium. Some general procedures to limit corrosion in magnesium structures are listed in Table 40.

Table 40           Procedures to Limit Corrosion in Magnesium Structures

Chapter 3.7: Zinc and Its Alloys – Corrosion

Zinc is not commonly used without supplemental corrosion protection for corrosion resistant applications since zinc is located near magnesium on the Galvanic Series, and is thus very susceptible to corrosion. It is therefore very anodic to most metals, and will corrode preferentially when galvanically coupled with metals that are more cathodic. Often it is used as an anodic coating to protect steel from corrosion. It is also used as sacrificial anodes for ship  hulls, pipelines and other applications. Furthermore, zinc can act as a barrier coating that is resistant to mechanical and electrochemical degradation.

Impurities have a significant impact on the corrosion resistance of zinc, as they often cause the surface to be more susceptible to corrosion. Some impurities, however, such as aluminum, may result in slight improvements in the corrosion resistance of zinc by forming a protective film on the surface.

3.7.1 Alloying for Corrosion Resistance

Neither alloying additions nor impurities significantly affect the corrosion resistance of zinc under normal conditions. Additions of antimony in amounts of 0.03 to 0.07 % do tend to  increase the corrosion rate of zinc in atmospheric environments, while copper in amounts less than 0.06% may increase the corrosion resistance of zinc. Variations in the content of lead, cadmium and iron, on the other hand, have little affect on the corrosion resistance of zinc.

3.7.2 Resistance to Forms of Corrosive Attack

Severe pitting is rarely a problem for zinc, since most instances of corrosion takes place uniformly along the surface of the metal. Stress corrosion cracking and corrosion fatigue also rarely occur in zinc, but it is somewhat susceptible to crevice corrosion.

3.7.3 Corrosion Resistance in Various Environments

Zinc has a good resistance to all types of atmospheres, but moist and acidic environments can be problematic for zinc in terms of corrosion. Weak and strong acids and strong bases tend to  attack zinc more readily, but it is generally resistant to weak bases. Industrial environments can be corrosive to zinc, especially if the contain sulfur dioxides. Zinc is susceptible to corrosion by sulfur dioxide, chlorides, and low-grade glycerin, but it is resistant to dry chlorine and hydrogen sulfide.

The corrosion rate of zinc is dependent on temperature, pH, and oxygen concentration. There seems to be a strong relationship between corrosion and oxygen content, as it increases with increasing oxygen content in the environment. Zinc is corroded 8 times faster in water with oxygen gas present than in water with no gases. In oxygen deficient environments, pitting tends to occur, while oxygen abundant environments lead to more uniform corrosion. The corrosion rate of zinc is temperature dependent, and it increases rapidly from room temperature to about 60°C, then decreases significantly at 100°C.

Zinc is susceptible to environments containing organic vapors, which tend to attack the metal’s surface. Organic substances therefore, may be very corrosive to zinc, if they produce organic vapors or other products such as sulfur or halogen compounds. Zinc does tend to be resistant, however, to anhydrous organic liquids with a neutral pH.

Zinc has a good resistance to water, but it is more susceptible to corrosion if oxygen or carbon dioxide is present, or if the water is at an elevated temperature, or if the water is strongly aerated or agitated. Furthermore, soft water attacks zinc more readily than hard water, and steam can  also be damaging, if zinc is exposed to it in a continuous manner.

3.8       Cobalt and Its Alloys

Cobalt and cobalt-based alloys are very similar to nickel and nickel-based alloys in terms of corrosion resistance, but typically they are slightly more susceptible to corrosion compared to their nickel counterparts. Cobalt-based alloys have an inherent wear and corrosion resistance. Cobalt is not considered an oxidation resistant metal, especially since the oxidation rate is generally about 25 times that of nickel. Cobalt-based superalloys, however, are resistant to oxidation and hot corrosion. Furthermore, the cobalt superalloys are more resistant to hot corrosion than are the nickel superalloys. Nickel and cobalt superalloys have a similar resistance to aqueous corrosion at lower temperatures.

3.8.1 Alloys and Alloying Elements

3.8.1.1 Chromium

Chromium is typically alloyed with cobalt in significant amounts to improve various properties of the metal, including corrosion and oxidation resistance. Cobalt superalloys generally contain 20-30% chromium, which contributes to their good oxidation and hot corrosion resistance. It  also provides resistance to corrosion at lower temperatures, as well as higher temperature resistance to oxidation and hot corrosion. Moreover, chromium additions provide enhanced protection of cobalt-based alloys against corrosion in dilute nitric acid environments, but may also decrease the resistance to corrosion in high concentrations of nitric acid. Cobalt-based  alloys with a significant chromium content are susceptible to corrosion in chromic acid environments. Cobalt-chromium alloys with a high carbon content also have good wear resistance, but carbon can also inhibit the beneficial effects of the chromium additions.

3.8.1.2 Nickel

Nickel additions improve the resistance of cobalt to corrosion in mineral acids, such as sulfuric and phosphoric acids. It also improves the resistance to SCC. Furthermore, nickel additions provide improved resistance to corrosion in caustic environments.

3.8.1.3 Tungsten

Tungsten additions can improve the resistance of cobalt to corrosion in general, but may lead to corrosion problems at temperatures above 980°C. Tungsten may also increase the corrosion resistance of cobalt-based alloys in chromic acid.

3.8.1.4 Other Alloying Elements

Copper additions improve the resistance of cobalt to corrosion in sulfuric and phosphoric acid conditions. Molybdenum additions can improve the resistance of cobalt to corrosion in general. Additions of vanadium and niobium can be detrimental to the cobalt alloys in terms of corrosion resistance, while additions of manganese, iron, yttrium and lanthanum can improve the alloy.

3.8.2 Resistance to Forms of Corrosive Attack

Cobalt is susceptible to pitting and crevice corrosion, and is usually very resistant to SCC in many environments, but at higher temperatures (>150 °C) in acid chlorides and strong bases it may experience stress corrosion cracking.⁷⁷ Cobalt based alloys have an excellent resistance to erosion corrosion, especially from cavitation, and they also typically have outstanding resistance to high temperature corrosion. The resistance of cobalt alloys to oxidation and carburization at high temperatures is generally very good, and the resistance to sulfidation is better than that of nickel-based alloys. Furthermore, cobalt-based alloys tend to be more resistant to hydrogen embrittlement compared to their nickel counterparts.

3.8.3 Corrosion Resistance to Acids and Alkalis

Cobalt-based alloys are generally resistant to corrosion in aqueous environments. Cobalt-based alloys are very susceptible to corrosion in phosphoric acid, but are resistant to corrosion in acetic acid environments. The corrosion resistance of cobalt-based alloys is improved in sulfuric acid environments when oxidizing agents are present. Cobalt-based alloys are typically more  resistant to nitric acid, but more susceptible to corrosion in caustic environments than their nickel counterparts.

3.9       Refractory Metals

Refractory metals have very high melting points, retain their strength at high temperatures, and accordingly are often used for jet engine and space applications.  Several of  the  refractory  metals and their corresponding melting points are given in Table 41. Refractory metals are typically susceptible to oxidation at high temperatures, but resistant to corrosion in many environments at lower temperatures.

Table 41           Melting Points of Several Refractory Metals

3.9.1 Molybdenum

At high temperatures (approximately 700°C) in air, molybdenum forms a volatile oxide (MoO₃). It does not perform well in the presence of oxidizing agents at temperatures greater than 500°C, and requires protective coatings in order to be used practically in such environments. Molybdenum does, however, have a good resistance to hydrofluoric, hydrochloric and sulfuric acids without the presence of oxidizing agents. It has a good resistance to corrosion in low to moderate concentrations of sulfuric acid at low to moderate temperatures. Molybdenum is susceptible to oxidizers (e.g. nitric acid), and are generally resistant to alkaline solutions, although not in the presence of oxygen or oxidizing agents.

3.9.2 Tantalum

Tantalum is a fairly inert and expensive metal that is durable and long lasting with a very good resistance to corrosion in many environments including severe ones. Its corrosion resistance can be at least partially attributed to the thin, protective oxide film (usually Ta₂O₅) that forms when exposed to air or another oxidizing environment at 300°C. Its corrosion resistance, in general, is better than that of niobium. It is, however, embrittled in oxygen at temperatures greater than 350°C.

Tantalum is resistant to fresh water, mine water, deionized water and seawater; it is also resistant to steam at high pressures. It is highly resistant to most acids (e.g. sulfuric (H₂SO₄), nitric (HNO₃), hydrochloric (HCl), hydrobromic (HBr), etc.), chemical solutions, salts and salt solutions, and organic compounds including alcohols, ketones, alkaloids and esters, and is fairly resistant to dilute alkaline solutions.

Tantalum reacts with gaseous oxygen, nitrogen, and hydrogen at higher temperatures. It is susceptible to corrosion in hydrofluoric acid, hot concentrated phosphoric acid, sulfite (SO₃), strong alkalis, and strong sulfuric acid at higher temperatures. Tantalum is also susceptible to hydrogen embrittlement, if it is not protected from becoming cathodic in an electrochemical cell that produces atomic hydrogen. The reason for this is because tantalum will absorb hydrogen when it is galvanically coupled with anodic metals.

3.9.3 Niobium

Niobium is a refractory metal that has characteristics very similar to tantalum, which is located below it on the Periodic Table. It oxidizes readily, especially in air above 200°C, and forms a protective oxide film that provides good corrosion resistance.

Embrittlement of niobium is a problem in hydrogen, nitrogen, oxygen, or carbon at temperatures greater than 300°C. Niobium reacts with nitrogen at temperatures greater than 350°C, water vapor at temperatures greater than 300°C, chlorine at temperatures greater than 200°C, and hydrogen, carbon monoxide and carbon dioxide at temperatures greater than 250°C.

In general, niobium has a good resistance to both mineral and organic acids, but is susceptible to alkaline solutions. Specifically, it is resistant to hydrochloric, hydroiodic, hydrobromic, nitric, sulfuric and phosphoric acids; it is susceptible, however, to corrosion in hydrofluoric acid, and strong sulfuric and hydrochloric acids at higher temperatures. It is also less resistant to hot mineral acids compared to tantalum.

Niobium exhibits a good resistance to most gases at temperatures up to 100°C. It is also resistant to liquid and vaporous metals and molten salts. Furthermore, neither salt solutions nor seawater readily attack niobium.

3.9.4 Tungsten

Tungsten is extraordinary because it has the highest melting point of any metal. Besides its high temperature capability, it is resistant to corrosion in weak acids and alkalis at lower temperatures, but it is somewhat susceptible to strong acids at lower temperatures. Moreover, tungsten is susceptible to corrosion by alkalis and strong acids at high temperatures, and the attack can be accelerated or possibly more severe in the presence of oxidizing agents. Tungsten is highly resistant to atmospheric corrosion and to corrosion in water. Oxidation is insignificant in air below 595°C or in oxygen at temperatures less than 510°C.⁹

3.9.5 Zirconium

Zirconium is an expensive and fairly reactive metal that is similar to hafnium, which is below it on the Periodic Table. A self-healing oxide film forms readily on the surface of zirconium in environments with available oxygen; this film protects against corrosion and wear. Zirconium is generally resistant to water and water vapor at regular and higher temperatures, although prolonged exposure to hot water may lead to rapid corrosion of the metal. It is also resistant to salt solutions, seawater, and polluted water. More specifically, zirconium is resistant to uniform, pitting and crevice corrosion in seawater.

Zirconium is also resistant to many acids and bases, including most mineral and organic acids and strong alkalis. Specifically, it is resistant to hydrochloric, nitric, acetic, formic, citric, lactic, and tannic acids, among others. Zirconium is susceptible to hydrofluoric acid, chromic acid and strong hydrochloric and sulfuric acids at higher temperatures. Its resistance to alkalis remains, even at higher temperatures, and has only a moderate corrosion rate when exposed to fused alkalis and liquid sodium.

Ferric chloride (FeCl₃) and cupric chloride (CuCl₂) environments will often cause pitting to occur on the surface of zirconium, but it is resistant to some molten salts. In general, zirconium  is resistant to oxidizers in the absence of halides, but it is susceptible to corrosion, for example, in a humidified chlorine gas. A further threat to zirconium is hydrogen embrittlement.

Impurities in the composition influence the corrosion resistance of zirconium. For example, impurities such as nitrogen, aluminum, titanium, and dissolved ferric and cupric chlorides) degrade the resistance of zirconium to water and steam. Nuclear grades of zirconium do not contain hafnium and have better corrosion resistance in water at higher temperatures.

3.10    Beryllium

Beryllium is used in the nuclear industry, jet and rocket propulsion systems, mirror and re-entry vehicle structures, and aircraft brakes. It is virtually unaffected in normal atmospheric conditions even at elevated temperatures. Condensation on Beryllium can pose a corrosive attack under certain circumstances.

3.10.1 Effect of Impurities

Beryllium is produced in several grades, although none include intentional alloy elements. The production of Beryllium is controlled to reduce impurities present. Commercial grade Beryllium typically contains between 1 and 4.5 % total impurity content. Impurities on the surface of Beryllium through fabrication, cleaning, and machining can increase rates of  corrosion.  Carbides (Be₂C), introduced through machining, as well as chlorides and sulfates, introduced during a drying process, have resulted in corrosive attack of Beryllium. Improper handling in the form of fingerprints left on dry Beryllium has also led to corrosion. It is essential to control the processing and handling of Beryllium to limit impurities in and on the surface of the finished product.

3.10.2 Resistance to Forms of Corrosive Attack

There is a limited amount of published literature on the corrosion of Beryllium with the following information found.

3.10.2.1 Pitting Corrosion

Pitting corrosion of Beryllium has been seen to occur in some components prompting an investigation of the source mechanisms. It was found that pitting occurred in areas rich in aluminum, silicon, and iron impurities on the material’s surface.

3.10.3Corrosion Resistance in Various Environments

The susceptibility of Beryllium is primarily a result of corrosive chemicals, namely chlorides, sulfates, and nitrites present under humid conditions. A controlled humidity environment for storage has been found to be effective in limiting corrosion of Beryllium components.

3.10.4 Corrosion Protection of Beryllium

Coatings used on beryllium for corrosion resistance include anodic coatings, chromate, fluoride, and organic paints.⁷⁸ Anodic coatings, similar to those used in anodizing aluminum alloys, have been found to increase corrosion resistance in aqueous solutions and for elevated atmospheric temperature environments. Chromate coatings provide protection during storage and handling periods and in marine type environments for moderate periods. Fluoride coatings are used in distilled water and saltwater environments. Organic paints are used to provide an electrical insulation layer. This limits galvanic attack and has been observed to provide long term storage protection when deposited on top of a passivation type coating. Plated coatings have also been used on beryllium, providing electrical contacts, improved wear resistance, and better polishing surfaces.

3.11    Uranium

Depleted uranium is primarily used in weapon systems for its high density. With some alloying, the corrosion resistance is increased in various environments. Two forms of corrosion of which uranium alloys have showed a higher susceptibility, are galvanic and stress corrosion cracking. Protective measures used for uranium alloys have been oxide coatings, organic films, and metal platings.

3.11.1 Alloys and Alloying Elements

The corrosion resistance of uranium is increased with the addition of some alloying elements. The most common alloying elements are titanium, molybdenum, niobium, and zirconium. The addition of these elements promotes the formation of g-phase (cubic) rather than the a-phase (orthorhombic) of unalloyed uranium, which increases the corrosion resistance.

3.11.2 Resistance to Forms of Corrosive Attack

3.11.2.1     Galvanic Corrosion

A measure of the electrode potentials of a few uranium alloys was obtained in both seawater and 0.1 N HCL. These values are used to determine  the  potential  for  galvanic  corrosion  in  similar environments, when in contact with dissimilar metals as covered in Section 2.2.  Table  42 gives the values measured in the two environments.

Table 42           Electrode Potentials of Uranium Alloys in Seawater and HCl

3.11.2.2 Stress Corrosion Cracking

Stress corrosion cracking has been found to be problematic with uranium alloys. The U-0.75Ti alloy has the highest susceptibility, with SCC also occurring for the U-Mo and U-Nb alloys. The study of U-0.75Ti alloy in varying environments showed water to be the primary variable responsible for SCC with oxygen deterring SCC. The U-Mo alloys revealed susceptibility for  Mo concentrations of 0.6-12%. From 0.6 to ~5%, metastable materials were produced  containing the a-phase, showing greater susceptibility to SCC. Above 5%, oxygen is  the primary variable responsible for SCC, just the opposite as for the U-0.75Ti alloy. Increasing carbon content in the U-Mo alloys also produces increased susceptibility. Heat treating  quenched alloys produced a more equilibrium microstructure, proving to be less susceptible. Uranium-niobium alloys showed water induced susceptibility for the lower content Nb alloys (2.3 and 4.5%) and oxygen induced susceptibility for the higher content alloys (6 and 8%).  Water vapor further increases the rate of attack for the U-6Nb and U-9Nb materials in an oxygen environment. The U-7.5Nb-2.5Zr alloy has been observed to form intergranular cracking which is easily propagated in the presence of oxygen, water, and chloride. Transgranular cracking has also been seen for U-7.5Nb-2.5Zr in oxygen environments, but propagates slowly. The standard aging temperature for this alloy is 150ºC, showing the slowest crack propagation rates.

3.11.3 Corrosion Resistance in Various Environments

The   methods   to   study   corrosion   of   uranium   alloys   have   primarily   been   through   the  thermodynamics and kinetics of corrosion science as covered in Section 6.0.  There has  been some limited corrosion testing on uranium, such as in seawater environments.

3.11.4 Atmospheric Environments

Corrosion of uranium and uranium alloys will react in humid air environments by the reaction:

U + (2+x) H₂O ® UO_2+x + (2+x)H₂                                                                                                                Equation 12

where 0 £ x £ 0.1. The generation of hydrogen of various uranium alloys in a 100% relative humidity, 75ºC environment, is presented in Figure 39.

Figure 39          Hydrogen Generation of Various Uranium Alloys

3.11.3.2 Water Environments

A similar dependence of alloying effect on the corrosion of uranium in water environments takes place.  Measurements  of  the  uniform  corrosion  rates  of  some  uranium  alloys  and  unalloyed uranium in seawater at 20ºC is shown in Figure 40.

Figure 40         Uranium Alloy Uniform Corrosion Rates in Seawater⁷⁹

3.11.3.3 Chemical Environments

The uranium-binary alloys show active-passive behavior in a number of chemical environments. The ion U³⁺ forms an active region, while UO₂²⁺ forms the passive region near the corrosion potential. Anodic polarization methods are used to study the active-passive transitions in uranium. The transition from active to passive generally represents a decrease in corrosion rate on the order of 10⁴ to 10⁶.

3.11.4 Corrosion Protection of Uranium

Materials and methods to provide corrosion protective coatings on uranium were studied primarily on unalloyed uranium. Ceramic oxides, organic films and metal platings have all been studied. The ceramic oxides and organic coatings tested have shown a minimal decrease up to increased uniform corrosion rates over unalloyed uranium. Metal platings, namely electroplated nickel and ion-plated aluminum have been found to decrease uniform corrosion rates in short- term slightly elevated temperature tests.

3.12    Cast Irons

Cast iron generally consists of alloying with >2% carbon and >1% silicon with various additional alloying elements dependent upon the application. Cast irons are among the lowest cost metals as they have low raw material costs and are more easily manufactured into product forms. They  may be alloyed for corrosion resistance obtaining levels similar to that of stainless steels and nickel-based alloys.

3.12.1 Alloying for Corrosion Resistance

Cast irons can be classified by the degree of alloying into unalloyed gray, ductile, malleable, and white cast irons, low to moderately alloyed cast irons, and high-nickel, high-chromium, and high-silicon cast irons. The unalloyed irons consist of £3% of carbon, £3% silicon, with no additional intentional alloying. The corrosion resistance of this class is slightly higher than that of the unalloyed steels. The low to moderate alloyed irons include additions of chromium,  nickel, copper, and/or molybdenum. They typically have two to three times the corrosion resistance of the unalloyed irons. The high alloyed cast irons have a high corrosion resistance to certain acid and alkali environments. High alloying for corrosion resistance may however compromise other material properties, such as a lower strength.

Alloying with silicon, nickel, chromium, copper, molybdenum, and to lesser extent, titanium and vanadium, will increase corrosion resistance. The alloying elements  along  with  their  associated effects on corrosion resistance are presented in Table 43.

Table 43           Cast Iron Alloying Elements and Their Effects

3.12.2 Resistance to Forms of Corrosion

Cast irons will exhibit the same forms of corrosion as other metals. Notable susceptibilities found in the literature, specific to cast irons will be covered in the following sections.

3.12.2.1 Uniform Corrosion

The corrosion resistance of unalloyed cast irons  slightly  exceeds  that  of  unalloyed  cast  steels,  with increased resistance dependent upon the extent of higher alloying content.  Figure  41 depicts the uniform corrosion rates of some cast irons in relation to a cast steel alloy.

Figure 41          Uniform Corrosion Rates for Ferrous Metals Exposed for Twelve Years

3.12.2.2 Galvanic Corrosion

Gray irons have microstructures conducive to galvanic attack in mild environments. The attack has been termed “graphitic corrosion” and has been classified as both galvanic corrosion and selective leaching. Graphite is cathodic to iron which leads to a localized galvanic cell within gray irons, which in turn leads to selective leaching of the iron. This form of attack only occurs in mild environments as more severe environments produce a more uniform corrosion where the graphite is also removed from the surface of the gray iron.

3.12.2.3 Fretting Corrosion

Fretting corrosion has been observed in  a  number  of  metals  when  in  contact  with  cast  irons. Table 44 summarizes field experience in fretting resistance of  cast  irons  to  other  various materials.

Table 44           Fretting Resistance of Cast Irons to Various Materials

3.12.2.4 Pitting Corrosion

Pitting of cast irons has been reported for environments that included chlorides, dilute alkylaryl sulfonates, antimony trichloride, and calm seawater. High silicon cast irons, especially those containing chromium and/or molybdenum have been found to exhibit a higher resistance to pitting. Nickel additions to cast irons increase pitting resistance for calm seawater environments.

3.12.2.5 Crevice Corrosion

The presence of chlorides in crevice areas of cast irons will increase the rate of crevice corrosion. High silicon cast irons with chromium and/or molybdenum provide higher resistances to crevice corrosion.

3.12.2.6 Intergranular Corrosion

Intergranular attack in cast irons is rare. The only instance found involves an attack of unalloyed cast iron by ammonium nitrate.

3.12.2.7 Erosion Corrosion

The resistance of cast irons to erosion corrosion may be enhanced by increasing the hardness and/or increasing some alloying elements. In relatively non corrosive environments, increasing the hardness through solid-solution or phase transformation induced hardening can increase erosion corrosion resistance. Higher alloying content combined with a higher hardness will increase resistance in more corrosive environments.

3.12.2.8 Stress Corrosion Cracking

Cast irons generally have less susceptibility to SCC due to their fabrication process which limits stresses in the material compared to other processes. However, cast irons still exhibit SCC in a number of environments. The flake graphite structure, found in gray irons and high silicon irons have been found to be more susceptible than other cast irons in the presence acid environments. The acids diffuse into the iron along graphite boundaries and the corrosive byproducts eventually produce enough pressure to crack the iron. Environments found to increase SCC in cast irons include the following:

• Sodium hydroxide solutions
• Calcium nitrite solutions
• Ammonium nitrate solutions
• Sodium nitrate solutions
• Mercuric nitrate solutions
• Hydrogen sulfide solutions
• Oleum (fuming hydrogen sulfide)
• Mixed acids
• Hydrogen cyanide solutions
• Seawater
• Molten sodium-lead alloys
• Acid chloride solutions

3.12.3 Corrosion Resistance in Various Environments

Cast irons find many applications in various environments, and are selected based upon the anticipated chemicals present. Unalloyed and low-alloy cast irons corrosion rates are increased by exposure to sulfur dioxide and similar industrial type atmospheres. They are also readily attacked by chlorides, typical in marine environments. In soils, increased rates of corrosion can be expected in poorly drained areas and where corrosive chemicals are present. The addition of ~3% nickel has been used to increase corrosion resistance of cast irons in poorly drained soils. The corrosion of unalloyed cast irons in water is lower for hard water conditions, where a protective scale of calcium carbonate will develop. Protective scales do not develop well in soft and deionized waters for unalloyed cast irons, resulting in some expected corrosion. Lower pH levels will increase the rate of attack while higher pH levels reduce corrosive effects. High alloy cast irons are not typically used in these environments as their increased cost versus performance does not warrant their use. High-nickel austenitic cast irons have been used for their resistance  to pitting in calm seawater conditions. High-silicon cast irons have been used for anodic protection in seawater and brackish water environments.

Cast irons also find applications in many of the common acid and alkali solutions used. They are generally attacked more in mineral acids than the organic acids. The cast irons find uses for varying concentration levels and temperatures, but impurities present can severely degrade their corrosion resistance. The resistance to mineral acids is summarized in Table 45. Unalloyed and low-alloyed cast irons have fair resistance to alkali solutions, but should be kept below 80ºC and 70% concentrations and are highly susceptible to hot solutions of ³30% concentration levels. The addition of 3-5% nickel increases corrosion resistance to alkali solutions. The high-silicon cast irons have generally the same corrosion resistance as the unalloyed cast irons. They may be used only when impurities are present which reduce the resistance of the unalloyed irons. High- chromium cast irons are more susceptible to alkali solutions and are therefore not recommended.

Table 45           Cast Iron Acid Resistance Properties

3.12.4 Corrosion Protection of Cast Irons

Metal, organic, conversion, and enamel coatings are used to protect unalloyed and low-alloyed cast irons. High-alloyed irons are rarely coated. Metal coatings may be  anodic  to  iron, providing a sacrificial protection, while other metals may be barrier type coatings. The  remaining coating material types  are  all  barrier  coatings.  The  various  coatings  and applicable environments are listed in Table 46.

Table 46           Coating Materials for Cast Irons

3.13    Tin

Tin is commonly used as a coating for metals (tin plate) to provide corrosion resistance. It is a relatively inert metal that is ductile and has a low strength, which degrades significantly with increasing temperature.

Oxygen or other oxidizers readily attack tin, but it has an excellent resistance to water having a high purity, and a good resistance to salt solutions and water containing carbon dioxide. A good resistance to atmospheric corrosion is also characteristic of tin. Tin is resistant to weak acids, but is very susceptible to corrosion in alkalis, strong acids and oxidizing acids. Tin is particularly susceptible to sulfuric, hydrochloric and nitric acids.

3.14      Cadmium

Cadmium is used mostly as an electroplated coating, especially on high-strength steels in aircraft, since it improves the resistance to corrosion fatigue. It has a favorable resistance to alkalis, but is susceptible to hydrogen embrittlement.

Chapter 3.15: Lead and Its Alloys – Corrosion

In general, lead has a very good resistance to corrosion in a number of environments including atmospheric, aqueous, and other chemical environments. Atmospheric corrosion poses almost no threat to lead due to its excellent resistance to corrosion in most types of atmospheric environments including those containing industrial pollutants (e.g. SO₂, SO₃, CO₂, H₂S, etc.). Lead’s inherent corrosion resistance is mostly due to the protective film on its surface, which can form in a wide variety of environments including those containing oxides, sulfates, carbonates, and chromates. An added benefit of this film is that it is insoluble in the corrosive medium in which it is formed, which consequently results in long-term protection in that environment.

Lead is generally resistant to corrosion in fresh water and seawater, except in those water environments containing dissolved oxygen. In soil, lead also typically has a good resistance to corrosion. The presence of organic acids in the soil from wood, usually results in an increased  rate of attack. Acetic, nitric and formic acids attack lead readily, but it has a good resistance to sulfuric, sulfurous, chromic and phosphoric acids and adequate resistance to hydrochloric and hydrofluoric acids. The presence of oxygen in acidic and soft water environments causes a significant increase in the  corrosion  rate.  In  the  presence  of  most  alkaline  environments,  lead  only has a fair resistance to corrosion.  Table 47 lists a number of corrosive media and  lead’s corresponding resistance to corrosion.

Lead is a very ductile metal with low strength and hardness properties, and due to its softness, it is especially susceptible to erosion corrosion. The corrosion resistance of lead does not vary much between the pure form and its alloys. Therefore, alloys are commonly chosen over pure lead based solely on strength properties. For instance, lead with 3-18% antimony has twice the  strength of pure lead, but the strength does decrease rapidly with temperature.

3.13    Noble Metals

The corrosion resistance of the noble metals is considered excellent although there are some discrepancies in corrosion testing results of these materials. A practical upper limit on  the uniform corrosion rates of noble metals is generally set at about 2 mils/yr due to their high costs. Some of the corrosion rates in various environments may extend higher than this “acceptable” limit as will be discussed. The high cost of the noble metals limits their use in functional devices to small scale applications such as electrical contacts due to their good conductivity and corrosion resistant linings where the combination of their properties make them cost effective.

Table 47           Resistance of Lead to Specific Corroding Agents

3.16.1 Silver

Silver is a ductile material, easily processed, with good corrosion resistance making it applicable as a coating material for corrosion protection in some environments. It is used throughout the food and pharmaceutical industries for lining processing equipment as it maintains the products purity without imparting a metallic flavor to the product.

Low alloying is done to improve the mechanical properties and has little effect on the corrosion resistance of silver. Sterling silver contains ³ 92.5% Ag with some Cu addition. The copper addition results in a duplex structure for most sterling silvers which can be a source of galvanic attack in strong electrolyte environments. The copper can also be selectively oxidized  in elevated temperature environments.  The corrosion resistance of silver in various environments  is listed in Table 48.

Table 48           Susceptibility of Silver to Various Acids

3.16.2 Gold

The high cost of gold in combination with its susceptibility to corrosion from halogens severely limits its use for corrosion properties. Gold is readily attacked by hot mixtures of HNO₃ and H₂SO₄, aqua regia, hydrogen cyanide, mixtures of HBr, HCl, HI, and HNO₃. Gold has been used as an autoclave lining for handling phosphate mixtures up to 500ºC and in the chemical industry for lining equipment used in hydrochlorinations and hydrofluorinations.

3.16.3 Platinum

Platinum has excellent corrosion resistance properties including resistance to industrial type atmospheres containing sulfur compounds. Like other noble metals, platinum finds uses as electrical contacts and for lining process equipment. Alloying platinum with rhodium, iridium, and ruthenium slightly increase corrosion resistance. A 10% rhodium addition increases the susceptibility of platinum to corrosion from dilute HCl at slightly elevated temperatures. Alloying with palladium has essentially no effect on the corrosion resistance while large additions of silver and gold will degrade corrosion resistance to certain chemicals. Platinum is more apt to be used as an alloying element to other materials such as titanium increasing        their resistance to some acids. Table 49 gives the  general  corrosion  susceptibilities  of  platinum to various chemicals.

Table 49           Chemicals that Attack Platinum

3.16.4 Palladium

Palladium is similarly used as other noble metals in electrical contact and as a coating in process equipment. Palladium has been extensively researched as an alloying element to titanium as well as other metals to increase corrosion resistance to acids such as HCl, HNO₃, and FeCl₃ salts.

Palladium is generally resistant to single acids and alkalis, and to most salt solutions. The susceptibilities of Pd are to the following compounds:⁸⁴

• Nitric acid, hydroiodic acid     Hot sulfuric acid
• FeCl₃ and hypochlorite solutions     Chlorine and bromine
• Iodine (very slightly)     Aqua regia
• Potassium cyanide

3.16.5 Ruthenium

Applications of ruthenium are limited due to the difficulty in producing wrought forms. Ruthenium is used as an alloying element to platinum and palladium as a hardening agent and to titanium for corrosion resistance. It has excellent resistance to acids at room temperature and at 100ºC, including aqua regia. Chlorine, bromine, and iodine solutions will attack ruthenium.

3.16.6 Rhodium

Rhodium is primarily used as an alloying element to palladium, platinum, and nickel. It hardens these materials as well as providing increased corrosion resistance. Platinum containing rhodium is used in crucibles, furnace windings, thermocouples, and oxidation catalysts for HNO₃ and ammonia production. Thin coatings of rhodium have been used on glass to produce high reflectivity mirrors and gray filters. Rhodium is slowly attacked by sodium hypochlorite solutions at room temperature but is resistant to most all other solutions including aqua regia and concentrated acids. At 100ºC, it is attacked by sulfuric and bromic acids.

3.16.7 Osmium

Osmium is the rarest of the noble metals with a worldwide annual production usually in the range of a few thousand ounces.⁸⁴ It is alloyed in conjunction with ruthenium to other noble metals for use in electrical contacts, non-rusting pivots for small instruments, and fountain pen tips. Osmium’s corrosion resistance is lower than most other noble metal, being attacked by halogens, some salt solutions, and hot acids. Osmium powder will slowly oxidize at room temperature to form osmium tetroxide.

3.16.8 Iridium

Iridium may be fabricated using conventional powder metallurgy techniques, although it is primarily used as an alloying element. Pure iridium is used in high performance spark plugs and very high temperature crucibles for single crystal preparation. It is used as a hardener and for increased corrosion resistance when added to palladium and platinum. Iridium containing 30% platinum have been used in chemical handling equipment for extremely corrosive materials and for electrical contacts in severe environments. Iridium is the most corrosion resistant metal known. It is highly resistant to virtually all acids at room temperature and at 100ºC. Iridium is slightly attacked by fused sodium, potassium hydroxides, fused sodium bicarbonate, and aqueous potassium cyanide. It has the highest resistance of the noble metals to halogen compounds, with a measured susceptibility only to moist bromine. Iridium can be dissolved in a hot aqua regia solution.

Chapter 4: Corrosion Protection and Control Methods – Corrosion

Even with the proper selection of base metals and well-designed systems or structures, there is no absolute way to eliminate all corrosion. Therefore, corrosion protection methods are used to additionally mitigate and control the effects of corrosion. Corrosion protection can be in a  number of different forms/strategies with perhaps multiple methods applied in severe environments. Forms of corrosion protection include the use of inhibitors, surface treatments, coatings and sealants, cathodic protection, and anodic protection. This section discusses many of the various forms of corrosion protection methods.

4.1       Inhibitors

Inhibitors are chemicals that react with the surface of a material decreasing the material’s corrosion rate, or interact with the operating environment to reduce its corrosivity. Inhibitors may be introduced into the environment in which the material is operating as solutions or dispersions to form a protective film. For instance, they can be injected into a completely aqueous recirculating system (e.g. automobile radiators) to reduce the corrosion rate in that system. They may also be used as additives in coating products, such as surface treatments, primers, sealants, hard coatings, and corrosion preventive compounds (CPCs). Furthermore, some inhibitors can be added to water that is used to wash a vehicle, system or component.

Corrosion inhibitors interact with the metal, slowing the corrosion process by:

• shifting the corrosion potential of the metal’s surface toward either the cathodic or anodic end
• preventing permeation of ions into the metal
• increasing the electrical resistance of the surface

The corrosion potential of a metal is shifted toward the anodic end by inhibiting the cathodic process. This is accomplished by using chemicals that inhibit the corrosion reactions taking place at the cathodic site of the corrosion cell, for example, blocking the hydrogen ions at the metal’s surface from combining to form hydrogen gas. Likewise, the corrosion potential of a metal is shifted toward the cathodic end by inhibiting the anodic process. This is accomplished by using chemicals that inhibit the corrosion reactions taking place at the anodic site of the corrosion cell for example, by keeping the metal from dissociating into ions.

Preventing the permeation of ions into the metal is accomplished by forming a protective film or layer on the metal surface. Inhibitors can form a protective barrier film, which effectively isolates the metal from the corrosive environment, or they can induce the formation of precipitates that block the corrosive agents from accessing the metal. Inhibitors can also increase the electrical resistance of the metal by passivating the surface.

Inhibitors are usually grouped into five different categories: passivating, cathodic, organic, precipitation, and vapor phase. Each of these groups is discussed separately in the following sections.

4.1.1 Passivating Inhibitors

Passivating inhibitors are the most common type of inhibitors mainly because they are very effective in reducing the rate of corrosion. They protect the material by aiding in the formation of a thin, inert film on the surface of a metal, thereby moving its corrosion potential toward the noble region, which effectively passivates the metal. This shift in corrosion potential can be significant, and sometimes on the order of a 100 mV.³ Passivating inhibitors can be either oxidizing, which  do not require oxygen to be present, or nonoxidizing, which do require oxygen to be present in the environment. Oxidizing inhibitors include nitrites and nitrates, and chromates were one of the most widely used inhibitors. Although chromate inhibitors are some of the most effective, they are currently being phased out by regulations from the Environmental Protection Agency in the United States due to health and environmental concerns. Nonoxidizing inhibitors include phosphates and molybdates. These can only be used for applications which encounter oxygen- containing environments. The primary disadvantage to passivating inhibitors is that they can actually accelerate localized corrosion on the material being protected if the concentration of inhibitors falls below a critical concentration. Therefore it may be necessary to periodically reapply the corrosion inhibitor or monitor the inhibitor concentration.¹⁰

4.1.2 Cathodic Inhibitors

Cathodic inhibitors specifically target the cathodic region of the metal or electrochemical cell and provide protection by inhibiting the rate of the cathodic reaction. This is generally accomplished by building a barrier layer to obstruct the corrosive agents from accessing the metal surface or by preventing the reagents in the cathodic process from forming their normal products (e.g. hydrogen gas). For example, certain inhibitors can precipitate on selected cathodic areas of the metal to form a barrier, effectively isolating the metal from the environment. Also, other inhibitors can preemptively occupy or react with hydrogen or oxygen, for example, and keep them from forming hydrogen gas or, in the case of oxygen, keep it from oxidizing the metal. Calcium bicarbonate, zinc compounds, and polyphosphates are some examples of cathodic inhibitors.

4.1.3 Organic Inhibitors

Unlike cathodic inhibitors, organic inhibitors tend be active over the entire metal by adsorbing to the surface to form a thin, water-displacing film. The strength of the adsorptive bond between the metal and the film is a key factor in determining the level of protection the inhibitor will provide. This bonding strength is primarily dependent on the relative ionic charge between the metallic surface and the organic inhibitor. Anionic inhibitors (inhibitors with a negative ionic charge), such as sulfonates, are used for positively charged metal. Cationic inhibitors (inhibitors with a positive ionic charge), such as amines, are used for a negatively charged metal.¹⁰

4.1.4 Precipitation Inhibitors

Precipitation inhibitors are chemicals that can induce the formation of precipitates on a metal.  The precipitates tend to cover the entire surface of the metal and act as somewhat of a barrier to the corrosive environment. Examples of precipitation inhibitors are silicates (e.g. sodium silicate) and phosphates.¹⁰

4.1.5 Vapor Phase Inhibitors

Vapor phase inhibitors are also known as volatile corrosion inhibitors are carried by a vapor phase product, such as water vapor, to the surface of the metal to be protected. When it reaches the metal surface it the vapor phase condenses, causing a release of the inhibitor ions.

4.1.6 Inhibitor Compounds

Inhibitors may be inorganic or organic materials. Inorganic inhibitors are usually crystalline salts including sodium chromates, phosphates and molybdates. The negative ions of these materials  are responsible for reducing corrosion. Organic inhibitors include sodium sulfonates, phosphonates, mercaptobenzotriazole (MBT), and aliphatic or aromatic compounds containing positively charged amine groups. Inhibitors may be produced into liquids, solids including hard and soft materials, or vapors to be used in numerous applications. Their greatest use comes in systems involved with liquid heating or cooling systems. Inhibitors are introduced into the liquid media and the concentration and/or the corrosion rate of the system monitored to maintain an optimal concentration level. Vapor phase inhibitors including morpholine and hydrazine are introduced into steam environments such as boilers, to increase the pH level in the system. The selection of inhibitors will depend upon the metal requiring protection, as well as the operating environment. Various inhibitors used to protect metals in  some  environments  are  listed  in Table 50.

4.2       Surface Treatments

A surface treatment is the modification of a material’s surface using various means to improve some characteristic of the material, in this case the corrosion resistance. Conversion coatings and anodizing involves a chemical reaction to create an improved corrosion resistant oxide film layer on the metal’s surface. Shot peening is a mechanical process to induce compressive residual stresses improving resistance to SCC and corrosion fatigue. Laser treatment uses heat to modify surface structure, aid a chemical reaction in modifying the surface, or to induce compressive residual stresses within a metal to increase its resistance to SCC and corrosion fatigue.

4.2.1 Conversion Coatings

Conversion coatings are used as a protective, or sometimes decorative, coating which  is  produced in-situ by a chemical reaction of a metal’s surface with a chosen environment.

4.2.2 Anodizing

Anodizing is an electrochemical process, most frequently used on aluminum, although it can be used with other metals, such as magnesium and titanium alloys. An electric current is passed through an electrolyte (usually chromic, phosphoric, or sulfuric acid) causing the surface of the anodic metal to form an oxide film. This film can be significantly thicker than the naturally occurring one, and thus can provide better corrosion protection. An advantage of anodizing over coating deposition methods is that the resultant coating is an integral part of the substrate rather than being a layer that is bonded to the substrate. Anodized coatings are, however, typically brittle and susceptible to strong acids and bases.

Table 50           Some Inhibitors Used to Protect Various Systems/Metals

4.2.3 Shot Peening

Shot peening is a cold working process originally implemented to increase fatigue strength. A stream of shot is used to bombard a metal’s surface, inducing compressive stresses and relieving tensile stresses within the material.⁸⁵ The depth of the shot peening effect is typically about 0.13 to 0.25 mm below the surface. The altering of residual stresses on the metal’s surface results in a higher fatigue resistant material, and also a higher resistance to corrosion fatigue and stress corrosion cracking.

4.2.4 Laser Treatment

There a four uses of laser technology to modify the surface properties of metals. One method is to harden the surface using laser heating which enhances thermal diffusion at the surface.⁸⁶ A second technique is to use laser heating to melt the surface which is then rapidly quenched to modify the surface structure. A third method uses a laser to melt the surface and alloying elements are added to the surface melt effectively creating a different material at the surface.  The fourth method makes use of a laser’s shock effect to induce compressive stresses within a metal’s surface.⁸⁷ This has the same effect as shot peening with the major difference that it can produce residual compressive stresses to a depth of approximately 1.0 mm, resulting in higher fatigue lives. This method is referred to as “laser peening” or “laser shock processing.”

Chapter 4.3: Coatings and Sealants – Corrosion

Metallic, inorganic and organic coatings are used frequently for providing long-term corrosion protection of metals in various types of corrosive media. There are two main types of coatings: barrier coatings and sacrificial coatings. A barrier coating acts as a shield and protects the metal from the surrounding environment, whereas a sacrificial coating  functions  as  a  sacrificial  anode  and  thus,  corrodes  preferentially.  Barrier  coatings  are  typically  unreactive,  resistant to  corrosion,  and  protective   against   wear.   Sacrificial   coatings   provide   cathodic protection by supplying electrons to the base metal. Sealants provide corrosion protection by completely securing the component from moisture penetration.

4.3.1 Metallic

Metallic coatings provide enhanced corrosion resistance of metals as either barrier coatings or sacrificial coatings. They are durable, usually easy to form, but sometimes porous, which can result in accelerated corrosion of the substrate metal. Some of the common metals used as coatings are nickel, lead, zinc, copper, cadmium, tin, chromium, and aluminum. Methods for applying metallic coatings include cladding, electrodeposition (electroplating), electroless plating, spraying, hot dipping, diffusion, chemical vapor deposition (CVD), and ion vapor deposition.

4.3.1.1 Nickel

Nickel is used as a coating for corrosion protection applications, and is also used as an undercoat for other coatings. Electrodeposition is the common method for applying nickel, but electroless plating can also be used. When nickel is used as a coating for steel, copper is sometimes used as an intermediate layer. Nickel is also used as an intermediate layer between steel and microcracked chromium to prevent the corrosion of steel. Nickel-phosphorous coatings have a superior corrosion resistance compared to nickel coatings, and can be electrodeposited or electrolessly deposited.

4.3.1.2 Aluminum

Hot-dipping, spraying, cementation, and ion vapor deposition processes are used to deposit aluminum coatings on steel. Hot-dipped aluminum coatings are used to protect the metal substrate from atmospheric corrosion and oxidation at elevated temperatures. Sprayed aluminum coatings are sometimes sealed with organic coatings to provide more uniform and impermeable protection. Ion vapor deposited aluminum coatings are soft and formable. Aluminum coatings have a minimum thickness of approximately 8-25 mm.⁶

4.3.1.3 Lead

Electrodeposition and hot dipping are usually employed to apply lead coatings on steel, with tin sometimes added to improve bonding. Of course, lead compounds are toxic and therefore, the use of lead coatings is limited.

4.3.1.4 Copper

Copper is susceptible to atmospheric corrosion, and thus, it is not very useful as a protective coating when used alone. It is, however, useful when used in conjunction with subsequent coatings, as it has a low porosity and can work as a barrier coating with porous, corrosion resistant coating to protect the base metal from corrosion. In addition corrosion inhibitors, such as benzotriazole, can also improve the performance of copper coatings.⁸⁸

4.3.1.5 Cadmium

Cadmium is usually a preferred coating for the corrosion protection of steel in moist and marine environments; it is anodic to steel and therefore, will act as a sacrificial anode on steel.  Cadmium coatings are smooth and conductive, and resist fretting and fatigue, but have been known to cause solid metal embrittlement of steel and titanium and exfoliation of susceptible aluminum alloys. Furthermore, the corrosion products of cadmium are toxic, so it should be avoided in applications that may contaminate the environment. There are alternatives to cadmium coatings however, such as zinc and tin coatings. Cadmium coatings are applied mostly by the electrodeposition process and are good for electrical applications. Minimum coating thickness is approximately 5 – 25 mm.⁶

4.3.1.6 Zinc

Galvanization denotes the application of a zinc coating to the surface of a metal by any method. Hot dipping, electrodeposition, and spraying are a few methods used to galvanize a metal. Zinc  is less expensive than cadmium, and is generally the preferred coating in industrial environments.

4.3.1.7 Chromium

Chromium coatings are hard and provide good wear resistance, but are typically used in conjunction with other coatings such as copper and nickel for corrosion protection applications.

4.3.1.7 Tin

Tin is another very common material used in coating applications and it provides good corrosion resistance to the metal substrates, either as a barrier or sacrificial coating. It is often used to coat steel and sometimes copper. Tin coatings are typically thin and porous; therefore, to achieve corrosion protection they should act as a sacrificial coating. Tin coatings are widely used in the food industry as coatings on steel containers.

4.3.1.7 Gold

Gold is often coated over other coatings to provide enhanced appearance or improved electrical properties. Gold coatings are used mostly for electrical applications (and jewelry) as they have a low contact resistance.

4.3.2 Ceramic

Ceramic coatings are inorganic, nonmetallic coatings that act as a barrier between the corrosive environment and the base material being protected. They often consist of an oxide film that is formed on the surface of a metal by chemical reaction, which can occur naturally on some metals; however, more effective corrosion resistant coatings can be produced. Ceramic coatings are especially useful for providing high temperature corrosion protection. Examples of ceramic coatings include chromate films and phosphate coatings.

4.3.2.3 Chromate Films

Although chromate films provide a significant improvement in the corrosion resistance of a metal substrate, it is mainly used as a precursor to other coatings and paints. Chromate coatings are often used on steel, copper, aluminum, magnesium, nickel, silver, tin, and cadmium substrates.⁸⁹ Thin chromate films can be applied by immersion, spraying, or brushing.

4.3.2.4 Phosphate Films

Metal phosphate coatings, which form on the surface of a metal when subjected to the appropriate environment by chemical reaction, are used mainly for corrosion protection, but in addition, they provide a good surface for other coatings to adhere to. When combined with corrosion inhibitors or other coatings, the corrosion protection is improved significantly. Phosphate coatings are usually applied either by spraying for larger components or by immersion in solution baths. Immersion is the preferred method as a more homogeneous coating is produced.

4.3.3 Organic

Organic coatings are widely used for corrosion protection applications on exterior surfaces and also for interior coatings and linings. In fact, organic coatings are used more for corrosion protection than any other protection method available; they can also provide enhanced appearance of a previously dull or unattractive metal. There are several types of  organic coatings, which include paints, varnishes, enamels, and lacquers, and numerous organic materials to choose from for corrosion protection applications.  The types of coatings are defined below   in Table 51.

Table 51           Organic Coating Types and Definitions

Organic coatings have three basic methods of protecting a metal substrate from corrosion: 1) by preventing the attacking agents from penetrating through to the metal (impermeability), 2) by inhibiting attacking agents, and 3) by functioning as a cathodically protective material. An impermeable coating will protect the metallic substrate from having to face otherwise harmful environments that contain corrosive agents. Organic coatings containing inhibitors can  neutralize the attacking corrosive agents by reacting with them and possibly forming a protective film on the metallic substrate. Cathodically protective organic coatings contain additives which decreases the corrosion potential between the metal and the surrounding corrosive environment.

An organic coating system will often have three components: 1) a primer, 2) an intermediate coat, and 3) a topcoat. The primer is very important to the integrity of the coating system. It is the fundamental layer of the system and thus provides the basic adhesion between the metal substrate and the intermediate or subsequent layer of the coating system, as well as corrosion protection. The intermediate coat provides corrosion resistance and thickness to the coating system. The top-coat is also very important since it provides the first level of protection against corrosion and acts as a seal over the intermediate coat and primer. Typically it is thinner than the priming coat, has good wear and abrasion resistance, and usually determines the appearance of the organic coating system.

Proper coating selection is clearly one of the most important aspects in protecting the metal from corrosion. There are, however, three other important factors that should be given proper consideration along with selecting the proper organic coating in order to provide the optimal service life. The first (1) is surface preparation, which is important for providing a strong bond between the coating and the substrate. The second (2) is proper selection and application of a priming coat, which should have good adherence to the substrate and should be compatible with the coating. Poor adhesion or incompatibility could lead to coating failure. The third (3) important factor is the proper selection of a topcoat; however, the topcoat is unimportant if the surface is prepared poorly or an improper primer is selected since the coating will fail anyway.

The ingredients of organic coatings usually include volatile and non-volatile components. The volatile components serve as thinners, while the non-volatile components act as the film-forming ingredients (e.g. resin, oil, wax etc.) and sometimes include pigments and plasticizers.⁹ The pigments have several functions; they provide protection against moisture penetration, resist corrosion, protect against sunlight, and add consistency and color to the coating. Plasticizers are used to keep the coating from cracking.

Organic coatings have some advantages and disadvantages when compared to metallic coatings.⁹ For instance, they are usually more economical, can be applied on top of metallic and inorganic coatings, come in various colors and have a broad range of physical characteristics. They are, however, more susceptible to mechanical damage, and they don’t offer any anodic protection to areas of the substrate that are exposed.

Table 52 provides a summary of various  organic  materials  used  in  coating  systems.  The table shows the advantages and disadvantages of the different resin materials, including properties and characteristics, compatibility with other materials, as well as their performance and compatibility in certain environments.

Table 52           Advantages and Limitations of Principal Organic Coating Materials

4.3.3.1 Corrosion Preventive Compounds (CPCs)

The organic coating system is typically expected to be a longer-term protection method for metals, but there are temporary protective organic materials that can provide short-term protection against corrosion. These are called corrosion preventive compounds (CPCs). CPCs  are generally separated into two categories: water displacing and non-water displacing compounds. They are often used on places where the protective coating has been damaged and the metal substrate is exposed until the coating can be reapplied. CPCs can be used on both interior and exterior surfaces for corrosion protection. Although some CPCs may appear to be a permanent film, they can usually be removed with an appropriate solvent, and are not expected  to be a long-term solution to corrosion.

The water displacing compounds are usually clear or translucent, soft, oily compounds, however some form hard, dry films. These can fill cracks and crevices and form a thin protective layer that is less than 1 mm thick.⁹¹ Non-water displacing compounds are typically thick, colored, and can be either hard or soft and are usually used for longer periods than the water displacing compounds. Generally, CPCs are applied as fluids by wiping, brushing, spraying or dipping.⁶

Three of the most common CPCs are described in military specifications. These are MIL-C- 16173, MIL-C-81309, and MIL-C-85054. MIL-C-16173 is a soft, water-displacing CPC that is sprayed on as a brown film. MIL-C-81309 is a very thin compound which forms a soft film after drying. MIL-C-85054, which is also known as Amlguard, forms a hard, clear film after drying and is of the most commonly used CPCs due to its superior protection capability.   Some of      the more common CPCs are categorized in Table 53.

Table 53           List of Some Common CPCs

4.3.3.2 Rubber

Rubber is not like most other organic coatings, since it is usually used as a lining material on pipes or tanks. They provide an excellent resistance to water.

Chapter 4.3.4: Coating Process – Corrosion

There are a number of methods, ranging from simple to sophisticated, that are used to apply coatings, and there are advantages and disadvantages to each method. The quality of the application of a coating is critical because any defect or significant porosity in the coating can result in severe localized corrosion. Selection of a coating application method is usually based  on the type of coating (i.e. metallic, ceramic, organic), the type of substrate to be coated, the amount of surface area that will be coated, and whether there are any environmental regulations or restrictions. Application methods for metallic coatings include cladding, electrodeposition (electroplating), flame spraying, vapor deposition, and hot dipping. Application methods for ceramic coatings include diffusion, spraying, and chemical conversion.¹² Application methods for organic coatings include brushing, rolling, and spraying. Materials and application methods of metallic, inorganic, and organic coatings will be described below.

4.3.4.1       Hot-Dipping

Hot dipping designates the coating application process of immersing a metal substrate in a molten metal bath, which is usually aluminum, zinc, tin, or lead. Since the applied coating consists of a molten metal, the melting temperature of the metal coating should be relatively low. Hot dipping can be either a continuous or batch process. Hot-dip galvanizing is the most common metal coating method; it involves the application of a thin layer of zinc to carbon steel. The zinc layer provides cathodic protection of the steel thereby protecting the steel from corrosion. Figure 42 shows the service life of hot-dip galvanized steel in different environments.

Figure 42         Service Life² for Hot-Dip Galvanized Coatings⁹²

4.3.4.2 Electrodeposition

Electrodeposition, also called electroplating, is a process where a thin metal layer is deposited on a metal substrate in order to enhance the surface properties, including its corrosion resistance. The metal substrate is placed in an electrolytic solution containing dissolved metal ions, which will ultimately become the coating. An electrical current is passed through the solution, between two electrodes, causing the ions to deposit on the cathode (metal substrate) resulting in a metallic coating.

Characteristics of the coating are dependent on control of the processing parameters including temperature, current density, residence time and composition of the solution.¹² The physical and mechanical properties of these coatings can be altered by varying the processing parameters. They can be made to be thick or thin, hard or soft, or have a layered composition.

A variety of metals are available for use as electrodeposited coatings and include aluminum, chromium, iron, cobalt, nickel, copper, zinc, rhodium, palladium, silver, cadmium, indium, tin, rhenium, platinum, gold, lead, brass, bronze and a number of other alloys. As with all coating application methods, electrodeposition has its advantages and disadvantages.

4.3.4.3 Electroless Plating

Electroless nickel plating is similar to the electrodeposition process except that it does not require an external electrical current to be applied. It is a chemical reduction process where

nickel ions are driven to the surface of the substrate metal by a reducing agent which is also present in the host solution. If processing conditions are properly maintained and the composition of the aqueous solution is uniform, the deposition of the nickel should be uniform over the entire surface of the substrate, even if it has a complex geometry.

4.3.4.4 Cladding

Metal claddings typically provide corrosion protection by acting as a barrier and a sacrificial coating. The cladding method involves a thin metal layer that is installed on the metal substrate by pressing, rolling or extrusion. This produces a metal layer with essentially zero porosity. An advantage is that this allows a thin piece of expensive, corrosion resistant material to be used on an inexpensive thicker piece of metal that is susceptible to corrosion instead of using the corrosion resistant material as the entire piece.

4.3.4.5 Thermal Spraying

Thermal spraying is a coating process in which a material feed is melted by a flame and sprayed by compressed gas onto a substrate; when the molten droplets/particles hit the substrate they flatten and adhere to the surface to form a coating. The process involves the build-up of these flattened particles which melt to form a cohesive coating that adheres to the substrate and covers the entire surface, while filling irregularities on the surface. Bonding between the coating and substrate usually results from mechanical interlock or diffusion and alloying. Therefore, surface preparation of the substrate is an important aspect in the quality of the coating. Often, it is required for the surface to be roughened in order to promote good mechanical adhesion between the coating and substrate. Thermal spraying can be performed using flame spraying, electric arc, or plasma arc.

4.3.4.6 Physical Vapor Deposition

There are several coating application methods which are subsets of the physical vapor deposition category. These include sputtering, evaporation, and ion plating. PVD processes involve plasma bombardment to deposit the metal over the entire area of the substrate.

4.3.4.7 Sputtering

Sputtering is the process where a target material is bombarded by gas ions causing atoms to be ejected and consequently deposited onto the substrate. Some advantages and disadvantages of this process are given in Table 54.

4.3.4.8 Evaporation

Evaporation is a relatively simple process that involves the vaporization of a metal, which is subsequently deposited on a substrate. The adhesion of coatings deposited by this method is  only marginal and uniformity is difficult to achieve. Therefore, the evaporation method is not typically used for corrosion prevention applications.

4.3.4.9 Ion Plating

Ion plating is a process in which ions are driven from a plasma by an electrical bias on the substrate where they are deposited. Alternatively, the coating can be applied using an ion beam deposition technique, where plasma ions bombard the substrate to create nucleation sites for a neutral ion species. The neutral species can then deposit onto the nucleation sites, resulting in  the formation of a coating.

4.3.4.10 Laser Surface Alloying

Laser surface alloying involves feeding the metal to be deposited into a laser beam. The laser beam melts the metal and deposits it on the surface of the substrate, where heat is transferred and a strong metallurgical bond is formed.

4.3.4.11 CVD

Chemical vapor deposition processes involve coating a substrate by chemical means, namely by reacting a precursor gas on the metal substrate. The gas is mixed in a chamber causing it to become reactive and is then sent to another chamber to be deposited onto the substrate. The gas mixture reacts at the surface of the substrate, which is heated in order to drive the endothermic reaction, to ultimately form the coating. It is important in this process to maintain a non-contaminated system. Table 55 lists some of the advantages and disadvantages corresponding to the various coating application methods.

4.3.4.12 Brushing

Brushing is perhaps the most intuitive coating application process, and is used to apply organic based coatings. It is a manual application method, and there are numerous types of brushes that can be used. It is very important to select the appropriate type of brush with the proper bristles in order to produce a high quality coating. The brush size, shape, and bristle type are all important considerations when selecting a brush for a specific coating application. This is because poor brush selection can lead to uneven or discontinuous coating application, runs, drips, or other unfavorable coating characteristics. A standard wall brush is often used for applying coatings to structural steel or similar surfaces. Oval-shaped brushes are used for other structural and marine applications, and are also used to apply coatings near rivets, boltheads, piping, railings and other difficult to reach areas.

Brushes are made with either synthetic, typically nylon, or natural fibers for bristles. The advantage to using a brush with synthetic bristles is that it has a very good resistance to abrasion and are good to use on rough surfaces such. Brushes with synthetic bristles are also less expensive than those employing natural fibers. One of the primary disadvantages to synthetic bristles is that they may be susceptible to strong solvents such as ketones. Natural bristles are more expensive and sensitive to water, but they have a good resistance to strong solvents and are capable of a much finer, uniform coating application.

An advantage to the application method of brushing is the ability to perform what’s called striping. Striping is used to apply the coating around irregular areas that cannot be easily or properly coated through a spraying or other coating technique. Areas that typically require striping include edges, rivets, fasteners, corners, boltheads, and welds. It is a recommended procedure because it can provide the proper coating thickness around these irregular areas, which would not be able to be achieved otherwise. Striping is not used, however, for coatings that have a solute that must remain in suspension, such as zinc-rich coatings. The brushing application method can also achieve complete coating penetration in particularly porous surface areas on a substrate.

A disadvantage of the brushing application method is that it is time consuming as opposed to the spraying methods. Also when applied over a large surface area it is very difficult to maintain a uniform coating thickness through brushing, and therefore it is not a practical method for components or systems with large areas. Furthermore, after the coating dries the surface may have brush marks or slight grooves left over from the bristles. This usually is only a detriment to the appearance rather than the functionality. Another disadvantage to using the brushing method is that it is a difficult technique to use for coatings containing a high solid content and also for fast drying coatings.

Brushing is most commonly used for applying oil-based or water-based coatings to surfaces with small or irregular areas. There are proper techniques in applying the coating that give the best results in the end product. Either an experienced professional or a well-trained technician should be used to apply coatings on critical assets or components.

4.3.4.13 Rolling

Rolling is another manual coating application process, and it requires a roller assembly consisting of a core roller and a cover to absorb and apply the coating material. The assembly can vary in diameter as well as length, and there are also various cover materials. Common  cover materials are polyester, nylon, mohair, and lambskin. Of course, the cover material is usually selected to suit the type of surface to be coated.

There are three types of roller cores: pipe rollers, fence rollers, and pressure rollers. Pipe rollers are used just as the name suggests: for coating surfaces such as pipes. The surfaces usually are contoured and need the roller to flex and cover the surface. Fence rollers use roller covers that have an extra long fiber length, which enables them to simultaneously coat both sides of a surface such as fence wire. Pressure rollers are more sophisticated and have a feed line that moves the coating material to the inside of the roller core from a pressurized tank. The core is a porous material which allows the coating to pass through to the surface of the cover, and thus pressure rollers can provide continuous application of the coating.

Rolling is a good application method for coating large, flat surfaces. A disadvantage is that it is much more difficult to achieve coating penetration into porous or cracked surfaces using the rolling coating application method, and is therefore not recommended for rough or irregular surfaces. Rolling does provide a fine quality finished surface on smooth surfaces. Rolling is a faster process than brushing, but is slower than other coating methods such as spraying.

The roller coating application method is typically used to apply oil-based and water-based coatings, and can also be used to apply epoxy and urethane coatings. This method is not recommended for applying coatings containing a high solids content, zinc rich coatings, or high performance coatings and linings. As with the brushing application method there are proper techniques that result in uniform and quality application of the coating on the substrate.

4.3.4.14 Spraying

There are several variations of the spray coating application method, including high volume-low pressure spraying, airless spray, air-assisted airless spray, plural component spray, and electrostatic spray. Conventional spraying simply uses compressed air to atomize coating particles and propel them toward the substrate. Though simple, the efficiency with which the coating successfully reaches the intended surface is low: ~ 25-30%. Conventional spraying is used to apply coatings such as latex paints, lacquers, stains, sealers, zinc-rich mixtures, alkyds, and epoxies.

An advantage of the spray application technique is that it requires significantly less time than brushing and rolling, and therefore it can be used to coat large surface areas. It also results in a smooth, uniformly coated surface compared to brushing and rolling, and does not leave brush or speckle marks or a textured appearance. Spraying equipment can also be used to clean off the surface prior to applying the coating. Spraying can produce a high quality, smooth surface.

A low efficiency for the amount of coating that is deposited on the substrate is one disadvantage to the spray application method.  Spraying can be a slower process than other coating methods.  It also is sometimes difficult to coat hard to reach areas, such as edges corners, and irregular surfaces with spraying. Since the equipment required for spraying is more expensive than that used for other coating methods, it must be cleaned after each use and properly maintained to ensure durability of the equipment.

High volume low pressure spraying is a spraying technique that uses approximately the same amount of compressed air as conventional spraying but requires less pressure to atomize the coating material. This results in a lower velocity air/coating stream and consequently improves the transfer efficiency from ~30% to up to 70%. This effectively reduces the coating costs by preserving more coating material. The negative side of high volume low pressure spraying is  that the application time needed to coat an equivalent surface area compared to conventional spraying is increased. Furthermore, this spraying technique may not be suitable for applying more viscous coatings due to the low pressure requirement.

Airless spraying is another spraying technique that uses a fluid pump to pressurize and propel the coating material onto the substrate. Advantages to using this technique include good surface penetration (e.g. cracks, porous surfaces), better irregular surface coverage (e.g. corners, edges), quick film buildup, rapid area coverage, and higher viscosity coating materials. The coating material transfer efficiency is usually between 30 and 50%. One of the disadvantages to airless spraying is that it is difficult to adjust and change the equipment configurations (e.g. nozzles, orifices) while in the field. It also does not atomize the coating material as well as the conventional spraying method. Poor application techniques using this particular method can result in coating deficiencies such as solvent entrapment, voids, runs, sags, pinholes, and wrinkles.

A variation of the airless spraying method is the air-assisted airless spray, which incorporates the advantages of the airless spray method and the conventional spray method. For instance, it combines the fine atomization abilities of the conventional spray with the improved production and surface penetration characteristics of the airless spray. This method allows the coating material to be joined with a compressed air jet after it has been atomized in the absence of air. This results in a further atomization of the coating material before it reaches the substrate. This combined method is useful for applying fillers, glazes, lacquers and polyurethanes.

Plural component spraying is a complex application method that mixes coating components immediately before the coating is propelled to the substrate. This method is used for high-solids coatings and for coatings with a short cure time, such as epoxies. This method can be performed by any of the spraying methods mentioned above. This method is used to apply polyesters, polyurethanes, vinyl esters, and epoxies.

Electrostatic spraying is also a coating application process that utilizes the various atomization methods mentioned above (i.e. conventional, airless, air assisted airless). It utilizes an electrostatic, high voltage supply to direct the atomized particles to the substrate by electrostatic attraction. This technique is used to coat irregularly shaped substrates such as cables, piping, and fencing. The advantages of this coating method are that it improves the coating material transfer efficiency, has a good rate of application, and has good atomization properties. A disadvantage  to this method is that it has a tendency for non-uniform deposition of the coating near irregular shaped objects on a surface. Furthermore, it requires special formulation of the coating material.

Proper application techniques are critical when using spraying techniques in order to achieve a high quality, uniform coating on the substrate. Therefore, it is very important that the applicator have either the necessary experience or training in order to produce acceptable coating results.

Table 55           Advantages and Disadvantages of Coating Application Methods

4.4       Cathodic Protection

Cathodic protection (CP) is a widely used electrochemical method for protecting a structure or important components of a system from corrosion. A CP system is essentially an electrochemical cell and must have a cathode, an anode, an electrical connection between them and an electrolyte. The principle behind CP is that dissolution of a metal (cathode) can be suppressed by supplying it with electrons, and in effect, controlling the corrosion. Corrosion is then targeted on the anode instead of the metal. Since an electrolyte is required for this method of protection, CP is not effective for systems in air or other environments that resist current flow between the anode and cathode.

There are two main classes of CP: active and passive. Active cathodic protection, also called impressed-current, requires the use of an external power supply. In this type of protection, the negative terminal of the power supply is connected to the metal to be protected, and the positive terminal is connected to an inert anode. The anode, however, is often not more anodic to the metal, and can be even more cathodic than the metal. The impressed-current ensures that current flows such that the metal acts as the cathode and is therefore protected from corrosion. Moreover, the anodes are not typically consumed by corrosion in impressed-current CP systems, since they do not undergo the typical corrosion reactions. It is possible to overprotect a system using impressed-current CP. If the voltage is too high, the metal can experience hydrogen  embrittlement (e.g. steel) or possibly accelerated corrosion (e.g. aluminum). Therefore, proper conditions for the system should be determined in order to optimize the protection.

Passive CP systems are simpler than impressed-current systems and involve the galvanic coupling of the metal to be protected to a sacrificial anode, which corrodes preferentially. The anode in  this type of system must be more anodic than the metal and must also readily corrode without passivation in order for the system to be successful. In some instances the sacrificial anode must be replaced after it has been consumed to ensure protection of the structure.  A comparison of   the characteristics of the active and passive CP systems is provided in Table 56.

Table 56           Comparison between Sacrificial Anode and Impressed-Current Cathodic Protection Systems

There are several anodes that are available for use in cathodic protection applications. For passive CP  systems  magnesium,  aluminum  and   zinc   are   commonly   used.   Characteristics   of some sacrificial anodes are given in Table 57. Furthermore, there is a  variety  of  anodes available for active CP systems. These include high-silicon cast iron, graphite,  polymers, precious metals, lead alloys, and ceramics.   Table 58 gives a comparison of the consumption   rate between various sacrificial and impressed-current anodes.

Table 57           Characteristics of Sacrificial Anodes

Table 58           Comparison of Sacrificial and Impressed-Current Anodes for Cathodic Protection

Impressed current cathodic protection is sometimes not practical, such as when the metal is in an extremely corrosive environment, which would require a prohibitively high current. Therefore, CP is sometimes used in conjunction with other protection methods in order to enhance the level of protection and avoiding an impractical system. It is common for pipelines, for example, to be coated with an organic coating, and CP is used to protect the structure from corrosion where there are weaknesses or defects in the coating, known as holidays.

A notable disadvantage of CP, specifically active CP, is the resulting stray-current effects it may impose on nearby systems or structures. Stray currents can be picked up by metallic components or structures that are in  close  proximity  to  the  CP  system,  potentially  resulting  in  accelerated corrosion of that metal component or system, as depicted in Figure 43.

Figure 43         Stray Currents Resulting from Cathodic Protection

Cathodic protection systems are usually designed and implemented by a company that specializes in this field. Choosing the right system and then designing it is not a straightforward process, and usually requires expert knowledge to determine what is best for a specific system in a specific environment. Therefore, it is generally recommended that an expert company be contracted or at least consulted to do such work.

4.4       Anodic Protection

Anodic protection is a method of corrosion control that was developed more recently than cathodic protection, but it is used less frequently. As its name implies, anodic protection shields the anodic electrode in the system from corrosion rather than the cathodic electrode as in CP. The principle behind anodic protection, however, is not quite analogous to that of CP. Essentially, instead of shifting corrosion potential from the metal to be protected to an anodic material as in CP, anodic protection involves passivation of the metal to be protected. A passive film forms on the surface of the metal with the application of an electrical current. Once this film is formed, it acts to protect the metal from dissolution, and the film itself is nearly insoluble in the environment which it formed. Passivation causes metals to become very non-reactive and consequently very resistant to corrosion. The limitation of this type of corrosion control is that not every metal can be protected this way; only certain metals in specific  environments  can  be  anodically  protected. These include the metals and solutions shown in Table 59.

Table 59           Metals and Solutions Capable of Being Anodically Protected

Anodic protection requires three electrodes, a potential controller (potentiostat), and a power source. The necessary electrodes are a cathode, a reference electrode and an anode, which is by definition the metal to be protected. The reference electrode monitors the voltage on the anode, and is very important since it is necessary to maintain proper protection and avoid accelerated corrosion. The cathode should be resistant to dissolution; it can be platinum on brass, steel,  silicon cast iron, copper, stainless steel, or nickel-plated steel, among others. The potential controller actively controls the potential on the anode.

A notable advantage of anodic protection is that after the passive film has formed, the amount of current required to maintain this protective film is very small. A further advantage is that the applied current is equal to the corrosion rate of the protected metal. This allows the instantaneous corrosion rate to be measured, which is not the case for CP. Moreover, anodic protection is effective in weak and strong corrosive media. Furthermore, the operating conditions for anodic protection systems can be determined accurately by laboratory-scale experiments, whereas, to do so  for  CP  is  hardly  a  scientific  procedure.  A  general  comparison  of  anodic  protection   and cathodic protection methods is provided in Table 60.

Table 60           Comparison of Anodic and Cathodic Protection

Throwing power indicates the distribution uniformity of the current density that is required. To achieve uniform protection, for example, electrodes need to be placed close together if the throwing power is low. On the other hand, if the throwing power is high, the electrodes can be placed farther apart. In anodic protection, for instance, a single cathode can protect a wider area  of metal because it has a high throwing power.

Chapter 5: Corrosion Monitoring and Inspection Technologies – Corrosion

There are numerous methods that may be used to monitor or inspect components for corrosion and corrosion related damage. The following section is aimed only at introducing available technologies and their applicability to the various forms of corrosion. Corrosion monitoring involves methodologies to assess the corrosivity of a system which may or may not be continuous (real-time monitoring) and to continuously monitor systems for defect formation. Corrosion inspection is the periodic checking of a system for corrosion and corrosion related defects. Since corrosion fatigue and stress corrosion cracking involves the formation and propagation of cracks, monitoring and inspection techniques to look for surface and subsurface cracks have been included.

5.1       Corrosion Monitoring

Corrosion monitoring is used to predict component wear out and to manage the corrosivity of the environment. There are a couple different methodologies used in the field of corrosion monitoring. One method uses probes (sensors) to monitor the chemical or electrochemical nature of the environment. The data collected is then used to relate to corrosion rates of materials, which is not always a direct method. Furthermore, probes can be adversely affected under certain conditions leading to erroneous corrosion rate determinations. A corrosion coupon is a second method, providing a low technology method to measure corrosion rates of materials. Acoustic emission is used to detect the formation of surface and subsurface damage in materials.

5.1.1 Coupon Testing

Coupon testing involves placing a sample within a system. The sample is removed periodically for inspection and weight loss measurements. Coupon testing is a simple procedure but is often overlooked as it is an old and low technology method. However, coupons provide the most reliable evidence, whereby information on the forms and location of corrosion on the samples, the average rate of corrosion, and the corrosion byproducts can be obtained. The downfalls are that it is time consuming and does not provide real time data.

5.1.2 Electrical Resistance Probes

Electrical resistance probes measure the change (increase) in electrical resistance due to a reduced cross-sectional area of the sensing element as a result of corrosion. The sensitivity of the sensor increases with a decrease in thickness of the sensing element; however this also results in a reduced lifetime. Any build up of deposits on the sensing element will affect the electrical resistance readings from the probe. Electrical resistance probes are also temperature dependent requiring an additional shielded probe for proper corrosion rate adjustments. They may be permanently installed within a system for continuous monitoring or potable for periodic measurements.

5.1.3 Inductive Resistance Probes

Inductive resistance probes measure a reduction in sensing element thickness by changes in the inductive resistance of a coil in the probe. By using sensing elements with a high magnetic permeability, the magnetic field around the coil is intensified. Any change in thickness of the elements will change the magnetic field encompassing the coil, and thus the corrosion rate may be obtained. Inductive resistance probes require a temperature adjustment similar to electrical resistance probes. The sensitivity of induction resistance probes is higher than with the electrical resistance probes.

5.1.4 Linear Polarization Resistance

The linear polarization resistance (LPR) monitoring method is an electrochemical method used to measure instantaneous rates of uniform corrosion and is widely applied under full immersion aqueous environments. A small potential, about 5-20mV, is applied to a sensor electrode with the direct current being measured. The solution resistance should be measured independently and subtracted from the measured resistance for accuracy. The polarization resistance obtained is inversely proportional to the corrosion rate. This method has been used for more than thirty years in virtually all types of water-based environments.

5.1.5 Electrical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS), like LPR, uses the polarization of electrodes to measure corrosion rates. The difference is that EIS uses alternating currents and measures the resulting phase shift relative to the applied current. The applied frequency is about 0.1 Hz to 100 kHz with more than one frequency required to obtain useful data. Typically, two frequencies are used; however, full frequency measurements may be used, producing the best data to identify the corrosion processes taking place.

5.1.6 Electrochemical Noise

The electrochemical noise method measures changes in the electric potential and current between freely corroding electrodes. Highly sensitive instrumentation is required as the fluctuations are on such a small scale. Three electrodes are needed to simultaneously measure both the potential and current noise. Different corrosion processes will produce different noise signatures. This data  may be used to identify pit initiation and growth before visible detection. However, the interpretation of signals is complex, with different strategies developed to help interpret results. Although this method is proven and can detect different corrosion processes, there is skepticism over the accuracy of corrosion rates derived from the measurements.

5.1.7 Zero Resistance Ammetry

Zero resistance ammetry is the measure of galvanic currents between two materials. This is accomplished by placing samples of the materials into a sensor unit, which is placed in the system environment. Deviation from actual component materials may occur due to slight differences in composition, heat treatment, surface condition and applied stresses. This technique can be used to monitor changes to the environment by using the same material for both sensor elements.

5.1.8 Thin Layer Activation

Thin layer activation involves inducing radioactive species on the surface layer of a material, and subsequently measuring gamma ray emission to determine the corrosion rate. A high energy beam of charged particles is used to bombard a material’s surface, producing radioactive elements in the surface area. One example is the formation of Co⁵⁶ within steel. This isotope will decay  into Fe⁵⁶, emitting gamma rays in the process. The change in gamma ray emission is used to determine the rate of material loss. A radioactive surface may be induced on system components or sample materials to be placed within the system.

5.1.9 Electric Field Method

This technique is used to look for corrosion across large structures by applying an electric current across the structure and measuring the resulting voltage distribution. Arrays of pins for the measurements are placed in specific areas across the structure. Increasing the distance between pins reduces the ability to detect localized corrosion. This method is widely used to detect corrosion on the interior of pipelines.

5.1.10 Corrosion Potential

Measuring the corrosion potential to determine the risk of corrosion is a direct result of corrosion kinetics (Appendix A). The corrosion potential of a material is measured relative to a reference electrode. This method is widely used to assess the corrosion of steel rebar in concrete and underground pipes incorporating cathodic protection. It is also used for structures containing anodic protection.

5.1.11 Hydrogen Probes

Hydrogen probes can be used to measure corrosion rate and to detect the diffusion of atomic hydrogen into materials. Many corrosion processes involve atomic hydrogen as a product of the corrosion reaction. Thus, by measuring the atomic hydrogen present, a corrosion rate may be determined. Hydrogen probes are more often used to detect the diffusion of atomic hydrogen into adjacent materials, such as pipe walls, as hydrogen-induced cracking may result. Hydrogen monitoring has been highly beneficial in oil refining and petrochemical industries due to the presence of hydrogen sulfide in such plants.

5.1.12 Chemical Analysis

Chemical analysis involves the inspection of materials, usually fluids, from a system for corrosion reaction by-products. This method is widely used to monitor the health of emergency generators through routine oil analysis. Particles in the waste oil are identified and quantified to monitor any abnormal wear and/or corrosion within the system.

5.1.13 Acoustic Emission

Acoustic emission is a monitoring technique where elastic waves are generated by the release of energy built up in stressed materials revealing the formation of a defect. The elastic waves must be continuously recorded for interpretation. This method is complicated by the normal emission of waves produced by thermal or mechanical stresses exhibited on the system. The interpreter must therefore be experienced to dismiss such “background noise” and diagnose abnormal events. This technique can be used to detect the formation of defects such as crack growth, material corrosion, surface rubbing, and leaking fluids. A summary of the corrosion monitoring methods is provided in Table 61.

Table 61           Advantages and Disadvantages of Corrosion Monitoring Methods

5.2       Corrosion Inspection

Corrosion Inspection Methods are periodic checks of materials and material systems to detect corrosion and corrosion related defects including cracks. The success of many of these methods lies with the operator and their experience with locating and identifying corrosion. Low technology methods are limited to surface damage only. They include visual inspection, liquid penetrant inspection, and magnetic particle inspection. High technology methods are used to detect subsurface defects, damage to hidden areas, and damage too small for visual inspection. These methods usually induce some form of energy into the material of interest, such as x-rays, sound waves, or heat, and measure the absorption/reflection of the energy. The data collected is then used to “map” defects found in the material. Experience with the various  equipment/methods and second or third inspection methods may be required to identify exactly what type of defect has been detected.

5.2.1 Visual

Visual Inspection involves the observation of light reflected from the surface of an object to the human eye. Although not a highly technical method, it’s the most widely used technique for corrosion inspection. The quality of inspection is directly related to the experience of  the inspector with the equipment and environmental conditions. Corrosion products are sometimes visible, which can lead to the identification of the corrosion problem. The appearance of  corrosion products from several alloys is provided in Table 62.

5.2.2 Enhanced Visual

Borescopes and fiberscopes provide a means to inspect interior areas of critical and corrosion prone components. A borescope is a thin rod shaped optical device that transmits an image from the components interior to the inspector’s eye. Critical areas are designed with access ports for borescope inspections. Fiberscopes work in the same way as borescopes, but they are flexible so that a wider area may be observed. Video imaging may also be incorporated into these devices so that the images can be viewed on a video monitor.

5.2.3 Liquid Penetrant Inspection

Liquid penetrant inspections provide a low cost option for locating surface cracks too small to be seen by visual inspection. An ultraviolet reflective liquid is first sprayed or wiped onto the  surface of the material. After a period of time, the liquid will enter cracks via capillary action.  The excess liquid is wiped from the surface and a powder is then applied. The powder draws the liquid back out of the crevices to the surface. An ultraviolet light is then used to illuminate the surface revealing the remaining liquid. The area of the liquid will be larger than the crack size.

5.2.4 Magnetic Particle Inspection

Magnetic particle inspection is used to find surface defects in ferromagnetic materials such as steel and iron. Magnetic particles, which may be dry or suspended in a liquid and colored or fluorescent, are dispersed over the material’s surface. A magnetic field is then induced in the material which produces flux lines which will be distorted by defects. Care must be taken in surface preparation as scratches and irregularities will also distort magnetic flux lines.

Table 62           Corrosion of Metals – Nature and Appearance of Corrosion Products

5.2.5 Eddy Current Inspection

Eddy currents are used to detect defects on and below the surface of a material. An alternating magnetic field is applied to the surface of the material. This induces eddy currents in the material producing a magnetic field which opposes the applied magnetic field. The measured impedance  is used to map the defects in the material.  Low frequency eddy current, roughly 100 Hz – 50  kHz, is used to penetrate deeper into a material.

5.2.6 Ultrasonic Inspection

Ultrasonic inspection uses high frequency sound waves transmitted through the material of interest. The transmitted sound wave will be reflected back to the source by both defects in the material and once the wave has reached the other side of the material. The recorded sound wave  is then used to map defects in the material and also to measure the thickness of the material. The identification of defects found in a material is left up to operator experience and/or additional inspection methods. Ultrasonic inspection is costly compared with other methods requiring large equipment and operator experience.

5.2.7 Radiography

Radiography is a method whereby x-rays, gamma rays, or neutrons are transmitted into a material and the absorption data recorded is then used to find any defects within the material. Again, the identification of defects is left up to operator experience or aided with the used of additional inspection methods. Neutron radiography is the most sensitive/most costly of these techniques.

5.2.8 Thermography

Thermography is a measure of the infrared radiation a material system emits. The underlying principal is that a good mechanical bond between materials is also a good thermal bond. It may  be used to detect corrosion, debonding, cracking, thinning, water absorption, among other defects. Thermography is not widely used due to its cost and limitation to surface defect detection.           A comparison between various nondestructive inspection techniques is given in Table 63.

Table 63           Nondestructive Inspection Methods

5.3       Corrosion Inspection Devices

Corrosion inspection devices that incorporate more than one inspection technique have been developed to improve the reliability of accurate detection of corrosion defects. These devices must be hand held for ease of use and cost effective. The two following devices have been developed for aircraft inspections to identify cracks and hidden corrosion by scanning the device across surface areas of the aircraft.

5.3.1 Mobile Automated Ultrasonic Scanner

The Mobile Automated Ultrasonic Scanner (MAUS) incorporates ultrasonic pulse-echo, ultrasonic resonance, and eddy current technologies into one unit for nondestructive inspection. MAUS IV is the forth generation device which is portable and applicable for detecting cracks and defects in materials for various components. The MAUS was developed for aircraft inspections, especially for lap joint evaluations.

5.3.2 Magneto-Optic Eddy Current Imaging

Magneto-optic eddy current imaging (MOI) is an inspection method which measures induced eddy currents using a Faraday magneto-optic sensor and displays the measurements on a video monitor. The images may be viewed in real time or recorded. The device is small enough to be hand held and may be easily moved about to scan large areas.

Chapter 6: Electrochemistry, Kinetics, and Thermodynamics of Typical Corrosion Processes – Corrosion

Corrosion is a process involving the deterioration of a material and is regulated by chemical or electrochemical reactions with the surrounding environment; consequently, it also results in a degradation of the material’s properties. The corrosion process occurs spontaneously when the environmental conditions thermodynamically favor a metal to be in its oxidized state. Therefore, thermodynamics is commonly used to determine the tendency of a material to corrode. To describe the chemical process of corrosion, however, electrochemistry and kinetics are used.

6.1       Electrochemistry and Kinetics

The principles of corrosion, based on electrochemistry, can be illustrated by an electrochemical cell, as shown in Figure 44. There are four necessary elements in order for cnorrosiAonotodeoccur:

• Cathode
• Electrolyte
• An electrical conducting path between the anode and cathode

Since corrosion requires all four of the elements listed above, it is readily obvious that corrosion prevention or control necessitates the elimination of just one of these elements and not necessarily all four.

Figure 44          Electrochemical Cell Showing Principles of Corrosion

The electrochemical  process  of  corrosion  involves  the  transfer  of  electrons  between  the  two electrodes resulting  in  a  flow  of  electrical  current  (indicated  as  “I”  in  Figure  44,  which  is  necessary to sustain the chemical reactions.   These electrochemical  reactions  occur   at both the anode and cathode  and  are  oxidation-reduction  reactions.  The  anode  is  host  to  the oxidation reaction, which generates electrons. This anodic (oxidation) reaction is described by Equation 13.

For the electrons to be transferred between the anode and cathode there must be an electrically conductive path, since the electrons are simultaneously consumed at the cathodic site where the reduction reaction takes place. An example of this cathodic reaction (reduction) is the evolution  of a hydrogen gas from the reduction of hydrogen ions as shown in Equation 14.   An example    of the oxidation-reduction process is illustrated in Figure 45.

Consumption of the electrons generated in Equation 1 could also occur by other mechanisms, as shown in Table 64.

An electrolyte is also necessary to sustain the electrochemical reactions, given by Equation 13 and Equation 14, because it contains the ions that aid in driving the reactions. The reactions shown in Equation 13 and Equation 14 are only partial reactions, and thus, together they occur simultaneously and at the same rate.

Of course, it is not necessary to always  have  two  distinct  and  separate  electrodes  for  corrosion to occur. In fact, the  most  common  form  of  corrosion  occurs  in  the  presence of one metal. It is still necessary, however to have the two electrodes (i.e. the anode and the cathode).   In this case, localized cells or electrodes exist on the surface of the metal, where    there is a relatively small difference in electrical potential ). This is usually the case where there  is compositional dissimilarities on the metal surface, for example, different metal phases,  different crystal orientations, crystal imperfections, grain boundaries, etc.⁹⁶ An example of this is given in the following section. Furthermore, it is not necessary that the electrolyte be in the form of a liquid. Instead, it may exist as ions in some vaporous media.

6.1.1    Example

A good example of the electrochemical corrosion process can be illustrated with the corrosion of zinc in a hydrochloric acid environment.  Zinc is removed from the surface of the metal and  enters the electrolytic solution in ionic form, as shown in Equation 15 and illustrated in Figure  45.

Simultaneously, the two electrons released from zinc are transferred to the hydrogen ion, which is supplied from the dissociation of the acid as shown in Equation 16. Equation 14 shows the association of hydrogen ions to form hydrogen gas upon the transfer of electrons from the zinc.

The overall reaction of zinc with a hydrochloric acid solution can be described as in Equation 17.

The corrosion process for other metals in like solutions is similar to that illustrated in this example; however there are other corrosion mechanisms for metals in different types of environments.

Figure 45        Illustration of the Oxidation-Reduction Process

6.2       Thermodynamics

The electrochemical reactions causing the physical corrosion of a material are spontaneous with no external driving forces, and thus, are driven only by nature’s tendency to seek lower energy states, as described by the Second Law of Thermodynamics. In other words, the electrochemical reactions occur to reduce the energy in the system. The surrounding system may, however, influence these reactions to occur at accelerated rates. For example, in an environment with an elevated temperature there is additional energy (from the heat) to drive the reactions at a faster rate. Thermodynamics, however, does not provide an indication of the rate of reaction, since it is independent of which path the reaction will take.

Thermodynamics is used primarily to determine, mathematically, the tendency for corrosion to occur, and can also be used to predict whether a metal will not experience corrosion. It cannot, however, be used to determine whether a metal will, in fact, experience corrosion or to what extent corrosion will occur.

Thermodynamics essentially quantifies the chemical stability of a system in terms of the Gibbs free energy. The amount of Gibbs free energy in a system represents the proximity of the system to equilibrium. That is, the lower the free energy, the closer the system is to equilibrium and conversely, the higher the free energy, the less stable the system is. (The free energy is at a minimum when the system is in equilibrium.) Gibbs free energy, G, at constant temperature, is given in terms of enthalpy, H, absolute temperature, T, and entropy, S as shown in Equation 18.

At equilibrium, when the free energy is at a minimum, the system has no tendency to undergo chemical change, and the free energy can be represented by Equation 19.

The equilibrium constant of a reaction can be determined for a range of conditions given the standard state free energy, which is commonly available or can be determined from the free energy of formations of the products.

The potential of an electrochemical cell can be given by Equation 20, if the system is thermodynamically reversible, and if the activities of the reactants and products remain approximately constant.

The electrochemical cell potential (E) is derived from Equation 21. The greater the difference between the electrochemical potentials of the electrodes (anode and cathode) the greater is the driving force for the corrosion reaction.

Ultimately, Equation 22 provides the means to predict the potential of an electrochemical cell. The more negative the cell potential, the more reactive the material, and thus the material is more susceptible to corrosion. Conversely, if the cell potential is less negative or even positive, then  the material is less susceptible to corrosion.