Corrosion

Corrosion Theory

Humans have most likely been trying to understand and control corrosion for as long as they have been using metal objects. The most important periods of prerecorded history are named for the metals that were used for tools and weapons (Iron Age, Bronze Age). With a few exceptions, metals are unstable in ordinary aqueous environments. Metals are usually extracted from ores through the application of a considerable amount of energy. Learning the processes that take place to make even the smallest corrosion reactions and how in turn it relates to human health are fascinating. Certain environments offer opportunities for these metals to combine chemically with elements to form compounds and return to their lower energy levels.

Corrosion is the primary means by which metals deteriorate. Most metals corrode on contact with water (and moisture in the air), acids, bases, salts, oils, aggressive metal polishes, and other solid and liquid chemicals. Metals will also corrode when exposed to gaseous materials like acid vapors, formaldehyde gas, ammonia gas, and sulfur containing gases. Corrosion specifically refers to any process involving the deterioration or degradation of metal components. The best known case is that of the rusting of steel. Corrosion processes are usually electrochemical in nature, having the essential features of a battery.

When metal atoms are exposed to an environment containing water molecules they can give up electrons, becoming themselves positively charged ions, provided an electrical circuit can be completed. This effect can be concentrated locally to form a pit or, sometimes a crack, or it can extend across a wide area to produce general wastage. Localized corrosion that leads to pitting may provide sites for fatigue initiation and, additionally, corrosive agents like seawater may lead to greatly enhanced growth of the fatigue crack. Pitting corrosion also occurs much faster in areas where microstructural changes have occurred due to welding operations.

Corrosion can be defined as the degradation of a material due to a reaction with its environment.

Degradation implies deterioration of physical properties of the material. This can be a weakening of the material due to a loss of cross-sectional area, it can be the shattering of a metal due to hydrogen embrittlement, or it can be the cracking of a polymer due to sunlight exposure.

Materials can be metals, polymers (plastics, rubbers, etc.), ceramics (concrete, brick, etc.) or composites-mechanical mixtures of two or more materials with different properties. Because metals are the most used type of structural materials most of this web site will be devoted to the corrosion of metals.                       

DEFINITION OF METAL CORROSION.

Metal Corrosion can be defined as the destructive attack of a metal through interaction with its environment.

            

        

DRIVING FORCE FOR CORROSION.

Most metals used in the construction of facilities are subject to corrosion. This is due to the high energy content of the elements in metallic form. In nature, most metals are found in chemical combination with other elements. These metallic ores are refined by man and formed into metals and alloys. As the energy content of the metals and alloys is higher than that of their ores, chemical re-combination of the metals to form ore like compounds is a natural process.

Metals corrode because we use them in environments where they are chemically unstable. Only copper and the precious metals (gold, silver, platinum, etc.) are found in nature in their metallic state. All other metals, to include iron-the metal most commonly used-are processed from minerals or ores into metals which are inherently unstable in their environments.

This golden statue in Bangkok, Thailand, is made of the only metal which is thermodynamically stable in room temperature air. All other metals are unstable and have a tendency to revert to their more stable mineral forms. Some metals form protective ceramic films (passive films) on their surfaces and these prevent, or slow down, their corrosion process. The woman in the picture below is wearing anodized titanium earrings. The thickness of the titanium oxide on the metal surface refracts the light and causes the rainbow colors on her earrings. Her husband is wearing stainless steel eyeglasses. The passive film that formed on his eyeglasses is only about a dozen atoms thick, but this passive film is so protective that his eyeglasses are protected from corrosion. We can prevent corrosion by using metals that form naturally protective passive films, but these alloys are usually expensive, so we have developed other means of corrosion control.

            

FUNDAMENTAL MECHANISM OF ATTACK.

Corrosion of metals takes place through the action of electrochemical cells. Although this single mechanism is responsible, the corrosion can take many forms. Through an understanding of the electrochemical cell and how it can act to cause the various forms of corrosion, the natural tendency of metals to suffer corrosion can be overcome and equipment that is resistant to failure by corrosion can be designed.

            

The Electrochemical Cell.

As in all chemical reactions, corrosion reactions occur through an exchange of electrons. In electrochemical reactions, the electrons are produced by a chemical reaction, the oxidation, in one area, the anode, travel through a metallic path and are consumed through a different chemical reaction in another area, the cathode. In some cases, such as the common dry cell battery, electrochemical reactions can be used to supply useful amounts of electrical current. In marine corrosion, however, the most common result is the transformation of complex and expensive equipment to useless junk.

          

Components.

In order for electrochemical reactions to occur, four components must be present and active. These components are the anode, cathode, electron path, and electrolyte.

            

      

Anode.

In an electrochemical cell, the anode is the site where electrons are produced through the chemical activity of the metal. The anode is the area where metal loss occurs. The metal loses electrons and migrates from the metal surface through the environment. The electrons remain in the metal but are free to move about in response to voltage gradients.

            

Cathode.

The cathode in an electrochemical cell is the site where electrons are consumed. For each electron that is produced at an anodic site, an electron must be consumed at a cathodic site. No metal loss occurs at sites that are totally cathodic.

            

Electron Path.

In order for electrons to flow from the anodic sites to cathodic sites, the electrons migrate through a metallic path. This migration occurs due to a voltage difference between the anodic and cathodic reactions. Electrons can move easily only through metals and some non-metals such as graphite. Electrons from electrochemical reactions cannot move through insulating materials such as most plastics nor can they directly enter water or air. In some cases, the electron path is the corroding metal itself, in other cases, the electron path is through an external electrical path.

Electrolyte.

Electrolytes are solutions that can conduct electrical currents through the movement of charged chemical constituents called ions. Positive and negative ions are present in equal amounts. Positive ions tend to migrate away from anodic areas and toward cathodic areas. Negative ions tend to migrate away from cathodic areas and towards anodic areas.

Anodic Reactions.

Metal loss at anodic sites in an electrochemical cell occurs when the metal atoms give up one or more electrons and move into the electrolyte as positively charged ions.

            

Typical Reactions.

The generic chemical formula for this metal loss at anodic sites is:

            

M ---> M+ + e-

            

where:

M = uncharged metal atom at the metal surface

M+ = positively charged metal ion in the electrolyte

e- = electron that remains in the metal

            

            

This type of chemical reaction is called oxidation even though it does not directly involve oxygen but only results in an increase in positive charge on the atom undergoing oxidation.

More than one electron can be lost in the reaction as in the case for iron where the most common anodic reaction is:

            

Fe ---> Fe+ + 2e-

where:

            

Fe = metallic iron

            

Fe+ = ferrous ion that carries a double negative charge

             

Correlation Between Current Flow and Weight Loss.

For each specific anodic reaction a characteristic number of electrons are produced in the reaction of one metal ions. Thus, all other things being equal, the metal loss is proportional to the number of electrons that are produced. As the electrons produced migrate to cathodic areas through the electron path, the metal loss is proportional to the current flow. In cases where more positively charged ions are produced, more electrons flow for a given number of corroding metal atoms but the current flow remains proportional to the metal loss.

            

Cathodic Reactions.

The electrons that are produced at anodic sites are consumed at cathodic sites. The type of chemical reactions that consume electrons are called reduction and have the generic chemical formula:

            

R+ + e- --> R

where:

R+ = a positive ion in solution

e- = an electron in the metal

Ro = the reduced chemical

            

In reduction, the chemical being reduced gains electrons and its charge is made more negative. In some cases, the where the ion in solution has a multiple positive charge, the total positive charge on the ion may not be neutralized. In other cases, the chemical which is reduced may not be a positive ion but is a neutral chemical which then becomes a negatively charged ion in solution in a reaction such as:

            

R + e- --> R-

What is Rust?

Rust is a generic term used to describe different iron hydroxides and oxides, Fe(OH)2, Fe(OH)3, FeO(OH), Fe2O3.H2O that form when iron corrodes.

The common form of Rust is a Red products Fe2O3 called hematite.

What causes Rust to form is the reaction between iron and water; for iron exposed to the atmosphere it could be either water condensing from air or rain.

The oxygen in the air dissolves in the water and causes rust to form.

There are always two distinct chemical reactions in a corrosion process, the basic chemistry of corrosionis:

1) Anodic Dissolution of Metal (Iron) that goes into solution (water)

Fe  ----->   Fe2+  +   2e-

2) Cathodic Reduction of Oxygen dissolved in water

O2  + 2H2O + 4e- ---->  4OH- 

The final reaction is:

Fe2+ + 2OH-  ----->  Fe(OH)2

This iron hydroxide will then further reacts with oxygen to give the final red product (Rust):

Fe2O3.H2O

Corrosion Types - Corrosion Forms

Although there is only one fundamental mechanism of corrosion, the electrochemical cell, there are several Corrosion forms or Corrosion Types  that can occur. Each form of attack has a specific arrangement of anodes and cathodes and the corrosion which occurs has a specific location and pattern. Each form of corrosion can be effectively controlled during design if it is anticipated. By understanding the various forms of corrosion, the conditions under which they occur, and how they are quantified, they can each be addressed and controlled. The most important Types of Corrosion are:

Uniform Corrosion                                             

Concentration Cell Corrosion.

Intergranular Corrosion

Atmospheric Corrosion

Erosion Corrosion

Fretting Corrosion

Dealloiyng

Formicary Corrosion

Microbiological Corrosion - MIC

Galvanic Corrosion or Bimetallic Corrosion

Pitting Corrosion - Crevice Corrosion

Stress Corrosion Cracking

Cavitation Corrosion

Hydrogen Embrittlement

Immunity

Uniform Corrosion.

If a metal is not immune to attack and corrosion cannot be completely eliminated, uniform corrosion is considered the form of corrosion that can be tolerated in marine structures and equipment. It is also relatively easy to control uniform corrosion to acceptable levels through judicious selection of materials, the application of corrosion control measures, and to allow for any corrosion which does occur.

Definition.

Uniform corrosion is the attack of a metal at essentially the same at all exposed areas of its surface. At no point is the penetration of the metal by corrosion twice as great as the average rate.

Mechanism.

Uniform corrosion occurs when there are local anodic and cathodic sites on the surface of the metal (see Galvanic Corrosion). Due to polarization effects, these locations shift from time to time and a given area on a metal will be act as both an anode and as a cathode over any extended period of time. The averaging effect of these shifting local action cells results in a rather uniform attack and general loss of material and roughening of the surface.

Examples.

Rusting steel in the atmosphere and the corrosion of copper alloys in seawater are common examples where uniform corrosion is usually encountered. Steel submerged in seawater also suffer uniform corrosion but can also suffer from non-uniform attack under some circumstances.

Appearance.

In uniform corrosion, the metal loss occurs at essentially the same rate over the entire metal surface. Smooth surfaces are usually roughened during uniform corrosion. This form of corrosion is characterized by the lack of any significant non-uniform attack such aspitting or crevice corrosion.

Corrosion products commonly remain on uniformly corroding surfaces but these can be removed by velocity, by mechanical action or by other mechanisms.

Significant Measurements.

Weight loss is the most commonly used method of measuring the corrosion rate of metals when uniform corrosion occurs. In this method, a test sample is cleaned, weighed, and its surface area is measured. It is then exposed for a specific period of time, re-cleaned and re-weighed.

The amount of metal loss as measured by the weight loss is used to calculate the loss in thickness of the metal assuming that the corrosion was absolutely uniform. In some cases this is further verified by thickness measurements.

These results are commonly expressed in “Mils perYear” or “Microns per Year.” It must be remembered that these rates are usually calculated from weight loss rather than thickness loss and are only valid if the attack was uniform. The maximum error of this measurement is theoretically a factor of two if the rule that attack can be no greater than twice the average rate at any given point is properly applied.

Concentration Cell Corrosion.

A difference in environment between sites on a single metal can also result in increased electrochemical activity. This difference in environment can be due to non-uniform deposits or fouling on the surface, or, more commonly, built in features which create significant difference in environment.

Crevices at joints are the most common cause of these built in environmental differences. This form of concentration cell corrosion called “crevice corrosion” is often the most difficult form of corrosion to avoid in design and also is one of the most common causes of failure of marine equipment.

Definition.

Concentration cell corrosion is corrosion that is accelerated by differences in environment between separated areas on a single metal.

Mechanism.

Any situation that creates a difference in environment between areas on a single metal can cause concentration cell attack. The basic mechanism is essentially the same as in galvanic corrosion but in the case of concentration cell corrosion the driving force is the difference in potential between a single metal exposed to different environments rather than the difference in potential between two different metals exposed to a single environment. The rates of attack experienced in concentration cell corrosion are affected by relative anode/cathode areas in the same manner as in galvanic corrosion. In crevice corrosion, the resistance of the electrolyte to the flow of ions can also be a significant factor in limiting attack in deep tight crevices.

Oxygen Concentration Cells.

Dissolved oxygen has a significant effect on the corrosion of many metals. This is particularly true for alloys such as stainless steelswhere the corrosion resistance of the alloy is dependent upon abundant oxygen for the stability and self-repair of protective films. Oxygen is also an active participant in the most predominant cathodic reaction in many environments. The oxygen content of the electrolyte inside a crevice is usually low as oxygen is consumed by both corrosion and biological activity and replacement of oxygen inside the crevice is limited. The crevice can be formed by metal-to-metal contact, by contact of a metal with a nonmetal or under deposits of debris or fouling. For a metal with a passive film, the metal tends to become active within the crevice where the lack of oxygen causes the passive film to be less stable and less easily repaired. The resulting active/passive cell has substantial driving potential as noted on the galvanic series. The anodic area within the crevice is normally small with respect to the cathodic area outside the crevice and with this adverse area ratio, the corrosion inside the crevice can be very rapid. Once initiated, crevice corrosion can also be accelerated by the formation of aggressive chemical compounds within the crevice which further accelerates the attack within the crevice. In the case of stainless steels, the chromium and nickel chlorides which are formed are very acidic and crevice corrosion can be very rapid once initiated.

Oxygen concentration cell corrosion can also occur on metals which do not have passive films. In this case, the difference in oxygen content makes the area with low oxygen content predominantly anodic with respect to more highly oxygenated areas. The reason for this is due to the effect of the law of mass action on the predominant cathodic reaction in neutral and alkaline environments.

This reaction is:

2 H2O + O2 + 4 e- ----> 4 OH-

The law of mass action indicates that, where the oxygen content is high, the cathodic reaction will occur more readily than when the oxygen content is low. Thus areas where the oxygen content is low will not be as effective a cathode, anodic reactions will predominate and the area will act as an anode. Where the oxygen content is high, the cathodic reactions will predominate and the area will act as a cathode. Thus, the area inside a crevice will be anodic with respect to the area outside and the same process as described above for active/passive oxygen concentration cells will occur. Oxygen concentration cell crevice corrosion is particularly insidious. First, many material are susceptible to this form of attack that have otherwise excellent performance in marine environments. Second, the attack often occurs deep inside crevices in sealed areas, in joints, and in fasteners where a very small amount of corrosion can result in a serious failure. Third, as it occurs deep within the crevice, it is difficult to detect. Crevice corrosion of this type often remains hidden until revealed by failure.

Intergranular Corrosion

Just as most engineering metals are mixtures of one or more metals, they consist of large numbers of individual metal crystals called grains that are joined together at their surfaces or grain boundaries. As there can be differences in composition at or adjacent to these grain boundaries, selective corrosion can occur at these sites.

Definition.

Intergranular corrosion is a selective attack of a metal at or adjacent to grain boundaries.

Mechanism.

There are three mechanisms that have been identified as causing intergranular corrosion in various situations.

1. The first mechanism is the selective attack of grain boundary material due to its high energy content. Metal crystals form in an ordered arrangement of atoms because this ordered arrangement has a lower energy content than a disordered arrangement. Grain boundaries are highly disordered as they are at the boundaries of crystals which, although they are internally ordered, have random orientation with respect to each other. The disordered grain boundary is often 10 to 100 atoms wide and these atoms have a higher energy than the surrounding atoms. Higher energy material can be more chemically active than lower energy material and thus, the grain boundary material can be anodic with respect to the surrounding grains. When this occurs, the anodic area is small and the cathodic area is large, thus, rapid attack can occur. The result is that the individual grains are no longer joined with the strong grain boundary “glue” and disintegrate leaving a powdery residue and rough grainy surface.

2. A second mechanism is selective attack of grain boundary material that has a different composition from the surrounding grains. When metals crystallize from the molten state, the crystals tend to be more pure than the molten material. This is because the pure metal crystals are more ordered and have a lower energy content than if they contained large amounts of impurities. In some cases, most of the impurities are concentrated at the grain boundaries. When the composition of this impure material causes it to be more anodic than the surrounding grains, rapid attack can occur with results similar to those described above. When the composition of the impure grain boundary material causes it to be more cathodic than the surrounding grains, the favorable anode/cathode area ratio makes this situation relatively innocuous. Contamination of grain boundaries can sometimes also occur after manufacture. Mercury on aluminum can penetrate and contaminate the grain boundaries and cause subsequent intergranular attack. This is why mercury and mercury compounds are prohibited aboard aluminum ships or on aircraft.

3. A third mechanism is selective attack adjacent to the grain boundaries due to the local depletion of an alloying element. This form of attack can occur in many stainless steels. It is called sensitization. Many stainless steels rely on a combination of nickel and chromium for their corrosion resistance. As both nickel and chromium are expensive, they are added only in amounts necessary to obtain the necessary corrosion resistance. Another element, which is commonly present in ail steels, is carbon. In stainless steels, carbon atoms tend to concentrate at the grain boundaries as an impurity during solidification. Chromium carbides can form adjacent to the grain boundaries during welding and heat treatment. When these compounds form, the chromium is removed from the alloy adjacent to the grain boundaries and the resulting alloy does not have enough chromium content to remain passive. Again, there is a very unfavourable anode/cathode area ratio and rapid attack can occur. Three different methods are used to avoid this type of attack in stainless steels during welding or other heating.

a. The first method to avoid sensitization is through heat treatment. At high temperatures (above 1,800°F), chromium carbides are unstable and will redissolve if they have formed. At low temperatures, (below 1,000°F) the chromium and carbon atoms cannot move and formation of chromium carbides is prevented. Formation of the chromium carbides is a problem primarily in the ranges of 1,100 to 1,600°F. When  welding stainless steel, some area adjacent to the weld is likely to reach this temperature range long enough to form amounts of chromium carbides. When this occurs, or when the alloy is otherwise sensitized, it should be heated to temperatures above 1,800°F to redissolve the carbides, then rapidly cooled to below 1,000°F to avoid carbide formation.

b. The second method used to avoid sensitization in stainless steels is to reduce the carbon content of the alloy to very low levels. These low carbon grades (such as 304 L and 316 L; L stands for low carbon) do not have enough carbon to form carbides and is thus resistant to sensitization during welding. Care must be taken, however, to not introduce additional carbon during welding from contamination, such as can be caused by oil or grease.

c. The third method used to avoid sensitization in the stainless steels is to intentionally add an element that will combine with the carbon but is not required for passivity of the alloy. Titanium and niobium have a greater affinity for carbon than chromium. They are added to the alloy during manufacture in amounts to combine with all of the carbon present in the alloy and thus inhibit sensitization. Type 321 stainless steel contains titanium and Type 347 stainless steel contains niobium. These alloys, or the low carbon grades, should be used when welding without heat treatment is required.

Examples.

Aluminum alloys are susceptible to intergranular attack, usually the type that is caused by segregation of impurities at the grain boundaries. In addition to the stainless steels, some nickel alloys are also subject to sensitization and subsequent intergranular attack.

Appearance.

Intergranular attack caused by high grain boundary energies or impurities at the grain boundaries results in attack with a grainy residue and rough surface. Under high magnification, the individual grains are often visible. Intergranular attack of aluminum alloys is associated with pitting or other localized attack. Sensitization in stainless steels has a similar grainy appearance. When caused by welding it is often localized in narrow bands adjacent to the weld and is sometimes called “knife line attack.”

Significant Measurements.

Microscopic examination of sectioned samples is often required to verify that intergranular attack has occurred. There are several standardized methods for determining the resistance of stainless steels to sensitization.

ATMOSPHERIC CORROSION

The atmospheric environment varies drastically with regards to corrosivity depending on the geographical location.

Electrolyte.

In atmospheric corrosion, the electrolyte is moisture from precipitation, fog or dew, sea spray, or other sources.

The three factors that have the most influence on the corrosivity of the atmosphere at a given site are:

(1) the amount of time that exposed surfaces remain wet at the site,

(2) the amount of chloride from the sea that reaches the surfaces

(3) the amount of industrial pollutants (mainly acids) that reach the surfaces.

In all atmospheric environments there is an excess of oxygen, thus the corrosion of most metals in atmospheric environments is not limited by the amount of oxygen present and can proceed rapidly when the electrolyte is present.

The corrosivity of the atmosphere varies drastically.

The corrosion rate of steel, for example, can vary by a factor of 100.

In general, the least corrosive atmospheres are found in dry inland (desert) sites and the most corrosive sites are industrial or industrialmarine sites.

Moist tropical locations are very corrosive due both to the time of wetness and the high temperatures experienced.

However, local conditions and features of design have an influence on corrosive attack that often exceeds the differences experienced due to geographical conditions.

Thus, generalizations regarding specific site corrosivity based on the corrosion of a single metal at a single location at a given site can be misleading.

It is not prudent to ignore the possibility of corrosion at a dry inland site nor to consider corrosion inevitable at a marine industrial site.

The rates of attack can vary at different sites, but the mechanisms of attack, features that cause accelerated attack and corrective measures for corrosion prevention and corrosion protection that can reduce the attack are similar.

Erosion-Corrosion

Erosion-corrosion is a general term that refers to a corrosion process enhanced by the action of flowing fluids. The process can also be characterized by whether the fluid contains solid particles, is in the form of impinging droplets, or is undergoing cavitation. Cavitation is the formation and sudden collapse of vapor bubbles in a liquid.

Manifestation and Quantification

Erosion-corrosion can result in general corrosion that occurs at a higher rate than would be expected under stagnant conditions. In this case the measure of erosion-corrosion would be the rate of surface waste or the rate of penetration. The more usual effect of erosion-corrosion, however, is localized attack, which can appear as grooves, gullies, waves, rounded holes, etc., and usually exhibits a directional pattern that correlates with the direction of flow.

The appearance of cavitation damage is similar to pitting. However, the pitted areas are closely spaced and the surface is usually considerably roughened. Severe cavitation damage can completely remove sections of metal.

Cavitation damage that results from fluid movement relative to the metal surface can usually be correlated with the direction of flow A condition conducive to erosion corrosion is the flow of corrosive fluids (gas or liquid) relative to a metal surface.

The rate of corrosion depends upon the flow rate of the fluid. Turbulent flow results in much higher corrosion rates than Iaminar flow. Hard particles entrained in the flowing fluid can enhance erosion corrosion.

Cavitation damage is caused by flow discontinuities that result in the formation and subsequent coUapse of vapor bubbles on a metal surface. High-velocity drop impingement, such as raindrops on a helicopter rotor, can also result in rapid surface damage.

Mechanism

Erosion-corrosion implies that the fluid medium is potentially corrosive to the metal. Erosion facilitates the corrosion process. This fact distinguishes erosion-corrosion from pure erosion or mechanical wear. Erosion corrosion influences the rate of corrosion by changing the conditions of local cell action. The corrosion process is accelerated if the fluid speed is sufficient to remove weakly adhered corrosion products from the surface.

Removal of these products reduces their polarizing or inhibitive effect.

At the breakaway speed the fluid begins to remove the corrosion film and the corrosion rate increases. A steady corrosion rate is achieved at the speed at which the fdm is completely removed. Fluid flow also maintains a uniform concentration of corrodent at the metal surfaces.

Impingement of suspended hard particles can accelerate the damage to the protective film. and can cause mechanical damage to the underlying metal.

Application Constraints and Protection Approaches

Several methods for preventing or minimizing damage resulting from erosion-corrosion are available.

They include

1. Material selection. Select materials with better resistance to erosion-corrosion.

2. Design considerations. Streamline the flow, avoid designs that create turbulence. Minimize abrupt changes in flow direction. Introduce smooth aerodynamic or hydrodynamic flow channels; avoid roughly textured surfaces. Carefully align pipe sections. Avoid flow obstructions in design or obstructions that can arise under operations, increase the thickness of material in vunerable areas, install renewable impingement plates or baffles, and design for easy repair by using interchangeable parts.

3. Aherarion of environment. Decrease fluid stream speed to achieve laminar flow, regulate the concentration of dissolved oxygen in the environment to achieve optimal film-forming characteristics, provide falters for removal of suspended solids, and provide condensed moisture traps in vapor lines.

4. Specification of suitable coatings or linings. Use of hard-facing may be helpful in some situations and resilient barriers may be helpful in others, e.g., cavitation.

5. Cathodic protection. Provide cathodic protection whenever possible.

When surfaces move in relation to each other, this relative motion can result in abrasion. This abrasion can increase the attack at these fraying surfaces.

Erosion corrosion is the result of a combination of an aggressive chemical environment and high fluid-surface velocities. This can be the result of fast fluid flow past a stationary object, such as the case with the oil-field check valve shown on the left below, or it can result from the quick motion of an object in a stationary fluid, such as happens when a ship's propeller churns the ocean.

Surfaces which have undergone erosion corrosion are generally fairly clean, unlike the surfaces from many other forms of corrosion.

Erosion corrosion can be controlled by the use of harder alloys (including flame-sprayed or welded hard facings) or by using a more corrosion resistant alloy. Alterations in fluid velocity and changes in flow patterns can also reduce the effects of erosion corrosion.

Erosion corrosion is often the result of the wearing away of a protective scale or coating on the metal surface. The oil field production tubing shown above on the right corroded when the pressure on the well became low enough to cause multiphase fluid flow. The impact of collapsing gas bubbles caused the damage at joints where the tubing was connected and turbulence was greater.

Many people assume that erosion corrosion is associated with turbulent flow. This is true, because all practical piping systems require turbulent flow-the fluid would not flow fast enough if lamellar (nonturbulent) flow were maintained. Most, if not all, erosion corrosion can be attributed to multiphase fluid flow. The check valve on the left above failed due to sand and other particles in an otherwise noncorrosive fluid. The tubing on the right failed due to the pressure differences caused when gas bubbles collapsed against the pipe wall and destroyed the protective mineral scale that was limiting corrosion.

Fretting Corrosion

Definition.

Fretting corrosion is an attack that is accelerated by the relative motion of contacting surfaces.

Mechanism.

Fretting corrosion is usually a combination of corrosion and abrasive wear. The motion between the surfaces removes protective films and results in accelerated attack. Also, most corrosion products are abrasive and their presence increases the removal of protective films and in direct abrasion of the metal.

Examples.

Fretting was common in riveted joints on ships and other riveted structures where cyclic loads were experienced, but this has largely been eliminated through welded construction. Fretting is, however, still encountered in bolted joints and flanges where there is not enough bolt tension to eliminate movement in the joint. Thermal expansion with frequent cycling can also result in fretting attack. Any combination of corrosion and wear will almost always be worse than the action of either one separately.

Appearance.

Fretting corrosion usually results in scuffed surfaces in joints or at other wear sites. If inspected soon after the relative motion ceases, the surfaces will often be bright and have corrosion products attached to the surfaces.

Significant Measurements.

There are no standard tests for fretting corrosion. When encountered, it is addressed through mechanical design rather than material selection. Where it cannot be eliminated it can sometimes be reduced by using inhibitive caulking compounds in the joints.

Dealloying Corrosion

Dealloying Corrosion

Most of the commonly used metallic material are alloys formed from mixing two or more metals. Pure metals are usually too soft and weak to be used structurally. In this form of corrosion,  dealloying, corrosion selectively attacks one or more constituent of the alloy mixture.

Definition of Dealloying.

Dealloying is the selective corrosive attack of one or more constituent of a metallic alloy.

Mechanism of Dealloying Corrosion.

As can be seen from the galvanic series, constituents of many common alloys have widely separated positions on the galvanic series. In the case of brass, the main constituents are zinc and copper. In the case of cast iron, the main constituents are iron and graphite. When the surface of such alloys is exposed to an electrolyte, galvanic action proceeds with the more anodic material being selectively attacked. In many cases, the cathodic material remains behind and is bound into its original shape by a residue of remaining anodic material and corrosion products. The strength of the remaining material is, however, greatly reduced and will often fail duringnormal handling. Single phase material, where the alloy constituents are well mixed, are often less susceptible to this form of attack than alloys where phases of largely different composition are present. In many alloys, heat treatments have been developed specifically to make the alloy more homogeneous and less susceptible to dealloying.

Examples of Dealloying Corrosion

The dezincification of brass and the graphitization of cast iron are common examples of dealloying.

Appearance of Dealloying Corrosion.

In dealloying, the size and shape of the original component is often retained. The remaining constituent is often a different color than the original alloy and the depth and location of attack can be easily identified by this color change. Dealloying can either occur over the entire surface (layering) or localized in pits (plug type).

Significant Measurements.

Weight loss is not a significant measurement of the impact of dealloying. The depth of attack must be measured by sectioning and microscopic examination. The impact of dealloying on the strength of the material can be assessed through mechanical testing. In many cases, the depth of attack is self-limiting, particularly in the plug type of attack but the limiting depth is significant, often in the order of 1/4 inch. The fact that there is a limiting depth is significant only for very thick walled sections.

Microbial Corrosion

Microbial corrosion (also called microbiologically-influenced corrosion or MIC) is corrosion that is caused by the presence and activities of microbes. This corrosion can take many forms and can be controlled by biocides or by conventional corrosion control methods.

There are a number of mechanisms associated with this form of corrosion, and detailed explanations are available at the web sites listed at the bottom of this section. Most MIC takes the form of pits that form underneath colonies of living organic matter and mineral and biodeposits. This biofilm creates a protective environment where conditions can become quite corrosive and corrosion is accelerated.

The picture below shows a biofilm on a metallic condenser surface. These biofilms can allow

(Courtesy of www.asm.org)

corrosive chemicals to collect within and under the films. Thus the corrosive conditions under a biofilm can be very aggressive, even in locations where the bulk environment is noncorrosive.

(Courtesy of www.micscan.com)

MIC can be a serious problem in stagnant water systems such as the fire-protection system that produced the pits shown above. (see Pitting Corrosion). The use of biocides and mechanical cleaning methods can reduce MIC, but anywhere where stagnant water is likely to collect is a location where MIC can occur.

Corrosion (oxidation of metal) can only occur if some other chemical is present to be reduced. In most environments, the chemical that is reduced is either dissolved oxygen or hydrogen ions in acids. In anaerobic conditions (no oxygen or air present), some bacteria (anaerobic bacteria) can thrive. These bacteria can provide the reducible chemicals that allow corrosion to occur. That's how the limited corrosion that was found on the hull of the Titanic occurred. The picture below shows a "rusticle" removed from the hull of Titanic. This combination of rust and organic debris clearly shows the location of rivet holes and where two steel plates overlapped.

(Couresy of www.dbi.sk.ca)

Much microbial corrosion involves anaerobic or stagnant conditions, but it can also be found on structures exposed to air. The pictures below show a spillway

(Courtesy of www.meic.com)

gate from a hydroelectric dam on the Columbia River. The stress corrosion cracks were caused by pigeon droppings which produced ammonia-a chemical that causes stress corrosion cracking on copper alloys like the washers used on this structure. Since it's impossible to potty train pigeons, a new alloy resistant to ammonia was necessary.

In addition to the use of corrosion resistant alloys, control of MIC involves the use of biocides and cleaning methods that remove deposits from metal surfaces. Bacteria are very small, and it is often very difficult to get a metal system smooth enough and clean enough to prevent MIC.

Galvanic Corrosion Bimetallic Corrosion

Galvanic corrosion is a localised corrosion mechanism by which metals can be preferentially corroded.

This type or form of corrosion has the potential to attack junctions of metals, or regions where one construction metal is changed to another. Frequently this condition arises because different metals are more easily fabricated into certain forms; an example might be a door frame manufactured from aluminium extrusions (aluminium extrudes extremely well into architectural sections), but with a door handle fabricated from stainless steel tube to exploit its higher strength and abrasion resistance.

Galvanic corrosion is well known to most designers, specifiers and fabricators, but often the only rule in force is "don't mix metals".

What Conditions are Needed

For galvanic corrosion to occur there are three conditions which must be met:

Condition 1. Metals must be far apart on the galvanic series

The galvanic or electrochemical series ranks metals according to their potential, generally measured with respect to the Standard Calomel Electrode (S.C.E.). The results are often viewed as a galvanic corrosion chart or galvanic corrosion table similar to that on the third page.

This chart says that the "anodic" or "less noble" metals at the negative end of the series - at the right of this diagram, such as magnesium, zinc and aluminium - are more likely to be attacked than those at the "cathodic" or "noble" end of the series such as gold and graphite.

The critical point is the difference in potential of the two materials being considered as a joined pair. A difference of hundreds of millivolts is likely to result in galvanic corrosion, but only a few tens of millivolts is unlikely to be a problem.

Condition 2. The metals must be in electrical contact

The two different metals must be in electrical contact with each other. This is of course very common. The two metals can be bolted, welded or clamped together, or even just resting against each other. 

Condition 3. The metal junction must be bridged by an electrolyte

An electrolyte is simply an electrically conducting fluid. Almost any fluid falls into this category, with distilled water as an exception. Even rain water is likely to become sufficiently conducting after contact with common environmental contaminants.

If the conductivity of the liquid is high (a common example is sea water) the galvanic corrosion of the less noble metal will be spread over a larger area; in low conductivity liquids the corrosion will be localised to the part of the less noble metal near to the junction.

 

The Area Effect

The relative area of the anode and cathode has a pronounced effect upon the amount of corrosion that occurs due to Galvanic Corrosion. A small anode (the less noble metal, such as aluminium) joined to a large cathode (the more noble metal, such as stainless steel) will result in a high current density on the aluminium, and hence a high rate of corrosion.

The corrosion is concentrated by the area difference. Conversely if the area of the anode is large compared to that of the cathode this dilutes the corrosive effect, in most cases to the extent that no problem occurs. It is common practice to use stainless steel fasteners to fix aluminium sheeting or signs, but if aluminium screws were used to fix stainless steel the screws may rapidly corrode.

An apparent contradiction of the area effect on Galvanic Corrosion occurs when the component comprised of the two metals is only partly wetted. Consider for instance a stainless steel bolt in an aluminium plate; if water collects in the corner at the edge of the bolt but the remainder of the plate remains dry, the effective area of the less noble aluminium is only the wetted region, which may be only a similar size to that section of the bolt that is wetted .... thus it is quite possible for the aluminium plate to be galvanically attacked in the region immediately surrounding the bolt. Only the wet “area” counts.

Crevices & Stagnant Conditions

As shown in the electrochemical series chart  there are two different potentials associated with each stainless steel grade. The less noble value shown in outlined boxes is that which applies if a crevice is formed between the two dissimilar metals or such as beneath bio-fouling.

Such a crevice could be from the design or fabrication of the component, and formation of biological films is more likely in stagnant or slow-flowing sea water. The result of these stagnant conditions is oxygen depletion and the less noble potential which can make the stainless steel susceptible to corrosion in conditions that might otherwise be considered non-corrosive.

Passive Surface Films and effect on Galvanic Corrosion

Stainless steels naturally form passive surface films this is what makes them “stainless”. This film also reduces the amount of current available for corrosion, so slows the corrosion rate down compared to some other galvanic pairs.

Avoidance of Galvanic Corrosion

The methods for avoidance of galvanic corrosion are in general suggested by the above descriptions of the conditions necessary for its occurrence.

Don’t Mix Metals. If only one material is used in a construction the problem is avoided (Condition 1 is not present – no mixed metals) andGalvanic Corrosion will not take place. Be particularly aware of zinc plated or galvanised fasters in stainless steel sheets – a common substitution because of perceived cost savings or better availability. These less noble fasteners in the galvanic series are likely to be rapidly attacked.

Prevent Electrical Contact. It is often practical to prevent electrical contact between the dissimilar metals (removal of Condition 2). This may be achieved by the use of nonconducting (eg rubber or plastic) spacers, spool pieces or gaskets, perhaps in conjunction with sleeves around bolts. For the same reason a gap may be left between galvanised roofing and a stainless steel down-pipe.

Corrosion potentials in flowing sea water at ambient temperature. The unshaded symbols show ranges exhibited by stainless steels in acidic water such as may exist in crevices or in stagnant or low velocity or poorly aerated water. The more Noble materials at the left side tend to be cathodic and hence protected; those at the right are less Noble and tend to be anodic and hence corroded in a galvanic couple.

Prevent the Wetted Junction.

The third Condition can be removed by ensuring that no electrolyte remains at the intermetallic junction - this may require extra attention to drainage or to protection from the weather. A good covering of paint or sealant over the junction can be effective to avoid Galvanic Corrosion.

Use the Area Effect to avoid Galvanic Corrosion.

The area effect should also be considered in avoiding corrosion damage, particularly in selection of fastener materials.

Stainless steel fasteners can be used to hold aluminium structures, but the area effect will not apply if the wetted area shrinks over time due to evaporation. In the situation of 304 or 316 fasteners used in conjunction with other less noble structural materials, the fastener will be galvanically protected by the surrounding large, less noble area.

Positively Use Galvanic Protection to avoid Galvanic Corrosion.

The galvanic effect can also be used to provide corrosion protection. For example it is prudent to guard against possible crevices, perhaps associated with marine fouling, or simply under bolt heads, by specifying slightly more noble bolt materials. An example is the use of 316 fasteners in conjunction with 304 structural materials – the minor galvanic protection afforded the fasteners improves their corrosion resistance.

Pitting & Crevice Corrosion of Stainless Steel

Stainless Steels are a family of alloys exhibiting good resistance to attack by many of the environments encountered in industry and in domestic, commercial and marine exposure. Their resistance is not perfect, however, and the large number of grades of stainless steel now available is largely because of this challenge of finding cost-effective resistance to these various environments.

The corrosion resistance of stainless steels to some environments can be described by corrosion resistance tables, as the corrosion which does occur is a fairly uniform metal thinning over time. This is termed "General Corrosion". "Localised Corrosion" by contrast results in attack at certain specific sites while other parts of the metal may remain totally unaffected.

Studies of corrosion failures of stainless steel have indicated that pitting and crevice corrosion types are major problems, and together account for perhaps 25% of all corrosion failures.

What is Pitting Corrosion?

Under certain specific conditions, particularly involving chlorides (such as sodium chloride in sea water) and exacerbated by elevated temperatures, small pits can form in the surface of the steel. Dependent upon both the environment and the steel itself these small pits may continue to grow, and if they do can lead to perforation, while the majority of the steel surface may still be totally unaffected.

What is Crevice Corrosion?

Crevice Corrosion can be thought of as a special case of pitting corrosion, but one where the initial "pit" is provided by an external feature; examples of these features are sharp re-entrant corners, overlapping metal surfaces, non-metallic gaskets or incomplete weld penetration. To function as a corrosion site a crevice has to be of sufficient width to permit entry of the corrodent, but sufficiently narrow to ensure that the corrodent remains stagnant. Accordingly crevice corrosion usually occurs in gaps a few micrometres wide, and is not found in grooves or slots in which circulation of the corrodent is possible.

Environmental Factors

The severity of the environment is very largely dependent upon two factors - the chloride (Cl-) content and the temperature - and the resistance of a particular steel to pitting and crevice corrosion is usually described in terms of what % Cl- (or ppm Cl- ) and °C it can resist. It should be noted that the most common grade of stainless steel, Type 304, may be considered susceptible to pitting corrosion in sea water (2% or 20,000 ppm chloride) above about 10°C, and even in low chloride content water may be susceptible at only slightly elevated temperatures. A safe chloride level for warm ambient temperatures is generally about 150ppm (150mg/l). Grade 316 is more resistant and is commonly used in ambient sea water, but can be attacked in crevices or if the temperature increases even slightly.

The velocity of the liquid is also significant; a stagnant solution is more likely to result in pitting and crevice attack, particularly if there are particles to settle out of the liquid. Note that there may also be a problem from stress corrosion cracking if austenitic stainless steels are used in chloride containing water at temperatures over about 60°C.

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Which Steels are Susceptible?

All stainless steels grades can be considered susceptible, but their resistances vary widely. Their resistance to attack is largely a measure of their content of chromium, molybdenum and nitrogen. Another factor of importance is the presence of certain metallurgical phases (in particular the grades 303, 416 and 430F containing inclusions of manganese sulphide) have very low resistances, and ferrite may be harmful in austenitic grades in severe environments. A clean and smooth surface finish improves the resistance to attack. Contamination by mild steel or other "free iron" greatly accelerates attack initiation.

Measurement of Resistance to Attack

Laboratory tests have been developed to measure the resistance of metals to both pitting and crevice corrosion. This testing has two main aims - firstly to enable ranking of each alloy in order of resistance, and secondly as a quality control measure, to ensure that particular batches of steel have been produced not just with correct composition, but also have been properly rolled and heat treated.

The most commonly used test is that in ASTM G48, which measures resistance to a solution of 6% ferric chloride, at a temperature appropriate for the alloy, shown in the graph above. If an artificial crevice is added to the sample the test measures crevice corrosion resistance rather than pitting resistance.

The temperature which is just high enough to cause failure of this test is termed the Critical Pitting Temperature (CPT) or the Critical Crevice Temperature (CCT). Alternative laboratory tests can be carried out using electrochemical cells with a variety of test solutions. The results obtained in laboratory tests are approximate only, as factors such as surface finish, water velocity, water contaminants and metallurgical condition of the steel are all important.

Pitting Resistance Equivalent Number (PRE)

From experience it has been found that an estimate of resistance to pitting can be made by calculation from the composition as the Pitting Resistance Equivalent Number:

PRE = % Cr + 3.3 x %Mo + 16 x %N

Various multipliers (up to 30) for Nitrogen have been used in this equation; with the higher values often used for the austenitic stainless steel grades; in any case the effect of nitrogen is very important, hence the requirement by many suppliers (including Atlas) that the highly resistant grade 2205 have a minimum nitrogen content of 0.14%. This also explains the trend in extremely high pitting resistant alloys for even higher nitrogen levels. The super duplex grade UR52N+ (UNS S32520/S32550) typically contains 0.2% nitrogen, while the super austenitic grade 4565S (UNS S34565) typically contains 0.45% nitrogen.

Effect of Welding

The welding process results in metallurgical changes in both fusion zone and heat affected zone. In most alloy systems some degradation in pitting and crevice corrosion resistance occurs in welding, but these effects can be minimised if proper materials and practices are used. Proper materials usually involves over-alloyed consumables and practices includes proper heat inputs. It is important that correct information be sought from suppliers. Again looking at the extremely high pitting resistant alloys it has been found that the high molybdenum alloys are particularly susceptible to fusion zone micro-segregation, leading to lowered pitting resistance. Alloys such as 4565S which achieve their pitting resistance by high nitrogen rather than very high molybdenum levels have been found to be less affected by weld segregation.

Measures to Reduce Pitting and Crevice Corrosion

Control the environment to low chloride content and low temperature if possible.

Fully understand the environment.

Use alloys sufficiently high in chromium, molybdenum and/or nitrogen to ensure resistance.

Prepare surfaces to best possible finish. Mirror-finish resists pitting best.

Remove all contaminants, especially free-iron, by passivation

Design and fabricate to avoid crevices.

Design and fabricate to avoid trapped and pooled liquids

Weld with correct consumables and practices and inspect to check for inadvertent crevices.

Pickle to remove all weld scale

Stress Corrosion Cracking

Metals are useful in engineering structures because of their strength, ductility, and durability. Ductility is extremely important as it allows the material to deform in response to loading thus redistributing the stresses. In some cases, however, chemical interactions with the environment can reduce the ductility of metals so that they behave more like brittle materials when subjected to stress.

-Caustic Stress Corrosion Cracking

-Chloride Stress Corrosion Cracking - CSCC

Definition.

Stress corrosion cracking is the intergranular or transgranular cracking of a material due to the combined action of tensile stress and a specific environment.

Mechanism.

Even after many years of intensive study, the exact mechanism of stress corrosion cracking remains a matter of extensive disagreement and study. It is commonly attributed to the rupture of protective films at the tips of pits or pre-existing cracks due to the applied stress. In many cases, the materials appear to be totally resistant to corrosion in a given environment until stresses are applied. They then crack catastrophically without any sign of other corrosion attack.

Examples.

Many materials, particularly high strength materials, are susceptible to stress corrosion cracking when exposed to a specific environment. For example, cold worked brass, which is found in ammunition cartridges, is susceptible to stress corrosion cracking when exposed to an environment containing ammonia. In chloride containing environments, titanium alloys, aluminum alloys, and high strength stainless steels are susceptible and specific alloys, which are resistant to stress corrosion cracking, should be used. The stresses required to initiate and propagate cracking are often low and many failures occur due to residual stresses rather than applied stress.

Caustic stress corrosion

Caustic stress corrosion, or caustic embrittlement, is another form of intergranular corrosion cracking.

The mechanism is similar to that of chloride stress corrosion.

Mild steels (steels with low carbon and low alloy content) and stainless steels will crack if they are exposed to concentrated caustic (high pH) environments with the metal under a tensile stress. In stress cracking that is induced by a caustic environment, the presence of dissolved oxygen is not necessary for the cracking to occur. Caustic stress corrosion cracking was first encountered in the operation of riveted steam boilers.

These boilers were found to fail on occasion along riveted seams. Failure was attributed to caustic-induced cracking at the highly stressed regions near and under the rivets.

Boiler water could easily flow into the crevices which existed under the rivets. Radiative heating would cause the water in the crevices to boil. As steam was formed, it would escape from the crevice.

More boiler water would then flow into the crevice, boil, and pass from the crevice as steam. The net result of this continuing process was concentration of caustic under the rivet.

The combination of high stress and high caustic concentrations eventually led to destructive cracking of the boiler vessel.

Where the rate of steam generation (boiling) is high, it is more difficult to eliminate the problem of solute concentration in regions of the boiler. Caustic stress corrosion may concentrate in such regions as the water evaporates rapidly, but sufficient concentration of caustic by such a mechanism to induce stress corrosion cracking is considered unlikely.

Available data indicates that caustic concentrations greater than 10,000 ppm, and probably up to 50,000 ppm, are required to induce caustic stress cracking (40,000 ppm NaOH is equivalent to 40 grams per liter or 1 mole per liter).

The pH of such a solution is on the order of 14. An alkaline environment is produced and controlled by use of a solution having some properties of a buffer, that is, one that tends to retard or slow a reaction or tends to force it in one direction or the other.

Chloride Stress Corrosion Cracking - CSCC

Chloride Stress Corrosion Cracking is a localized corrosion mechanisms like pitting and crevice corrosion. The three conditions that must be present for chloride stress corrosion to occur are as follows.

- Chloride ions are present in the environment

- Dissolved oxygen is present in the environment

- Metal is under tensile stress

Austenitic stainless steel is a non-magnetic stainless steel grades consisting of iron, chromium, and nickel, with a low carbon content. This alloy is highly corrosion resistant and has desirable mechanical properties. One type of corrosion which can attack austenitic stainless steel is chloride stress corrosion.

Chloride stress corrosion is a type of intergranular corrosion. Chloride stress corrosion involves selective attack of the metal along grain boundaries.

In the formation of the steel, a chromium-rich carbide precipitates at the grain boundaries leaving these areas low in protective chromium, and thereby, susceptible to attack.

It has been found that this is closely associated with certain heat treatments resulting from welding. This can be minimized considerably by proper annealing processes.

This form of corrosion is controlled by maintaining low chloride ion and oxygen content in the environment and the use of low carbon steels. Environments containing dissolved oxygen and chloride ions can readily be created in auxiliary water systems.

Chloride ions can enter these systems via leaks in condensers or at other locations where auxiliary systems associated with the nuclear facility are cooled by unpurified cooling water. Dissolved oxygen can readily enter these systems with feed and makeup water. Thus, chloride stress corrosion cracking is of concern, and controls must be used to prevent its occurrence.

Figure 1 Intergranular Corrosion Cracking

Figure 1 illustrates intergranular stress corrosion cracking. The pressure of a tensile stress opens up intergranular cracks and accelerates further corrosion.

Chloride stress corrosion is a particularly significant problem in the operation of nuclear facilities because of the wide use of austenitic stainless steel, and the inherent presence of high tensile stresses associated with pressurization.

Chloride stress corrosion cracks have been known to propagate in austenitic stainless steel at stresses of about one-fifth yield strength with chloride concentrations of less than 50 ppm.

 Tests show that the 18-8 stainless steels grade are susceptible to chloride stress attack when both the chloride ion concentration and dissolved oxygen concentration are above certain values.

The region of susceptibility for austenitic stainless steel is illustrated in Figure 2.

Figure 2 Austenitic Stainless Steel Chloride Stress Corrosion Cracking

Note that when dissolved oxygen is present at about 1 ppm, chloride stress corrosion cracking can be initiated at chloride ion concentrations near 1 ppm.

However, when the concentration of dissolved oxygen is very low, susceptibility to chloride stress corrosion cracking is reduced.

High temperature tends to decrease the time required for chloride-induced cracking to occur, but there appears to be no practical temperature limit below which cracking will not occur, given sufficient time and severe conditions.

The curve in Figure 2 is valid for temperatures in the range 470°F to 500°F.

Cavitation Corrosion.

Under high velocity flow conditions, particularly when the flow is turbulent, areas of high and low pressure will be induced. In areas of low pressure, gas and vapor bubbles will be produced. When these bubbles move to an area of higher pressure, they collapse and their implosion creates a pressure wave that can remove protective films and cause increased corrosion.

Definition.

Cavitation corrosion is corrosion that is enhanced through the formation and collapse of gas or vapor bubbles at or near the metal surface.

Mechanism.

As described above, the formation and collapse of gas or vapor bubbles in a liquid can cause localized damage to the films responsible for limiting corrosion. Once this localized corrosion is established, the local roughening can often serve as a new site for further turbulence and more cavitation attack.

Examples.

Cavitation is commonly encountered in pumps and in high speed propellers. It is enhanced when entrained air is present in systems with high velocity flow. Cavitation can also occur on or near high intensity sound generators. Stainless steels, some nickel alloys, and titanium alloys are highly resistant to cavitation damage but even these will be attacked under severe conditions.

Appearance.

Cavitation corrosion is similar to erosion corrosion and pitting is usually encountered. Cavitation can often be verified by a hydrodynamic analysis that can be used to locate and minimize bubble formation or move the area of bubble collapse to an area where the attack will have a minimal effect.

Significant Measurements.

There are no standard tests for cavitation attack. Only through actual full scale tests or from experience can failure due to cavitation be avoided. High velocity flow should be avoided in the design of all systems.

Appearance.

Stress corrosion cracking must be evaluated using microscopic examination of the cracked sections. The cracking is often branched. Stress corrosion cracking can occur in the presence of other forms of corrosion attack or without the presence of other visible attack.

Significant Measurements.

In general, alloys known to be susceptible to stress corrosion cracking should be avoided. In some cases, special heat treatments can minimize the susceptibility to stress corrosion cracking. Many tests have been developed to test the susceptibility of metals to stress corrosion cracking. All of these combine mechanical loading, often in the presence of a pre-existing crack, and exposure to the specific environment of interest. For alloys with limited sensitivity to stress corrosion cracking, critical stresses can be defined below which stress corrosion will not occur. The structural analysis and manufacturing processes required to accommodate these critical stress criteria are often very complex and the use of highly resistant materials is recommended.

Hydrogen Embrittlement.

Hydrogen can enter most metals. Due to the small size of the hydrogen atom, it can migrate through the metal structure and cause a loss of ductility similar to that experienced in stress corrosion cracking.

Definition.

Hydrogen embrittlement is the severe loss of ductility of a metal when hydrogen has been introduced into the metal structure.

Mechanism.

Hydrogen atoms can enter a metal either from hydrogen gas, usually at elevated temperatures, or from atomic hydrogen that is electrolytically formed on its surface. This hydrogen can either reduce the energy required for forming cracks under stress or can accumulate at areas of high stress, such as crack tips, and cause pressure, which directly assists crack propagation. High strength materials in general are the most susceptible to hydrogen embrittlement. Hydrogen can be formed electrolytically during electroplating, during welding when hydrogen is present in the electrode material, in the electrode coating, in the shielding gas, or simply as moisture on the metal surface, or when excessive cathodic protection is applied (potentials more negative than minus 1.2 volts are normally required for significant hydrogen formation by cathodic protection.)

Examples.

Ferritic and martensitic (magnetic) steels, particularly those with a yield strength in excess of 130 ksi, are particularly prone to hydrogen embrittlement. Austenitic (non-magnetic) stainless steels are less susceptible. When hydrogen pickup is suspected, such as in electroplating or welding, the hydrogen can be removed by baking at 200 to 300°F. Hydrogen pickup during welding is normally prevented by using low hydrogen electrodes and mild preheating to remove water from the surfaces being welded. Appearance. Other than catastrophic failure by cracking, there is often no visible evidence of hydrogen embrittlement. In extreme cases, where hydrogen gas bubbles are formed inside the metal, shinny internal blisters are visible at the fracture surface.

Significant Measurements.

Analysis of the metal for untrained hydrogen can be used to verify hydrogen embrittlement if heating subsequent to failure has not driven off the hydrogen gas. Due to the difficulty in verifying this form of attack, it is often blamed for other forms of cracking failure, often when simple overload is the actual cause of failure.

Corrosion Type Immunity.

The first form of corrosion described is the lack of attack, or immunity. This can result from the action of two basic mechanism. Corrosion test measurements that are used to measure very low corrosion rates must be used to validate that corrosion activity is completely absent.

Definition.

Immunity is the lack of measurable attack on a metal when exposed to operational environments.

Mechanism.

Immunity can result from two basic mechanisms. In the first case, the energy content of the metal is lower (more stable) than any of the corrosion products that could possibly form. Such metals are commonly found in nature as metals that indicates the stability of the metallic state for these elements. Corrosion of such metals where an increase of energy is required will not take place naturally just as a ball will not roll uphill unless pushed. In the second case, there is an energy tendency for the metal to suffer corrosion, but corrosion activity is prevented by the presence of a highly stable passive film. Not only must this film be stable in the environment, but it must be able to repair itself by reaction with the environment if it is damaged.

Examples.

Gold and platinum are examples of metals that are normally immune in a wide variety of environments due to the low energy content of their metallic state. Titanium and some highly alloyed materials, such as the complex nickelchromium- molybdenum alloy Hastelloy C-276, have an extremely stable passive film that will repair itself when damaged.

 Appearance.

A metal that is immune to corrosion in a given environment will not show any change due to corrosion after exposure.

Significant Measurements.

Metals that do not suffer corrosion are unchanged by exposure to their environment. Their strength, weight, size, shape, and surface finish are unaffected by exposure. To measure very low rates of corrosion, weight loss and other material property measurements may not be sufficiently sensitive. In these cases, the metal is exposed to a small quantity of the electrolyte and the electrolyte is analyzed for the presence of metal atoms from the corrosion process.

Corrosion Fatigue.

Many materials will exhibit a substantial reduction in fatigue life when exposed to a corrosive environment. In some cases, the reduction is severe, in other cases it is less dramatic, but only a very few materials show a fatigue resistance in a corrosive environments as great as that in dry air.

Definition.

Corrosion fatigue is the reduced ability of a metal to withstand repeated stress when exposed to the combined action of stress and a corrosive environment as compared to the effects of stress alone.

Mechanism of Corrosion Fatigue.

Fatigue resistance can be reduced by corrosion activity in many ways. In materials that are susceptible to stress corrosion, fatigue resistance is probably lowered by the rapid propagation of fatigue cracks after they reach the size required for stress corrosion cracking. In materials not susceptible to stress corrosion cracking, corrosion probably enhances crack propagation through direct attack at the crack tips, or by the formation of stress risers such as pits. Corrosion fatigue is usually 4-20 more severe at low cycling frequency where the longer time to failure allows more corrosion activity to occur.

Examples of Corrosion Fatigue.

High strength steels are susceptible to substantial reduction in fatigue resistance in many environments. The endurance limit (stress below which fatigue failure will not occur) is often reduced by a factor of ten from that measured in air. Cathodic protection can increase the resistance of steels to corrosion fatigue, but care must be taken not to overprotect them ashydrogen embrittlement would then occur. Titanium alloys, which are not subject to stress corrosion cracking, are particularly resistant to corrosion fatigue as are some of the more corrosion resistance nickel alloys, such as Inconel 625 and Inconel 718.Copper alloys and stainless steels are also susceptible to corrosion fatigue with a reduction of one-half in their endurance limit being common.

Appearance of Corrosion Fatigue.

Corrosion fatigue gives a fracture surface similar to ordinary fatigue except that in some cases, corrosion products are present in the outer sections of the cracks.

Significant Measurements.

In the simplest corrosion fatigue test, the electrolyte is simply dripped over the surface of a rotating beam fatigue test specimen. In more sophisticated tests, flat specimens are stressed as cantilever beams and only tensile stresses are induced on the surface exposed to the corrosive environment. When cyclic loading is a factor in design, fatigue data from tests that include the corrosive environment must be used.

Corrosion Control

There are a number of means of controlling corrosion. The choice of a means of corrosion control depends on economics, safety requirements, and a number of technical considerations. The NASA Kennedy Space Center has a group of trained corrosion professionals who can provide guidance on corrosion control.

Design

Materials Selection

Protective Coatings

Inhibitors and Other Means of Environmental Alteration

Corrosion Allowances

Cathodic Protection

Design

Engineering design is a complicated process that includes design for purpose, manufacturability, inspection, and maintenance.  One of the considerations often overlooked in designing manufactured products is drainage.  The corrosion of the automobile side panel above could have been minimized by providing drainage to allow any water and debris to fall off of the car instead of collecting and causing corrosion from the far side of the panel.

All of the other methods of corrosion control should be considered in the design process.

Materials Selection

Carbon Steel

Stainless Steel

Aluminum

Copper Alloys

Titanium

Carbon Steel

Most large metal structures are made from carbon steel-the world's most useful structural material. Carbon steel is inexpensive, readily available in a variety of forms, and can be machined, welded, and formed into many shapes.

This large statue by Pablo Picasso in front of the Chicago city hall is made from a special form of carbon steel known as weathering steel. Weathering steel does not need painting in many boldly exposed environments. Unfortunately, weathering steel has been misused in many circumstances where it could not drain and form a protective rust film. This has given the alloy a mixed reputation in the construction industry.

Where other means of corrosion control are not practical, other alloys can be substituted for carbon steel. This normally doubles or more the material cost for a structure, and other corrosion control methods must be considered before deciding on the use of more expensive alternates to carbon steel.

Some forms of carbon steel are subject to special types of corrosion such as hydrogen embrittlement, etc. It is common practice to limit the allowable strength levels of carbon steel to avoid brittle behavior in environments where environmental cracking may occur. High strength bolts cannot be galvanized for the same reason-a concern that they may hydrogen embrittle due to corrosion on the surface.

Protective coatings, cathodic protection, and corrosion inhibitors are all extensively used to prolong the life of carbon steel structures and to allow their use in environments such as the Kennedy Space Center where the environment would otherwise be too corrosive for their use.

Stainless Steels

The stainless steel body on this sports car is one example of how stainless steels can be used. The stainless steel is virtually immune to corrosion in this application-at least in comparison to the corrosion that would be experienced by conventional carbon steel or aluminum auto bodies.

Stainless steels are a common alternative to carbon steels. There are many kinds of stainless steels, but the most common austenitic stainless steels (300-series stainless steels) are based on the general formula of iron with approximately 18% chromium and 8% nickel. These austenitic stainless steels are frequently immune to general corrosion, but they may experience pitting and crevice corrosion and undergo stress corrosion cracking in some environments.

Aluminum

Aluminum alloys are widely used in aerospace applications where their favorable strength-to-weight ratios make them the structural metal of choice. They can have excellent atmospheric corrosion capabilities. Unfortunately, the protective properties of the aluminum oxide films that form on these alloys can break down locally and allow extensive corrosion. This is discussed further in the section on intergranular corrosion.

The highway guardrail shown on the right is located near the ocean in Florida. The aluminum alloy maintains a silvery shine except in locations where the passive film has suffered mechanical damage. The wear caused by the rail touching the wooden post at this location destroyed the passive film on the edges of the rail and allowedintergranular corrosion to proceed and cause the exfoliation corrosion shown above. While the corrosion above is very interesting and makes for an interesting web site, it is important to note that the railing is decades old and would have never lasted as long in this location if it were made of carbon steel.

Intergranular corrosion is a major problem on airplanes and other structures made from aluminum alloys. It frequently occurs at bolt and rivet holes or at cutouts where the small grain boundaries perpendicular to the metal surface are exposed.

Copper Alloys

Brasses and bronzes are commonly used piping materials, and they are also used for valves and fittings. They are subject to stress corrosion cracking in the presence of ammonia compounds. They also suffer from dealloying and can cause galvanic corrosion when coupled with steel and other structural metals. Most copper alloys are relatively soft and subject to erosion corrosion.

The dezincification shown above could have been controlled by using inhibited brasses which have been commercially available since the 1930's.

Titanium

Titanium is one of the more common metals in nature, but its limited use means that small-scale production operations result in a relatively expensive metal. In the United States it finds extensive use in the aerospace industry. The Japanese make extensive use of titanium in the chemical process industries.

There are two general types of titanium alloys-aerospace alloys and corrosion resistant alloys. The crevice corrosion of an aerospace alloy flange in a saltwater application is a classic example of how titanium gets misused.

Preventing Corrosion in New Mexico Oilfields

Corrosion control methods include:

Cathodic Protection

Protective Coatings

Chemical Inhibitors

Plastic or cement liners

Use of special alloys

Solids removal

Removal of corrosive gases

Dehydration

In addition, monitoring corrosion and scale formation is an important part of the prevention process, allowing producers to better understand the environment and to be able to predict problems. Field experience indicates that a systematic, integrated process of monitoring and failure analysis will, over time, reduce failure rates, and effectively lower production costs.

Cathodic Protection

In general, cathodic protection is an approach where the metal surface to be protected is made into the cathode of a corrosion cell. Since corrosion and material loss occurs at the anode, this approach protects the metal.

There are two types of cathodic protection, the sacrificial anode and the impressed-current method. The sacrificial anode method is the simpler method, and utilizes galvanic corrosion. Sacrificial anodes are pieces of metal usually electrically connected by a wire or steel strap to the structure to be protected. The metals used must be less noble than steel (the common oil-field material), such as magnesium, zinc, or aluminum. The sacrificial anodes are preferentially corroded, protecting the (cathodic) steel from corrosion. Magnesium and zinc are usually used in soils, and zinc can also be used in brine environments. Sacrificial anodes are most often used when current requirements are relatively low, electric power is not readily available, and when system life is short, which calls for a low capital investment.

Impressed-current method uses an external energy source to produce an electric current that is sent to the impressed-current anodes, which can be composed of graphite, high-silicon cast iron, lead-silver alloy, platinum, or even scrap steel rails. Impressed-current cathodic protection is used when current requirements are high, electrolyte conductivity is high, fluctuations in current requirements will occur, and when electrical power is readily available.

Cathodic protection interferes with the natural action of the electrochemical cells that are responsible for corrosion. Cathodic protection can be effectively applied to control corrosion of surfaces that are immersed in water or exposed to soil.

Cathodic protection in its classical form cannot be used to protect surfaces exposed to the atmosphere. The use of anodic metallic coatings such as zinc on steel (galvanizing) is, however, a form of cathodic protection, which is effective in the atmosphere. There are two basic methods of supplying the electrical currents required to interfere with the electrochemical cell action.

The first method, cathodic protection with galvanic anodes, uses the corrosion of an active metal, such as magnesium or zinc, to provide the required electrical current. In this method, called sacrificial or galvanic anode cathodic protection, the active metal is consumed in the process of protecting the surfaces where corrosion is controlled and the anodes must be periodically replaced.

In the second method, impressed current cathodic protection, an alternative source of direct electrical current, usually a rectifier that converts alternating current to direct current, is used to provide the required electrical current. In this system, the electrical circuit is completed through an inert anode material that is not consumed in the process.

Coatings

Protective coatings can be used to protect tubing, downhole equipment, wellhead components, Christmas trees, and various flowlines and pressure vessels. Coatings work by reducing the cathodic area available for corrosion reaction. Since no coat can be 100% holiday-free (without pinholes or defects), coatings are often used in conjunction with cathodic protection or chemical inhibition.

Quality control parameters for coating include surface finish/preparation, application techniques, coating thickness, holiday detection, joint condition, and inspection. Coated equipment must be carefully handled after coating to prevent defects in the coat.

Corrosion Inhibitors

There is a wide variety of corrosion inhibitor formulations available that can be selected to handle most environments in oil and gas production systems except those where oxygen is present. The application technique must match the systems mechanical and process considerations in order to assure that the inhibitor will reach the metal surface where needed. Most corrosion inhibitors, which are typically organic amine-based compounds, function by establishing a film that protects metal from corrosive fluids.

The choice of a specific inhibition program is a combination of technical and economic considerations. The programs should be modified or adjusted periodically to optimize the program for cost effectiveness. Additionally, programs should be monitored and periodically reviewed because systems are continually changing.

Types of inhibitors

There are many types of corrosion inhibitors for various applications. Generally they can be grouped into two broad categories, organic and inorganic. Inorganic inhibitors are most often used in cooling tower water, heating/cooling mechanisms, dehydration glycol, and sweetening amine solutions. Organic film formers are used in oil, gas, and water wells, water and gas systems, and flowlines.

Inorganic corrosion inhibitors are usually metal salts, which act to passivate the metal surface. They have limited use because they require constant concentrations, are often pH sensitive, and usually don't work if there are chlorides present.

Organic corrosion inhibitors are the most common corrosion inhibitors in use in oilfield systems. The majority of these are "organic film forming inhibitors". These are organic chemicals with a polar ("water loving") head and a long hydrocarbon ("oil loving") tail. When applied, these compounds align with the polar head towards the metal and the tail towards the outside, effectively establishing an oil-wet film on the metal surface. This inhibitor film breaks the corrosion cell by separating the metal surface and the electrolyte containing water.

Inhibitor selection

The first step in selecting an inhibitor is to review the system, its physical layout, mechanical considerations, and fluids being handled, locating any special or unique factors.

The second step is to select the application method(s) that assure the inhibitor gets to where it will be effective, i.e., the metal surface. This step is very important - large numbers of "inhibitor failures" were actually application failures.

Common application methods for wells with packered annuli include formation squeeze, tubing displacement, partial tubing displacement and yo-yo, and treating strings. Less common application methods include weighted liquids, dump bailers, wash bailers, inhibitor sticks, chemical injector valves, and gas lift gas addition.

Application methods for wells with open annuli include annular batch - operator applied or by treater truck, and continuous, with a chemical injector pump.

The third step is to review the properties required for the application technique. Inhibitor properties that should be taken into consideration during selection include solubility and dispersability, both in the carrier and in produced fluids (water, oil, gas); emulsification properties, viscosity, freeze point, thermal stability, corrosiveness, foaming properties, partitioning (between oil and water), compatibility with other chemicals, mobility of individual components, compatibility with downstream process, and environmental concerns.