Corrosion Metals - Chapter 1.1 Corrosion of Metals James F Jenkins and Richard W Drisko Introduction This chapter describes in basic terms the

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Unformatted text preview: Chapter 1.1 Corrosion of Metals James F. Jenkins and Richard W. Drisko Introduction This chapter describes in basic terms the causes and mechanisms of corrosion. Corrosion is defined as “the chemical or electrochemical reaction between a metal and its environment resulting in the loss of the material and its properties.” 1 Various types of corrosion are discussed and the basic principles behind the use of protective coatings and cathodic protection for corrosion control are also covered. The strategies used in corrosion control by design are briefly discussed as well. This basic knowledge helps in understanding how protective coatings, cathodic protection, and other corrosion control methods can best be used as part of a total corrosion control program. Further information on these corrosion control methods can be found in subsequent chapters. and strength. Rust is also unsightly and can cause contamination of the environment and industrial products. It is further detrimental in that it is not a stable base for coatings. Why Metals Corrode With few exceptions, metallic elements are found in nature in chemical combination with other elements. For example, iron is usually found in nature in the form of an ore, such as iron oxide. This combined form has a low chemical energy content and is very stable. Iron can be produced from iron ore by a high temperature smelting process. The heat that is added during smelting breaks the chemical bond between the iron and the oxygen. As a result, the iron and other metals used in structural applications have a higher energy content than they do in their original state, and are relatively unstable. Corrosion is a natural process. Just like water flows to seek the lowest level, all natural processes tend towards the lowest possible energy states. Thus, iron and steel have a natural tendency to combine with other chemical elements to return to their lower energy states. In order to do this, iron and steel will frequently combine with oxygen, present in most natural environments, to form iron oxides, or “rust,” similar chemically to the original iron ore. Figure 1 illustrates this cycle of refining and corrosion of iron and steel. When rust forms on an iron or steel structure, metal is lost from the surface, reducing cross section Figure 1. The corrosion cycle. Immunity and Passivity Some metals such as gold and platinum have lower energy levels in their metallic form than when combined with other chemical elements. These metals are often found in nature in the metallic form and do not tend to combine with other elements. They are thus highly resistant to corrosion in most natural environments. These materials are said to be immune to corrosion in those natural environments. Other metals and alloys, while in a high energy state in their metallic forms, are resistant to corrosion due to formation of passive films (usually oxides) on their surfaces. These films form through a natural process similar to corrosion, and are usually invisible to the naked eye. They are, however, tightly adherent and continuous and serve as a barrier between the underlying metal and the environment. Stainless steels, aluminum alloys, and titanium are examples of metals that are in a high energy state in their metallic forms, but are relatively resistant to corrosion due to the formation of passive films on their surfaces. However, particularly in the case of stainless steels and aluminum alloys, this film is not resistant to all natural environments and can break down in one or more particular environments. This breakdown of the passive film often results in rapid, localized corrosion, due to the electrochemical activity of the parts of the surface that remain passive. Figure 2 shows an example of such rapid, localized corrosion. (Note: This type of rapid, localized corrosion does not occur when paint coatings break down. Although paints provide a similar type of protection to the underlying metal, they are usually not electrochemically active.) where Mo is a neutral metal atom, M+ is a positively charged metal ion, and e- is an electron. Corrosion occurs as the positively charged ions enter the electrolyte and are thus effectively removed from the metal anode surface. The electrons remain in the bulk metal and can move through the metal to complete other reactions. In the case of iron (Fe) two electrons are usually lost, and the equation is: Feo → Fe++ + 2 e- where Feo is an iron atom and Fe++ is an iron (ferrous) ion. After the iron ions (Fe++) enter the electrolyte, they usually combine with oxygen in a series of reactions that ultimately form rust. Figure 2. Corroded low-alloy steel bridge where protective outside film has been lost. The Mechanism of Corrosion The combination of metals with other chemical elements in the environment—what is commonly called corrosion—occurs through the action of the electrochemical cell. The electrochemical cell consists of four components: an anode, a cathode, an electrolyte, and a metallic path for the flow of electrons. When all four of these components are present as shown in Figure 3 “cyclic reaction” occurs that results in corrosion at the anode. The key to understanding corrosion and corrosion control is that all of the components of this electrochemical cell must be present and active for corrosion to occur. If any one of the components is missing or inactive, corrosion will be arrested. Anode At the anode in an electrochemical cell, metal atoms at the surface lose one or more electrons and become positively charged ions. The generic chemical equation for this type of reaction is: Mo → M+ + e- Figure 3. The basic components of the electrochemical cell. Cathode At the surface of the cathode in an electrochemical cell, the electrons produced by the reactions at the anode are “consumed,” i.e., used up by chemical reactions. The generic chemical equation for this type of reaction is: R+ + e- → Ro or o R + e- → RIn this equation, R stands for any of a number of possible compounds that can exist in an oxidized form (R+) and in a reduced form (Ro). Many cathodic reactions are possible in 2 natural environments. The cathodic reactions that actually occur are dependent on the chemical composition of the electrolyte. In many instances where the electrolyte is water, the cathodic reaction is: 2 H2O + O2 + 4 e- → 4 OHIn this reaction, two water molecules (H2O) combine with one oxygen molecule (O2) and four electrons to form four hydroxide ions (OH-). In this case, the water and oxygen are reduced as in the generic cathodic reaction above. These hydroxide ions tend to create an alkaline environment at active cathodic areas. Metallic Path A metallic path between the anode and the cathode allows electrons produced at the anode to flow to the cathode. A metallic path is required in the corrosion cell because the electrolyte cannot carry free electrons. In many cases, where the anode and cathode are on the same piece of metal, the metal itself is the “metallic path” that carries the electrons from the anode to the cathode. Electrolyte The electrolyte serves as an external conductive media and a source of chemicals for reactions at the cathode, and as a reservoir for the metal ions and other corrosion products formed at the anode. Within the electrolyte, a flow of charged ions balances the flow of electrons through the metallic path. Under atmospheric conditions, the electrolyte consists of just a thin film of moisture on the surface, and the electrochemical cells responsible for corrosion are localized within this thin film. Under immersion conditions, however, much more electrolyte is present, and the electrochemical cells responsible for corrosion can involve much larger areas. Rate of Reaction Many factors can affect corrosion, but the bottom line is that the rate at which corrosion occurs is limited by the rate of reaction at the least active component of the electrochemical cell. For example, if there is an incomplete metallic path, this may be the limiting factor in the overall corrosion reaction. In this case, the electrochemical cell responsible for corrosion is similar to that in a flashlight battery when the flashlight is switched off (see Figure 4). When a battery is installed in a circuit such as a flashlight, no current flows until the flashlight is switched on. The high effective resistance of the open switch prevents current flow and the electrochemical discharge of the battery. Similarly, an incomplete metallic path prevents corrosion. The nature of the electrolyte may also affect the overall corrosion reaction. If the available electrolyte is very pure water that has relatively few ions, the ion flow can be the limiting factor. In many cases of corrosion under immersion conditions, the amount of oxygen available for the cathodic reaction is the limiting factor. Many methods for controlling corrosion target only one component of the overall electrochemical cell. By controlling the rate of just one of the reactions involved in the overall electrochemical cell, the overall rate of corrosion can be controlled. It should be noted that temperature has an effect on the rate of the corrosion reaction. However, this effect is very complex, and is beyond the scope of this text. In the case of dissimilar metal corrosion, the potential difference between the metals also has an effect on reaction rate. This is discussed in the galvanic corrosion section of this chapter. Figure 4. The dry cell battery. Measuring Corrosion There are many methods of measuring corrosion: Weight Loss Weight loss is one of the most widely used methods of measuring corrosion. A sample is first carefully cleaned to remove all surface contamination. After cleaning, it is weighed. It is then exposed to the 3 environment in question and then recleaned and reweighed after a given period of time. If no corrosion has occurred, there will be no weight loss. Size Measurement The dimensions of the sample are measured before and after exposure. No change in dimensions indicates that no corrosion has occurred. Visual Observation Even minor amounts of corrosion are readily visible due to roughening of the surface. Chemical Analysis Surface deposits and the environments are tested for corrosion products. If surface deposits and the environment test negative for corrosion products (i.e., none present), it can be assumed that no corrosion has occurred. Forms of Corrosion No Attack As stated in section immunity and passivity, some metals and alloys are essentially unaffected by corrosion in certain environments. This may be either because they are more stable in their metallic forms than in a combined forms or because they form natural protective films on their surfaces that provide completely effective passivity. However, just because a given metal or alloy is essentially unaffected by corrosion in one or more environments does not mean that it is resistant to corrosion in all environments. That no corrosion has occurred can be verified by one of the methods described in the previous section. corrosion is shown in Figure 5, where anodic and cathodic sites periodically reverse. In this case, the metallic path is through the metal itself. The electrolyte may either be a thin film of moisture in atmospheric exposure, a liquid in which the surface is immersed, or water contained in moist earth. The amount of uniform corrosion is usually measured by weight loss. If weight loss is determined over a given period of time, it can also be used to calculate an average rate of metal loss over the entire surface. This corrosion rate is usually expressed in mils (0.001 inch) per year (mpy) or millimeters per year (mm/yr). This is a good way to measure the amount and rate of corrosion if the corrosion is truly uniform; however, these average rates can give misleading results if the corrosion is not uniform over the entire surface. (See the section on pitting for further information.) Direct measurement of metal loss through metal thickness is also sometimes performed and can be used to determine corrosion rate in mpy or mm/yr. Figure 5. The corrosion cell on a metal surface. Uniform Corrosion Uniform corrosion is a form of corrosion in which a metal is attacked at about the same rate over the entire exposed surface. While considerable surface roughening can take place in uniform corrosion, when the depth of attack at any point exceeds twice the average depth of attack, the corrosion is no longer considered to be uniform. When a metal is attacked by uniform corrosion, the location of anodic and cathodic areas shifts from time to time, i.e., every point on the surface acts as both an anode and a cathode at some time during the exposure. A schematic representation of uniform Since corrosion rates commonly vary with time (e.g., slower as corrosion products form protective films), they are usually measured over several different intervals. Corrosion rates can also be measured continuously for extended periods, using electrochemical techniques to determine how the rates are affected by time. A coating is a very effective tool in combating uniform corrosion because corrosion usually proceeds slowly at local sites where the coating breaks down or is damaged. These areas can therefore be repaired before significant damage occurs, assuming that 4 inspection identifies the defects at an early stage. Galvanic Corrosion When two or more dissimilar metals are connected by a metallic path and exposed to an electrolyte, galvanic corrosion can occur as shown in Figure 6. This dissimilar metal corrosion is driven by the difference in electrical potential between the metals. An electrochemical cell is formed in which the more active metal acts as an anode and the less active metal acts as a cathode. In galvanic corrosion, the more active metal corrodes more than if it were not electrically coupled, and the less active metal corrodes less than if it were not electrically coupled. A “galvanic series” table that lists metals in order of their electrical potential in a given environment can be used to determine which metal in a given combination will act as an anode and which will act as a cathode. Table 1 is a galvanic series derived from exposure of common metals to seawater. The galvanic activity of metals in other environments is similar to that in seawater, but significant differences may occur. It should be noted that in North America, galvanic series are listed with the most active metals at the top, but the opposite may be true in other parts of the world. To determine which convention has been used in a particular galvanic series table, look for active metals like zinc, magnesium or aluminum and see if they are listed at the top or at the bottom. It should also be noted that some metals, such as the 300 Series stainless steel, are listed twice. In atmospheric exposures, the anodic area and cathodic area involved in galvanic corrosion are usually about equal in size. This is because the electrical resistance of the thin film of moisture acting as the electrolyte is very large over distances much more than 1/8 inch or so (1-2 mm). Under immersion conditions, however, the effective resistance of the electrolyte is much less and galvanic corrosion effects have a much greater range. The cathodic reaction is often the limiting factor in corrosion under immersion conditions due to the limited availability of dissolved oxygen. As described in cathode section, in many instances where the electrolyte is water, the cathodic reaction is: 2 H O + O + 4 e- → 4 OH2 2 Thus, the rate at which electrons can be consumed at the cathode limits the rate of galvanic attack in these situations. Table 1. Galvanic Series Derived from Exposure of Common Metals to Seawater. Figure 6. Galvanic corrosion cell. The amount of galvanic corrosion that occurs in a given situation can be measured indirectly by monitoring the current flow between the anodes and cathodes. It can also be measured directly by determining the weight loss of the anodic and cathodic materials, or by some other direct means of measurement such as pitting depths or thickness measurements as appropriate to the form of attack. Relative rates of galvanic attack can be 5 assessed by looking at the distance between the metals in a galvanic series. For example, steel is farther from copper than it is from lead in the galvanic series, so the rate of galvanic attack on a piece of steel would be expected to be higher if coupled to a piece of copper than if coupled to a piece of lead, all other things being equal. Actual rates of galvanic attack are difficult to predict. They depend on the potential difference between the metals involved and the relative areas of affected anodic and cathodic surface. However, the relative areas of affected anode and cathode surface can, and often do, have a greater effect on galvanic corrosion than the potential difference between the metals involved. If the anode is large and the cathode is small, the low rate at which electrons can be consumed at the cathode results in little acceleration of corrosion on the larger anodic surface. (Figure 7) On the other hand, if the anode is small and the cathode is large, a relatively large number of electrons can be consumed at the cathode and this effect is concentrated over a smaller anode, resulting in a substantial acceleration of corrosion at the small anodic area. In this case, there is a large acceleration of corrosion at the anode. The effect of area ratio on galvanic corrosion is shown more graphically in Figure 8. can effectively isolate most of the surface of a metal from the electrolyte and can therefore be used to control galvanic corrosion. If galvanic corrosion is active, coating of the anode alone can result in having a small anode and large cathode with catastrophic results. This is because a small break in the coating on the anode will create a small anode-large cathode situation. Even though the cathodic material may be highly corrosion resistant, it is the galvanic corrosion of the anodic material that is important in such cases. When in doubt, the entire system should be coated; the mistake should not be made of coating only the anodic material and thereby creating an adverse area ratio. When only the cathode is coated, the effective anode/cathode area ratio is increased thus reducing corrosion at the anode. Figure 7. Rate of corrosion. Figure 8. The area effect in galvanic corrosion. Top: “Benign” area ratio—small cathode has little effect on large anode. Bottom: “Adverse” area ratio—large cathode has great effect on small anode. The area ratio effect is important when using coatings as a means of corrosion control. Coatings 6 Pitting Pitting corrosion (also called simply “pitting”) occurs when the amount of corrosion at one or more points on a metal is much greater than the average amount of corrosion. In some cases, the entire surface is corroded, but unevenly. In other cases, some areas are essentially unattacked. Figure 9 shows an example of pitting corrosion being measured. Pitting can occur through several mechanisms. Metals are not chemically or physically homogeneous. Some areas may have more of a tendency to be anodic than others and the shifting of anodic and cathodic areas that is necessary for uniform corrosion does not occur. This lack of homogeneity may be due to inclusions within the metal or to the combination of metallurgical phases that are naturally present in many alloys. Figure 9. Diver using a depth gauge to measure pit depths. Courtesy Underwater Engineering Services, Inc. measurement of pitting corrosion rates. In some cases, uniform corrosion rates in mpy or mm/yr are given for metals that actually have corroded by localized attack such as pitting. Such corrosion rates often greatly understate the actual depth of penetration of corrosion into the metal. In some applications, such as a structural beam, scattered pitting may not cause too much trouble, but a single pit through a tank wall or pipe handling a hazardous liquid can be disastrous even though most of the surface may be relatively unaffected. The amount of pitting is established by direct measurement of the depth of pits and the number of pits that occur in a given surface area. Pitting is essentially a random process; therefore, statistical sampling and analysis are often performed. Pit depths may be measured in several ways. One of the simplest ways is with a pit depth gauge that uses a dial micrometer and a pointed probe. For pitting corrosion, weight losses are only determined to establish that the deepest pit has more than twice the average metal loss based on weight loss, which is the point where uneven uniform corrosion becomes, by definition, pitting corrosion. Where pitting occurs at a significant rate, localized corrosion can have disastrous effects (e.g., in the case of a tank). In such cases, coatings alone are seldom effective in controlling corrosion as coating defects and degradation are inevitable. However, when coatings are combined with other forms of corrosion control, particularly cathodic protection, effective control of pitting corrosion is possible. Concentration Cell Corrosion Concentration cell corrosion is often called crevice corrosion because the differences in environment that drive this type of corrosion are often located in and adjacent to crevices. These crevices commonly occur at joints and attachments. Crevices can be formed at metal-to-metal joints or metal to non-metal joints. Deposits of debris or corrosion products can also form crevices. Concentration cell corrosion commonly occurs by one of two different mechanisms. Figure 10 illustrates these two types of concentration cell corrosion. The most common is oxygen concentration cell corrosion. In this type of corrosion, the availability of oxygen is less inside the crevice than it is outside the crevice. Another mechanism of pitting occurs by local breakdown of passive films on a metal. In this case, the area with the passive film is cathodic to the area without the passive film and a type of galvanic (dissimilar metal) corrosion occurs. The potential difference between areas with the passive films and sites lacking the passive film allows active corrosion to occur. This can be seen in Table 1 for 300 Series stainless steel where the 300 Series stainless steels occupy two positions, one much more active than the other. The more active position is occupied by material that is not protected by a passive film and the less active position is occupied by material that is protected by a passive film. Since pitting attack is, by definition, nonuniform, weight loss is not a suitable method for 7 This affects the cathodic reaction: 2 H2O + O2 + 4e→ 4 OH- Low oxygen concentration inhibit this reaction by limiting the availability of one of the reactants. Any factor that inhibits the cathodic reactions on a surface will make the anodic reactions on that surface more prevalent. Thus, in oxygen concentration cell corrosion, the surfaces inside the crevice are exposed to a lower oxygen environment and become anodic with respect to the surfaces outside the crevice and corrosion occurs inside the crevice area. In some cases, the corrosion of the surface outside the crevice is reduced. together. Like galvanic corrosion, concentration cell corrosion is normally accelerated under immersion conditions. Another possible mechanism of concentration cell corrosion is based on differences in metal ion concentration. In this case, the limited circulation inside the crevice causes a buildup of corrosion products. A buildup of metal ions (M+) will inhibit the generic anodic reaction: Mo → M++ eThis is because a buildup of reaction products (M ) inhibits the reaction. Any factor that inhibits the anodic reaction will cause the area to become more cathodic. In metal ion concentration cell corrosion, the area inside the crevice becomes the cathode and the area outside becomes the anode. This is opposite to the distribution of attack in oxygen concentration cell crevice attack. This form of crevice attack is usually less severe than oxygen concentration cell corrosion because the anode/cathode area ratio is not adverse in this case. There is a large anodic area outside the crevice and only a small cathodic area inside the crevice. The type of crevice corrosion that occurs in a given situation depends on the metals involved and the environments to which they are exposed. Stainless steels are particularly sensitive to oxygen concentration cell attack and copper alloys are commonly susceptible to metal ion concentration cell attack. Iron and steel show relatively minor effects of crevice corrosion. For iron and most other steels, crevices corrode more than adjacent surfaces under atmospheric conditions primarily because they remain wet more of the time. Sealants, which are intended to keep the environments out of crevice areas, are sometimes successful in preventing crevice corrosion under atmospheric conditions, but are relatively ineffective in preventing crevice corrosion under immersion conditions. Coating of the external surfaces (the area surrounding the crevice), however, can reduce the intensity of oxygen concentration cell attack by reducing the cathodic area. + Figure 10. Concentration cell corrosion. Top: Oxygen concentration cell. Bottom: Metal ion concentration cell. As in galvanic corrosion, oxygen concentration cell corrosion is accelerated by the adverse area ratio between the anode and the cathode. For example, the crevice area formed under a bolt head is usually small with respect to the area of the material being fastened Stray Current Corrosion Stray current corrosion is most commonly encountered in underground environments but can 8 also occur under immersion conditions. In stray current corrosion, an electrical current flowing in the environment adjacent to a structure causes one area on the structure to act as an anode and another area to act as a cathode. Direct current (DC) is the more damaging type of stray current, but alternating current (AC) can also cause stray current attack. In underground soil environments, stray current corrosion can be caused by currents arising from direct current railway systems, mining operations using direct current, welding operations, and underground cathodic protection systems. Stray currents can also be induced naturally on long underground pipelines. This is due to the interaction between the electrically conductive pipeline and the earth’s magnetic field. Stray currents can also be induced through improper grounding of electrical systems in buildings. Figure 11 shows a typical stray current situation caused by an electric railway. flow. Coatings are very useful in controlling stray currents as they can effectively electrically isolate the buried structure from the environment so that it does not become a low resistance path. If the structure is coated only in the more positive (anodic) areas, corrosion may become concentrated at defects in these areas, as in the case of galvanic corrosion. This is because the effective cathodic area will be large and the effective anodic areas at coating defects will be small. Very rapid corrosion can occur if stray currents are present and only the anodic areas are coated. Other Forms of Corrosion There are many other forms of corrosion, such as: • Dealloying • Intergranular attack • Stress corrosion cracking • Hydrogen embrittlement • Corrosion fatigue • Erosion corrosion • Cavitation corrosion • Fretting Corrosion However, these forms of corrosion are not commonly controlled or affected by the application of protective coatings. More information on these forms of corrosion can be found in References 1 through 3. Methods for Corrosion Control Many different methods can be used to control corrosion. By combining some of these methods, the cost of corrosion and its effect on the function of the structure can be minimized. Figure 11. Stray current caused by electric railway. In this example, the pipeline becomes a low resistance path for the current returning from the train to the power source. Wherever the pipeline is caused to be more positive by the stray current, corrosion occurs at a higher rate. Stray currents can be detected by electrical measurements. If stray currents are found to be a problem, they can be reduced or eliminated by several techniques including: reducing the current flow in the ground by modifying the current source; electrical bonding to control the current flow; and application of cathodic protection to counterbalance the stray current 9 Protective Coatings Protective coatings are widely used to control corrosion. In the broadest sense, any material that forms a continuous film on the surface of a substrate can be considered to be a protective coating. Protective coatings control corrosion primarily by providing a barrier between the metal and its environments. This barrier reduces the activity of the chemical reactions responsible for corrosion by slowing the movement of the reactants and reaction products involved. Organic Coatings. Organic coatings are usually liquid applied coatings that are converted to a solid film after application. The barrier action responsible for the primary protective action of organic coatings is often enhanced by the addition of chemicals that inhibit corrosion, or by loading with zinc to provide galvanic (cathodic) protection to the underlying metal. Metallic Coatings. Metallic coatings are thin films of metal applied to a substrate. These coatings can be applied by dipping the metal to be coated in a molten metal bath (e.g., galvanizing), by electroplating, and by thermal spray. There are two generic types of metallic coatings, those that are anodic to the underlying metal (called here “anodic metallic coatings”) and those that are cathodic to the underlying metal (called here “cathodic metallic coatings”). Both of these generic types provide barrier protection, but they differ in their ability to provide corrosion protection when they are damaged or defective. Cathodic Protection Cathodic protection can provide effective control of corrosion in underground and immersion conditions. In its simplest form, (a sacrificial anode system), cathodic protection is essentially an intentional galvanic corrosion cell designed so that the structure to be protected acts as a cathode. It therefore has a reduced corrosion rate. The anodic material that is intentionally added to the system corrodes at an accelerated rate. Impressed current systems are similar, but instead of using sacrificial anodes, they provide protection by inducing a current in the system from an external power supply. Cathodic protection, combined with the use of appropriate protective coatings, can provide better control of corrosion than either method used alone. The barrier action provided by the coating reduces the surface area to be protected by cathodic protection. This in turn reduces the cost of the cathodic protection system by decreasing the amount of anodic material that is consumed in sacrificial anode systems, or the amount of current that must be supplied in an impressed current system. It should be noted that the effectiveness of the coating system is also improved because corrosion does not occur at coating defects or damaged areas. Good Design Many of the factors that affect how corrosion will attack a given system can be addressed at the design stage. For example, corrosion can be con- trolled to some degree by avoiding structural features that trap and hold moisture, by avoiding joints that cannot be effectively protected by coatings, and by avoiding sharp edges where coatings are to be used. Particularly in cases where protective coatings are used as a part of the total corrosion control system, another important design factor is to allow for easy coating maintenance. Good design also provides for easy access for coating inspection, surface preparation, and coating application. Materials Selection. The compatibility of materials with their environments should be a basic consideration in any engineering design. However, it is not always practical or possible to use materials that are highly resistant to corrosion. Materials selection is only one aspect of the overall design process. Other design considerations besides materials selection include the ability of the various types of corrosion control measures to reduce the effects of corrosion and the effect of corrosion on overall system function. A good design balances all of these factors to obtain the desired system performance and lifetime at the least cost. Figure 12. A Munters rental dehumidifier setup to protect the hotwell of the condenser in a power generation plant. Dry air circulates through the equipment, preventing corrosion from occuring. Courtesy Munters Moisture Control Services. Corrosion Allowance. Except in cases where special 10 highly corrosion-resistant materials are used, some corrosion is always inevitable. Therefore, successful designs will consider the type and extent of corrosion anticipated and will make allowances for the metal loss that will occur. Particularly where uniform corrosion is anticipated, this corrosion allowance is often provided by making the components thicker. While this is often considered to be a “factor of safety,” it actually provides extra metal to compensate for metal losses due to corrosion that is likely to occur when and where the corrosion control methods used are not completely effective. The overall system design must be based on the type and amount of corrosion that will occur. Periodic inspections must be performed to verify that the amount of corrosion is within safe limits. This is a frequent practice in chemical process industries. One way is to select materials that are resistant to attack in the specific exposure environment. Another is to use cathodic protection and/or protective coatings. The application of protective coatings is one of the most important means of corrosion control. In most cases, the best way to control corrosion is to use a combination of two or more appropriate corrosion control methods. References 1. ASTM G15-83. Standard Terminology Related to Corrosion and Corrosion Testing; ASTM: West Conshokoen, PA. 2. Van Delinder, L.S. Corrosion Basics: An Introduction; NACE: Houston, 1984. 2. Fontana, Mars G. Corrosion Engineering, 3 rd Edition; McGraw Hill: New York, 1986. 3. Atkinson, J.T.N; Van Droffelaar, H. Corrosion and Its Change of Environment. In some circumstances, corrosion is controlled by changing the environment. In liquid handling systems, this may be accomplished by removing oxygen from the system by deaeration, or by the addition of corrosion inhibitors. In other cases, the environment is changed by controlling atmospheric conditions, e.g., dehumidification may be used to control corrosion in interior spaces. An example of a dehumidification system is shown in Figure 12. Such corrosion control measures may be required during manufacture of critical equipment or may be used as a temporary means to control corrosion until other corrosion control methods can be applied. Dehumidification of the interior of tanks during and after blast cleaning and prior to the application of a protective coating is one example of this type of environmental control. Control: An Introduction to the Subject, 2nd Edition; NACE: Houston, 1994. 4. Uhlig, Herbert H. Corrosion and Corrosion Control: An Introduction to Corrosion Science and Engineering, 3 rd Edition; John Wiley & Sons, Inc.: New York, 1985. 5. Munger, Charles G. Corrosion Prevention by Protective Coatings; NACE: Houston, 1984. About the Authors James F. Jenkins James F. Jenkins retired in 1995 after 30 years of service to the U.S. Navy in corrosion control for shore and ocean-based facilities. Now a consultant, he is a registered corrosion engineer in the state of California. Mr. Jenkins received his BS degree in metallurgical engineering from the University of Arizona. Dr. Richard W. Drisko Dr. Richard W. Drisko has been the senior technical advisor to SSPC: The Society for Protective Coatings since January 1995. Prior to this, he was employed for over 40 years at the Naval Civil Engineering Laboratory, Port Hueneme, California, where he conducted research, evaluation, and testing, and served as the Navy’s center of expertise on coatings for shore structures. He is a professional corrosion engineer in the state of California, an SSPC certified protective coatings specialist (PCS), and a NACE International certificated corrosion specialist. Dr. Drisko received his BS, MS, and PhD degrees from Stanford. Summary Corrosion is an electrochemical process that naturally occurs on most metals when they are exposed to aggressive environments. Rusting of steel in atmospheric or immersion conditions is a common example of corrosion. The electrochemical process responsible for corrosion involves four components: an anode, a cathode, a metallic path, and an electrolyte. The rate of the overall corrosion reaction can be controlled by limiting the activity of any one of these components. There are many forms of corrosion, which all depend on the activity of electrochemical cells, but differ in the location and distribution of attack. There are many ways to control corrosion. 11 ...
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