Chapter 2 IL. INTERGRANULAR CORROSION Intergranular corrosion is a localized form of corrosion taking place at the grain boundaries of a metal with little or no attack on the grain boundaries themselves. This results in a loss of strength and ductility. The attack is often rapid, penetrating deeply into the metal and failure In the case of austenitic stainless steels the attack is the result of car- bide precipitation during welding operations. Carbide precipitation can be prevented by using alloys containing less than 0. 03%o carbon, by using alloys that have been stabilized with columbium or titanium, or by specifying So- lution heat treatment followed by a rapid quench that will keep carbides in solution. The most practical approach is to use either a low carbon content or stabilized austenitic stainless steel Nickel-based alloys can also be subjected to carbide precipitation and precipitation of intermetallic phases when exposed to temperatures lower than their annealing temperatures. As with austenitic stainless steels, low carbon content alloys are recommended to delay precipitation of carbides In some alloys, such as alloy 625, niobium, tantalum, or titanium is added to stabilize the alloy against precipitation of chromium or molybdenum car- bides. These elements combine with carbon instead of the chromium or IIL. GALVANIC CORROSION This form of corrosion is sometimes referred to as dissimilar metal corrosion and is found in the most unusual places most headaches. Galvanic corrosion is also often experienced in older homes where modern copper water tubing is connected to the older existing carbor steel water lines. The coupling of the copper to the carbon steel causes the carbon steel to corrode. The galvanic series of metals provides details of how galvanic current will flow between two metals and which metal will corrode when they are in contact or near each other and an electrolyte is present(e. g, water). Table 2.2 lists the galvanic series. When two different metallic materials are electrically connected and placed in a conductive solution(electrolyte), an electric potential exists. This potential difference will provide a stronger driving force for the dissolution of the less noble(more electrically negative) material. It will also reduce the tendency for the more noble material to dissolve. Notice in Table 2.2 precious metals of gold and plati (more noble, or cathodic) end of the series (protected end), while zinc and magnesium are at the lower potential (less noble, or anodic)end. It is this principle that forms the scientific basis for using such materials as zinc to sacrificially protect a stainless steel drive shaft on a pleasure boat MARCEL DEKKER. INC 270 Madison Avenue. New York. New York 10016
18 Chapter 2 II. INTERGRANULAR CORROSION Intergranular corrosion is a localized form of corrosion taking place at the grain boundaries of a metal with little or no attack on the grain boundaries themselves. This results in a loss of strength and ductility. The attack is often rapid, penetrating deeply into the metal and causing failure. In the case of austenitic stainless steels the attack is the result of carbide precipitation during welding operations. Carbide precipitation can be prevented by using alloys containing less than 0.03% carbon, by using alloys that have been stabilized with columbium or titanium, or by specifying solution heat treatment followed by a rapid quench that will keep carbides in solution. The most practical approach is to use either a low carbon content or stabilized austenitic stainless steel. Nickel-based alloys can also be subjected to carbide precipitation and precipitation of intermetallic phases when exposed to temperatures lower than their annealing temperatures. As with austenitic stainless steels, low carbon content alloys are recommended to delay precipitation of carbides. In some alloys, such as alloy 625, niobium, tantalum, or titanium is added to stabilize the alloy against precipitation of chromium or molybdenum carbides. These elements combine with carbon instead of the chromium or molybdenum. III. GALVANIC CORROSION This form of corrosion is sometimes referred to as dissimilar metal corrosion and is found in the most unusual places, often causing professionals the most headaches. Galvanic corrosion is also often experienced in older homes where modern copper water tubing is connected to the older existing carbon steel water lines. The coupling of the copper to the carbon steel causes the carbon steel to corrode. The galvanic series of metals provides details of how galvanic current will flow between two metals and which metal will corrode when they are in contact or near each other and an electrolyte is present (e.g., water). Table 2.2 lists the galvanic series. When two different metallic materials are electrically connected and placed in a conductive solution (electrolyte), an electric potential exists. This potential difference will provide a stronger driving force for the dissolution of the less noble (more electrically negative) material. It will also reduce the tendency for the more noble material to dissolve. Notice in Table 2.2 that the precious metals of gold and platinum are at the higher potential (more noble, or cathodic) end of the series (protected end), while zinc and magnesium are at the lower potential (less noble, or anodic) end. It is this principle that forms the scientific basis for using such materials as zinc to sacrificially protect a stainless steel drive shaft on a pleasure boat
Corrosion of metallic materials TABLE 2.2 Galvanic Series of Metals and Alloys Corroded end(anodic Muntz metal Magnesium alloys Naval bronze Nickel(active) alvanized stee Aluminum 6053 Hastelloy C(active) Aluminum 3003 Yellow brass Aluminum 2024 Admiralty brass Aluminum bronze Alclad Red brass Cadmium Copp Mild steel Silicon bro /30Cu Cast iron Nickel (passive) 13% chromium stainless steel Monel 18-8 Stainless steel type 304 0/50 lead tin solder Ferretic stainless steel 400 series 18-8-3 stainless steel type 316 18-8 stainless steel type 304 (active) Silver 18-8-3 Stainless steel type 316 Graphite Gold Protected end(cathodic) You will note that several materials are shown in two places in the galvanic series, being indicated as either active or passive. This is the result of the tendency of some metals and alloys to form surface films, especially in oxidizing environments. These films shift the measured potential in the noble direction. In this state the material is said to be passive The particular way in which metals will react can be predicted from the relative positions of the materials in the galvanic series. When it is ecessary to use dissimilar metals, two materials should be selected which are relatively close in the galvanic series. The further apart the metals are in the galvanic series, the greater the rate of corrosion The rate of corrosion is also affected by the relative areas between the anode and the cathode. since the fow of current is from the anode to the cathode, the combination of a large cathodic area and a small anodic area is undesirable. Corrosion of the anode can be 100-1000 times greater than MARCEL DEKKER. INC 270 Madison Avenue. New York. New York 10016
Corrosion of Metallic Materials 19 TABLE 2.2 Galvanic Series of Metals and Alloys Corroded end (anodic) Magnesium Muntz metal Magnesium alloys Naval bronze Zinc Nickel (active) Galvanized steel Inconel (active) Aluminum 6053 Hastelloy C (active) Aluminum 3003 Yellow brass Aluminum 2024 Admiralty brass Aluminum Aluminum bronze Alclad Red brass Cadmium Copper Mild steel Silicon bronze Wrought iron 70/30 Cupro-nickel Cast iron Nickel (passive) Ni-resist Iconel (passive) 13% chromium stainless steel (active) 50/50 lead tin solder Ferretic stainless steel 400 series 18-8 stainless steel type 304 (active) 18-8-3 Stainless steel type 316 (active) Lead Tin Monel 18-8 Stainless steel type 304 (passive) 18-8-3 stainless steel type 316 (passive) Silver Graphite Gold Platinum Protected end (cathodic) You will note that several materials are shown in two places in the galvanic series, being indicated as either active or passive. This is the result of the tendency of some metals and alloys to form surface films, especially in oxidizing environments. These films shift the measured potential in the noble direction. In this state the material is said to be passive. The particular way in which metals will react can be predicted from the relative positions of the materials in the galvanic series. When it is necessary to use dissimilar metals, two materials should be selected which are relatively close in the galvanic series. The further apart the metals are in the galvanic series, the greater the rate of corrosion. The rate of corrosion is also affected by the relative areas between the anode and the cathode. Since the flow of current is from the anode to the cathode, the combination of a large cathodic area and a small anodic area is undesirable. Corrosion of the anode can be 100–1000 times greater than
if the two areas were equal. Ideally the anode area should be larger than the The passivity of stainless steel is the result of the presence of a cor- rosion-resistant oxide film on the surface. In most material environments it will remain in the passive state and tend to be cathodic to ordinary iron or steel. When chloride concentrations are high, such as in seawater or in ducing solutions, a change to the active state will usually take place. Oxygen starvation also causes a change to the active state. This occurs when there is no free access to oxygen, such as in crevices and beneath contamination of partially fouled surfaces Differences in soil concentrations. such as moisture content and resis- tivity, can be responsible for creating anodic and cathodic areas. Where there is a difference in concentrations of oxygen in the water or in moist soils contact with metal at different areas, cathodes will develop at relatively high oxygen concentrations, and anodes at points of low concentrations. Stained portions of metals tend to be anodic and unstrained portions cathodic When joining two dissimilar metals together, galvanic corrosion can be prevented by insulating the two metals from each other. For example, when bolting flanges of dissimilar metals together, plastic washers can be used to separate the two metals V. CREVICE CORROSION Crevice corrosion is a localized type of corrosion occurring within or ad- jacent to narrow gaps or openings formed by metal-to-metal or metal-to- nonmetal contact. It results from local differences in oxygen concentrations associated deposits on the metal surface, gaskets, lap joints, or crevices under a bolt or around rivet heads where small amounts of liquid can collect and be The material responsible for the formation of the crevice need not be metallic. Wood, plastics, rubber, glass, concrete, asbestos, wax, and livin organisms have been reported to cause crevice corrosion. Once the attack begins within the crevice, its progress is very rapid. It is frequently more ntense in chloride environments Prevention can be accomplished by proper design and operating pro- cedures. Nonabsorbant gasketing material should be used at flanged joint while fully penetrated butt welded joints are preferred to threaded joints. In the design of tankage, butt welded joints are preferable to lap joints. If lap joints are used, the laps should be filled with fillet welding or a suitable caulking compound designed to prevent crevice corrosion The critical crevice corrosion temperature of an alloy is that temp ature at which crevice corrosion is first observed when immersed in a ferric MARCEL DEKKER. INC 270 Madison Avenue. New York. New York 10016
20 Chapter 2 if the two areas were equal. Ideally the anode area should be larger than the cathode area. The passivity of stainless steel is the result of the presence of a corrosion-resistant oxide film on the surface. In most material environments it will remain in the passive state and tend to be cathodic to ordinary iron or steel. When chloride concentrations are high, such as in seawater or in reducing solutions, a change to the active state will usually take place. Oxygen starvation also causes a change to the active state. This occurs when there is no free access to oxygen, such as in crevices and beneath contamination of partially fouled surfaces. Differences in soil concentrations, such as moisture content and resistivity, can be responsible for creating anodic and cathodic areas. Where there is a difference in concentrations of oxygen in the water or in moist soils in contact with metal at different areas, cathodes will develop at relatively high oxygen concentrations, and anodes at points of low concentrations. Stained portions of metals tend to be anodic and unstrained portions cathodic. When joining two dissimilar metals together, galvanic corrosion can be prevented by insulating the two metals from each other. For example, when bolting flanges of dissimilar metals together, plastic washers can be used to separate the two metals. IV. CREVICE CORROSION Crevice corrosion is a localized type of corrosion occurring within or adjacent to narrow gaps or openings formed by metal-to-metal or metal-tononmetal contact. It results from local differences in oxygen concentrations, associated deposits on the metal surface, gaskets, lap joints, or crevices under a bolt or around rivet heads where small amounts of liquid can collect and become stagnant. The material responsible for the formation of the crevice need not be metallic. Wood, plastics, rubber, glass, concrete, asbestos, wax, and living organisms have been reported to cause crevice corrosion. Once the attack begins within the crevice, its progress is very rapid. It is frequently more intense in chloride environments. Prevention can be accomplished by proper design and operating procedures. Nonabsorbant gasketting material should be used at flanged joints, while fully penetrated butt welded joints are preferred to threaded joints. In the design of tankage, butt welded joints are preferable to lap joints. If lap joints are used, the laps should be filled with fillet welding or a suitable caulking compound designed to prevent crevice corrosion. The critical crevice corrosion temperature of an alloy is that temperature at which crevice corrosion is first observed when immersed in a ferric
Corrosion of metallic materials chloride solution. Table 2.3 lists the critical crevice corrosion temperature of several alloys in 10% ferric chloride solution V. PITTING Pitting is a form of localized corrosion that is primarily responsible for the ailure of iron and steel hydraulic structures. Pitting may result in the per foration of water pipe, making it unusable even though a relatively small percentage of the total metal has been lost due to rusting. Pitting can also cause structural failure from localized weakening effects even though there is considerable sound material remaining The initiation of a pit is associated with the breakdown of the protec- tive film on the surface. The main factor that causes and accelerates pitting is electrical contact between dissimilar metals, or between what are termed concentration cells(areas of the same metal where oxygen or conductive salt concentrations in water differ). These couples cause a difference of potential that results in an electric current flowing through the water or across moist steel, from the metallic anode to a nearby cathode. The cathode may be brass or copper, mill scale, or any other portion of the metal surface that is cathodic to the more active metal areas however. when the anodic rea is relatively large compared with the cathodic area, the damage is spread out and usually negligible. When the anodic area is relatively small, the metal loss is concentrated and may be serious. For example, it can be ex pected when large areas of the surface are generally covered by mill scale applied coatings, or deposits of various kinds, but breaks exist in the con- TABLE 2.3 Critical Crevice Corrosion Temperatures in 10% Ferric Chloride solution Al Temperature(F/C) Type 316 27/-3 Type 317 Alloy 904L Alloy 220s -Brite oy 100/38 AL-6XI 100/38 Alloy 276 130/55 MARCEL DEKKER. INC 270 Madison Avenue. New York. New York 10016
Corrosion of Metallic Materials 21 TABLE 2.3 Critical Crevice Corrosion Temperatures in 10% Ferric Chloride Solution Alloy Temperature (F/C) Type 316 27/3 Alloy 825 27/3 Type 317 36/2 Alloy 904L 59/15 Alloy 220S 68/20 E-Brite 70/21 Alloy G 86/30 Alloy 625 100/38 AL-6XN 100/38 Alloy 276 130/55 chloride solution. Table 2.3 lists the critical crevice corrosion temperature of several alloys in 10% ferric chloride solution. V. PITTING Pitting is a form of localized corrosion that is primarily responsible for the failure of iron and steel hydraulic structures. Pitting may result in the perforation of water pipe, making it unusable even though a relatively small percentage of the total metal has been lost due to rusting. Pitting can also cause structural failure from localized weakening effects even though there is considerable sound material remaining. The initiation of a pit is associated with the breakdown of the protective film on the surface. The main factor that causes and accelerates pitting is electrical contact between dissimilar metals, or between what are termed concentration cells (areas of the same metal where oxygen or conductive salt concentrations in water differ). These couples cause a difference of potential that results in an electric current flowing through the water or across moist steel, from the metallic anode to a nearby cathode. The cathode may be brass or copper, mill scale, or any other portion of the metal surface that is cathodic to the more active metal areas. However, when the anodic area is relatively large compared with the cathodic area, the damage is spread out and usually negligible. When the anodic area is relatively small, the metal loss is concentrated and may be serious. For example, it can be expected when large areas of the surface are generally covered by mill scale, applied coatings, or deposits of various kinds, but breaks exist in the con-
tinuity of the protective material. Pitting may also develop on bare clean metal surfaces because of irregularities in the physical or chemical structure of the metal. localized dissimilar soil conditions at the surface of steel can also create conditions that promote pitting. Figure 2.1 shows diagrammat cally how a pit forms when a break in mill scale occurs. If an appreciable attack is confined to a small area of metal acting as an anode, the developed pits are described as deep. If the area of attack is elatively large, the pits are called shallow. The ratio of deepest metal pen- etration to average metal penetration, as determined by weight loss of the specimen, is known as the pitting factor. A pitting factor of l represents uniform corrosion Performance in the area of pitting and crevice corrosion is often sured using critical pitting temperature(CPT), critical crevice temper (CCT), and pitting resistance equivalent number(PREN). As a general the higher the pren, the better the resistance. The pren will be discussed further in Chapter 7, which deals with the corrosion resistance of stainless steel since it is determined by the chromium, molybdenum, and nitrogen contents Prevention can be accomplished by proper materials selection, fol- lowed by a design that prevents stagnation of material and alternate wetting and drying of the surface. Also, if coatings are to be applied, care should be taken that they are continuous, without"holidays VI. EROSION CORROSION Erosion corrosion results from the movement of a corrodent over the surface of a metal. the movement is associated with the mechanical wear. the ncrease in localized corrosion resulting from the erosion process is usually Fe2+ (rust) ← Current flow IGURE 2.1 Formation of pit from break in mill scale MARCEL DEKKER. INC 270 Madison Avenue. New York. New York 10016
22 Chapter 2 FIGURE 2.1 Formation of pit from break in mill scale. tinuity of the protective material. Pitting may also develop on bare clean metal surfaces because of irregularities in the physical or chemical structure of the metal. Localized dissimilar soil conditions at the surface of steel can also create conditions that promote pitting. Figure 2.1 shows diagrammatically how a pit forms when a break in mill scale occurs. If an appreciable attack is confined to a small area of metal acting as an anode, the developed pits are described as deep. If the area of attack is relatively large, the pits are called shallow. The ratio of deepest metal penetration to average metal penetration, as determined by weight loss of the specimen, is known as the pitting factor. A pitting factor of 1 represents uniform corrosion. Performance in the area of pitting and crevice corrosion is often measured using critical pitting temperature (CPT), critical crevice temperature (CCT), and pitting resistance equivalent number (PREN). As a general rule, the higher the PREN, the better the resistance. The PREN will be discussed further in Chapter 7, which deals with the corrosion resistance of stainless steel since it is determined by the chromium, molybdenum, and nitrogen contents. Prevention can be accomplished by proper materials selection, followed by a design that prevents stagnation of material and alternate wetting and drying of the surface. Also, if coatings are to be applied, care should be taken that they are continuous, without ‘‘holidays.’’ VI. EROSION CORROSION Erosion corrosion results from the movement of a corrodent over the surface of a metal. The movement is associated with the mechanical wear. The increase in localized corrosion resulting from the erosion process is usually