《材料失效分析 Materials Failure Analysis》课程教学资源（参考书籍）METALLIC MATERIALS Physical, Mechanical, and Corrosion Properties

Corrosion of metallic materials Inder the service conditions in which they are placed that they are expected to survive for long periods without protection. When these same metals are placed into contact with more aggressive corrodents, they suffer attack and re degraded. Corrosion of structural grades of iron and steel, however, proceeds rapidly unless the metal is amply protected. Ordinarily iron and steel corrode in the presence of both oxygen and water. If either of these ingredients are absent, corrosion will not take place Rapid corrosion may take place in water, the rate of corrosion being accel- erated by the velocity or the acidity of the water, by the motion of the metal by an increase in temperature, by the presence of certain bacteria, or by other factors. Conversely, corrosion is retarded by protective layers(films) consisting of corrosion products or absorbed oxygen. High alkalinity of the water also retards the rate of corrosion on steel surfaces There are nine basic forms of corrosion that metallic materials may be 1. Uniform corrosion 2. Intergranular corrosion 3. Galvanic corrosion 5 tins 6. Erosion corrosion 7. Stress corrosion cracking 8. Biological corrosion In addition there are other forms that specific metals or alloys are subject to. Prevention or control of corrosion can usually be achieved by use of a suitable material of construction, use of proper design and installation tech niques, and by following specific in-plant procedures, or a combination of L. UNIFORM CORROSION A metal resists corrosion by forming a passive film on the surface. This film is formed naturally when the metal is exposed to air for a period of time. It can also be formed more quickly by a chemical treatment. For example nitric acid if applied to an austenitic stainless steel will form this protective film. Such a film is actually a form of corrosion, but once formed it prevents uture degradation of the metal, as long as the film remains intact. It does not provide an overall resistance to corrosion, since it may be subject to chemical attack. The immunity of the film to attack is a function of the film composition, the temperature, and the aggressiveness of the chemical. Ex- MARCEL DEKKER. INC 270 Madison Avenue. New York. New York 10016

Corrosion of Metallic Materials 13 under the service conditions in which they are placed that they are expected to survive for long periods without protection. When these same metals are placed into contact with more aggressive corrodents, they suffer attack and are degraded. Corrosion of structural grades of iron and steel, however, proceeds rapidly unless the metal is amply protected. Ordinarily iron and steel corrode in the presence of both oxygen and water. If either of these ingredients are absent, corrosion will not take place. Rapid corrosion may take place in water, the rate of corrosion being accel￾erated by the velocity or the acidity of the water, by the motion of the metal, by an increase in temperature, by the presence of certain bacteria, or by other factors. Conversely, corrosion is retarded by protective layers (films) consisting of corrosion products or absorbed oxygen. High alkalinity of the water also retards the rate of corrosion on steel surfaces. There are nine basic forms of corrosion that metallic materials may be subject to: 1. Uniform corrosion 2. Intergranular corrosion 3. Galvanic corrosion 4. Crevice corrosion 5. Pitting 6. Erosion corrosion 7. Stress corrosion cracking 8. Biological corrosion 9. Selective leaching In addition there are other forms that specific metals or alloys are subject to. Prevention or control of corrosion can usually be achieved by use of a suitable material of construction, use of proper design and installation tech￾niques, and by following specific in-plant procedures, or a combination of these. I. UNIFORM CORROSION A metal resists corrosion by forming a passive film on the surface. This film is formed naturally when the metal is exposed to air for a period of time. It can also be formed more quickly by a chemical treatment. For example, nitric acid if applied to an austenitic stainless steel will form this protective film. Such a film is actually a form of corrosion, but once formed it prevents future degradation of the metal, as long as the film remains intact. It does not provide an overall resistance to corrosion, since it may be subject to chemical attack. The immunity of the film to attack is a function of the film composition, the temperature, and the aggressiveness of the chemical. Ex-

Chapter 2 amples of such films are the patina formed on copper, the rusting of ir the tarnishing of silver, the fogging of nickel, and the high temperature oxidation of metals A. Passive Films There are two theories regarding the formation of these films. The first the- ory states that the film formed is a metal oxide or other reaction compound This is known as the oxide film theory. The second theory states that oxygen is adsorbed on the surface forming a chemisorbed film. However all chem- sorbed films react over a period of time with the underlying metal to form metal oxides. Oxide films are formed at room temperature Metal oxides can be classified as network formers intermediates or modifiers This division can be related to thin oxide films on metals, The metals that fall into ne work-forming or intermediate classes tend to grow protective oxides that support anion or mixed anion/cation movement. The network formers are noncrystalline, while the intermediates tend to be microcrystalline at low temperatures B. Passive film on lron Iron in iron oxides can assume a valence of two or three. The former acts as a modifier and the latter as a network former. The iron is protected from the corrosion environment by a thin oxide film 1-4 mm in thickness with a composition of VFe2O,/Fe3O4. This is the same type of film formed by the reaction of clean iron with oxygen or dry air. The VFe2O3 layer is responsible for the passivity, while the Fe3O4 provides the basis for the formation of a higher oxidizing state. Iron is more difficult to passivate than nickel, because with iron it is not possible to go directly to the passivation species VFe2O3. Instead, a lower oxidation state of Fe O is required, and his film is highly susceptible to chemical dissolution. The VFe2O3 layer will not form until the Fe] O4 phase has existed on the surface for a reason- able period of time. During this time the Fe3 O4 layer continues to form. C. Passive Film on nickel The passive film on nickel can be achieved quite readily in contrast to the formation of the passive film on iron. Differences in the nature of the oxide film on iron and nickel are responsible for this phenomenon. The film thick ess on nickel is between 0.9 and 1.2 mm. while the iron oxide film between 1.5 and 4.5 mm. There are two theories as to exactly what the passive film on nickel is. It is either entirely Nio with a small amount of nonstoichiometry giving rise to Ni and cation vacancies, or it consists on MARCEL DEKKER. INC 270 Madison Avenue. New York. New York 10016

14 Chapter 2 amples of such films are the patina formed on copper, the rusting of iron, the tarnishing of silver, the fogging of nickel, and the high temperature oxidation of metals. A. Passive Films There are two theories regarding the formation of these films. The first the￾ory states that the film formed is a metal oxide or other reaction compound. This is known as the oxide film theory. The second theory states that oxygen is adsorbed on the surface forming a chemisorbed film. However, all chem￾isorbed films react over a period of time with the underlying metal to form metal oxides. Oxide films are formed at room temperature. Metal oxides can be classified as network formers, intermediates, or modifiers. This division can be related to thin oxide films on metals. The metals that fall into net￾work-forming or intermediate classes tend to grow protective oxides that support anion or mixed anion/cation movement. The network formers are noncrystalline, while the intermediates tend to be microcrystalline at low temperatures. B. Passive Film on Iron Iron in iron oxides can assume a valence of two or three. The former acts as a modifier and the latter as a network former. The iron is protected from the corrosion environment by a thin oxide film 1–4 mm in thickness with a composition of This is the same type of film formed by Fe O /Fe O .
23 34 the reaction of clean iron with oxygen or dry air. The layer is
Fe O2 3 responsible for the passivity, while the Fe3O4 provides the basis for the formation of a higher oxidizing state. Iron is more difficult to passivate than nickel, because with iron it is not possible to go directly to the passivation species Instead, a lower oxidation state of Fe3O4
Fe O . 2 3 is required, and this film is highly susceptible to chemical dissolution. The layer
Fe O2 3 will not form until the Fe3O4 phase has existed on the surface for a reason￾able period of time. During this time the Fe3O4 layer continues to form. C. Passive Film on Nickel The passive film on nickel can be achieved quite readily in contrast to the formation of the passive film on iron. Differences in the nature of the oxide film on iron and nickel are responsible for this phenomenon. The film thick￾ness on nickel is between 0.9 and 1.2 mm, while the iron oxide film is between 1.5 and 4.5 mm. There are two theories as to exactly what the passive film on nickel is. It is either entirely NiO with a small amount of nonstoichiometry giving rise to Ni3 and cation vacancies, or it consists on

Corrosion of metallic materials an inner layer of NiO and an outer layer of anhydrous Ni(OH)2. The passive oxide film on nickel once formed cannot be easily removed by either ca thodic treatment or chemical dissolution Passive Film on austenitic stainless steel The passive film formed on stainless steel is duplex in nature, consisting of an inner barrier oxide film and an outer deposit hydroxide or salt film Passivation takes place by the rapid formation of surface-absorbed hydrated complexes of metals, which are sufficiently stable on the alloy surface that further reaction with water enables the formation of a hydroxide phase that rapidly deprotonates to form an insoluble surface oxide film. The three most commonly used austenite stabilizers, nickel, manganese, and nitrogen, all contribute to the passivity. Chromium, a major alloying ingredient, is in itself very corrosion resistant and is found in greater abundance in the passive film than iron, which is the majority element in the alloy E. Passive Film on Copper When exposed to the atmosphere over long periods of time, copper will form a coloration on the surface known as patina, which in reality is a corrosion product that acts as a protective film against further corrosion. When first formed the patina has a dark color that gradually turns green The length of time required to form the patina depends on the atmosphere. because the coloration is given by copper hydroxide compounds. In a marine atmosphere, the compound is a mixture of copper/hydroxide/chloride and in urban or industrial atmospheres copper/hydroxide/sulfate. These compound will form in approximately 7 years. When exposed in a clean rural atmo- sphere, tens or hundreds of years may be required to form patina. Passive film on aluminum Aluminum forms a thin, compact, and adherent oxide film on the which limits further corrosion. When formed in air at atmospheric atures it is approximately 5 mm thick. If formed at elevated temperatures or in the presence of water or water vapor it will be thicker. This oxide film is stable in the pH range of 4-9. with a few exceptions the film will dissolve at lower or higher pH ranges. Exceptions are concentrated nitric acid ( 1)and concentrated ammonium hydroxide(pH 13). In both cases the oxide film is stable The oxide film is not homogeneous and contains weak points. Break down of the oxide film at weak points leads to localized corrosion. With increasing alloying content and on heat-treatable alloys the oxide film be- MARCEL DEKKER. INC 270 Madison Avenue. New York. New York 10016

Corrosion of Metallic Materials 15 an inner layer of NiO and an outer layer of anhydrous Ni(OH)2. The passive oxide film on nickel once formed cannot be easily removed by either ca￾thodic treatment or chemical dissolution. D. Passive Film on Austenitic Stainless Steel The passive film formed on stainless steel is duplex in nature, consisting of an inner barrier oxide film and an outer deposit hydroxide or salt film. Passivation takes place by the rapid formation of surface-absorbed hydrated complexes of metals, which are sufficiently stable on the alloy surface that further reaction with water enables the formation of a hydroxide phase that rapidly deprotonates to form an insoluble surface oxide film. The three most commonly used austenite stabilizers, nickel, manganese, and nitrogen, all contribute to the passivity. Chromium, a major alloying ingredient, is in itself very corrosion resistant and is found in greater abundance in the passive film than iron, which is the majority element in the alloy. E. Passive Film on Copper When exposed to the atmosphere over long periods of time, copper will form a coloration on the surface known as patina, which in reality is a corrosion product that acts as a protective film against further corrosion. When first formed the patina has a dark color that gradually turns green. The length of time required to form the patina depends on the atmosphere, because the coloration is given by copper hydroxide compounds. In a marine atmosphere, the compound is a mixture of copper/hydroxide/chloride and in urban or industrial atmospheres copper/hydroxide/sulfate. These compounds will form in approximately 7 years. When exposed in a clean rural atmo￾sphere, tens or hundreds of years may be required to form patina. F. Passive Film on Aluminum Aluminum forms a thin, compact, and adherent oxide film on the surface which limits further corrosion. When formed in air at atmospheric temper￾atures it is approximately 5 mm thick. If formed at elevated temperatures or in the presence of water or water vapor it will be thicker. This oxide film is stable in the pH range of 4–9. With a few exceptions the film will dissolve at lower or higher pH ranges. Exceptions are concentrated nitric acid (pH 1) and concentrated ammonium hydroxide (pH 13). In both cases the oxide film is stable. The oxide film is not homogeneous and contains weak points. Break￾down of the oxide film at weak points leads to localized corrosion. With increasing alloying content and on heat-treatable alloys the oxide film be￾comes more nonhomogeneous

Chapter 2 G. Passive film on nickel The passive film formed on nickel will not protect the nickel from corrosive attack in oxidizing environments, such as nitric acid. When alloyed with chromium a much improved stable film results, producing a greater corrosion resistance to a variety of oxidizing media. However, these alloys are subject to attack in environments containing chloride or other halides, especially if oxidizing agents are present. Corrosion will be in the form of pitting. The addition of molybdenum or tungsten will improve the corrosion resistance H. Passive Film on Titanium Titanium forms a stable, protective, strongly adherent oxide film. This film forms instantly when a fresh surface is exposed to air or moisture. Addition of alloying elements to titanium affect the corrosion resistance because these elements alter the composition of the oxide film The oxide film of titanium is very thin and is attacked by only a fev substances, most notable of which is hydrofluoric acid. Because of its strong affinity for oxygen, titanium is capable of healing ruptures in this film almost instantly in any environment when a trace of moisture or oxygen is present Passive Film on tantalum When exposed to oxidizing or slightly anodic conditions tantalum forms a thin, impervious layer of tantalum oxide. This passivating oxide has the broadest range of stability with regard to chemical attack or thermal break- down compared to other metallic films. Chemicals or conditions which at- tack tantalum, such as hydrofluoric acid, are those which penetrate or dis- solve the oxide film J. Uniform Corrosion rates When exposed to a corrosion medium, metals tend to enter into a chemical union with the elements of the corrosion medium, forming stable compounds similar to those found in nature when metal loss occurs in this manner. the compound formed is referred to as the corrosion product and the metal that of halogens, particularly chlorides. They will react with and penetrate the film on stainless steel, resulting in general corrosion. Corrosion tables developed to indicate th This type of attack is termed uniform corrosion. It is one of the most easily report average or typical rates of corrosion for various metals in common MARCEL DEKKER. INC 270 Madison Avenue. New York. New York 10016

16 Chapter 2 G. Passive Film on Nickel The passive film formed on nickel will not protect the nickel from corrosive attack in oxidizing environments, such as nitric acid. When alloyed with chromium a much improved stable film results, producing a greater corrosion resistance to a variety of oxidizing media. However, these alloys are subject to attack in environments containing chloride or other halides, especially if oxidizing agents are present. Corrosion will be in the form of pitting. The addition of molybdenum or tungsten will improve the corrosion resistance. H. Passive Film on Titanium Titanium forms a stable, protective, strongly adherent oxide film. This film forms instantly when a fresh surface is exposed to air or moisture. Addition of alloying elements to titanium affect the corrosion resistance because these elements alter the composition of the oxide film. The oxide film of titanium is very thin and is attacked by only a few substances, most notable of which is hydrofluoric acid. Because of its strong affinity for oxygen, titanium is capable of healing ruptures in this film almost instantly in any environment when a trace of moisture or oxygen is present. I. Passive Film on Tantalum When exposed to oxidizing or slightly anodic conditions tantalum forms a thin, impervious layer of tantalum oxide. This passivating oxide has the broadest range of stability with regard to chemical attack or thermal break￾down compared to other metallic films. Chemicals or conditions which at￾tack tantalum, such as hydrofluoric acid, are those which penetrate or dis￾solve the oxide film. J. Uniform Corrosion Rates When exposed to a corrosion medium, metals tend to enter into a chemical union with the elements of the corrosion medium, forming stable compounds similar to those found in nature. When metal loss occurs in this manner, the compound formed is referred to as the corrosion product and the metal surface is referred to as being corroded. An example of such an attack is that of halogens, particularly chlorides. They will react with and penetrate the film on stainless steel, resulting in general corrosion. Corrosion tables are developed to indicate the interaction between a chemical and a metal. This type of attack is termed uniform corrosion. It is one of the most easily measured and predictable forms of corrosion. Many references exist which report average or typical rates of corrosion for various metals in common

Corrosion of metallic materials media. One such reference is Schweitzer, Philip A(Ed ) Corrosion Resis- tance Tables, Fourth Edition, Vols. 1-3(Marcel Dekker, New York, 1995) Since corrosion is so uniform corrosion rates for materials are often expressed in terms of metal thickness lost per unit of time. One common expression is mils per year(mpy); sometimes millimeters per year is used. Because of its predictability, low rates of corrosion are often tolerated and catastrophic failures are rare if planned inspection and monitoring is imple mented. For most chemical process equipment and structures, general cor- rosion rates of less than 3 mpy are considered acceptable. Rates between 2 nd 20 mpy are routinely considered useful engineering materials for the given environment. In severe environments, materials exhibiting high gen- eral corrosion rates of between 20 and 50 mpy might be considered eco- nomically justifiable. Materials which exhibit rates of general corrosion be- yond this are usually unacceptable. It should be remembered that not only does the metal loss need to be considered, but where the metal is goin nust also be considered. Contamination of product, even at low concentra- tions, can be more costly than the replacement of the corroded component to h Uniform corrosion is generally thought of in terms of metal loss due emical attack or dissolution of the metallic component onto metallic ions. In high temperature situations, uniform loss is more commonly pre- ceded by its combination with another element rather than its oxidation to a metallic ion Combination with oxygen to form metallic oxide, or scale results in the loss of the material in its useful engineering form as it ulti mately flakes off to return to nature To determine the corrosion rate a prepared specimen is exposed to the test environment for a period of time and then removed to determine how nuch metal has been lost. The exposure time, weight loss, surface area exposed, and density of the metal are used to calculate the corrosion rate of the metal using the formula :73 WL mpy DAT WL= weight loss, g D=density, g/cr The corrosion rates calculated from the formula or taken from the tables will assist in determining how much corrosion allowance should be included n the design based on the expected lifetime of the equipment. MARCEL DEKKER. INC 270 Madison Avenue. New York. New York 10016

Corrosion of Metallic Materials 17 media. One such reference is Schweitzer, Philip A. (Ed.) Corrosion Resis￾tance Tables, Fourth Edition, Vols. 1–3 (Marcel Dekker, New York, 1995). Since corrosion is so uniform, corrosion rates for materials are often expressed in terms of metal thickness lost per unit of time. One common expression is mils per year (mpy); sometimes millimeters per year is used. Because of its predictability, low rates of corrosion are often tolerated and catastrophic failures are rare if planned inspection and monitoring is imple￾mented. For most chemical process equipment and structures, general cor￾rosion rates of less than 3 mpy are considered acceptable. Rates between 2 and 20 mpy are routinely considered useful engineering materials for the given environment. In severe environments, materials exhibiting high gen￾eral corrosion rates of between 20 and 50 mpy might be considered eco￾nomically justifiable. Materials which exhibit rates of general corrosion be￾yond this are usually unacceptable. It should be remembered that not only does the metal loss need to be considered, but where the metal is going must also be considered. Contamination of product, even at low concentra￾tions, can be more costly than the replacement of the corroded component. Uniform corrosion is generally thought of in terms of metal loss due to chemical attack or dissolution of the metallic component onto metallic ions. In high temperature situations, uniform loss is more commonly pre￾ceded by its combination with another element rather than its oxidation to a metallic ion. Combination with oxygen to form metallic oxide, or scale, results in the loss of the material in its useful engineering form as it ulti￾mately flakes off to return to nature. To determine the corrosion rate a prepared specimen is exposed to the test environment for a period of time and then removed to determine how much metal has been lost. The exposure time, weight loss, surface area exposed, and density of the metal are used to calculate the corrosion rate of the metal using the formula 22.273 WL mpy = DAT where WL = weight loss, g D = density, g/cm3 A = area, in2 T = time, days The corrosion rates calculated from the formula or taken from the tables will assist in determining how much corrosion allowance should be included in the design based on the expected lifetime of the equipment