Corrosion of metallic materials related to the removal or damage of the protective film. The mechanism is usually identified by localized corrosion which exhibits a pattern that follows the flow of the corrodent This type of corrosion is also referred to as impingement attack and is caused by contact with high velocity liquids resulting in a pitting type of corrosion. It is most prevalent in condenser tubes and pipe fittings such elbows and tees. Prevention can be accomplished by one or more means: 1. Reduce the velocity. 2. Select a harder material 3. Properly design the piping system or the condensers. Erosion corrosion also occurs when abrasive materials destroy or dam- age the passive film on the surface of the metal, thereby permitting corrosion to take place An additional subset of erosion corrosion is the case of cavitation prevalent in pump impellers. This form of attack is caused by the formation and collapse of tiny vapor bubbles near a metallic surface in the presence of a corrodent. The protective film is again damaged, in this case by the high pressures caused by the collapse of the bubbles When two metal surfaces are in contact and experience a very slight relative motion causing damage to one or both surfaces, fretting corrosion, a special form of erosion corrosion, takes place. The movement causes me- chanical damage to the protective film which can lead to erosion corrosion when a corrodent is present. This corrosion usually takes the form of a VIL. STRESS CORROSION CRACKING Certain alloys(or alloy systems) in specific environments may be su stress corrosion cracking (SCC). Stress corrosion cracking occurs of stress. Usually the metal or alloy is virtually free of corrosion over most of its surface, yet fine cracks penetrate through the surface at the points of stress. Depending on the alloy system and corrodent combination, the crack ing can be intergranular or transgranular. The rate of propagation can vary greatly and is affected by stress levels, temperature, and concentration of the corrodent. This type of attack takes place in certain media. All metals are potentially subject to SCC. The conditions necessary for stress corrosion cracking 1. Suitable environment 2. Tensile stress Sensitive metal 4. Appropriate temperature and pH values MARCEL DEKKER. INC 270 Madison Avenue. New York. New York 10016
Corrosion of Metallic Materials 23 related to the removal or damage of the protective film. The mechanism is usually identified by localized corrosion which exhibits a pattern that follows the flow of the corrodent. This type of corrosion is also referred to as impingement attack and is caused by contact with high velocity liquids resulting in a pitting type of corrosion. It is most prevalent in condenser tubes and pipe fittings such as elbows and tees. Prevention can be accomplished by one or more means: 1. Reduce the velocity. 2. Select a harder material. 3. Properly design the piping system or the condensers. Erosion corrosion also occurs when abrasive materials destroy or damage the passive film on the surface of the metal, thereby permitting corrosion to take place. An additional subset of erosion corrosion is the case of cavitation, prevalent in pump impellers. This form of attack is caused by the formation and collapse of tiny vapor bubbles near a metallic surface in the presence of a corrodent. The protective film is again damaged, in this case by the high pressures caused by the collapse of the bubbles. When two metal surfaces are in contact and experience a very slight relative motion causing damage to one or both surfaces, fretting corrosion, a special form of erosion corrosion, takes place. The movement causes mechanical damage to the protective film which can lead to erosion corrosion when a corrodent is present. This corrosion usually takes the form of a pitting type attack. VII. STRESS CORROSION CRACKING Certain alloys (or alloy systems) in specific environments may be subject to stress corrosion cracking (SCC). Stress corrosion cracking occurs at points of stress. Usually the metal or alloy is virtually free of corrosion over most of its surface, yet fine cracks penetrate through the surface at the points of stress. Depending on the alloy system and corrodent combination, the cracking can be intergranular or transgranular. The rate of propagation can vary greatly and is affected by stress levels, temperature, and concentration of the corrodent. This type of attack takes place in certain media. All metals are potentially subject to SCC. The conditions necessary for stress corrosion cracking are 1. Suitable environment 2. Tensile stress 3. Sensitive metal 4. Appropriate temperature and pH values
An ammonia-containing environment can induce SCC in copper-con- taining alloys, while with low alloy austenitic stainless steels a chloride ning environment is necessary. It is not necessary to have a high con- centration of corrodent to cause SCC. A solution containing only a few parts per million of the critical ion is all that is necessary. Temperature and pH are also factors. There is usually a threshold temperature below which SCC will not take place and a maximum or minimum pH value before cracking will start Normally stress corrosion cracking will not occur if the part is in com- pression. Failure is triggered by a tensile stress that must approach the yield stress of the metal. The stresses may be induced by faulty installation or they may be residual stresses from welding, straightening, bending, or ac- cidental denting of the component. Pits, which act as stress concentration sites, will often initiate SCc Alloy content of stainless steels, particularly nickel, determine the sen- itivity of the metal to SCC. Ferritic stainless steels, which are nickel free Ind the high nickel alloys are not subject to stress corrosion cracking.An alloy with a nickel content greater than 30%o is immune to SCC. The most common grades of stainless steel(304, 304L, 316, 316L, 321, 347, 303, and 601) have nickel contents in the range of 7-10% and are the most suscep- tible to stress corrosion cracking Examples of stress corrosion cracking include the cracking of austen itic stainless steels in the presence of chlorides, caustic embrittlement crack ng of steel in caustic solutions, cracking of cold-formed brass in ammonia environments, and cracking of monel in hydrofluorosilicic acid. Table 2. 4 is a partial listing of alloy systems subject to stress corrosion cracking In severe combinations, such as type 304 stainless steel in a boilin magnesium chloride solution, extensive cracking can be generated in a mat- ter of hours Fortunately in most industrial applications the progress of SCC is much slower. However, because of the nature of the cracking it is difficult to detect until extensive corrosion has developed, which can lead to unexpected failure Tensile stresses can assist in other corrosion processes, such as the simple mechanical fatigue process. Corrosion fatigue is difficult to differ- entiate from simple mechanical fatigue, but is recognized as a factor when the environment is believed to have accelerated the normal fatigue process. Such systems can also have the effect of lowering the endurance limit such that fatigue will occur at a stress level below which it would be normally expected. It is important that any stresses which may have been induced the fabrication be removed by an appropriate stress-relief operation. The MARCEL DEKKER. INC 270 Madison Avenue. New York. New York 10016
24 Chapter 2 An ammonia-containing environment can induce SCC in copper-containing alloys, while with low alloy austenitic stainless steels a chloridecontaining environment is necessary. It is not necessary to have a high concentration of corrodent to cause SCC. A solution containing only a few parts per million of the critical ion is all that is necessary. Temperature and pH are also factors. There is usually a threshold temperature below which SCC will not take place and a maximum or minimum pH value before cracking will start. Normally stress corrosion cracking will not occur if the part is in compression. Failure is triggered by a tensile stress that must approach the yield stress of the metal. The stresses may be induced by faulty installation or they may be residual stresses from welding, straightening, bending, or accidental denting of the component. Pits, which act as stress concentration sites, will often initiate SCC. Alloy content of stainless steels, particularly nickel, determine the sensitivity of the metal to SCC. Ferritic stainless steels, which are nickel free, and the high nickel alloys are not subject to stress corrosion cracking. An alloy with a nickel content greater than 30% is immune to SCC. The most common grades of stainless steel (304, 304L, 316, 316L, 321, 347, 303, and 301) have nickel contents in the range of 7–10% and are the most susceptible to stress corrosion cracking. Examples of stress corrosion cracking include the cracking of austenitic stainless steels in the presence of chlorides, caustic embrittlement cracking of steel in caustic solutions, cracking of cold-formed brass in ammonia environments, and cracking of monel in hydrofluorosilicic acid. Table 2.4 is a partial listing of alloy systems subject to stress corrosion cracking. In severe combinations, such as type 304 stainless steel in a boiling magnesium chloride solution, extensive cracking can be generated in a matter of hours. Fortunately in most industrial applications the progress of SCC is much slower. However, because of the nature of the cracking it is difficult to detect until extensive corrosion has developed, which can lead to unexpected failure. Tensile stresses can assist in other corrosion processes, such as the simple mechanical fatigue process. Corrosion fatigue is difficult to differentiate from simple mechanical fatigue, but is recognized as a factor when the environment is believed to have accelerated the normal fatigue process. Such systems can also have the effect of lowering the endurance limit such that fatigue will occur at a stress level below which it would be normally expected. It is important that any stresses which may have been induced during the fabrication be removed by an appropriate stress-relief operation. The
Corrosion of metallic materials TABLE 2.4 Alloy Systems Subject to Stress Corrosion Cracking Environment Carbon steel Anhydrous liquid ammonia, HCN, ammonium nitrate, sodium nitrite, sodium hydroxide Aluminum base Air. seawater, salt and chemical combination Magnesium base Nitric acid. caustic, HF solutions, salts coastal atmospheres Copper base Primarily ammonia and ammonium hydroxide, amines, mercury Martensitic and precip- Seawater, chlorides, H2s solutions itation hardening Austenitic stainless Chlorides(organic and inorganic), caustic solutions, sulfurous and polythiuric acids Nickel base Caustic above600°F(315°C) fused caustic hydrofluoric acid Titanium Seawater, salt atmosphere, fused salt Zirconium FeCl3 or CuCl solutions design should also avoid stagnant areas that could lead to pitting and the initiation of stress concentration sites VIIL. BIOLOGICAL CORROSION Corrosive conditions can be developed by living microorganisms as a result of their infuence on anodic and cathodic reactions. This metabolic activity can directly or indirectly cause deterioration e corrosion process. This activity can 1. Produce a corrosive environment 2. Create electrolytic concentration cells on the metal surface 3. Alter the resistance of surface films 4. Have an influence on the rate of anodic or cathodic reaction 5. Alter the environmental composition Because this form of corrosion gives the appearance of pitting it is first necessary to diagnose the presence of bacteria. This is also referred to as microbial corrosion The term microorganism covers a wide variety of life forms including bacteria, blue-green cyanobacteria, algae, lichens, fungi, and protozoa. All MARCEL DEKKER. INC 270 Madison Avenue. New York. New York 10016
Corrosion of Metallic Materials 25 TABLE 2.4 Alloy Systems Subject to Stress Corrosion Cracking Alloy Environment Carbon steel Anhydrous liquid ammonia, HCN, ammonium nitrate, sodium nitrite, sodium hydroxide Aluminum base Air, seawater, salt and chemical combination Magnesium base Nitric acid, caustic, HF solutions, salts, coastal atmospheres Copper base Primarily ammonia and ammonium hydroxide, amines, mercury Martensitic and precipitation hardening stainless steels Seawater, chlorides, H2S solutions Austenitic stainless steels Chlorides (organic and inorganic), caustic solutions, sulfurous and polythiuric acids Nickel base Caustic above 600F (315C) fused caustic, hydrofluoric acid Titanium Seawater, salt atmosphere, fused salt Zirconium FeCl3 or CuCl2 solutions design should also avoid stagnant areas that could lead to pitting and the initiation of stress concentration sites. VIII. BIOLOGICAL CORROSION Corrosive conditions can be developed by living microorganisms as a result of their influence on anodic and cathodic reactions. This metabolic activity can directly or indirectly cause deterioration of a metal by the corrosion process. This activity can 1. Produce a corrosive environment 2. Create electrolytic concentration cells on the metal surface 3. Alter the resistance of surface films 4. Have an influence on the rate of anodic or cathodic reaction 5. Alter the environmental composition Because this form of corrosion gives the appearance of pitting it is first necessary to diagnose the presence of bacteria. This is also referred to as microbial corrosion. The term microorganism covers a wide variety of life forms including bacteria, blue-green cyanobacteria, algae, lichens, fungi, and protozoa. All
microorganisms may be involved in the biodeterioration of materials. Pure Of the mixed cultures only a few actually become actively involved in the process of corrosion. The other organisms support the active ones by ad- ting the environmental conditions in such a manner as to support their growth. For example, in the case of metal corrosion caused by sulfate re lucing bacteria(SRB), the accompanying organisms remove oxygen and produce simple carbon compounds like acetic acid and/or lactic acid nutrients for srB Bacteria are the smallest living organisms on this planet. Some can only live with and others without oxygen. Some can adapt to changin conditions and live either aerobically or anaerobically. There is a wide di versity with regard to their metabolisms. They are classified as to their source of metabolic energy as follows Energy so Classification Phototrophs Chemical reactors Chemotroph organic hydrogen donators Lithotrophs Organic hydrogen donators Organotrophs Carbon dioxide(cell source Autotrophs Organic molecules(cell source) Heterotrophs These six terms may be combined to describe easily the nutritional require ments of a bacterium. For example, if energy is derived from inorganic hydrogen donators and biomass is derived from organic molecules, they are called mirotrophs(chemolithoorganotrophs) An important feature of microbial life is the ability to degrade any naturally occurring compound. Exceptions to this rule are a few manmade materials like highly polymerized and halogenated compounds In addition to energy and carbon sources, nitrogen, phosphorus, and trace elements are needed by microorganisms. Nitrogen compounds may inorganic ammonium nitrate as well as organically bound nitrogen(e.g amino acids, nucleotides). With the help of an enzyme called nitrogenuse bacteria are able to fix nitrogen from atmospheric nitrogen, producing am- monia, which is incorporated in cell constituents Phosphorus is taken up as inorganic phosphate or as organically bound phosphoroxylated compounds, such as phosphorus-containing sugars and lipids. Phosphorus in the form of adenosine triphosphate(ATP)is the main rgy MARCEL DEKKER. INC 270 Madison Avenue. New York. New York 10016
26 Chapter 2 microorganisms may be involved in the biodeterioration of materials. Pure cultures never occur under natural conditions; rather mixed cultures prevail. Of the mixed cultures only a few actually become actively involved in the process of corrosion. The other organisms support the active ones by adjusting the environmental conditions in such a manner as to support their growth. For example, in the case of metal corrosion caused by sulfate reducing bacteria (SRB), the accompanying organisms remove oxygen and produce simple carbon compounds like acetic acid and/or lactic acid as nutrients for SRB. Bacteria are the smallest living organisms on this planet. Some can only live with and others without oxygen. Some can adapt to changing conditions and live either aerobically or anaerobically. There is a wide diversity with regard to their metabolisms. They are classified as to their source of metabolic energy as follows: Energy source Classification Light Phototrophs Chemical reactors Chemotrophs Inorganic hydrogen donators Lithotrophs Organic hydrogen donators Organotrophs Carbon dioxide (cell source) Autotrophs Organic molecules (cell source) Heterotrophs These six terms may be combined to describe easily the nutritional requirements of a bacterium. For example, if energy is derived from inorganic hydrogen donators and biomass is derived from organic molecules, they are called mirotrophs (chemolithoorganotrophs). An important feature of microbial life is the ability to degrade any naturally occurring compound. Exceptions to this rule are a few manmade materials like highly polymerized and halogenated compounds. In addition to energy and carbon sources, nitrogen, phosphorus, and trace elements are needed by microorganisms. Nitrogen compounds may be inorganic ammonium nitrate as well as organically bound nitrogen (e.g., amino acids, nucleotides). With the help of an enzyme called nitrogenuse, bacteria are able to fix nitrogen from atmospheric nitrogen, producing ammonia, which is incorporated in cell constituents. Phosphorus is taken up as inorganic phosphate or as organically bound phosphoroxylated compounds, such as phosphorus-containing sugars and lipids. Phosphorus in the form of adenosine triphosphate (ATP) is the main energy storing compound
Corrosion of metallic materials For many of the metabolic purposes trace elements are needed. Cobalt aids in the transfer of methyl groups from/to organic or inorganic molecules (vitamin B12, cobalamine, is involved in the methylation of heavy metals such as mercury). Iron as Fe or Fe' is required for the electron transport system, where it acts as an oxidizable/reducible central atom in cytochromes or in nonheme iron sulfur proteins. Magnesium acts in a similar manner in the chlorophyll molecule. Copper is an essential part of a cytachrome at the terminal end of the electron transport system, is responsible for the reduction of oxygen to water. Since life cannot exist without water, water is an essential requirement for microbial life and growth. Different microorganisms have different re- quirements as to the amount of water needed. A solid material is surrounded by three types of water: hygroscopic, pellicular, and gravitational. Only pel licular and gravitational water are biologically available and can be used by microorganisms. The biologically available water is usually measured as the water activity, aw, of a sample where V, is the vapor pressure of the solution and Pw is the vapor pressure of pure water, at the same temperature. Most bacteria require an aw value in excess of 0.90 Hydrogen ion concentration is another important factor affectin growth. Microorganisms are classified as to their ability to grow under acidic, neutral, or alkaline conditions, being given such titles as acidophiles neutrophiles, or alkalopholes. Most microorganisms thrive in the neutral ph ange of 6-8 Microbial growth is also affected by redox potential. Under standard conditions hydrogen is assumed to have a redox potential of -421 mV, and oxygen has a redox potential of 820 mV. Metabolism can take place within Available oxygen is another factor that influences microbial growth. Microbial growth is possible under aerated as well as under totally oxygen free conditions. Those organisms living with the amount of oxygen con- tained in the air are called aerobes, while those that perform their metabolism without any free oxygen are called anaerobes. These latter are able to bound oxygen(sulfate, carbon dioxide)or to ferment organic compounds Temperature is another important factor affecting microbial growth. Microbial life is possible within the range of -5 to 110C Microorganisms are also classified as to the temperature range in which they thrive, as in the ollowing table. MARCEL DEKKER. INC 270 Madison Avenue. New York. New York 10016
Corrosion of Metallic Materials 27 For many of the metabolic purposes trace elements are needed. Cobalt aids in the transfer of methyl groups from/to organic or inorganic molecules (vitamin B12, cobalamine, is involved in the methylation of heavy metals such as mercury). Iron as Fe2 or Fe3 is required for the electron transport system, where it acts as an oxidizable/reducible central atom in cytochromes or in nonheme iron sulfur proteins. Magnesium acts in a similar manner in the chlorophyll molecule. Copper is an essential part of a cytachrome which, at the terminal end of the electron transport system, is responsible for the reduction of oxygen to water. Since life cannot exist without water, water is an essential requirement for microbial life and growth. Different microorganisms have different requirements as to the amount of water needed. A solid material is surrounded by three types of water: hygroscopic, pellicular, and gravitational. Only pellicular and gravitational water are biologically available and can be used by microorganisms. The biologically available water is usually measured as the water activity, aw, of a sample: Vs a = w Pw where Vs is the vapor pressure of the solution and Pw is the vapor pressure of pure water, at the same temperature. Most bacteria require an aw value in excess of 0.90. Hydrogen ion concentration is another important factor affecting growth. Microorganisms are classified as to their ability to grow under acidic, neutral, or alkaline conditions, being given such titles as acidophiles, neutrophiles, or alkalopholes. Most microorganisms thrive in the neutral pH range of 6–8. Microbial growth is also affected by redox potential. Under standard conditions hydrogen is assumed to have a redox potential of 421 mV, and oxygen has a redox potential of 820 mV. Metabolism can take place within that range. Available oxygen is another factor that influences microbial growth. Microbial growth is possible under aerated as well as under totally oxygen free conditions. Those organisms living with the amount of oxygen contained in the air are called aerobes, while those that perform their metabolism without any free oxygen are called anaerobes. These latter are able to use bound oxygen (sulfate, carbon dioxide) or to ferment organic compounds. Temperature is another important factor affecting microbial growth. Microbial life is possible within the range of 5 to 110C. Microorganisms are also classified as to the temperature range in which they thrive, as in the following table: