Practices in Failure Analysis Corrosion failures Corrosion is traditionally defined as the destructive chemical or electrochemical reaction of a metal with its environment. A broader, more modern definition of corrosion is the deterioration of a material or its properties due to reaction with its environment. The latter definition makes three important points. It does not limit corrosion to chemical or electrochemical processes, because some forms of corrosion do not involve these processes. It also characterizes corrosion damage as the deterioration of the material or its properties, because some forms of corrosion weaken the material without visible changes in appearance or measurable weight loss. Finally, the modern definition of corrosion acknowledges that nonmetallic materials may corrode. Examples of corrosion of nonmetallic materials are the aging of rubber due to the effects of heat and/or oils; swelling, oxidation, stress cracking, and ultraviolet deterioration of plastics; destruction of the binder in concrete by environmental agents; and the biological attack or rotting of wood This section focuses on analysis of metal corrosion There are many forms of metallic corrosion. These include: uniform corrosion, localized attack(e.g, crevice corrosion and pitting), galvanic attack, cracking phenomena(e.g, stress-corrosion cracking, hydrogen embrittlement, liquid metal embrittlement, and corrosion fatigue), velocity phenomena (e.g, erosion, cavitation, and impingement), fretting, ntergranular attack, and dealloying or selective leaching. These forms are discussed in more detail in the article Forms of Corrosion''in this volume The type of corrosion, the corrosion rate, and the severity and extent of corrosion are influenced by the nature of the environment, the metal surface in contact with the environment, and the mechanical stresses(magnitude and direction) These factors do not necessarily remain constant as corrosion progresses. They are affected by externally imposed changes and those resulting from the corrosion process itself. Other factors that greatly affect corrosion processes include temperature and temperature gradients at the metal/environment interface, crevices in the metal part or assembly, relative motion between the environment and the metal part, and the presence of dissimilar metals in an electrically conductive environment. Processing and fabrication operations such as surface grinding, heat treating, welding, cold working, forming, drilling, and shearing produce local or general changes on metal parts that, to varying degrees, affect their usceptibility to corrosion. When a corrosion failure has occurred, several means of preventing or minimizing future failures are available. Very often more than one method is used at the same time. The more important corrective and preventive measures are Change in alloy, heat treatment, or product form(e.g, solution annealing of austenitic stainless steels minimizes the risk of intergranular attack and stress- corrosion cracking) Use of resin coatings(e.g, acrylics, epoxies, phenolics, furanes and urethanes in the form of paints, potting compounds, adhesives, coatings, and linings Use of inert lubricants(chemically inert resins such as silicones, esters, and fluorocarbons that sometimes can serve both as effective lubricants and as corrosion-resistant coatings and linings Use of electrolytic and chemical coatings and surface treatments(e.g, anodizing of aluminum and aluminum alloys for protection in natural,"nonaggressive environments) Use of metallic coatings(e.g, zinc-rich coatings) Use of galvanic protection(cathodic or anodic) Design changes for corrosion control Use of inhibitors Changes in pH and applied potentia Continuous monitoring of variables Corrosion Failure analys Complete investigations of corrosion failures can be very complex, although not all corrosion failures require a comprehensive, detailed failure analysis. Often the preliminary examination will determine the extent of investigation required. In general, the investigation should consider various possibilities without being unnecessarily costly or time consuming. Routine checks to determine that the specified material is used is important. Such checks have shown that preferentially at the welds. Forgings that failed in service actually were castings in which failure was initiated by
Practices in Failure Analysis Corrosion Failures Corrosion is traditionally defined as the destructive chemical or electrochemical reaction of a metal with its environment. A broader, more modern definition of corrosion is the deterioration of a material or its properties due to reaction with its environment. The latter definition makes three important points. It does not limit corrosion to chemical or electrochemical processes, because some forms of corrosion do not involve these processes. It also characterizes corrosion damage as the deterioration of the material or its properties, because some forms of corrosion weaken the material without visible changes in appearance or measurable weight loss. Finally, the modern definition of corrosion acknowledges that nonmetallic materials may corrode. Examples of corrosion of nonmetallic materials are the aging of rubber due to the effects of heat and/or oils; swelling, oxidation, stress cracking, and ultraviolet deterioration of plastics; destruction of the binder in concrete by environmental agents; and the biological attack or rotting of wood. This section focuses on analysis of metal corrosion. There are many forms of metallic corrosion. These include: uniform corrosion, localized attack (e.g., crevice corrosion and pitting), galvanic attack, cracking phenomena (e.g., stress-corrosion cracking, hydrogen embrittlement, liquid metal embrittlement, and corrosion fatigue), velocity phenomena (e.g., erosion, cavitation, and impingement), fretting, intergranular attack, and dealloying or selective leaching. These forms are discussed in more detail in the article “Forms of Corrosion” in this Volume. The type of corrosion, the corrosion rate, and the severity and extent of corrosion are influenced by the nature of the environment, the metal surface in contact with the environment, and the mechanical stresses (magnitude and direction). These factors do not necessarily remain constant as corrosion progresses. They are affected by externally imposed changes and those resulting from the corrosion process itself. Other factors that greatly affect corrosion processes include temperature and temperature gradients at the metal/environment interface, crevices in the metal part or assembly, relative motion between the environment and the metal part, and the presence of dissimilar metals in an electrically conductive environment. Processing and fabrication operations such as surface grinding, heat treating, welding, cold working, forming, drilling, and shearing produce local or general changes on metal parts that, to varying degrees, affect their susceptibility to corrosion. When a corrosion failure has occurred, several means of preventing or minimizing future failures are available. Very often more than one method is used at the same time. The more important corrective and preventive measures are: · Change in alloy, heat treatment, or product form (e.g., solution annealing of austenitic stainless steels minimizes the risk of intergranular attack and stress-corrosion cracking) · Use of resin coatings (e.g., acrylics, epoxies, phenolics, furanes and urethanes in the form of paints, potting compounds, adhesives, coatings, and linings) · Use of inert lubricants (chemically inert resins such as silicones, esters, and fluorocarbons that sometimes can serve both as effective lubricants and as corrosion-resistant coatings and linings) · Use of electrolytic and chemical coatings and surface treatments (e.g., anodizing of aluminum and aluminum alloys for protection in natural, “nonaggressive” environments) · Use of metallic coatings (e.g., zinc-rich coatings) · Use of galvanic protection (cathodic or anodic) · Design changes for corrosion control · Use of inhibitors · Changes in pH and applied potential · Continuous monitoring of variables. Corrosion Failure Analysis Complete investigations of corrosion failures can be very complex, although not all corrosion failures require a comprehensive, detailed failure analysis. Often the preliminary examination will determine the extent of investigation required. In general, the investigation should consider various possibilities without being unnecessarily costly or time consuming. Routine checks to determine that the specified material is used is important. Such checks have shown that “seamless” tubes that failed in service by developing longitudinal splits were actually welded tubes that corroded preferentially at the welds. Forgings that failed in service actually were castings in which failure was initiated by
corrosion at porous areas exposed at the surface of the metal. Also, Monel metal parts that corroded rapidly in an environment to which Monel is highly resistant actually were strongly magnetic and made of carbon steel. In another instance, failure of a braided copper wire was traced to the substitution of a carbon-black filler for the usual silica filler in a sheath covering the wire. Thus, galvanic action occurred between the carbon and the copper in the presence of moisture While knowledge of the part and its application is desirable in any failure analysis, accurate history is especially important in the investigation of corrosion failures Information about the type of environment to which the failed part was exposed is essential. For instance, corrosion behavior in plants along rivers is affected by both local and upstream chemical composition in the system. Other determining factors include the temperature, whether exposure to the environment is continuous or intermittent and whether these and other factors varied during the service life of the part If available, engineering drawings and material and manufacturing specifications for the part should be examined Particular attention should be given to any part changes that may have been made. Missing information should be obtained from operating and inspection personnel, if possible. At the same time, the accuracy of any relevant documentary information, such as daily log sheets or inspection reports, should be verified. The investigator should determine if any tests or changes affected the physical evidence of the failure On-Site Examination and Sampling On-site examination is generally the same for corrosion failures as for other types of failures. The region of failure itself should be examined visually using hand magnifiers and any other suitable viewing equipment that is available The areas immediately adjacent to and near the failure, as well as related components of the system, should be examined for possible effects on the failure. Remote, related equipment should be examined, especially in complex systems and where liquid or gases flow. Also, the possibility of introducing chemicals or other contaminants from upwind or upstream areas should be checked. In one case, for example, ammonia released to the atmosphere by a neighboring plant lead to the SCC failure of a carbon steel boiler If possible, the failed component and related components of the system should be photographed before samples are practice is to calibrate the color at the beginning of every session by taking a picture of the film box, for example.good Ion Accessories useful in on-site examinations include plastic bags for holding samples, a stainless steel spatula for digging out soft corrosion products, and a file capable of cutting through hard scales. a magnet can distinguish austenitic from martensitic and ferritic stainless steels as well as steels from nonferrous alloys Sampling. When on-site sampling is done, the investigator should be guided by the information already obtained about the history of the failure. The bulk environment to which the failed part was exposed should be sampled, and suitable techniques should be used to obtain samples and make observations(such as pH, for instance)on the local environment at the point of failure In addition to taking samples from the failed area, samples from adjacent areas or apparently noncorroded regions should be obtained for comparison purposes. New or unused parts can provide evidence of the initial or unexposed condition of the part Removal of specimens and samples of corrosion products from the failed part or area requires the utmost caution. Care must be taken to avoid destroying valuable evidence or damaging the part and its related components. For example, torch cutting often is used for removal of specimens because it is fast and convenient. With torch cutting, cuts should be made away from the failure site to prevent alteration of the microstructure, thermal degradation of residues, and introduction of contaminants. If an abrasive cutoff wheel or a saw is used, the same precautions to avoid overheating apply. Also coolants or lubricants that can contaminate or alter the part or any deposits present should not be used After the samples have been extracted, they must be suitably protected during transportation to the laboratory, such as ith glass vials and polyethylene bags. One way to retain deposited material is to tape a covering of inert plastic over the critical area Preliminary laboratory examination The procedures followed in the preliminary laboratory examination will vary, depending on whether an on-site examination has already been performed by the failure analyst and its completeness. An on-site examination by a well- equipped investigator will include much of the work that otherwise would have to be done in the preliminary laboratory examination When no on-site examination has been done, a failure analyst will be aided by records on the part and environment and the remainder of the failed part(or at the very minimum a good photographic record of it). Also helpful are undamaged or unused parts, related components, and samples of the environment Thefileisdownloadedfromwww.bzfxw.com
corrosion at porous areas exposed at the surface of the metal. Also, Monel metal parts that corroded rapidly in an environment to which Monel is highly resistant actually were strongly magnetic and made of carbon steel. In another instance, failure of a braided copper wire was traced to the substitution of a carbon-black filler for the usual silica filler in a sheath covering the wire. Thus, galvanic action occurred between the carbon and the copper in the presence of moisture. While knowledge of the part and its application is desirable in any failure analysis, accurate history is especially important in the investigation of corrosion failures. Information about the type of environment to which the failed part was exposed is essential. For instance, corrosion behavior in plants along rivers is affected by both local and upstream chemical composition in the system. Other determining factors include the temperature, whether exposure to the environment is continuous or intermittent and whether these and other factors varied during the service life of the part. If available, engineering drawings and material and manufacturing specifications for the part should be examined. Particular attention should be given to any part changes that may have been made. Missing information should be obtained from operating and inspection personnel, if possible. At the same time, the accuracy of any relevant documentary information, such as daily log sheets or inspection reports, should be verified. The investigator should determine if any tests or changes affected the physical evidence of the failure. On-Site Examination and Sampling On-site examination is generally the same for corrosion failures as for other types of failures. The region of failure itself should be examined visually using hand magnifiers and any other suitable viewing equipment that is available. The areas immediately adjacent to and near the failure, as well as related components of the system, should be examined for possible effects on the failure. Remote, related equipment should be examined, especially in complex systems and where liquid or gases flow. Also, the possibility of introducing chemicals or other contaminants from upwind or upstream areas should be checked. In one case, for example, ammonia released to the atmosphere by a neighboring plant lead to the SCC failure of a carbon steel boiler. If possible, the failed component and related components of the system should be photographed before samples are removed. Color photographs are highly desirable, especially when colored products of corrosion are present. A good practice is to calibrate the color at the beginning of every session by taking a picture of the film box, for example, using the same lighting conditions. Accessories useful in on-site examinations include plastic bags for holding samples, a stainless steel spatula for digging out soft corrosion products, and a file capable of cutting through hard scales. A magnet can distinguish austenitic from martensitic and ferritic stainless steels, as well as steels from nonferrous alloys. Sampling. When on-site sampling is done, the investigator should be guided by the information already obtained about the history of the failure. The bulk environment to which the failed part was exposed should be sampled, and suitable techniques should be used to obtain samples and make observations (such as pH, for instance) on the local environment at the point of failure. In addition to taking samples from the failed area, samples from adjacent areas or apparently noncorroded regions should be obtained for comparison purposes. New or unused parts can provide evidence of the initial or unexposed condition of the part. Removal of specimens and samples of corrosion products from the failed part or area requires the utmost caution. Care must be taken to avoid destroying valuable evidence or damaging the part and its related components. For example, torch cutting often is used for removal of specimens because it is fast and convenient. With torch cutting, cuts should be made away from the failure site to prevent alteration of the microstructure, thermal degradation of residues, and introduction of contaminants. If an abrasive cutoff wheel or a saw is used, the same precautions to avoid overheating apply. Also, coolants or lubricants that can contaminate or alter the part or any deposits present should not be used. After the samples have been extracted, they must be suitably protected during transportation to the laboratory, such as with glass vials and polyethylene bags. One way to retain deposited material is to tape a covering of inert plastic over the critical area. Preliminary Laboratory Examination The procedures followed in the preliminary laboratory examination will vary, depending on whether an on-site examination has already been performed by the failure analyst and its completeness. An on-site examination by a wellequipped investigator will include much of the work that otherwise would have to be done in the preliminary laboratory examination. When no on-site examination has been done, a failure analyst will be aided by records on the part and environment and the remainder of the failed part (or at the very minimum a good photographic record of it). Also helpful are undamaged or unused parts, related components, and samples of the environment. The file is downloaded from www.bzfxw.com
Preservation of Evidence. Whether or not an on-site examination has been done, the samples should be handled so that maximum information can be gained before any sample is damaged, destroyed, or contaminated, preventing further tests Also, a complete written and photographic record should be kept through all investigation stages Visual Examination and Cleaning. First, the sample is examined visually, most often with the aid of a low-power hand magnifier. At this stage, important features include the extent of damage, general appearance of the damage zone, and the color, texture, and quantity of surface residues. If substantial amounts of foreign matter are visible, cleaning is necessary before further examination. The residues can be removed in some areas, leaving portions of the failure region in the as- received condition to preserve evidence. When only small amounts of foreign matter are present, cleaning can be deferred so that the surface can be examined with a stereomicroscope before and after cleaning. Cleaning also can be deferred until necessary for surface examination at higher magnifications or for the preparation of metallographic specimens. Small amounts of residues can be removed using transparent tape or acetate replicas and retained for later analysis ashing with water or solvent, with or without the aid of an ultrasonic bath, usually adequately removes soft residues that obscure the view. Inhibited pickling solutions will remove adherent rust or scale. Usually, the cleaning solutions should be saved for later analysis and identification of the substance removed Nondestructive Tests. For parts in which internal damage may have resulted from corrosion or from the combined effects of corrosion, stress, and imperfections, nondestructive testing is desirable. Radiography and ultrasonics can be used to locate internal discontinuities, and magnetic-particle and liquid-penetrant techniques are used to locate surface imperfections. Methods such as eddy current and holography are used less frequently, mainly because these methods require the use of standards to accurately interpret the data Microscopic Examination. Examination by both light microscopy and electron microscopy can be used to observe minute features on corroded surfaces, to evaluate microstructure of the metallic parts, and to observe the manner and extent to which the metal was attacked by the corrodent Viewing the cleaned surface with a stereomicroscope clearly shows gross topographic features such as pitting, cracking, or surface patterns that can provide information about the failure mechanism. This information includes whether corrosion was the sole phenomenon involved, the type of corrosion, and whether other mechanisms, such as wear and fracture, also were operative If the features cannot be observed clearly using a stereomicroscope, instruments such as deep-field photographic microscopes or a SEM may be used. These instruments produce images with a greater depth of field and, therefore, can resolve the topographical features of very rough surfaces. Transmission electron microscopy, using replicas, can resolve extremely fine features Microscopic examination of polished or polished-and-etched specimens can reveal microstructural features as well as damage such as cracking. If the corrosion products possess sufficient coherence and hardness to be polished, they should be retained. One way to keep the surface material in place is to impregnate the sample with a casting-type resin and allow it to harden before cutting samples. Usually, it is helpful to vacuum impregnate the sample during the casting process to be sure that any surface-connected voids are filled with casting plastic. To secure maximum quality of retention, polishing on napless cloths with diamond abrasives is used Chemical analysis of the Corrosion Products The job of the failure analyst is to establish what role, if any, the corrosion products played in the failure and to identif and analyze the metal or metals of which the failed part was made, the environment to which the failed part was exposed inhomogeneities in the part surface, and foreign matter and metal surfaces Both conventional techniques(such as wet chemical analysis, emission spectroscopy, x-ray diffraction, infrared spectrophotometry, gas chromatography, and x-ray fluorescence spectrography) and special techniques(such as energy dispersive x-ray spectrometry, electron-microprobe analysis, ion-microprobe analysis, Auger-electron spectrometry Mossbauer spectrometry, and electron diffraction) may be needed to define the composition and structure of various substances completely Identification and analysis of the metals of which the failed part was made usually is routine. Ordinarily, the purpose is not to look for minor deviations from the chemical composition specified for the part, but rather to check for possible major deviations in composition and to make sure that the correct alloy was used. However, for some austenitic stainless steels, the corrosion resistance and other properties of welded joints require close control over the composition of the stainless steel and the weld metal The bulk composition of the failure environment as well as the local composition of the environment at the metal interface are important in determining whether, or how, corrosion contributed to failure. Composition of the environment usually can be obtained chemically or spectroscopically Serious corrosion damage can result from the presence of inhomogeneities in the surface of a metal part used in a corrosive environment. A classic example is severe local attack on a stainless steel because of embedded particles of tramp" iron in the surface of the stainless steel. Establishing the nature of a layer of material on the metal surface may be more difficult than analyzing the material, especially when a thorough on-site investigation cannot be made. The surfac
Preservation of Evidence. Whether or not an on-site examination has been done, the samples should be handled so that maximum information can be gained before any sample is damaged, destroyed, or contaminated, preventing further tests. Also, a complete written and photographic record should be kept through all investigation stages. Visual Examination and Cleaning. First, the sample is examined visually, most often with the aid of a low-power hand magnifier. At this stage, important features include the extent of damage, general appearance of the damage zone, and the color, texture, and quantity of surface residues. If substantial amounts of foreign matter are visible, cleaning is necessary before further examination. The residues can be removed in some areas, leaving portions of the failure region in the asreceived condition to preserve evidence. When only small amounts of foreign matter are present, cleaning can be deferred so that the surface can be examined with a stereomicroscope before and after cleaning. Cleaning also can be deferred until necessary for surface examination at higher magnifications or for the preparation of metallographic specimens. Small amounts of residues can be removed using transparent tape or acetate replicas and retained for later analysis. Washing with water or solvent, with or without the aid of an ultrasonic bath, usually adequately removes soft residues that obscure the view. Inhibited pickling solutions will remove adherent rust or scale. Usually, the cleaning solutions should be saved for later analysis and identification of the substance removed. Nondestructive Tests. For parts in which internal damage may have resulted from corrosion or from the combined effects of corrosion, stress, and imperfections, nondestructive testing is desirable. Radiography and ultrasonics can be used to locate internal discontinuities, and magnetic-particle and liquid-penetrant techniques are used to locate surface imperfections. Methods such as eddy current and holography are used less frequently, mainly because these methods require the use of standards to accurately interpret the data. Microscopic Examination. Examination by both light microscopy and electron microscopy can be used to observe minute features on corroded surfaces, to evaluate microstructure of the metallic parts, and to observe the manner and extent to which the metal was attacked by the corrodent. Viewing the cleaned surface with a stereomicroscope clearly shows gross topographic features such as pitting, cracking, or surface patterns that can provide information about the failure mechanism. This information includes whether corrosion was the sole phenomenon involved, the type of corrosion, and whether other mechanisms, such as wear and fracture, also were operative. If the features cannot be observed clearly using a stereomicroscope, instruments such as deep-field photographic microscopes or a SEM may be used. These instruments produce images with a greater depth of field and, therefore, can resolve the topographical features of very rough surfaces. Transmission electron microscopy, using replicas, can resolve extremely fine features. Microscopic examination of polished or polished-and-etched specimens can reveal microstructural features as well as damage such as cracking. If the corrosion products possess sufficient coherence and hardness to be polished, they should be retained. One way to keep the surface material in place is to impregnate the sample with a casting-type resin and allow it to harden before cutting samples. Usually, it is helpful to vacuum impregnate the sample during the casting process to be sure that any surface-connected voids are filled with casting plastic. To secure maximum quality of retention, polishing on napless cloths with diamond abrasives is used. Chemical Analysis of the Corrosion Products The job of the failure analyst is to establish what role, if any, the corrosion products played in the failure and to identify and analyze the metal or metals of which the failed part was made, the environment to which the failed part was exposed, inhomogeneities in the part surface, and foreign matter and metal surfaces. Both conventional techniques (such as wet chemical analysis, emission spectroscopy, x-ray diffraction, infrared spectrophotometry, gas chromatography, and x-ray fluorescence spectrography) and special techniques (such as energy dispersive x-ray spectrometry, electron-microprobe analysis, ion-microprobe analysis, Auger-electron spectrometry, Mössbauer spectrometry, and electron diffraction) may be needed to define the composition and structure of various substances completely. Identification and analysis of the metals of which the failed part was made usually is routine. Ordinarily, the purpose is not to look for minor deviations from the chemical composition specified for the part, but rather to check for possible major deviations in composition and to make sure that the correct alloy was used. However, for some austenitic stainless steels, the corrosion resistance and other properties of welded joints require close control over the composition of the stainless steel and the weld metal. The bulk composition of the failure environment as well as the local composition of the environment at the metal interface are important in determining whether, or how, corrosion contributed to failure. Composition of the environment usually can be obtained chemically or spectroscopically. Serious corrosion damage can result from the presence of inhomogeneities in the surface of a metal part used in a corrosive environment. A classic example is severe local attack on a stainless steel because of embedded particles of “tramp” iron in the surface of the stainless steel. Establishing the nature of a layer of material on the metal surface may be more difficult than analyzing the material, especially when a thorough on-site investigation cannot be made. The surface
layer can be merely a trace of innocuous soil or a residue of corrodents or of corrosion products. It also can differ in composition from the bulk metal Corrosion Testing Various types of corrosion-testing techniques are used to investigate corrosion failures and to evaluate the resistance to corrosion of metals and alloys for service in specific applications. They include accelerated tests, simulated-use tests, and electrochemical tests. Other techniques include monitoring performance in pilot-plant operations and in actual service Some accelerated-test methods have been accepted as standard by both the military and industry. To shorten testing time corrosion is accelerated in relation to naturally occurring corrosion, usually by increasing temperature, using a more ggressive environment, or increasing stress(e.g, SCC) Because various factors influence natural corrosion processes and differ widely in time dependence, the results laboratory tests must be interpreted carefully. They can be related to expected actual service behavior only when close correlation with long-term service results has been established Simulated-use tests are used frequently to analyze corrosion behavior of metals and alloys in specific applications. In these tests, either actual parts or test specimens are exposed to a synthetic or natural service environment Electrochemical tests provide data that can establish criteria for passivity or anodic protection against corrosion and determine critical breakdown or pitting potentials. The two general methods of electrochemical corrosion testing are controlled current and controlled potential. For either test method, AstM G 3 provides useful guidance and standardization of the manner of recording and reporting electrochemical measurements In the controlled-current method the current(a measure of the corrosion rate) is controlled and the resulting corrosion potential is measured. Several instruments are available for such tests, in which either logarithmic or linear polarization curves are developed. Both galvanostatic and galvanodynamic polarization measurements are employed to plot anodic and cathodic polarization curves In the controlled-potential method of electrochemical testing, the corrosion potential(oxidizing power) is controlled and the resulting corrosion current is measured. Equipment is available for both constant-potential (potentiostatic) and varying-potential(potentiodynamic)testing to determine overall corrosion- rate profiles for metal-electrolyte systems over a range of potentials For the most part, electrochemical tests are more valuable for evaluating the corrosion resistance of materials per se rather than for direct use as a failure analysis tool Tests for Corrective Action Once the failure cause and mechanism have been established the conclusions and the effectiveness of recommended corrective actions can be confirmed through testing. These tests should simulate as closely as possible the environmental and mechanical conditions to which the failed part was subjected in service. Ideally, the part should be tested in service however, such testing frequently is not feasible because of factors such as length of time to failure or extraneous damage caused by failure of the part. Thus, simulated-service tests may have to be performed in the laboratory. The following parameters should be carefully controlled Environmental factors: Temperature, which may be steady or fluctuating, and also may affect stress Single-phase or two-phase environment, which may involve alternate wetting and drying Environmental composition, including major and minor constituents, concentration and changes thereof, dissolved gases, and pH Electrochemical conditions, which may involve galvanic coupling or applied cathodic protection Mechanical factors Loading, which may be static or cyclic--if cyclic, mean stress may be zero, tensile, or compressive; also, stress- wave shape and period must be defined Surface damage, which may occur by fretting, abrasion, cavitation, or liquid-impingement corrosion Practices in Failure Analysis Thefileisdownloadedfromwww.bzfxw.com
layer can be merely a trace of innocuous soil or a residue of corrodents or of corrosion products. It also can differ in composition from the bulk metal. Corrosion Testing Various types of corrosion-testing techniques are used to investigate corrosion failures and to evaluate the resistance to corrosion of metals and alloys for service in specific applications. They include accelerated tests, simulated-use tests, and electrochemical tests. Other techniques include monitoring performance in pilot-plant operations and in actual service. Some accelerated-test methods have been accepted as standard by both the military and industry. To shorten testing time, corrosion is accelerated in relation to naturally occurring corrosion, usually by increasing temperature, using a more aggressive environment, or increasing stress (e.g., SCC). Because various factors influence natural corrosion processes and differ widely in time dependence, the results of laboratory tests must be interpreted carefully. They can be related to expected actual service behavior only when close correlation with long-term service results has been established. Simulated-use tests are used frequently to analyze corrosion behavior of metals and alloys in specific applications. In these tests, either actual parts or test specimens are exposed to a synthetic or natural service environment. Electrochemical tests provide data that can establish criteria for passivity or anodic protection against corrosion and determine critical breakdown or pitting potentials. The two general methods of electrochemical corrosion testing are controlled current and controlled potential. For either test method, ASTM G 3 provides useful guidance and standardization of the manner of recording and reporting electrochemical measurements. In the controlled-current method, the current (a measure of the corrosion rate) is controlled and the resulting corrosion potential is measured. Several instruments are available for such tests, in which either logarithmic or linear polarization curves are developed. Both galvanostatic and galvanodynamic polarization measurements are employed to plot anodic and cathodic polarization curves. In the controlled-potential method of electrochemical testing, the corrosion potential (oxidizing power) is controlled and the resulting corrosion current is measured. Equipment is available for both constant-potential (potentiostatic) and varying-potential (potentiodynamic) testing to determine overall corrosion-rate profiles for metal-electrolyte systems over a range of potentials. For the most part, electrochemical tests are more valuable for evaluating the corrosion resistance of materials per se rather than for direct use as a failure analysis tool. Tests for Corrective Action Once the failure cause and mechanism have been established, the conclusions and the effectiveness of recommended corrective actions can be confirmed through testing. These tests should simulate as closely as possible the environmental and mechanical conditions to which the failed part was subjected in service. Ideally, the part should be tested in service; however, such testing frequently is not feasible because of factors such as length of time to failure or extraneous damage caused by failure of the part. Thus, simulated-service tests may have to be performed in the laboratory. The following parameters should be carefully controlled: Environmental factors: · Temperature, which may be steady or fluctuating, and also may affect stress · Single-phase or two-phase environment, which may involve alternate wetting and drying · Environmental composition, including major and minor constituents, concentration and changes thereof, dissolved gases, and pH · Electrochemical conditions, which may involve galvanic coupling or applied cathodic protection Mechanical factors: · Loading, which may be static or cyclic—if cyclic, mean stress may be zero, tensile, or compressive; also, stresswave shape and period must be defined · Surface damage, which may occur by fretting, abrasion, cavitation, or liquid-impingement corrosion Practices in Failure Analysis The file is downloaded from www.bzfxw.com
Wear failures Wear, friction, and lubrication are complex, interwoven subjects that may all affect the tendency of a part to fail or to ease being able to perform its intended function. While all three are very important, only wear receives emphasis in this ection,while friction and lubrication are covered less extensively. The science of wear is called tribology, from the Greek word tribos, meaning wear, and it forms the basis for a systems approach to wear failure analysis A systems approach to wear is necessary to understand the interactions among The system material components-the bulk and surface properties of the wearing parts, the properties of the material causing the wear, and the interface medium The operating variables The interactions among the material components of the system The operating environment It should already be clear that wear failures are not easy to analyze, describe, or prevent. There is no general agreement even as to the various types or forms of wear because they can be described in several different ways. One relativel This section discusses these eight form of wear, as follow. Sed by wear is to describe eight major forms or mechanisms simple way to classify the mechanical surface damage ca abrasive wear Erosive wea Adhesive wear Frett Cavitation Liquid-droplet impingement Corrosive wear About half of all wear failures are caused by abrasive wear, 15% by adhesive wear, and about 8% each by erosion and fretting. The other four mechanisms account for the remaining wear failures the characteristics and mitigation of each are briefly described in the following section, along with lubricant failures Types of Wear Abrasive Wear. The principal mechanism of abrasive wear is indicated by cutting. Abrasive wear is sometimes referred to as grinding wear. High-stress, low-speed particles or projections from hard materials cut and plow small grooves in softer materials. This type of wear is characteristic of damage to many ground contact tools, such as plows and cultivators, as well as other parts that dig into or rub against hard, abrasive materials. Again, it can lead to complete or partial failure and destruction of the part, particularly with respect to cutting tools, which can be badly dulled Abrasive wear can be mitigated by changing material to High-carbon steel 20 to 30% Cr white cast iron wC or TiC composites and by increasing surface hardness using Hard chrome plate Hardfacing alloy weld overlay Electroless nickel-phosphorus alloy plate High-velocity oxyfuel(HVOF)ceramic coatings(WC, CrC, aluminum oxide) Lubrication is of marginal use in combating abrasive wear, unless a lubricant layer thicker than the particle size can be maintained. Further examples and details are given in the article" Abrasive Wear Failures""in this volume Erosive wear is similar to abrasive wear, except that the force is provided by the kinetic energy of the particles as they are carried in a fluid. Low-stress, high-speed particles impinge upon the surface at some angle, tending to cut( ductile metal) or fracture(brittle material)very small wear chips, or particles from the surface. Erosion is a particular problem with various types of impellers, propellers, fans, and other parts where particles in a fluid, such as air or water, strike the
Wear Failures Wear, friction, and lubrication are complex, interwoven subjects that may all affect the tendency of a part to fail or to cease being able to perform its intended function. While all three are very important, only wear receives emphasis in this section, while friction and lubrication are covered less extensively. The science of wear is called tribology, from the Greek word tribos, meaning wear, and it forms the basis for a systems approach to wear failure analysis. A systems approach to wear is necessary to understand the interactions among: · The system material components—the bulk and surface properties of the wearing parts, the properties of the material causing the wear, and the interface medium · The operating variables · The interactions among the material components of the system · The operating environment It should already be clear that wear failures are not easy to analyze, describe, or prevent. There is no general agreement even as to the various types or forms of wear because they can be described in several different ways. One relatively simple way to classify the mechanical surface damage caused by wear is to describe eight major forms or mechanisms. This section discusses these eight form of wear, as follows: · Abrasive wear · Erosive wear · Adhesive wear · Fretting · Cavitation · Liquid-droplet impingement · Rolling-contact fatigue · Corrosive wear About half of all wear failures are caused by abrasive wear, 15% by adhesive wear, and about 8% each by erosion and fretting. The other four mechanisms account for the remaining wear failures. The characteristics and mitigation of each are briefly described in the following section, along with lubricant failures. Types of Wear Abrasive Wear. The principal mechanism of abrasive wear is indicated by cutting. Abrasive wear is sometimes referred to as grinding wear. High-stress, low-speed particles or projections from hard materials cut and plow small grooves in softer materials. This type of wear is characteristic of damage to many ground contact tools, such as plows and cultivators, as well as other parts that dig into or rub against hard, abrasive materials. Again, it can lead to complete or partial failure and destruction of the part, particularly with respect to cutting tools, which can be badly dulled. Abrasive wear can be mitigated by changing material to: · High-carbon steel · 20 to 30% Cr white cast iron · WC or TiC composites and by increasing surface hardness using: · Hard chrome plate · Hardfacing alloy weld overlay · Electroless nickel-phosphorus alloy plate · High-velocity oxyfuel (HVOF) ceramic coatings (WC, CrC, aluminum oxide) Lubrication is of marginal use in combating abrasive wear, unless a lubricant layer thicker than the particle size can be maintained. Further examples and details are given in the article “Abrasive Wear Failures” in this Volume. Erosive wear is similar to abrasive wear, except that the force is provided by the kinetic energy of the particles as they are carried in a fluid. Low-stress, high-speed particles impinge upon the surface at some angle, tending to cut (ductile metal) or fracture (brittle material) very small wear chips, or particles from the surface. Erosion is a particular problem with various types of impellers, propellers, fans, and other parts where particles in a fluid, such as air or water, strike the