Laboratory photography in a controlled setting should also be performed to guarantee accurate reproduction of color tones and surface textures The type and extent of on-site sampling depends on the environment and its availability. Piping corrosion in a domestic water service would require water samples from the source(incoming water supply )and from the end use(for example, a faucet). Microbiologically influenced corrosion may apply in certain cases. On-site testing or sample removal is necessary to retain the most accurate information regarding the type and number of bacteria involved. In most cases, removal of corrosion deposits from the surface may be performed in the field If the sample is undergoing extensive laboratory examination, it may be prudent to carefully retain the corrosion in place for laboratory documentation and removal If samples are removed on site, care must be taken to avoid any contamination. Sealed bags, latex gloves, tools for sample removal, buffered containers for water samples, MIC kits, and adhesive tape are useful for sample removal. (See the article"Biological Corrosion Failures" in this Volume for listings of commercially available kits. The analyst should avoid touching any corrosion product directly with bare hands to prevent contamination Physical removal of samples must be done in a manner to avoid further damage to the failed component and to avoid disturbance of corrosion products. In general, cutting must be done with care to avoid alterations of the metallurgical condition of the material and corrosion deposits. Saw cutting is generally preferred over torch cutting, since heating of the sample can affect the material and the corrosion product. If torch cutting is performed, a distance of 75 to 150 mm(3 to 6 in. )should be maintained from the area of interest. Saw cutting should be performed at a slow rate to avoid overheating. Use of lubricants and coolants should be avoided, if possible, to minimize contamination The proper shipment and storage of samples lessens the possibility of destruction of pertinent evidence Wrapping and sealing of the failed component will generally retain evidence and prevent further oxidation or contamination. A neutral environment may be required to reduce activity. Any fracture surfaces should be otected from the potential of rubbing and contact Laboratory Examination. While each type of failure may have unique tests, some general steps should be taken in investigating all corrosion failures All samples must be properly identified and their origin, handling, and processing within the laboratory Photographs should document samples in the "as-received"condition Stereomicroscopic examination of the involved areas should follow. Photographic documentation during stereomicroscopy should also be performed Nondestructive test methods should now be considered. The key to performing nondestructive testing is to avoid any disturbance of scale product until corrosion samples have been removed. Radiography to document casting quality or to evaluate cracking is usually acceptable. However, the use of liquids or dyes is not acceptable until the corrosion samples have been removed The removal of corrosion deposits for evaluation is the next step. The samples should be removed with a noncontaminating tool such as a stainless steel pick. Corrodent samples should be placed in clean, clearly marked containers The corrosion deposits should be analyzed. One of the most common analysis techniques is energy dispersive spectroscopy(EDS). This method, used in conjunction with scanning electron microscopy (SEM), provides information regarding the elemental composition of the corrosion deposit Based on the visual examination, a corrosion sample may need to be subjected to microbiological nal These steps may be followed by cleaning and/or other tests After the corrosion deposits selected for examination have been secured, the failure sample may be cleaned However, in some cases it may be desirable to retain the corrosion product intact for metallographic examination. For instance, a component subject to hydrogen damage or caustic corrosion may benefit from an analysis of the corrosion product layering effect. Cross sections should be taken prior to removal of corrosion p Precautions must be taken during the cleaning process to avoid any destruction to the base metal. Generally, the least aggressive cleaning method is initiated first, such as brushing with a soft brush or light air pressure Thefileisdownloadedfromwww.bzfxw.com
Laboratory photography in a controlled setting should also be performed to guarantee accurate reproduction of color tones and surface textures. The type and extent of on-site sampling depends on the environment and its availability. Piping corrosion in a domestic water service would require water samples from the source (incoming water supply) and from the end use (for example, a faucet). Microbiologically influenced corrosion may apply in certain cases. On-site testing or sample removal is necessary to retain the most accurate information regarding the type and number of bacteria involved. In most cases, removal of corrosion deposits from the surface may be performed in the field. If the sample is undergoing extensive laboratory examination, it may be prudent to carefully retain the corrosion in place for laboratory documentation and removal. If samples are removed on site, care must be taken to avoid any contamination. Sealed bags, latex gloves, tools for sample removal, buffered containers for water samples, MIC kits, and adhesive tape are useful for sample removal. (See the article “Biological Corrosion Failures” in this Volume for listings of commercially available kits.) The analyst should avoid touching any corrosion product directly with bare hands to prevent contamination. Physical removal of samples must be done in a manner to avoid further damage to the failed component and to avoid disturbance of corrosion products. In general, cutting must be done with care to avoid alterations of the metallurgical condition of the material and corrosion deposits. Saw cutting is generally preferred over torch cutting, since heating of the sample can affect the material and the corrosion product. If torch cutting is performed, a distance of 75 to 150 mm (3 to 6 in.) should be maintained from the area of interest. Saw cutting should be performed at a slow rate to avoid overheating. Use of lubricants and coolants should be avoided, if possible, to minimize contamination. The proper shipment and storage of samples lessens the possibility of destruction of pertinent evidence. Wrapping and sealing of the failed component will generally retain evidence and prevent further oxidation or contamination. A neutral environment may be required to reduce activity. Any fracture surfaces should be protected from the potential of rubbing and contact. Laboratory Examination. While each type of failure may have unique tests, some general steps should be taken in investigating all corrosion failures: · All samples must be properly identified and their origin, handling, and processing within the laboratory documented. · Photographs should document samples in the “as-received” condition. · Stereomicroscopic examination of the involved areas should follow. Photographic documentation during stereomicroscopy should also be performed. · Nondestructive test methods should now be considered. The key to performing nondestructive testing is to avoid any disturbance of scale product until corrosion samples have been removed. Radiography to document casting quality or to evaluate cracking is usually acceptable. However, the use of liquids or dyes is not acceptable until the corrosion samples have been removed. · The removal of corrosion deposits for evaluation is the next step. The samples should be removed with a noncontaminating tool such as a stainless steel pick. Corrodent samples should be placed in clean, clearly marked containers. · The corrosion deposits should be analyzed. One of the most common analysis techniques is energydispersive spectroscopy (EDS). This method, used in conjunction with scanning electron microscopy (SEM), provides information regarding the elemental composition of the corrosion deposit. · Based on the visual examination, a corrosion sample may need to be subjected to microbiological analysis. · These steps may be followed by cleaning and/or other tests. After the corrosion deposits selected for examination have been secured, the failure sample may be cleaned. However, in some cases it may be desirable to retain the corrosion product intact for metallographic examination. For instance, a component subject to hydrogen damage or caustic corrosion may benefit from an analysis of the corrosion product layering effect. Cross sections should be taken prior to removal of corrosion products. Precautions must be taken during the cleaning process to avoid any destruction to the base metal. Generally, the least aggressive cleaning method is initiated first, such as brushing with a soft brush or light air pressure. The file is downloaded from www.bzfxw.com
Ultrasonic cleaning in acetone is considered a nonaggressive approach. This cleaning method will remove some light surface deposits. Deposits that have been exposed to elevated temperatures will generally require a more aggressive approach. Inhibited dilute acid solutions and citric acid cleaners can be used to clean adherent corrosion deposits. In cases where the protection of the fracture features is critical, softened acetate tape can be used to remove adherent deposits. This method also retains the deposits removed for further examination if necessar If the failure analysis does not involve preservation of a fracture surface, fine sandblasting of the base metal may be useful to remove scale deposits as in the evaluation of the pitting After the corrosion deposits have been removed, additional nondestructive testing may prove useful. Magnetic particle examination, penetrant examination, eddy current testing, and ultrasonic testing are a few techniques that can be employed to explore the quality and condition of the failure and material Microscopic Examination. As noted previously, stereomicroscopy is performed to document the corrosion product, and it also is an appropriate tool for analysis of the fracture surface(if one exists)or other surface after cleaning. This analysis will provide information regarding the failure initiation, cracking, and corrosion-surface patterns such as pitting and wear/erosion. Photographic documentation of the surface should be performed Based on the results of the stereomicroscopy areas can be selected for sem to characterize the fracture features. A determination of the fracture features, such as intergranular cracking, cleavage, and ductile dimples will provide valuable information pointing to the likely cause of failure and the corrosion mechanism Metallography is an essential tool for the examination of the failure specimens. Selection of the most informative cross-section locations is important. Metallurgical examination of a cross section requires mounting, surface grinding, polishing, and examining the sample in the unetched and etched condition under a microscope. Microstructural features and conditions such as cracking and crack progression, pit morphology, selective leaching, surface features, and other characteristics can all be examined. These features provide key evidence regarding the cause of failure and extent of damage As discussed earlier, in some cases it is useful to examine a cross section of the corrosion product with the base material. There are various techniques to retain the scale without pullout during the polishing process. In some cases, plating over the scale prior to mounting the sample or impregnating the mount with resin after preparation can retain the scale for examination. Additional information is provided in the articles"Practices in Failure Analysis"and"Metallographic Techniques in Failure Analysis"in this Volume Corrosion testing includes several categories of tests. Normally, corrosion testing is considered a long-term pproach to investigations regarding material selection. Simulated testing or in situ testing is often performed when a given environment may be unique or where materials may experience unique flow conditions. For ample, piping containing fluid may experience unique flow conditions that may result in erosion-corrosio failures. In that case, sections of pipe of different alloys may be placed on line and monitored. Other common methods of testing include accelerated tests, simulated or pilot testing, and electrochemical tests Typically, a variety of materials are selected for evaluation. A standard, such as carbon steel, may be used to verify the corrosiveness of the environment. Multiple samples of the same material should be tested to verify reproducibility. Sources for corrosion test methods are ASTM International, NACE, and internationally, ISo, EN, and JIS standards. Other testing methods and inspection procedures have been developed on an industry specific basis by industrial and government organizations, such as in the pulp and paper and the electric power industries, to specifically address corrosion issues relating to their specific operating conditions and environments Accelerated tests are performed when an expedited answer is needed to solve a serious corrosion problem and demonstrate an acceptable corrosion-resisting lifetime, provided that correlations can be established Accelerated testing usually involves the exposure to extreme conditions, relative to the actual servie environment,to accelerate the corrosion process. The process may be accelerated with high-stress conditions, elevated temperatures, or highly aggressive solutions While accelerated testing does provide test data sooner, extrapolation of results may be misleading. The acceleration of certain factors during the testing may cause different corrosion mechanisms to become active, and thus the results may not reflect those that would be obtained under actual service conditions. Care must be taken when evaluating results so that economical and acceptable choices are not eliminated due to their rformance in unrealistic environments Simulated-use tests are primarily set up in the laboratory with an environment as close as possible to the environment of interest. In these cases, sufficient time must be allowed for an appropriate response; a short time
Ultrasonic cleaning in acetone is considered a nonaggressive approach. This cleaning method will remove some light surface deposits. Deposits that have been exposed to elevated temperatures will generally require a more aggressive approach. Inhibited dilute acid solutions and citric acid cleaners can be used to clean adherent corrosion deposits. In cases where the protection of the fracture features is critical, softened acetate tape can be used to remove adherent deposits. This method also retains the deposits removed for further examination if necessary. If the failure analysis does not involve preservation of a fracture surface, fine sandblasting of the base metal may be useful to remove scale deposits as in the evaluation of the pitting. After the corrosion deposits have been removed, additional nondestructive testing may prove useful. Magnetic particle examination, penetrant examination, eddy current testing, and ultrasonic testing are a few techniques that can be employed to explore the quality and condition of the failure and material. Microscopic Examination. As noted previously, stereomicroscopy is performed to document the corrosion product, and it also is an appropriate tool for analysis of the fracture surface (if one exists) or other surface after cleaning. This analysis will provide information regarding the failure initiation, cracking, and corrosion-surface patterns such as pitting and wear/erosion. Photographic documentation of the surface should be performed. Based on the results of the stereomicroscopy, areas can be selected for SEM to characterize the fracture features. A determination of the fracture features, such as intergranular cracking, cleavage, and ductile dimples, will provide valuable information pointing to the likely cause of failure and the corrosion mechanism. Metallography is an essential tool for the examination of the failure specimens. Selection of the most informative cross-section locations is important. Metallurgical examination of a cross section requires mounting, surface grinding, polishing, and examining the sample in the unetched and etched condition under a microscope. Microstructural features and conditions such as cracking and crack progression, pit morphology, selective leaching, surface features, and other characteristics can all be examined. These features provide key evidence regarding the cause of failure and extent of damage. As discussed earlier, in some cases it is useful to examine a cross section of the corrosion product with the base material. There are various techniques to retain the scale without pullout during the polishing process. In some cases, plating over the scale prior to mounting the sample or impregnating the mount with resin after preparation can retain the scale for examination. Additional information is provided in the articles “Practices in Failure Analysis” and “Metallographic Techniques in Failure Analysis” in this Volume. Corrosion testing includes several categories of tests. Normally, corrosion testing is considered a long-term approach to investigations regarding material selection. Simulated testing or in situ testing is often performed when a given environment may be unique or where materials may experience unique flow conditions. For example, piping containing fluid may experience unique flow conditions that may result in erosion-corrosion failures. In that case, sections of pipe of different alloys may be placed on line and monitored. Other common methods of testing include accelerated tests, simulated or pilot testing, and electrochemical tests. Typically, a variety of materials are selected for evaluation. A standard, such as carbon steel, may be used to verify the corrosiveness of the environment. Multiple samples of the same material should be tested to verify reproducibility. Sources for corrosion test methods are ASTM International, NACE, and internationally, ISO, EN, and JIS standards. Other testing methods and inspection procedures have been developed on an industryspecific basis by industrial and government organizations, such as in the pulp and paper and the electric power industries, to specifically address corrosion issues relating to their specific operating conditions and environments. Accelerated tests are performed when an expedited answer is needed to solve a serious corrosion problem and to demonstrate an acceptable corrosion-resisting lifetime, provided that correlations can be established. Accelerated testing usually involves the exposure to extreme conditions, relative to the actual service environment, to accelerate the corrosion process. The process may be accelerated with high-stress conditions, elevated temperatures, or highly aggressive solutions. While accelerated testing does provide test data sooner, extrapolation of results may be misleading. The acceleration of certain factors during the testing may cause different corrosion mechanisms to become active, and thus the results may not reflect those that would be obtained under actual service conditions. Care must be taken when evaluating results so that economical and acceptable choices are not eliminated due to their performance in unrealistic environments. Simulated-use tests are primarily set up in the laboratory with an environment as close as possible to the environment of interest. In these cases, sufficient time must be allowed for an appropriate response; a short time
exposure likely will not produce enough corrosion activity to evaluate the corrosion behavior of the material. Nevertheless, by simulating the exact conditions of operation, an accurate assessment of the material can generally be produced with an adequate exposure time ASTM publishes standard test methods and analytical procedures for corrosion and wear testing (Ref 2) Electrochemical testing is performed for general information regarding the passivity or anodic protection of material against corrosion and to determine the critical breakdown or pitting potential. This type of testing is performed by two methods: controlling the current or controlling the potential. Standard methods for electrochemical testing are published in Ref 2 As the name suggests in the controlled-current test method the current is controlled and the resulting corrosion potential is measured. Polarization curves are generated. Galvanostatic and galvanodynamic polarization measurements are used to plot anodic and cathodic polarization curves. The assumption that corrosion rates remain constant with time can produce inaccurate results with this test method In the controlled-potential method, instrumentation is available for both constant-potential(potentiostatic)and variable-potential(potentiodynamic) testing to determine overall corrosion-rate profiles for metal-electrolyte stems over a range of potentials Corrosion Rates and Types. Knowing and understanding corrosion rates and the types of corrosion is essential to the evaluation of corrosion failures and to the communication of the results to others the articles that follow in this Section discuss the forms mechanisms and relative rates of corrosion. Corrosion Volume 13 of the ASM Handbook, provides detailed information regarding types of corrosion, corrosion testing, corrosion failures, and industry- and alloy-Specific corrosion considerations. Volume 13 also provides information regarding the use of specific alloys in given environments, corrosion prevention, and the use of nonmetallic materials. The Handbook of Corrosion Data, 2nd edition(ASM International, 1995)is a compilation of corrosion data from published sources. Corrosion rates of various alloys are provided with a general discussion of the corrosion resistance of alloy groups in particular environments Internet web sites published by ASM International, ASTM, NACE, the Nickel Development Institute, and the Copper Development Association provide the ability to search libraries of data for a given request Analysis of Incomplete Data. Incomplete or inconsistent data may occur in certain instances when the failure piece has been contaminated by an unknown source. Improper handling of the failure sample can introduce contamination on the sample. Testing of contaminated samples may produce misleading data and erroneous results. For example, the sampling of a deposit removed from a fracture surface that experienced stress corrosion may not reveal the corrodent that caused the scc. often the fracture surface is flushed with water or cleaned prior to testing. Liquid penetrants, cleaning fluids, cutting fluids, and solvents may alter the chemical composition of the surface deposits Results from laboratory testing may provide extraneous results. The tests may not model the service conditions Laboratory testing cannot easily model flow conditions such as turbulence, erosion, and localized attack. Care must be taken when evaluating the laboratory data to be certain that the conclusions drawn are an accurate assessment of the operating environment ASTMG 16, "Applying Statistics to Analysis of Corrosion Data"(Ref 3), provides a guide for handling data; it refers to ASTM E 178, Practice for Dealing with Outlying Observations"(Ref 4) for the treatment of data that appear inconsistent with the bulk of the findings References cited in this section 2. Wear and Corrosion, Vol 03.02, Annual Book of AsTM Standards, ASTM 3.Applying Statistics to Analysis of Corrosion Data, G 16, Annual Book of ASTM Standards, ASTM 4."Practice for Dealing with Outlying Observations, " E 178, Annual Book of ASTM Standards, ASTM Thefileisdownloadedfromwww.bzfxw.com
exposure likely will not produce enough corrosion activity to evaluate the corrosion behavior of the material. Nevertheless, by simulating the exact conditions of operation, an accurate assessment of the material can generally be produced with an adequate exposure time. ASTM publishes standard test methods and analytical procedures for corrosion and wear testing (Ref 2). Electrochemical testing is performed for general information regarding the passivity or anodic protection of a material against corrosion and to determine the critical breakdown or pitting potential. This type of testing is performed by two methods: controlling the current or controlling the potential. Standard methods for electrochemical testing are published in Ref 2. As the name suggests, in the controlled-current test method, the current is controlled and the resulting corrosion potential is measured. Polarization curves are generated. Galvanostatic and galvanodynamic polarization measurements are used to plot anodic and cathodic polarization curves. The assumption that corrosion rates remain constant with time can produce inaccurate results with this test method. In the controlled-potential method, instrumentation is available for both constant-potential (potentiostatic) and variable-potential (potentiodynamic) testing to determine overall corrosion-rate profiles for metal-electrolyte systems over a range of potentials. Corrosion Rates and Types. Knowing and understanding corrosion rates and the types of corrosion is essential to the evaluation of corrosion failures and to the communication of the results to others. The articles that follow in this Section discuss the forms, mechanisms, and relative rates of corrosion. Corrosion, Volume 13 of the ASM Handbook, provides detailed information regarding types of corrosion, corrosion testing, corrosion failures, and industry- and alloy-specific corrosion considerations. Volume 13 also provides information regarding the use of specific alloys in given environments, corrosion prevention, and the use of nonmetallic materials. The Handbook of Corrosion Data, 2nd edition (ASM International, 1995) is a compilation of corrosion data from published sources. Corrosion rates of various alloys are provided with a general discussion of the corrosion resistance of alloy groups in particular environments. Internet web sites published by ASM International, ASTM, NACE, the Nickel Development Institute, and the Copper Development Association provide the ability to search libraries of data for a given request. Analysis of Incomplete Data. Incomplete or inconsistent data may occur in certain instances when the failure piece has been contaminated by an unknown source. Improper handling of the failure sample can introduce contamination on the sample. Testing of contaminated samples may produce misleading data and erroneous results. For example, the sampling of a deposit removed from a fracture surface that experienced stress corrosion may not reveal the corrodent that caused the SCC. Often the fracture surface is flushed with water or cleaned prior to testing. Liquid penetrants, cleaning fluids, cutting fluids, and solvents may alter the chemical composition of the surface deposits. Results from laboratory testing may provide extraneous results. The tests may not model the service conditions. Laboratory testing cannot easily model flow conditions such as turbulence, erosion, and localized attack. Care must be taken when evaluating the laboratory data to be certain that the conclusions drawn are an accurate assessment of the operating environment. ASTM G 16, “Applying Statistics to Analysis of Corrosion Data” (Ref 3), provides a guide for handling data; it refers to ASTM E 178, “Practice for Dealing with Outlying Observations” (Ref 4) for the treatment of data that appear inconsistent with the bulk of the findings. References cited in this section 2. Wear and Corrosion, Vol 03.02, Annual Book of ASTM Standards, ASTM 3. “Applying Statistics to Analysis of Corrosion Data,” G 16, Annual Book of ASTM Standards, ASTM 4. “Practice for Dealing with Outlying Observations,” E 178, Annual Book of ASTM Standards, ASTM The file is downloaded from www.bzfxw.com
Analysis and Prevention of Corrosion-Related Failures S.R. Freeman, Millennium Metallurgy Ltd Examples of Corrosion Failure Analy Example 1: Analysis of Pitting and MIC of Stainless Steel Piping. Type S31603)austenitic stainless steel piping was installed as part of a collection system for a storm nt system used in a manufacturing facility. Within six months of start-up, leaks were discovered On-Site Examination. A of tests were performed on site to eliminate stray currents as a possible cause of failure in the piping system. The piping system was inspected for possible sources of ac or dc current flow.An external battery was used to impress a voltage. Voltage and current were recorded with the external power supply applied and removed There was no evidence of stray currents or electrical discharge It was noted that the ambient temperature and slow or stagnant flow conditions present in the piping were ideal for bacteria growth On-Site Sampling. Water samples were obtained for corrosivity and MIC testing. Commercially available field MIC kits were used. Samples of the damaged pipe were removed total dissolved solids level of 10,000 ppm was also high. The MIC water samples were shipped to the laboratory within 24 h of removal for viable culture testing. The testing showed high levels of aerobic, acid producing, and low-nutrient bacteria Laboratory Examination. Perforated pipe samples were provided for metallurgical evaluation. Figure 2(a) shows the leak area as viewed from the outside-diameter surface. The sample was cut dry to avoid contamination. The inside-diameter surface is shown in Fig. 2(b). The pit appeared larger on the inside diameter surface, indicating pit initiation occurred at the inside surface and at the bottom of the pipe. a rusty discoloration was apparent along the bottom length of the pipe. This discoloration corresponded to the area of the pitting. There was no corrosion deposit associated with the pitting. The discolored areas and other areas were evaluated using EDs. The EDS revealed contaminants consisting of chlorine, sulfur, sodium, silicon, and potassium. The area of discoloration revealed iron and oxygen only Fig. 2 Pitting corrosion of 316L stainless steel pipe. (a) view of pitting on the outside- diameter surface at the leak location.(b) View of the inside-diameter surface where the pit size was larger at the leak location. There was a rusty discoloration along the bottom of the pipe. (c) Cross section of pipe wall through the perforation. The sample was etched in AstM 89 reagent to delineate the microstructure. Uniform wall thickness is approximately 2.9 mm(0.1l in. ) 5x Courtesy of s.R. Freeman, Millennium Metallurgy, Ltd A metallurgical cross section was prepared through the pitted region. Figure 2(c)shows a 10X magnification of the cross section through the pitted region after etching with AStM 89 reagent. The pit was not associated with a welded region, and the microstructure appeared normal. There was no evidence of general wall loss(uniform corrosion)
Analysis and Prevention of Corrosion-Related Failures S.R. Freeman, Millennium Metallurgy, Ltd. Examples of Corrosion Failure Analysis Example 1: Analysis of Pitting and MIC of Stainless Steel Piping. Type 316L (UNS S31603) austenitic stainless steel piping was installed as part of a collection system for a storm sewer treatment system used in a manufacturing facility. Within six months of start-up, leaks were discovered. On-Site Examination. A series of tests were performed on site to eliminate stray currents as a possible cause of failure in the piping system. The piping system was inspected for possible sources of ac or dc current flow. An external battery was used to impress a voltage. Voltage and current were recorded with the external power supply applied and removed. There was no evidence of stray currents or electrical discharge. It was noted that the ambient temperature and slow or stagnant flow conditions present in the piping were ideal for bacteria growth. On-Site Sampling. Water samples were obtained for corrosivity and MIC testing. Commercially available field MIC kits were used. Samples of the damaged pipe were removed. Laboratory Testing. High levels of chlorides, as high as 20,000 ppm, were reported in the water sample. The total dissolved solids level of 10,000 ppm was also high. The MIC water samples were shipped to the laboratory within 24 h of removal for viable culture testing. The testing showed high levels of aerobic, acidproducing, and low-nutrient bacteria. Laboratory Examination. Perforated pipe samples were provided for metallurgical evaluation. Figure 2(a) shows the leak area as viewed from the outside-diameter surface. The sample was cut dry to avoid contamination. The inside-diameter surface is shown in Fig. 2(b). The pit appeared larger on the insidediameter surface, indicating pit initiation occurred at the inside surface and at the bottom of the pipe. A rusty discoloration was apparent along the bottom length of the pipe. This discoloration corresponded to the area of the pitting. There was no corrosion deposit associated with the pitting. The discolored areas and other areas were evaluated using EDS. The EDS revealed contaminants consisting of chlorine, sulfur, sodium, silicon, and potassium. The area of discoloration revealed iron and oxygen only. Fig. 2 Pitting corrosion of 316L stainless steel pipe. (a) View of pitting on the outsidediameter surface at the leak location. (b) View of the inside-diameter surface, where the pit size was larger at the leak location. There was a rusty discoloration along the bottom of the pipe. (c) Cross section of pipe wall through the perforation. The sample was etched in ASTM 89 reagent to delineate the microstructure. Uniform wall thickness is approximately 2.9 mm (0.11 in.). 5×. Courtesy of S.R. Freeman, Millennium Metallurgy, Ltd. A metallurgical cross section was prepared through the pitted region. Figure 2(c) shows a 10× magnification of the cross section through the pitted region after etching with ASTM 89 reagent. The pit was not associated with a welded region, and the microstructure appeared normal. There was no evidence of general wall loss (uniform corrosion)
Conclusion. The pitting in the austenitic stainless steel pipe is believed to be caused by damage to the passive layer brought about by a combination of MIC, high chloride levels, and high total dissolved solids. The low flow and stagnant conditions present in the piping are primary contributors to the pit progression. This type of material will perform significantly better when there is more flow of water Retesting of the water indicated similar high levels of aerobic bacteria, so these must be considered a part of th design basis Recommendations. Due to the extensive amount of piping, the pinhole size of the leaks, and environmental consequences of leaks, repair of the existing pitted pipe was impractical. Replacement of the pipe was recommended Several alloys, nonmetallic materials, and lined materials were proposed for coupon testing to determine which is the best in this particular environment Example 2: Analysis of a Corrosion Failure of an Aboveground Storage Tank. A failure of an aboveground storage tank occurred due to external corrosion of the tank floor. The liquid asphalt tank operated at elevated temperatures(approximately 177C, or 350F)and had been in service for six years. Cathodic protection (rectifiers) had been installed since start-up of the tank operation. It was noted, however, that some operational problems with the rectifier may have interrupted its protection On-Site Examination. The underside of the floor plates showed extensive localized wall thinning. Figure 3 shows a pit in one of the tank floor plates. While it was expected that the ground under the tank was near operational temperatures, it was determined that the temperatures were below 104C(220F)at most areas evaluated in the ground. Thus, it was possible for moisture to accumulate at the tank floor, producing intermittent wetting and drying conditions. This provided an explanation of the circumferential corrosion attern observed several feet in from the periphery of the tank. Heating was provided near the center of the tank and water in the backfill below the tank floor should be minimal. Perforations of the tank were large, and thinning was apparent in the adjacent areas PIT Fig3 View of an isolated pit on the outside of the floor tank of a liquid asphalt tank. the asphalt had been leaking for some time as the surrounding area was covered with deposits and the wall thinning was significant. Courtesy of s.R. Freeman, Millennium Metallurgy, Ltd On-Site Sampling Soil samples were taken and tested, but did not indicate high corrosivity. Sample of the scale deposits from the outer surface of the tank floor were taken Laboratory Examination. Scale samples analyzed by EDS showed the deposits consisted of primarily Fe203 (iron oxide). The rapid exfoliation, scale buildup, and pitting of the tank floor from the external (underside) surface also prompted MIC testing of the soil and of the deposits in the area of wall thinning. Although the temperatures were somewhat high for MIC to thrive, the results of the samples confirmed high levels of various Thefileisdownloadedfromwww.bzfxw.com
Conclusion. The pitting in the austenitic stainless steel pipe is believed to be caused by damage to the passive layer brought about by a combination of MIC, high chloride levels, and high total dissolved solids. The lowflow and stagnant conditions present in the piping are primary contributors to the pit progression. This type of material will perform significantly better when there is more flow of water. Retesting of the water indicated similar high levels of aerobic bacteria, so these must be considered a part of the design basis. Recommendations. Due to the extensive amount of piping, the pinhole size of the leaks, and environmental consequences of leaks, repair of the existing pitted pipe was impractical. Replacement of the pipe was recommended. Several alloys, nonmetallic materials, and lined materials were proposed for coupon testing to determine which is the best in this particular environment. Example 2: Analysis of a Corrosion Failure of an Aboveground Storage Tank. A failure of an aboveground storage tank occurred due to external corrosion of the tank floor. The liquid asphalt tank operated at elevated temperatures (approximately 177 °C, or 350 °F) and had been in service for six years. Cathodic protection (rectifiers) had been installed since start-up of the tank operation. It was noted, however, that some operational problems with the rectifier may have interrupted its protection. On-Site Examination. The underside of the floor plates showed extensive localized wall thinning. Figure 3 shows a pit in one of the tank floor plates. While it was expected that the ground under the tank was near operational temperatures, it was determined that the temperatures were below 104 °C (220 °F) at most areas evaluated in the ground. Thus, it was possible for moisture to accumulate at the tank floor, producing intermittent wetting and drying conditions. This provided an explanation of the circumferential corrosion pattern observed several feet in from the periphery of the tank. Heating was provided near the center of the tank and water in the backfill below the tank floor should be minimal. Perforations of the tank were large, and thinning was apparent in the adjacent areas. Fig. 3 View of an isolated pit on the outside of the floor tank of a liquid asphalt tank. The asphalt had been leaking for some time as the surrounding area was covered with deposits and the wall thinning was significant. Courtesy of S.R. Freeman, Millennium Metallurgy, Ltd. On-Site Sampling. Soil samples were taken and tested, but did not indicate high corrosivity. Sample of the scale deposits from the outer surface of the tank floor were taken. Laboratory Examination. Scale samples analyzed by EDS showed the deposits consisted of primarily Fe2O3 (iron oxide). The rapid exfoliation, scale buildup, and pitting of the tank floor from the external (underside) surface also prompted MIC testing of the soil and of the deposits in the area of wall thinning. Although the temperatures were somewhat high for MIC to thrive, the results of the samples confirmed high levels of various The file is downloaded from www.bzfxw.com