Materials and Corrosion 2011, 62, No. 10 Do:10.1002/maco.20100574 967 Corrosion Concepts In this forum readers will be able to present practical problems experience will become a permanent feature of this periodical. for discussion it is d that these contributions will We are particularly anxious that both Senior Scientists and those include not only discussion of general problems and incidents of with more practical experience will make use of this forum to on but that suggested remedies will also be presented and exchange information, problems and potential remedies. sed. It is hoped that this exchange of knowledge and Acidic/caustic alternating corrosion on carbon steel pipes in heat exchanger of ethylene plant Y Gong, C. Yang, C. Yao and Z.-G. Yang Caustic embrittlement, a kind of stress corrosion cracking(SCC), is always encountered on materials under stresses amid caustic environment. acidic corrosion is another familiar degradation on materials contacting acidic media However it has been seldom studied what effect would be resulted in on materials that are exposed to an acidic/caustic alternating environment. In this paper, failure events were discovered on the carbon steel pipes under such an alternating service condition due to frequent sharp fluctuations of the heat medium's(process water) pH values in a heat exchanger. What is more, even chloride ions and sulfur element were detected, i.e pitting corrosion was involved as well. In order to identify the causes of the failure, matrix materials of the pipes were examined, failure defects on pipe surfaces were investigated, particularly the process water was thoroughly inspected via a series of characterization methods. Based on the analysis results, a novel four-level mechanism from microscopic scale to macroscopic scale was tentatively proposed to explain such an acidic/caustic alternating corrosion 1 Introduction according to the differences in configuration styles of cracking furnaces applied in the cracking stage. Among them, the tubular Ethylene is generally one of the most produced base chemicals in cracking furnace is the most widely used one, thanks to its petrochemical industry, and its extensive applications predomi- superiorities like low energy consumption, short residence time, nantly lie in synthesizing lots of downstream organic compounds high efficiency, and so on. Further detailed, ABB Lummus Global such as ethylene oxide(C2H4O), styrene(CgHg), polyethylene Stone& Webster, Kellogg Brown& Root (KBR), Linde, etc, are all PE), and so on. With respect to its manufacturing process, it is the corporations that design and fabricate tubular cracking usually divided into two primary stages, respectively, nominated furnaces under their own technologies and patents, and their cracking and separation. The former one pyrolyzes hydrocarbon whole ethylene plants are accordingly named after their brands feedstocks like naphtha, liquefied petroleum gas(LPG), gas oil, In fact, compared with the tubular cracking furnaces which etc, into smaller ones as C1-C4 and simultaneously introduces have always been paid special attention to by the above mentioned unsaturation[ 1]. the latter one subsequently refines the pyrolysis corporations, the complex systems and the equipments employed gas through a series of separation systems to eliminate the by. in the separation usually attract relatively less interests products like diesel oil, aromatics, propylene(C3 H6), butadiene research, nevertheless which also play a critical role in safe service (CH6), etc, aiming to obtain the purified ethylene. Actually, of a whole ethylene plant. As one mature process with nearly ethylene plants are cor nly distinguished into various types 60-year application, the Lummus ethylene plant holds the largest share of ethylene plants market in China currently [2 ] Particular Y Gong, C Yang, C Yao, Z-G. Yang in its separation stage, the process water stripper system is Department of Materials Science, Fudan University, No 220 Handan actually one of the most important systems, whose detailed Road, Shanghai 200433(P R China operation procedures are displayed in Fig. 1. As is shown in this E-mail:ziyang@fudan.edu.cn schematic diagram, the process water stripper system is the www.matcorr.com o 2011 WILEY-VCH Verlag GmbH& Co KGaA, Weinheim
Acidic/caustic alternating corrosion on carbon steel pipes in heat exchanger of ethylene plant Y. Gong, C. Yang, C. Yao and Z.-G. Yang* Caustic embrittlement, a kind of stress corrosion cracking (SCC), is always encountered on materials under stresses amid caustic environment. Acidic corrosion is another familiar degradation on materials contacting acidic media. However, it has been seldom studied what effect would be resulted in on materials that are exposed to an acidic/caustic alternating environment. In this paper, failure events were discovered on the carbon steel pipes under such an alternating service condition due to frequent sharp fluctuations of the heat medium’s (process water) pH values in a heat exchanger. What is more, even chloride ions and sulfur element were detected, i.e., pitting corrosion was involved as well. In order to identify the causes of the failure, matrix materials of the pipes were examined, failure defects on pipe surfaces were investigated, particularly the process water was thoroughly inspected via a series of characterization methods. Based on the analysis results, a novel four-level mechanism from microscopic scale to macroscopic scale was tentatively proposed to explain such an acidic/caustic alternating corrosion. 1 Introduction Ethylene is generally one of the most produced base chemicals in petrochemical industry, and its extensive applications predominantly lie in synthesizing lots of downstream organic compounds such as ethylene oxide (C2H4O), styrene (C8H8), polyethylene (PE), and so on. With respect to its manufacturing process, it is usually divided into two primary stages, respectively, nominated cracking and separation. The former one pyrolyzes hydrocarbon feedstocks like naphtha, liquefied petroleum gas (LPG), gas oil, etc., into smaller ones as C1–C4 and simultaneously introduces unsaturation [1], the latter one subsequently refines the pyrolysis gas through a series of separation systems to eliminate the byproducts like diesel oil, aromatics, propylene (C3H6), butadiene (C4H6), etc., aiming to obtain the purified ethylene. Actually, ethylene plants are commonly distinguished into various types according to the differences in configuration styles of cracking furnaces applied in the cracking stage. Among them, the tubular cracking furnace is the most widely used one, thanks to its superiorities like low energy consumption, short residence time, high efficiency, and so on. Further detailed, ABB Lummus Global, Stone & Webster, Kellogg Brown & Root (KBR), Linde, etc., are all the corporations that design and fabricate tubular cracking furnaces under their own technologies and patents, and their whole ethylene plants are accordingly named after their brands. In fact, compared with the tubular cracking furnaces which have always been paid special attention to by the above mentioned corporations, the complex systems and the equipments employed in the separation stage usually attract relatively less interests in research, nevertheless which also play a critical role in safe service of a whole ethylene plant. As one mature process with nearly 60-year application, the Lummus ethylene plant holds the largest share of ethylene plants market in China currently [2]. Particularly in its separation stage, the process water stripper system is actually one of the most important systems, whose detailed operation procedures are displayed in Fig. 1. As is shown in this schematic diagram, the process water stripper system is the Materials and Corrosion 2011, 62, No. 10 DOI: 10.1002/maco.201005741 967 Y. Gong, C. Yang, C. Yao, Z.-G. Yang Department of Materials Science, Fudan University, No. 220 Handan Road, Shanghai 200433 (P. R. China) E-mail: zgyang@fudan.edu.cn In this forum readers will be able to present practical problems for discussion. It is envisaged that these contributions will include not only discussion of general problems and incidents of corrosion but that suggested remedies will also be presented and discussed. It is hoped that this exchange of knowledge and experience will become a permanent feature of this periodical. We are particularly anxious that both Senior Scientists and those with more practical experience will make use of this forum to exchange information, problems and potential remedies. Corrosion Concepts www.matcorr.com 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
968 Gong, Yang, Yao and Yang Materials and Corrosion 2011.62. No. Hrc light oil T-150/beam& gasoline Dl601 T-1401 E1605A-H 严=一址如城 Figure 1 Schematic diagram of operation procedures of the process water stripper system in Lummus ethylene plant beginning one in separation stage right after cracking stage, out. Concretely, photoelectric direct reading spectrometer and aiming to separate the oil phase, and the steam phase in pyrolysis metallurgical microscope(MM)were used to inspect the gas, and meanwhile utilizes the waste heat from the pyrolysis gas chemical compositions and the metallographic structures of as well by means of heat exchange. Concretely, the pyrolysis gas is the pipes' matrix materials; ion chromatography(IC),mass firstly transported into the steam/oil fractionator T-1401, in which spectrometry(MS), inductively coupled plasma-atomic emission the diesel oil is collected for recycling, and the remanent quench conducted to examine the chemical compositions of the proces oil (Qo) is prepared for heat exchange. After successive cooling water; three-dimensional (3D) stereomicroscope, scanning and separation procedures, oil phase in the light-weight electron microscope (SEM), and energy disperse spectroscope constituents is distilled and recycled for further refinement to (EDS) were employed to analyze the macro and micromorphol- acquire purified ethylene, while the steam phase (virtually in ogies along with microarea chemical compositions of the defects liquid state after cooling, hence also called the process water)is on the failure pipes. Based on the analysis results, causes of the stripped to eliminate acidic impurities and then sent into several failure were confirmed, meanwhile a novel term defined as successive steam equipments to produce diluted steam(DS )with acidic/caustic alternating corrosion was proposed and wa lowtemperature. In fact, the process water inevitably still explained in a four-level mechanism from microscopic scale to contains some residual acidic substances like SO2, H2S, and other macroscopic scale. Achievements of this paper have critical organic compounds after stripping, which may lead to degrada- significances not only in a better grasp of features of several tions on materials used in the following equipments. Under this individual corrosions simultaneously occurring together under situation,alkali liquor(20% NaOH solution) is adopted to be complex service conditions in engineering practice, but also in added into the process water subsequent to stripping for corrosion prevention of the pipes with same matrix materials neutralization, and four inspection sites(accurate locations are under similar service conditions marked with red alphabets also in Fig. 1)are especially set up to real-time monitor the pH values for avoiding abnormality. As fo the QO, after a series of recycling and refining procedures, it is 2 Experimental methods and results conveyed into the QO/DS heat exchanger E-1605 with"high temperature to heat the DS carried from DS generator D-1601. 2.1 Visual observation Finally, the cooled Qo is collected in recycling system, while the heated DS is sent back to the dS generator, some is distributed to The Qo/ds heat exchanger is a horizontal ient for supplying heat, and the other is still for heat exchange dimension of about c2400 x 9000 mm(Fig. 2a) which circulation the DS that is produced in the dS generator is transported into In this event, some localized defects including concaves and through the tee pipe on the middle part of its bottom, and then perforations were periodically discovered on the pipes in the Qo/ sent out through the two tee pipes, respectively, on the two ends of DS heat exchanger of the process water stripper system of one its roof after heat exchange, see Fig. 2b. The Qo is fed into the Lummus ethylene plant in a petrochemical works in Shanghai. heat exchanger through its bottom-left tee pipe, and delivered out There were totally eight sets of heat exchangers in this system, through the top-left one after heating the DS, see the stream respectively, called E-1605 A-H, among which failure predomi- direction in Fig. 2b. Figure 2c displays the side view of the heat nantly occurred within the set of B. The expected life of the pipes exchanger(after detaching the fixing plate), in which there exist in heat exchangers was about 3 years, but the latest failure was 5925 pipes(carbon steel, p19 x 9000 x 2 mm)in four arrays with engendered no more than just 9 months(from March 2009 to total heat exchange area of 3005 m. As for the concrete heat December 2009). In order to determine the causes, investigations exchange mode, the dS outside the pipes(called the shell side) is into four aspects including matrix materials, process media, heated by the Qo inside the pipes(called the tube side), seen in service conditions, and maintenance management were carried Fig. 2d. The working parameters including temperatures and o 2011 WILEY-VCH Verlag Gmbh Co KGaA, Weinheim www.matcorr.com
beginning one in separation stage right after cracking stage, aiming to separate the oil phase, and the steam phase in pyrolysis gas, and meanwhile utilizes the waste heat from the pyrolysis gas as well by means of heat exchange. Concretely, the pyrolysis gas is firstly transported into the steam/oil fractionator T-1401, in which the light-weight constituents as steam and light oil are vaporized, the diesel oil is collected for recycling, and the remanent quench oil (QO) is prepared for heat exchange. After successive cooling and separation procedures, oil phase in the light-weight constituents is distilled and recycled for further refinement to acquire purified ethylene, while the steam phase (virtually in liquid state after cooling, hence also called the process water) is stripped to eliminate acidic impurities and then sent into several successive steam equipments to produce diluted steam (DS) with ‘‘low’’ temperature. In fact, the process water inevitably still contains some residual acidic substances like SO2, H2S, and other organic compounds after stripping, which may lead to degradations on materials used in the following equipments. Under this situation, alkali liquor (20% NaOH solution) is adopted to be added into the process water subsequent to stripping for neutralization, and four inspection sites (accurate locations are marked with red alphabets also in Fig. 1) are especially set up to real-time monitor the pH values for avoiding abnormality. As for the QO, after a series of recycling and refining procedures, it is conveyed into the QO/DS heat exchanger E-1605 with ‘‘high’’ temperature to heat the DS carried from DS generator D-1601. Finally, the cooled QO is collected in recycling system, while the heated DS is sent back to the DS generator, some is distributed to client for supplying heat, and the other is still for heat exchange circulation. In this event, some localized defects including concaves and perforations were periodically discovered on the pipes in the QO/ DS heat exchanger of the process water stripper system of one Lummus ethylene plant in a petrochemical works in Shanghai. There were totally eight sets of heat exchangers in this system, respectively, called E-1605 A–H, among which failure predominantly occurred within the set of B. The expected life of the pipes in heat exchangers was about 3 years, but the latest failure was engendered no more than just 9 months (from March 2009 to December 2009). In order to determine the causes, investigations into four aspects including matrix materials, process media, service conditions, and maintenance management were carried out. Concretely, photoelectric direct reading spectrometer and metallurgical microscope (MM) were used to inspect the chemical compositions and the metallographic structures of the pipes’ matrix materials; ion chromatography (IC), mass spectrometry (MS), inductively coupled plasma-atomic emission spectrometry (ICP–AES), and pH values inspection were conducted to examine the chemical compositions of the process water; three-dimensional (3D) stereomicroscope, scanning electron microscope (SEM), and energy disperse spectroscope (EDS) were employed to analyze the macro and micromorphologies along with microarea chemical compositions of the defects on the failure pipes. Based on the analysis results, causes of the failure were confirmed, meanwhile a novel term defined as acidic/caustic alternating corrosion was proposed and was explained in a four-level mechanism from microscopic scale to macroscopic scale. Achievements of this paper have critical significances not only in a better grasp of features of several individual corrosions simultaneously occurring together under complex service conditions in engineering practice, but also in corrosion prevention of the pipes with same matrix materials under similar service conditions. 2 Experimental methods and results 2.1 Visual observation The QO/DS heat exchanger is a horizontal cylinder with dimension of about w2400 9000 mm2 (Fig. 2a), within which the DS that is produced in the DS generator is transported into through the tee pipe on the middle part of its bottom, and then sent out through the two tee pipes, respectively, on the two ends of its roof after heat exchange, see Fig. 2b. The QO is fed into the heat exchanger through its bottom-left tee pipe, and delivered out through the top-left one after heating the DS, see the stream direction in Fig. 2b. Figure 2c displays the side view of the heat exchanger (after detaching the fixing plate), in which there exist 5925 pipes (carbon steel, w19 9000 2 mm3 ) in four arrays with total heat exchange area of 3005 m2 . As for the concrete heat exchange mode, the DS outside the pipes (called the shell side) is heated by the QO inside the pipes (called the tube side), seen in Fig. 2d. The working parameters including temperatures and 968 Gong, Yang, Yao and Yang Materials and Corrosion 2011, 62, No. 10 Figure 1. Schematic diagram of operation procedures of the process water stripper system in Lummus ethylene plant 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.matcorr.com
Materials and Corrosion 2011, 62, No. 10 Acidic/caustic alternating corrosion on carbon steel pipes 969 1708° quench oil (Qo) diluted steam(DS) 195C,0725MPa 1702°C,0.706MPa QO Figure 2. External appearances of the Qo/dS heat exchanger:(a)total appearance of eight sets, (b)streamline diagram of two heat media, (c)four arrays of pipes, and(d) heat exchange mode pressures of these two heat media before and after heat exchanges more, the failure pipes, whose inlets had already been blocked are also listed in Fig. 2b with plugs after being detected perforation to avoid further In this event, failure mainly occurred in the heat exchanger leakage of QO, dominantly accumulated in the lower two arrays B. Figure 3a presents the scaling morphology of leaked Qo(Fig. 3c and d), while the upper two arrays of pipes were nearly companied with yellow sulfur sublimating from it on the pipes' free of failure, seen in Fig. 3b. With regard to the concrete failure surfaces caused by perforation of some specific pipes. What is pipes, Fig. 4a and b, respectively, displays the macroscopic Figure 3. External appearances of the failure Qo/DS heat exchanger B: (a)scaling of leaked Qo, (b)upper two arrays of pipes, (c)lower two arrays of pipes, and(d)locations of failure pipes www.matcorr.com o 2011 WILEY-VCH Verlag GmbH& Co KGaA, Weinheim
pressures of these two heat media before and after heat exchanges are also listed in Fig. 2b. In this event, failure mainly occurred in the heat exchanger B. Figure 3a presents the scaling morphology of leaked QO accompanied with yellow sulfur sublimating from it on the pipes’ surfaces caused by perforation of some specific pipes. What is more, the failure pipes, whose inlets had already been blocked with plugs after being detected perforation to avoid further leakage of QO, dominantly accumulated in the lower two arrays (Fig. 3c and d), while the upper two arrays of pipes were nearly free of failure, seen in Fig. 3b. With regard to the concrete failure pipes, Fig. 4a and b, respectively, displays the macroscopic Materials and Corrosion 2011, 62, No. 10 Acidic/caustic alternating corrosion on carbon steel pipes 969 Figure 2. External appearances of the QO/DS heat exchanger: (a) total appearance of eight sets, (b) streamline diagram of two heat media, (c) four arrays of pipes, and (d) heat exchange mode Figure 3. External appearances of the failure QO/DS heat exchanger B: (a) scaling of leaked QO, (b) upper two arrays of pipes, (c) lower two arrays of pipes, and (d) locations of failure pipes www.matcorr.com 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
970 Gong, Yang, Yao and Yang Materials and Corrosion 2011. 62. No a) 89 234 b) b) Figure 4. Macroscopic morphologies of defects on failure pipes: (a) three neighboring concaves, and(b) peanut-like concave morphologies of (1)three neighboring concaves, as well as(2 )one peanut-like concave, on two failure pipes, and the diameters of these concaves were all not exceeding 10 mm. Subsequent investigations would be particularly carried out on the pip bearing the special peanut-like concave, whose location was 5.8m away from the floating head, ie, the rightmost part of the heat Figure 5. Metal structures of the failure pipe: (a)100x;(b)500x 2.2 Matrix material examination speaking, the matrix material of the failure pipe could be regarded qualified Chemical compositions of the matrix material of the failure pipe with peanut-like concave are listed in Table 1, which were in 2.3 3-D stereomicroscopy accordance with the requirements of 10 carbon steel specifica- tions in GB/T 699-1999 standard of China[3](corresponding to By using the Hirox KH-7700 3D digital microscope, morphologies ASTM 1010 steel (4). Etched in agent of HNO3 2 mL and ethanol of the peanut-like concave were further investigated. Figure 6a 98 mL, the metallographic structures of the matrix material are presents the total morphology of this concave with dimension of presented in Fig. 5. It is obvious in Fig 5a that the microstructure nearly 3 x 6mm. It can be easily inferred from Fig. 6b, which consisted of ferrites and pearlites, and the average ASTM grain shows the ridge in the middle part of the"peanut, "that this size of the ferrites was about seven. Furthermore, inclusions in special-shape concave may have been resulted from connection of the ferritic grains were fairly uniform, nevertheless the pearlites two neighboring round concaves. Further magnified, more ad already dissolved to some extent and consequently led to detailed morphologies of the left and the right parts of the increase of cementites content, seen in Fig 5b, which may act as "peanut"were, respectively, revealed. As is shown in Fig. 6c, the susceptible initiating sites of corrosion. However totally densely distributed pits with diameters not exceeding 0. 1mm Table 1. Chemical compositions of the failure pipe(wt%) Elemen P S Failure pipe 0.1 0018 007-0.13 0.17-0.37 <0.030 008-0. 0.050 denotes the content is undefined o 2011 WILEY-VCH Verlag Gmbh Co KGaA, Weinheim www.matcorr.com
morphologies of (1) three neighboring concaves, as well as (2) one peanut-like concave, on two failure pipes, and the diameters of these concaves were all not exceeding 10 mm. Subsequent investigations would be particularly carried out on the pipe bearing the special peanut-like concave, whose location was 5.8 m away from the floating head, i.e., the rightmost part of the heat exchanger in Fig. 2b. 2.2 Matrix material examination Chemical compositions of the matrix material of the failure pipe with peanut-like concave are listed in Table 1, which were in accordance with the requirements of 10 carbon steel specifications in GB/T 699-1999 standard of China [3] (corresponding to ASTM 1010 steel [4]). Etched in agent of HNO3 2 mL and ethanol 98 mL, the metallographic structures of the matrix material are presented in Fig. 5. It is obvious in Fig. 5a that the microstructure consisted of ferrites and pearlites, and the average ASTM grain size of the ferrites was about seven. Furthermore, inclusions in the ferritic grains were fairly uniform, nevertheless the pearlites had already dissolved to some extent and consequently led to increase of cementites content, seen in Fig. 5b, which may act as the susceptible initiating sites of corrosion. However totally speaking, the matrix material of the failure pipe could be regarded qualified. 2.3 3-D stereomicroscopy By using the Hirox KH-7700 3D digital microscope, morphologies of the peanut-like concave were further investigated. Figure 6a presents the total morphology of this concave with dimension of nearly 3 6 mm2 . It can be easily inferred from Fig. 6b, which shows the ridge in the middle part of the ‘‘peanut,’’ that this special-shape concave may have been resulted from connection of two neighboring round concaves. Further magnified, more detailed morphologies of the left and the right parts of the ‘‘peanut’’ were, respectively, revealed. As is shown in Fig. 6c, densely distributed pits with diameters not exceeding 0.1 mm 970 Gong, Yang, Yao and Yang Materials and Corrosion 2011, 62, No. 10 Figure 4. Macroscopic morphologies of defects on failure pipes: (a) three neighboring concaves, and (b) peanut-like concave Table 1. Chemical compositions of the failure pipe (wt%) Element C Si Mn P S Cr Ni Cu Failure pipe 0.109 0.258 0.436 0.018 0.005 0.027 0.007 0.016 10 0.07–0.13 0.17–0.37 0.35–0.65 0.030 0.020 0.15 0.30 0.25 ASTM 1010 0.08–0.13 – 0.30–0.60 0.040 0.050 ‘‘–’’ denotes the content is undefined. Figure 5. Metallographic structures of the failure pipe: (a) 100; (b) 500 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.matcorr.com
Materials and Corrosion 2011, 62, No. 10 Acidic/caustic alternating corrosion on carbon steel pipes 971 Figure 6. Morphologies and 3d profiles of the peanut-like concave: (a)total morphology, ( b)the ridge, (c) the left part, ( d)3D profile of the left part, (e) the right part, and( 3d profile of the right part c) Figure 7. Morphologies and 3D profiles of another two concaves: (a)another concave, b)3D profile, (c)the heart-like concave, and ( d)3D profile o 2011 WILEY-VCH Verlag GmbH& Co KGaA, Weinheim
Materials and Corrosion 2011, 62, No. 10 Acidic/caustic alternating corrosion on carbon steel pipes 971 Figure 6. Morphologies and 3D profiles of the peanut-like concave: (a) total morphology, (b) the ridge, (c) the left part, (d) 3D profile of the left part, (e) the right part, and (f) 3D profile of the right part Figure 7. Morphologies and 3D profiles of another two concaves: (a) another concave, (b) 3D profile, (c) the heart-like concave, and (d) 3D profile www.matcorr.com 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
972 Gong, Yang, Yao and Yang Materials and Corrosion 2011.62 Ne existed in the left part, and their average depth was about 0.04 mm peculiar shape of heart was also detected, whose diameter was with largest value of 0.06 mm for some specific one, seen from the only 0. 2 mm. It should be particularly pointed out that a localized 3D profile image in Fig. 6d. As for the right part, pits with pit with diameter of only 0.06 mm and depth of just 0.02mm diameters around 0. 1 mm were also detected( Fig. 6e), while their existed in the midst of the"heart'as well(Fig. 7d). It could be average and largest depths were, respectively, 0.02 and 0.04 mm, inferred that the two concaves were actually the morphologies in seen in Fig. 6f. To sum up the morphologies of the peanut-like two different growth stages of an eventually large and deep concave, from Fig 6a-f, it can be inferred that far smaller pits with concave just like the"peanut: the heart-like concave in Fig. 7c similar diameters(about 0. 1 mm) and depths(about 0.05 mm) was in the initial stage, while the concave in Fig. 7a was in the were prone to be connected with each other to engender larger and mid-term stage deeper concaves, while connection of two neighboring concaves was the cause of formation of the"peanut. 2.4 SEM and EDS analysis Besides the most distinct defect, i. e, the peanut-like concave another two minor defects were also observed on the failure pipe In order to thoroughly study the formation causes of the peanut surface. Figure 7a and b present the morphologies of a concave like concave, SEM was employed to observe its microscopic with diameter of approximately 0.8 mm and depth of 0.04 mm. morphologies. Figure 8a displays the total morphology. After Meanwhile, as is shown in Fig. 7c, an even smaller concave with a magnification, randomly and densely distributed white spots 9) 8. SEM of the peanut-like concave bottom surface: (a)total morphology, (b)densely distributed spots, (c)magnification of spots, (d)three oring"coins"with cavities, (e)three neighboring"coins"with cracks, ()coin"with both a cavity and a crack, (g) coral structure, and (h) of cracks o 2011 WILEY-VCH Verlag Gmbh Co KGaA, Weinheim www.matcorr.com
existed in the left part, and their average depth was about 0.04 mm with largest value of 0.06 mm for some specific one, seen from the 3D profile image in Fig. 6d. As for the right part, pits with diameters around 0.1 mm were also detected (Fig. 6e), while their average and largest depths were, respectively, 0.02 and 0.04 mm, seen in Fig. 6f. To sum up the morphologies of the peanut-like concave, from Fig. 6a–f, it can be inferred that far smaller pits with similar diameters (about 0.1 mm) and depths (about 0.05 mm) were prone to be connected with each other to engender larger and deeper concaves, while connection of two neighboring concaves was the cause of formation of the ‘‘peanut.’’ Besides the most distinct defect, i.e., the peanut-like concave, another two minor defects were also observed on the failure pipe surface. Figure 7a and b present the morphologies of a concave with diameter of approximately 0.8 mm and depth of 0.04 mm. Meanwhile, as is shown in Fig. 7c, an even smaller concave with a peculiar shape of heart was also detected, whose diameter was only 0.2 mm. It should be particularly pointed out that a localized pit with diameter of only 0.06 mm and depth of just 0.02 mm existed in the midst of the ‘‘heart’’ as well (Fig. 7d). It could be inferred that the two concaves were actually the morphologies in two different growth stages of an eventually large and deep concave just like the ‘‘peanut’’: the heart-like concave in Fig. 7c was in the initial stage, while the concave in Fig. 7a was in the mid-term stage. 2.4 SEM and EDS analysis In order to thoroughly study the formation causes of the peanutlike concave, SEM was employed to observe its microscopic morphologies. Figure 8a displays the total morphology. After magnification, randomly and densely distributed white spots 972 Gong, Yang, Yao and Yang Materials and Corrosion 2011, 62, No. 10 Figure 8. SEM of the peanut-like concave bottom surface: (a) total morphology, (b) densely distributed spots, (c) magnification of spots, (d) three neighboring ‘‘coins’’ with cavities, (e) three neighboring ‘‘coins’’ with cracks, (f) ‘‘coin’’ with both a cavity and a crack, (g) coral structure, and (h) group of cracks 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.matcorr.com
Materials and Corrosion 2011, 62, No. 10 Acidic/caustic alternating corrosion on carbon steel pipes 973 were obviously found on the concave bottom surface, seen in Fig. 8b. Further magnified, these spots uniformly exhibited a coin-like shape with average diameters of around 10 um, shown in Fig. 8c. Figure 8d and e both present the morphologies of three neighboring"coins"with different arrangements, in the former ne(Fig. 8d )each" coinpossessed a cavity with even smaller diameter of 2 um in its center, while in the latter one(Fig. 8e)each "coint was cleaved across its whole length by a main microcrack with branches. Figure 8f displays the representative morphology of the"coinhere, which had both a cavity in its center and a main microcrack across it. Moreover, a common ground was detected within all the"coins" that they were filled with a coral structure whose whisker width had already reached in near nanoscale, approximately 100 nm. Such a strange structure deserved more detailed analysis to understand its formation mechanisms. Meanwhile, a group of tiny cracks were observed in the corrosion products on the concave bottom surface as well(Fig. 8h), which may be abrased block by block during corrosion procedure Then the sample was cut from the peanut-like concave to observe its cross-section under SEM. Figure 9a is the total morphology. After magnification, pits with several feature morphologies of pitting corrosion including elliptical, wide, and optical undercutting shallow, subsurface and undercutting were found, seen in Fig. 9b wide& shallow Figure 9c was the further magnified morphology of the subsurface type. It should be mentioned that widths of these pits were nearly the same of those pits shown in Fig. 6d-f. In other words, pits detected in both Fig. 6 and Fig. 9 were virtually the merely viewed from different directions. Consequently, it could be inferred now that pitting corrosion may be partly ascribed to 2s 00 vl erts wmm 1ooxk formation of the peanut-like concave on the failure pipe. For purpose of investigating the chemical compositions of the marked sites(A-F) within the microscopic defects in Fig. 8 and 9, EDS was utilized. As is shown in Fig. 10a-e and Table 2, chemical compositions of the coral structures in all the "coins were almost the same: approximately equaled to the chemical formula of NaFe20SgO4o, calculated from the mass ratios between these elements. This complex compound with multiple elements may be actually the mixture of several simple compounds like Na3 FeO3, Na2 FeO2, FeSO4, and iron oxides, i.e., formation of the coins"was possibly relevant to Na and s elements Considering the engineering media in the system, NaOH as well as the residual SO2 and H2S in the process water may be the sources of these two elements. Moreover, it is a little surprising b o w Erols wm that the chlorine element was detected within the pit(Fig. 10f and Table 2)as well, which further confirmed that pits in the peanut- Figure 9. SEM of the cross-sections of the peanut-like concave: (a)total like concave could have been resulted from pitting corrosion. morphology, (b) pits with different morphologies, and (c) subsurface 2.5 Process water inspection corrosion. Figure 11 displays the changing situation of process water pH values from the four inspection sites(co 2.5.1 pH value locations could be referred from Fig. 1)in the year 2009, during According to the designed parameters from Lummus, which inspection was ceased for over half a month in March to appropriate PH values of the process water in stripper system install new pipes for substituting the failure ones within the Qo/ should be confined in an alkalescent range of 7.5-9.5, above Ds heat exchanger, while the interval lasting for nearly 2 months which the process water would be emulsified by the organic from May to July was the routine downtime. Before entering the compounds in it, and/ or cause the equipments with matrix stripper T-1601, the process water had the lowest pH values materials of steels be transpassivated and consequently corroded, among the four(site A, the top-left in Fig. 11)due to the high i.e., the caustic corrosion; below which the equipments would content of acidic substances like SO2, H2S. After stripping and also be corroded due to the low ph values, i.e., the acidic adding alkali liquor, pH values of the process water before the ds www.matcorr.com o 2011 WILEY-VCH Verlag GmbH& Co KGaA, Weinheim
were obviously found on the concave bottom surface, seen in Fig. 8b. Further magnified, these spots uniformly exhibited a coin-like shape with average diameters of around 10 mm, shown in Fig. 8c. Figure 8d and e both present the morphologies of three neighboring ‘‘coins’’ with different arrangements, in the former one (Fig. 8d) each ‘‘coin’’ possessed a cavity with even smaller diameter of 2mm in its center, while in the latter one (Fig. 8e) each ‘‘coin’’ was cleaved across its whole length by a main microcrack with branches. Figure 8f displays the representative morphology of the ‘‘coin’’ here, which had both a cavity in its center and a main microcrack across it. Moreover, a common ground was detected within all the ‘‘coins’’ that they were filled with a coral structure, whose whisker width had already reached in near nanoscale, approximately 100 nm. Such a strange structure deserved more detailed analysis to understand its formation mechanisms. Meanwhile, a group of tiny cracks were observed in the corrosion products on the concave bottom surface as well (Fig. 8h), which may be abrased block by block during corrosion procedure. Then the sample was cut from the peanut-like concave to observe its cross-section under SEM. Figure 9a is the total morphology. After magnification, pits with several feature morphologies of pitting corrosion including elliptical, wide, and shallow, subsurface and undercutting were found, seen in Fig. 9b. Figure 9c was the further magnified morphology of the subsurface type. It should be mentioned that widths of these pits were nearly the same of those pits shown in Fig. 6d–f. In other words, pits detected in both Fig. 6 and Fig. 9 were virtually the same ones, merely viewed from different directions. Consequently, it could be inferred now that pitting corrosion may be partly ascribed to formation of the peanut-like concave on the failure pipe. For purpose of investigating the chemical compositions of the marked sites (A–F) within the microscopic defects in Fig. 8 and 9, EDS was utilized. As is shown in Fig. 10a–e and Table 2, chemical compositions of the coral structures in all the ‘‘coins’’ were almost the same: approximately equaled to the chemical formula of NaFe20S8O40, calculated from the mass ratios between these elements. This complex compound with multiple elements may be actually the mixture of several simple compounds like Na3FeO3, Na2FeO2, FeSO4, and iron oxides, i.e., formation of the ‘‘coins’’ was possibly relevant to Na and S elements. Considering the engineering media in the system, NaOH as well as the residual SO2 and H2S in the process water may be the sources of these two elements. Moreover, it is a little surprising that the chlorine element was detected within the pit (Fig. 10f and Table 2) as well, which further confirmed that pits in the peanutlike concave could have been resulted from pitting corrosion. 2.5 Process water inspection 2.5.1 pH value According to the designed parameters from Lummus, appropriate pH values of the process water in stripper system should be confined in an alkalescent range of 7.5–9.5, above which the process water would be emulsified by the organic compounds in it, and/or cause the equipments with matrix materials of steels be transpassivated and consequently corroded, i.e., the caustic corrosion; below which the equipments would also be corroded due to the low pH values, i.e., the acidic corrosion. Figure 11 displays the changing situation of the process water pH values from the four inspection sites (concrete locations could be referred from Fig. 1) in the year 2009, during which inspection was ceased for over half a month in March to install new pipes for substituting the failure ones within the QO/ DS heat exchanger, while the interval lasting for nearly 2 months from May to July was the routine downtime. Before entering the stripper T-1601, the process water had the lowest pH values among the four (site A, the top-left in Fig. 11) due to the high content of acidic substances like SO2, H2S. After stripping and adding alkali liquor, pH values of the process water before the DS Materials and Corrosion 2011, 62, No. 10 Acidic/caustic alternating corrosion on carbon steel pipes 973 Figure 9. SEM of the cross-sections of the peanut-like concave: (a) total morphology, (b) pits with different morphologies, and (c) subsurface www.matcorr.com 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
974 Gong, Yang, Yao and Yang Materials and Corrosion 2011.62. No. Figure 10. EDS results within the peanut-like concave: (a)site A, (b)site B, (c)site C,(d)site D, (e) site E, and(D site F Table 2. Chemical compositions within the peanut-like concave(wt%) O eeee r'' denotes the content lower than 0.5 wt%o o 2011 WILEY-VCH Verlag Gmbh Co KGaA, Weinheim www.matcorr.com
974 Gong, Yang, Yao and Yang Materials and Corrosion 2011, 62, No. 10 Figure 10. EDS results within the peanut-like concave: (a) site A, (b) site B, (c) site C, (d) site D, (e) site E, and (f) site F Table 2. Chemical compositions within the peanut-like concave (wt%) Element O Na S Fe Cl Site A 36.12 2.03 4.90 56.95 / Site B 31.01 0.87 7.43 60.69 / Site C 36.50 1.40 4.83 57.27 / Site D 33.82 1.15 5.51 59.53 / Site E 29.71 0.56 2.38 67.35 / Site F 11.05 5.21 0.57 60.73 1.49 ‘‘/’’ denotes the content lower than 0.5 wt%. 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.matcorr.com
Materials and Corrosion 2011, 62, No. 10 Acidic/caustic alternating corrosion on carbon steel pipes 975 wPP个一7解产 Figure 11. Changing situation of the ter pH values from four inspection sites in the year 2009 Table 3. Statistical data of the pH values from the four inspection sites H value Lowest Highest 9.5(times) Site A 1041 12 Site B 5.26 11.50 Site C 11.55 Site D 11 generator(site B, the top-right in Fig. 11), and after the QO/Ds concentrations were, respectively, 3.89, 0.71, 0.33, and heat exchanger(site C, the bottom-left in Fig. 11) had already (mg/L). Although their contents were all not pretty high, but converged within the expected alkalescent range. However, ph corrosion would still be engendered when these ions accumu values of these two sites also exceeded over 9.5 for many times, some specific pre-existing defects on the pipes surfaces even reached as high as around 11.5, seen in Table 3. But in fact, the most distinct phenomenon in Fig. 11 is that all the three 2.5.3 /CP-AES particularly in some specific points the pH value jumped from determine the cations in the proce ofIC, ICP-AES was used to ater. The results showed 5 to 9, or dropped from 9 to 6 in just 1 day. Such a violent and that the predominant cation was sodium ion Nat*,whose frequent acidic/caustic alternating environment would exert concentration reached 28.54 ppm. This result verified that a more serious corrosion effect on the equipments here, especially relatively high content of naoH was contained in the process the QO/DS heat exchanger, than either the acidic or the caustic water, which may be the major factor of the failure. However, one did solely alue of the iron ions Fe"f(including Fe and Fe't) was just 0.16 ppm, which manifested that corrosion in this event only took 2.5.2 lon chromatograph place in some specific small areas. In other words, it was the In order to testify the EDS results that formation of the pits in the localized corrosion rather than the uniform corrosion occurred peanut-like concave may have been caused by Cl elements, IC was on the pipes in the Qo/DS heat exchanger pplied. It can be learned from Fig. 12 that the major anions in the process water were F, CI, PO4, and soz-, and thei 3 Discussion Based on the above analysis results, chemical compositions, and metallographic structures of the failure pipe were both qualified what is more, no inappropriate operations were investigated during service of the Qo/DS heat exchanger according to the designed requirements as well. Thus, it can be now concluded that among the four possible factors of the failure including matrix materials, process media, service conditions, and maintenance 120140 management, the first and the third one could be ruled out. On the other hand, it is quite clear that the frequent sharp fluctuations of Figure 12. lon chromatograph results of the process wate the process water pH values, the concentrated content of NaoH www.matcorr.com o 2011 WILEY-VCH Verlag GmbH& Co KGaA, Weinheim
generator (site B, the top-right in Fig. 11), and after the QO/DS heat exchanger (site C, the bottom-left in Fig. 11) had already converged within the expected alkalescent range. However, pH values of these two sites also exceeded over 9.5 for many times, even reached as high as around 11.5, seen in Table 3. But in fact, the most distinct phenomenon in Fig. 11 is that all the three curves behaved sharp fluctuations during the whole year, particularly in some specific points the pH value jumped from 5 to 9, or dropped from 9 to 6 in just 1 day. Such a violent and frequent acidic/caustic alternating environment would exert more serious corrosion effect on the equipments here, especially the QO/DS heat exchanger, than either the acidic or the caustic one did solely. 2.5.2 Ion chromatograph In order to testify the EDS results that formation of the pits in the peanut-like concave may have been caused by Cl elements, IC was applied. It can be learned from Fig. 12 that the major anions in the process water were F, Cl, PO3 4 , and SO2 4 , and their concentrations were, respectively, 3.89, 0.71, 0.33, and 0.28 ppm (mg/L). Although their contents were all not pretty high, but localized corrosion would still be engendered when these ions accumulated in some specific pre-existing defects on the pipes surfaces. 2.5.3 ICP–AES After examining the anions by means of IC, ICP–AES was used to determine the cations in the process water. The results showed that the predominant cation was sodium ion NaR, whose concentration reached 28.54 ppm. This result verified that a relatively high content of NaOH was contained in the process water, which may be the major factor of the failure. However, value of the iron ions FenR (including Fe2R and Fe3R) was just 0.16 ppm, which manifested that corrosion in this event only took place in some specific small areas. In other words, it was the localized corrosion rather than the uniform corrosion occurred on the pipes in the QO/DS heat exchanger. 3 Discussion Based on the above analysis results, chemical compositions, and metallographic structures of the failure pipe were both qualified, what is more, no inappropriate operations were investigated during service of the QO/DS heat exchanger according to the designed requirements as well. Thus, it can be now concluded that among the four possible factors of the failure including matrix materials, process media, service conditions, and maintenance management, the first and the third one could be ruled out. On the other hand, it is quite clear that the frequent sharp fluctuations of the process water pH values, the concentrated content of NaOH Materials and Corrosion 2011, 62, No. 10 Acidic/caustic alternating corrosion on carbon steel pipes 975 Figure 11. Changing situation of the process water pH values from four inspection sites in the year 2009 Table 3. Statistical data of the pH values from the four inspection sites pH value Lowest Highest 9.5 (times) Site A 5.07 10.41 103 12 Site B 5.26 11.50 6 247 Site C 6.68 11.55 5 141 Site D 7.21 11.94 1 7 Figure 12. Ion chromatograph results of the process water www.matcorr.com 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
976 Gong, Yang, Yao and Yang Materials and Corrosion 2011.62. No. solution as well as the presence of chloride ions and sulfur element it can be inferred that pitting corrosion was favored when the ph the process water, were actually the true causes of the failure In values dropped to the acidic region below 7, especially the extreme other words, the process media, i.e., the process water should be value near 5. Under this condition, the aggressive chloride ions mainly blamed for. In fact, the unqualified process water was penetrated into and then accumulated within the pre- existin virtually resulted from improper maintenance management. Then, defects of the passive films on the pipes surfaces, such as discussion will be especially put forward on these two aspects. inclusions, imperfections, pores, etc, even the cementites on the With regard to the process water, the most distinct ferritic grain boundaries resulted from pearlites dissolution. characteristic was the sharp fluctuations of its PH values lasting Consequently, cavities in size of 1-2 um, as shown in Fig. d-f, were for the whole year of 2009. As was discussed above that an engendered. This is exactly the initial stage of the acidic/ caustic alkalescent range of 7.5-9.5 of the process water pH values could alternating corrosion, i.e., the first level of the four-level ensure the pipes in the Qo/DS heat exchanger free of acidic and mechanism, whose schematic diagram, causes of formation, caustic corrosion. However in this event, the frequent and violent and other features are all summarized in Table 4. With respect to fluctuations of the pH values beyond the range would induce a the detailed procedures of pitting corrosion caused by chloride kind of alternating corrosion effect between the acidic and the ions, our previous work [5] can be referenced, hence they would not caustic corrosion on the pipes, which we nominated the term of be repeatedly discussed. As for the sources of the chloride ions in acidic/caustic alternating corrosion. This effect commonly brings the process water, insufficient desalination could be ascribed to about corrosion fatigue on materials and is consequently severer However, it was indeed because of the low content of these chloride than the added result of the two independent ones. In order to ions, extent of the pitting corrosion in this event was actually not detailedly explain the concrete procedures of this acidic/caustic severe, and the affected zones only scattered in some specific alternating corrosion, a novel four-level mechanism from sites on the pipe surface. Besides this, the sulfur element was microscopic scale to macroscopic scale was put forward introduced by the residual SO2 and H2S, and was predominantly in In engineering practice, carbon steel pipes should usually form of sulfate radical Soa- in the process water, i.e., virtually the undergo a caustic treatment procedure before application to sulfuric acid H2SO4 under acidic environment. Consequently produce a protective passive film of Fe3 O4 on the surface, seen in the pitting corrosion effect was aggravated to some extent, and the the Equation(1). However, it is a common sense that steels bearing corrosion products were involved with sulfur element as well, seen assive films, mainly refer to the stainless steels, are prone to be from the EDS results in Fig. 10 and Table 2 attacked by pitting corrosion with the presence of halide ions. In this event, chloride element/ions were detected in the process Fe+2H-→Fe(OH)2+ rater through analyses of EDS and IC. Although it may be 4Fe(0H),+2H20+02-4Fe(OH challenged that the content of them was not high enough, failure Fe(OH),+2Fe(OH)2+Fe304+4H,0 case of pitting corrosion caused by localized accumulation of low oncentration chloride ions was actually investigated in our previous work [5, 6]. Figure 9 is the obvious evidence for the Actually, it could be easily learned from Table 3 that duration occurrence of pitting corrosion on the failure pipe in this event. of the process water pH values in the caustic region was far longer What is more, associated with the service conditions of the pipes, than that in the acidic region, in other words, the caustic i.e., the frequent sharp fluctuations of the process water pH values, corrosion was severer than the acidic (pitting) corrosion on the Table 4. Summary of the four-level mechanisms Schematic diagram Causes of formation erm Feature size Level Connection of pit Concave o mm Caustic embrittlement Connection of coins pH>7: anodic dissolution pH<7: hydrogen embrittlement pitting com Cavity o 2011 WILEY-VCH Verlag Gmbh Co KGaA, Weinheim www.matcorr.com
solution as well as the presence of chloride ions and sulfur element in the process water, were actually the true causes of the failure. In other words, the process media, i.e., the process water should be mainly blamed for. In fact, the unqualified process water was virtually resulted from improper maintenance management. Then, discussion will be especially put forward on these two aspects. With regard to the process water, the most distinct characteristic was the sharp fluctuations of its pH values lasting for the whole year of 2009. As was discussed above that an alkalescent range of 7.5–9.5 of the process water pH values could ensure the pipes in the QO/DS heat exchanger free of acidic and caustic corrosion. However in this event, the frequent and violent fluctuations of the pH values beyond the range would induce a kind of alternating corrosion effect between the acidic and the caustic corrosion on the pipes, which we nominated the term of acidic/caustic alternating corrosion. This effect commonly brings about corrosion fatigue on materials and is consequently severer than the added result of the two independent ones. In order to detailedly explain the concrete procedures of this acidic/caustic alternating corrosion, a novel four-level mechanism from microscopic scale to macroscopic scale was put forward. In engineering practice, carbon steel pipes should usually undergo a caustic treatment procedure before application to produce a protective passive film of Fe3O4 on the surface, seen in the Equation (1). However, it is a common sense that steels bearing passive films, mainly refer to the stainless steels, are prone to be attacked by pitting corrosion with the presence of halide ions. In this event, chloride element/ions were detected in the process water through analyses of EDS and IC. Although it may be challenged that the content of them was not high enough, failure case of pitting corrosion caused by localized accumulation of lowconcentration chloride ions was actually investigated in our previous work [5, 6]. Figure 9 is the obvious evidence for the occurrence of pitting corrosion on the failure pipe in this event. What is more, associated with the service conditions of the pipes, i.e., the frequent sharp fluctuations of the process water pH values, it can be inferred that pitting corrosion was favored when the pH values dropped to the acidic region below 7, especially the extreme value near 5. Under this condition, the aggressive chloride ions penetrated into and then accumulated within the pre-existing defects of the passive films on the pipes surfaces, such as inclusions, imperfections, pores, etc., even the cementites on the ferritic grain boundaries resulted from pearlites dissolution. Consequently, cavities in size of 1–2mm, as shown in Fig. d–f, were engendered. This is exactly the initial stage of the acidic/caustic alternating corrosion, i.e., the first level of the four-level mechanism, whose schematic diagram, causes of formation, and other features are all summarized in Table 4. With respect to the detailed procedures of pitting corrosion caused by chloride ions, our previous work [5] can be referenced, hence they would not be repeatedly discussed. As for the sources of the chloride ions in the process water, insufficient desalination could be ascribed to. However, it was indeed because of the low content of these chloride ions, extent of the pitting corrosion in this event was actually not severe, and the affected zones only scattered in some specific sites on the pipe surface. Besides this, the sulfur element was introduced by the residual SO2 and H2S, and was predominantly in form of sulfate radical SO2 4 in the process water, i.e., virtually the sulfuric acid H2SO4 under acidic environment. Consequently, the pitting corrosion effect was aggravated to some extent, and the corrosion products were involved with sulfur element as well, seen from the EDS results in Fig. 10 and Table 2. Fe þ 2OH ! FeðOHÞ2 þ 2e 4FeðOHÞ2 þ 2H2O þ O2 ! 4FeðOHÞ3 FeðOHÞ2 þ 2FeðOHÞ3 ! Fe3O4 þ 4H2O (1) Actually, it could be easily learned from Table 3 that duration of the process water pH values in the caustic region was far longer than that in the acidic region, in other words, the caustic corrosion was severer than the acidic (pitting) corrosion on the 976 Gong, Yang, Yao and Yang Materials and Corrosion 2011, 62, No. 10 Table 4. Summary of the four-level mechanisms Schematic diagram Causes of formation Term Feature size Level Scale Connection of pits Concave 3 mm 4 Macroscopic Connection of coins Caustic embrittlement Pit 0.1 mm 3 pH > 7: anodic dissolution Coin 10mm 2 Microscopic pH < 7: hydrogen embrittlement Acidic environment Cl: pitting corrosion Cavity 2mm 1 S: acidic corrosion 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.matcorr.com
