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