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, WeinheimAcidic/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 predomi￾nantly 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 by￾products 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