Materials and Corrosion 2009. 60. No. 1: Do:101002/mac0.200805198 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 ed 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 corrosion but that suggested remedies will also be presented and exchange information, problems and potential remedies. discussed. It is hoped that this exchange of knowledge and Pitting corrosion on 316L pipes in terephthalic acid(TA)dryer Y. Gong, J. Cao, X.-H. Meng and Z -G. Yang Grade 316L is a type of austenitic stainless steel with ultra-low carbon content and it exhibits superior corrosion resistance. However, pitting is always observed in 316L steel when it is exposed to media containing halide ions In the present study, we found that in the presence of acetate acid( HAc)containing chloride or bromide ions, pitting occurred on the surface of the rotary steam pipes with the matrix material of 316L steel terephthalic acid(TA)dryer. In order to identify the causes of the failure metallographic structures and chemical compositions of the matrix material were inspected by an optical microscope(OM)and a photoelectric direct reading spectrometer. Beside these, scanning electron microscopy(SEM) and energy dispersive spectroscopy(EDS)as well as ion chromatography(IC) were used to analyze the micromorphologies of the corrosion pits and the chemical compositions of the corrosion deposits within them. Analysis of the results revealed the sources of halide ions and the factors accelerating the corrosion rate. Beside these, detailed mechanisms of pitting were discussed and six out of all the seven theoretical morphologies of pitting features were obtained in practice 1 Introduction etc, it is still the most widely applied method currently available for Purified terephthalic acid(PTA) has wide applications in a variety products produced by the Witten process is not ide urity of the manufacturing terephthalic acid (TA). However, the of fields in our daily life, such as chemical fiber industry, light lack of purification procedures. The Amoco process is a relatively dustry, electronics industry, and so on. Besides polyester bottle new method that can purify Ta products immediately after chips, polyester films, and polyester fibers, over 90% of the obtaining the crude TA (CTA). Hence, the Amoco process has application of PTA is as the raw material to manufacture presently become more and more popular in many countries due polyethylene terephthalate(PEt(1], one of the most widely used to its simplified purification procedures [ 3 engineering plastics in the world. There are now two primary The Amoco process has three major stages [4 methods for manufacturing PTA, which are namely Witten process and Amoco process [2]. The Witten process was developed in the Manufacturing CTA 50s of the 20th century, and thanks to its superior qualities such as simple and mature process, reliable technique, low corrosiveness, At this stage, the raw material para-xylene(PX) is oxidized in the atmosphere of oxygen or oxygen-rich air to produce Y Gong, Cao, X-H. Meng, Z-G. Yang TA, with cobalt acetate (Co(Ac)2)and manganese acetate epartment of Materials Science, Fudan University, No. 220 Handan (Mn(Ac)z)as catalysts, tetrabromoethane (C2H2Br4)as the Road, Shanghai 200433(P R China) co-catalyst, and acetate acid(HAc)as the medium. Owing to E-mail:ziyang@fudan.edu.cn ularly the byproducts, the www.matcorr.com c 2009 WILEY-VCH Verlag GmbH & Co KGaA, WeinheimPitting corrosion on 316L pipes in terephthalic acid (TA) dryer Y. Gong, J. Cao, X.-H. Meng and Z.-G. Yang* Grade 316L is a type of austenitic stainless steel with ultra-low carbon content and it exhibits superior corrosion resistance. However, pitting is always observed in 316L steel when it is exposed to media containing halide ions. In the present study, we found that in the presence of acetate acid (HAc) containing chloride or bromide ions, pitting occurred on the surface of the rotary steam pipes with the matrix material of 316L steel in terephthalic acid (TA) dryer. In order to identify the causes of the failure, metallographic structures and chemical compositions of the matrix material were inspected by an optical microscope (OM) and a photoelectric direct reading spectrometer. Beside these, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) as well as ion chromatography (IC) were used to analyze the micromorphologies of the corrosion pits and the chemical compositions of the corrosion deposits within them. Analysis of the results revealed the sources of halide ions and the factors accelerating the corrosion rate. Beside these, detailed mechanisms of pitting were discussed and six out of all the seven theoretical morphologies of pitting features were obtained in practice. 1 Introduction Purified terephthalic acid (PTA) has wide applications in a variety of fields in our daily life, such as chemical fiber industry, light industry, electronics industry, and so on. Besides polyester bottle chips, polyester films, and polyester fibers, over 90% of the application of PTA is as the raw material to manufacture polyethylene terephthalate (PET) [1], one of the most widely used engineering plastics in the world. There are now two primary methods for manufacturing PTA, which are namely Witten process and Amoco process [2]. The Witten process was developed in the 50s of the 20th century, and thanks to its superior qualities such as simple and mature process, reliable technique, low corrosiveness, etc., it is still the most widely applied method currently available for manufacturing terephthalic acid (TA). However, the purity of the products produced by the Witten process is not ideal owing to the lack of purification procedures. The Amoco process is a relatively new method that can purify TA products immediately after obtaining the crude TA (CTA). Hence, the Amoco process has presently become more and more popular in many countries due to its simplified purification procedures [3]. The Amoco process has three major stages [4]:
Manufacturing CTA At this stage, the raw material para-xylene (PX) is oxidized in the atmosphere of oxygen or oxygen-rich air to produce TA, with cobalt acetate (Co(Ac)2) and manganese acetate (Mn(Ac)2) as catalysts, tetrabromoethane (C2H2Br4) as the co-catalyst, and acetate acid (HAc) as the medium. Owing to its high content of impurities, particularly the byproducts, the Materials and Corrosion 2009, 60, No. 11 DOI: 10.1002/maco.200805198 899 Y. Gong, J. Cao, X.-H. Meng, Z.-G. Yang Department of Materials Science, Fudan University, No. 220 Handan Road, Shanghai 200433 (P. R. China) E-mail: zgyang@fudan.edu.cn Corrosion Concepts 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. www.matcorr.com
2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim