Engineering Failure Analysis 31(2013)203-210 Contents lists available at SciVerse Science Direct ENGINEERING ANALYSIS Engineering Failure Analysis ELSEVIER journalhomepagewww.elsevier.com/locate/engfailanal Failure analysis of one peculiar Yin-Yang corrosion morphology on heat exchanger tubes in purified terephthalic acid(pta)dryer Yi Gong, Zhen-Guo Yang, Xin-Hao Meng Department of Materials Science, Fudan University, Shanghai 200433, PR China ARTICLE INFO A B STRACT rticle histor common knowledge that localized corrosions occur on austenitic stainless steels Received 11 June 2012 uch as 316L, exposed to halide ions. This paper will discuss such a localized failure that took place not long after beginning of service on the heat exchanger tubes inside a purified Available online 13 February 2013 terephthalic acid(PTA)dryer in a petrochemical works. Particularly, one peculiar corrosion morphology termed"Yin-Yangcorrosion was observed, i. e the upside surface of the failed tubes was severely corroded while the downside was intact. Consequently, in order to Heat-exchanger failures ascertain the actual causes of this premature failure nd the process media were investigated by a variety of characterization methods. Metal- Pitting corrosion graphic structures and chemical compositions of the tube matrix materials were Failure analysis nspected by optical microscope(OM)and photoelectric direct reading spectrometer: both the surfaces and the cross-sections of the corroded areas were microscopically analyzed through scanning electron microscope (SEM)and disperse spectroscopy (EDS): and the chemical constituents of the process media were detected via the gas chromatog- raphy-mass spectrometric(GC-MS). Finally, the localized corrosion mechanisms were dis cussed in detail and the pertinent countermeasures were proposed. e 2013 Elsevier Ltd. All rights reserved. 1 Introduction Purified terephthalic acid(Pta)is the raw material mainly for synthesizing the significant organic compounds as poly chylene terephthalate(Pet) and poly-trimethylene terephthalate(Prr)-the matrix materials of the widely-used polyester films, fibers, bottle chips, and so on. In terms of its manufacturing process, there now exist two primary ways, the witten and the Amoco. Comparatively, since the Amoco process consumes fewer feedstocks but yields more products, as well as facili- tates higher purity of products but employs simpler manufacturing procedures, 70% PTA are produced in this way nowadays. A> The Amoco process generally consists of three major steps: manufacturing CTA(crude terephthalic acid ) purifying CTA to A, and post treatment of PTA [1]. The first step is also called the oxidation unit, and the second is the refining unit. If only focusing on the latter one, its flow chart is shown in Fig. 1. In detail, the Cta from the oxidation unit is firstly conveyed into gasifiable impurities, and the dissolved P-TA is collected for recycling as wl oa o the hydrogen atmosphere to hydrogenate the dominant impurity 4-carboxyl benzaldehyde (4-CBa)into the soluble para-to uic acid (P-TA), seen in Eg. (1). Then, in order to eliminate such P-TA, the products including both PtA and P-TA are sent into he hot water. As a result, PtA in form of the wet filter cake is obtained and transported into a dryer to evaporate the Corresponding author. Tel :+86 21 65642523: fax: +86 21 65103056 1350-6307/s- see front matter o 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2013.01.038Failure analysis of one peculiar ‘Yin-Yang’ corrosion morphology on heat exchanger tubes in purified terephthalic acid (PTA) dryer Yi Gong, Zhen-Guo Yang ⇑ , Xin-Hao Meng Department of Materials Science, Fudan University, Shanghai 200433, PR China article info Article history: Received 11 June 2012 Received in revised form 8 January 2013 Accepted 15 January 2013 Available online 13 February 2013 Keywords: Heat-exchanger failures Heat pipes Pitting corrosion Failure analysis abstract It is a common knowledge that localized corrosions occur on austenitic stainless steels such as 316L, exposed to halide ions. This paper will discuss such a localized failure that took place not long after beginning of service on the heat exchanger tubes inside a purified terephthalic acid (PTA) dryer in a petrochemical works. Particularly, one peculiar corrosion morphology termed ‘Yin-Yang’ corrosion was observed, i.e. the upside surface of the failed tubes was severely corroded while the downside was intact. Consequently, in order to ascertain the actual causes of this premature failure, samples including the failed tubes and the process media were investigated by a variety of characterization methods. Metal￾lographic structures and chemical compositions of the tube matrix materials were inspected by optical microscope (OM) and photoelectric direct reading spectrometer; both the surfaces and the cross-sections of the corroded areas were microscopically analyzed through scanning electron microscope (SEM) and energy disperse spectroscopy (EDS); and the chemical constituents of the process media were detected via the gas chromatog￾raphy–mass spectrometric (GC–MS). Finally, the localized corrosion mechanisms were dis￾cussed in detail and the pertinent countermeasures were proposed. 2013 Elsevier Ltd. All rights reserved. 1. Introduction Purified terephthalic acid (PTA) is the raw material mainly for synthesizing the significant organic compounds as poly￾ethylene terephthalate (PET) and poly-trimethylene terephthalate (PTT) – the matrix materials of the widely-used polyester films, fibers, bottle chips, and so on. In terms of its manufacturing process, there now exist two primary ways, the Witten and the Amoco. Comparatively, since the Amoco process consumes fewer feedstocks but yields more products, as well as facili￾tates higher purity of products but employs simpler manufacturing procedures, 70% PTA are produced in this way nowadays. The Amoco process generally consists of three major steps: manufacturing CTA (crude terephthalic acid), purifying CTA to PTA, and post treatment of PTA [1]. The first step is also called the oxidation unit, and the second is the refining unit. If only focusing on the latter one, its flow chart is shown in Fig. 1. In detail, the CTA from the oxidation unit is firstly conveyed into the hydrogen atmosphere to hydrogenate the dominant impurity 4-carboxyl benzaldehyde (4-CBA) into the soluble para-to￾luic acid (P-TA), seen in Eq. (1). Then, in order to eliminate such P-TA, the products including both PTA and P-TA are sent into the hot water. As a result, PTA in form of the wet filter cake is obtained and transported into a dryer to evaporate the gasifiable impurities, and the dissolved P-TA is collected for recycling as well. 1350-6307/$ - see front matter 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2013.01.038 ⇑ Corresponding author. Tel.: +86 21 65642523; fax: +86 21 65103056. E-mail address: zgyang@fudan.edu.cn (Z.-G. Yang). Engineering Failure Analysis 31 (2013) 203–210 Contents lists available at SciVerse ScienceDirect Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal