Y Gong et aL/Engineering Failure Analysis 31(2013)203-210 than rotational. Consequently, long-term stagnation of the pretty wet PTA filter cakes was favored on the tubes upside surfaces due to these three factors, and then pitting corrosion was induced. with the progres orrosion, the upside urfaces were pitted and became rougher and rougher gradually, further facilitating the sta the filter cakes in re- turn. What a vicious cycle On the contrary, it was undoubtedly that the downside was to hold the wet filter cakes for a long time, so it was intact. As a result, the peculiar"Yin-Yang corrosion morphology was eventually formed with respect to the reason why this corrosion only occurred on the 4-in. tubes rather than the 3-and the 5-in. ones, it may be ascribed to their different surface roughness. It could be inferred that the 4-in. tubes were not manufactured in the same batch with the other two, and coincidentally its surfaces were rougher due to unqualified treatment, then increasing the pos- ibility of scaling for the wet PTA filter cakes. 4. Conclusions 1. Pitting corrosion occurred on the heat exchanger tubes of one PTA dryer, and even resulted in a peculiar "Yin-Yangco rosion morphology i.e. the tubes upside surfaces were severely corroded while the downside were intact. 2. Matrix materials of the heat exchanger tubes were qualified 316L stainless steels, in other words, the corrosion was not related to the incorrect selection of materials 3. Totally six, out of all the seven typical pitting corrosion morphologies in theory were observed on the failed tubes in this 4. The inappropriate operations in maintenance management, the inherent service conditions and parameters, and the unqualified surface treatment, were all the factors leading to the 'Yin-Yang corrosion on 4-in. tub 5. Recommendations 1. Concentration of the chloride ions in Naoh alkaline wash liquor should be limited under 30 ppm to avoid pitting. 2. Amount of the catalysts that contain bromine element in the oxidation unit should be strictly controlled. 3. Surface conditions of the 4-in. tubes must be inspected, and adequate treatment should be carried out if necessary. Acknowledgments le he work was supported by both Shanghai Petrochemical Co, Ltd and Shanghai Leading Academic Discipline Project ect Number: B113). to two-stage ong Y, Yuan JZ Failure analysis of leakage on titanium tubes within heat exchangers in a nuclear power plant. Part I: Electrochemical [3 Gong Y, Yang ZG,Yuan nalysis of leakage on titanium tubes within heat exchangers in a nuclear power plant. Part ll: Mechanical [4] Gong Y, Yang C, Yao C, Yang ZG. Acidic/caustic alternating corrosion on carbon steel pipes in heat exchanger of ethylene plant. Mater Corros 2011:62(10:967-78 er. Mater Des2011:32(1):671-81 izi gong y. zhong J. Yang ZG. Failure analysis of bursting on the inner pipe of a jacketed pipe in a tubular heat exchanger. Mater Des 2010:31(9): 4258-68 ng Y, Cao, Ji Assessment of creep rupture properties for dissimilar steels welded joints between 192 and HR3C Fatigue Fract Eng Mater Struct 2011: 34(2): 83-96. on and microstructures he welded joints of one China-made t91 steel. J Mater Eng [91 Gong Y, Yang ZG, Wang YF Impact simulation on ductile metal pipe with polymer coating by a coupled finite element and meshfree method. J Fail Anal [10 ASME CA osion Eng Fail Anal 2010: 17: 886-93 Oltra R, vignal v, et al. SvET usions by microinjection Corros Sci 2003:45:1143-59 n3i Pe indust ia astis i C Ga200 1: 2801 50-6: L eotanti G. Lamberti C 0.5 wtx Pd/c catalyst for purification of terephthalic acid: irreversible deactivation [14 ASTM G46-94. Standard guide for examination and evaluation of pitting corrosion. USA: ASTM International: 2005. [15 Gong Y, Cao J Meng XH, Yang ZG. Pitting corrosion on 316L pipes in terephthalic acid (ta) dryer. Mater Corros 2009: 60(11): 899-908.than rotational. Consequently, long-term stagnation of the pretty wet PTA filter cakes was favored on the tubes’ upside surfaces due to these three factors, and then pitting corrosion was induced. With the progress of pitting corrosion, the upside surfaces were pitted and became rougher and rougher gradually, further facilitating the stagnation of the filter cakes in re￾turn. What a vicious cycle! On the contrary, it was undoubtedly that the downside was impossible to hold the wet filter cakes for a long time, so it was intact. As a result, the peculiar ‘Yin-Yang’ corrosion morphology was eventually formed. With respect to the reason why this corrosion only occurred on the 4-in. tubes rather than the 3- and the 5-in. ones, it may be ascribed to their different surface roughness. It could be inferred that the 4-in. tubes were not manufactured in the same batch with the other two, and coincidentally its surfaces were rougher due to unqualified treatment, then increasing the pos￾sibility of scaling for the wet PTA filter cakes. 4. Conclusions 1. Pitting corrosion occurred on the heat exchanger tubes of one PTA dryer, and even resulted in a peculiar ‘Yin-Yang’ cor￾rosion morphology, i.e. the tubes’ upside surfaces were severely corroded while the downside were intact. 2. Matrix materials of the heat exchanger tubes were qualified 316L stainless steels, in other words, the corrosion was not related to the incorrect selection of materials. 3. Totally six, out of all the seven typical pitting corrosion morphologies in theory were observed on the failed tubes in this incident. 4. The inappropriate operations in maintenance management, the inherent service conditions and parameters, and the unqualified surface treatment, were all the factors leading to the ‘Yin-Yang’ corrosion on 4-in. tubes. 5. Recommendations 1. Concentration of the chloride ions in NaOH alkaline wash liquor should be limited under 30 ppm to avoid pitting. 2. Amount of the catalysts that contain bromine element in the oxidation unit should be strictly controlled. 3. Surface conditions of the 4-in. tubes must be inspected, and adequate treatment should be carried out if necessary. Acknowledgments The work was supported by both Shanghai Petrochemical Co., Ltd. and Shanghai Leading Academic Discipline Project (Project Number: B113). References [1] Pophali GR, Khan R, Dhodapkar RS, Nandy T, Devotta S. Anaerobic–aerobic treatment of purified terephthalic acid (PTA) effluent; a techno-economic alternative to two-stage aerobic process. J Environ Manage 2007;85:1024–33. [2] Yang ZG, Gong Y, Yuan JZ. Failure analysis of leakage on titanium tubes within heat exchangers in a nuclear power plant. Part I: Electrochemical corrosion. Mater Corros 2012;63(1):7–17. [3] Gong Y, Yang ZG, Yuan JZ. Failure analysis of leakage on titanium tubes within heat exchangers in a nuclear power plant. Part II: Mechanical degradation. Mater Corros 2012;63(1):18–28. [4] Gong Y, Yang C, Yao C, Yang ZG. Acidic/caustic alternating corrosion on carbon steel pipes in heat exchanger of ethylene plant. Mater Corros 2011;62(10):967–78. [5] Gong Y, Yang ZG. Corrosion evaluation of one dry desulfurization equipment – circulating fluidized bed boiler. Mater Des 2011;32(1):671–81. [6] Gong Y, Zhong J, Yang ZG. Failure analysis of bursting on the inner pipe of a jacketed pipe in a tubular heat exchanger. Mater Des 2010;31(9):4258–68. [7] Gong Y, Cao J, Ji LN, Yang C, Yao C, Yang ZG, et al. Assessment of creep rupture properties for dissimilar steels welded joints between T92 and HR3C. Fatigue Fract Eng Mater Struct 2011;34(2):83–96. [8] Gong Y, Yang ZG, Yang FY. Heat strength evaluation and microstructures observation of the welded joints of one China-made T91 steel. J Mater Eng Perform 2012;21(7):1313–9. [9] Gong Y, Yang ZG, Wang YF. Impact simulation on ductile metal pipe with polymer coating by a coupled finite element and meshfree method. J Fail Anal Prev 2012;12(3):267–72. [10] ASME CASE N-708-2010. Use of JIS G4303, Grades SUS304, SUS304L, SUS316, and SUS316L Section III, Division 1. [11] Deen KM, Virk MA, Haque CI, Ahmad R, Khan IH. Failure investigation of heat exchanger plates due to pitting corrosion. Eng Fail Anal 2010;17:886–93. [12] Vuillemin B, Philippe X, Oltra R, Vignal V, et al. SVET, AFM and AES study of pitting corrosion initiated on MnS inclusions by microinjection. Corros Sci 2003;45:1143–59. [13] Pellegrini R, Agostini G, Groppo E, Piovano A, Leofanti G, Lamberti C. 0.5 wt% Pd/C catalyst for purification of terephthalic acid: irreversible deactivation in industrial plants. J Catal 2011;280:150–60. [14] ASTM G46-94. Standard guide for examination and evaluation of pitting corrosion. USA: ASTM International; 2005. [15] Gong Y, Cao J, Meng XH, Yang ZG. Pitting corrosion on 316L pipes in terephthalic acid (TA) dryer. Mater Corros 2009;60(11):899–908. 210 Y. Gong et al. / Engineering Failure Analysis 31 (2013) 203–210