复旦大学：《材料失效分析 Materials Failure Analysis》课程教学资源（教学案例）06. failure analysis of one peculiar ‘yin-yang’ corrosion morphology on heat exchanger tubes in purified terephthalic acid（pta）dryer

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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.2013.01.038

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 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. Metallographic 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 chromatography–mass spectrometric (GC–MS). Finally, the localized corrosion mechanisms were discussed 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 polyethylene 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 facilitates 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-toluic 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

204 Y Gong et aL/Engineering Failure Analysis 31(2013)203-210 hy crystallizer PTA device Fig. 1. Flow chart of the refining unit of the Amoco process. COOH COOH +2H +h. mPR (1) CHO Apparently, the pta dryer plays a critical role in purification of the whole manufacturing process. In our incident, such a dryer was indeed applied within a Pta plant with Amoco process in one petrochemical company in Shanghai. As for its con- figuration and parameters, it was a rotating cylinder( 2. 2 r/min) with three arrays of 3 mm-thick heat exchanger tubes inside around the circumference, and the tube diameters were 3, 4, and 5 in respectively. From the inlet of the dryer, the high-tem- perature(135C) steam vapors were conveyed in the tube side, and the wet filter cakes with PTA concentration of 38.7% were transported and heated in the shell side. From the outlet, the nitrogen gas was imported to take away the H,o and other small molecules that were all evaporated from the wet filter cakes lowever in fact, after capacity expanding of the whole Pta plant, severe corrosion occurred on the 316L stainless steel heat exchanger tubes of the Pta dryer not long after beginning of service. Particularly, the corrosion even exhibited a pecu liar morphology, i.e. the upside of the failed tubes was corroded but the downside was intact. In order to vividly depict this interesting appearance, we termed it 'Yin-Yang corrosion, the well-known terminology referenced from the Chinese tradi- tional philosophy. Correspondingly the 'Yin' face was denoted as the corroded area, while the 'Yang face was the intact one Thus, for purpose of investigating the actual causes of this premature failure(designed lifetime was 8 years, failed just after 2 years). a variety of pertinent characterization measures referring to our previous experiences of failure analysis and struc tural integrity evaluation of heat exchanger tubes [2-9] were comprehensively employed for the samples, including matrix materials examination of the tubes, chemical constituents inspection of the process media, and macro/microscopic analysis of the defects. Results showed that the interaction between factors from the process media, the service conditions, and the operation parameters, was the se of this 'Yin-Yang'corrosion. Then, the relevant corrosion mechanisms were dis- cussed and the countermeasures were proposed. This paper actually presents a model case of applying such comprehensive analysis methods for failure analysis in practical engineering, and its achievement will have a reference value for corrosion prevention of heat exchanger tubes operating under similar service conditions. 2. Experimental and results 2.1. Visual observation As revealed in Fig. 2a, corrosion failure only occurred on the second array, i.e. the 4-in. tubes, of all the three arrays of heat exchanger tubes inside the Pta dryer. After sampling, Fig. 2b displayed the external appearance of the above-mentioned 'Yin-Yang, corrosion morphology on one failed tube, and on its surface the horizontal boundary line between the 'Yin and the 'Yang faces could be clearly observed. Further magnified, the corroded'Yin' face was actually covered with densely distributed corrosion pits, seen in Fig. 2C, implying a localized corrosion mechanism. 2.2. Microscopic observation For purpose of obviously comparing the two opic morphologies of its cross-section As show Yin'and"Yang faces of the failed tube, Fig 3 pre een the left'Yin' ented the micro- 3a, a fictitious boundary line could be drawn betw and the right "Yang faces Further magnified, tl face was composed of corrosion concaves and pits, seen in Fig. 3b while contrarily, the'Yang' face was relatively smooth without any significant defects, seen in Fig. 3c

ð1Þ Apparently, the PTA dryer plays a critical role in purification of the whole manufacturing process. In our incident, such a dryer was indeed applied within a PTA plant with Amoco process in one petrochemical company in Shanghai. As for its con- figuration and parameters, it was a rotating cylinder (2.2 r/min) with three arrays of 3 mm-thick heat exchanger tubes inside around the circumference, and the tube diameters were 3, 4, and 5 in. respectively. From the inlet of the dryer, the high-temperature (135 C) steam vapors were conveyed in the tube side, and the wet filter cakes with PTA concentration of 38.7% were transported and heated in the shell side. From the outlet, the nitrogen gas was imported to take away the H2O and other small molecules that were all evaporated from the wet filter cakes. However in fact, after capacity expanding of the whole PTA plant, severe corrosion occurred on the 316L stainless steel heat exchanger tubes of the PTA dryer not long after beginning of service. Particularly, the corrosion even exhibited a peculiar morphology, i.e. the upside of the failed tubes was corroded but the downside was intact. In order to vividly depict this interesting appearance, we termed it ‘Yin-Yang’ corrosion, the well-known terminology referenced from the Chinese traditional philosophy. Correspondingly, the ‘Yin’ face was denoted as the corroded area, while the ‘Yang’ face was the intact one. Thus, for purpose of investigating the actual causes of this premature failure (designed lifetime was 8 years, failed just after 2 years), a variety of pertinent characterization measures referring to our previous experiences of failure analysis and structural integrity evaluation of heat exchanger tubes [2–9] were comprehensively employed for the samples, including matrix materials examination of the tubes, chemical constituents inspection of the process media, and macro/microscopic analysis of the defects. Results showed that the interaction between factors from the process media, the service conditions, and the operation parameters, was the main cause of this ‘Yin-Yang’ corrosion. Then, the relevant corrosion mechanisms were discussed and the countermeasures were proposed. This paper actually presents a model case of applying such comprehensive analysis methods for failure analysis in practical engineering, and its achievement will have a reference value for corrosion prevention of heat exchanger tubes operating under similar service conditions. 2. Experimental and results 2.1. Visual observation As revealed in Fig. 2a, corrosion failure only occurred on the second array, i.e. the 4-in. tubes, of all the three arrays of heat exchanger tubes inside the PTA dryer. After sampling, Fig. 2b displayed the external appearance of the above-mentioned ‘Yin-Yang’ corrosion morphology on one failed tube, and on its surface the horizontal boundary line between the ‘Yin’ and the ‘Yang’ faces could be clearly observed. Further magnified, the corroded ‘Yin’ face was actually covered with densely distributed corrosion pits, seen in Fig. 2c, implying a localized corrosion mechanism. 2.2. Microscopic observation For purpose of obviously comparing the two different ‘Yin’ and ‘Yang’ faces of the failed tube, Fig. 3 presented the microscopic morphologies of its cross-section. As shown in Fig. 3a, a fictitious boundary line could be drawn between the left ‘Yin’ and the right ‘Yang’ faces. Further magnified, the ‘Yin’ face was composed of corrosion concaves and pits, seen in Fig. 3b, while contrarily, the ‘Yang’ face was relatively smooth without any significant defects, seen in Fig. 3c. Fig. 1. Flow chart of the refining unit of the Amoco process. 204 Y. Gong et al. / Engineering Failure Analysis 31 (2013) 203–210

Y Gong et al/Engineering Failure Analysis 31 (2013)203-210 2.4. Process media inspection Then, in order to find out the corrosive sources of this ' corrosion, the process media were inspected. Since the catalysts of the hydrogenation reaction were not so aggressive(Eq (1), attention was only paid to the process media before the refining unit, i. e in the oxidation unit, particularly the Cta dryer. Fig. 5 showed the GC-MS result of the exhaust gas that evaporated from the CTA filter cake in the Cta dryer. In it, the peaks at 1.34, 1.53, 2.17 and 3. 46 corresponded to CH3Br. CH3COOCH3, benzene(c6h6)and toluene(csHsch3) respectively, while the peaks at 3. 15 and 3.62 represented C2H2Br2 nd the peak at 6. 25 was C2HBr3. In fact, all these substances were the by-products generated in the oxidation unit, especially because of the catalysts containing bromine element in the oxidation reaction [13. However in terms of the refining unit, this GC-MS result positively demonstrated that most of the corrosive factors, i.e. the aggressive bromine element(ions)con- tained substances, had been ruled out. In other words, the "Yin-Yang,' corrosion inside the pta dryer was not relevant to the process media in the service environment. 25. SEM S eDs At first, surface of the failed tube with corrosion was observed under SEM. As shown in Fig 6a, a distinct boundary line, just like that in Fig 3a, could be easily marked between the "Yin'and the'Yang'faces. After magnification, lots of corrosion pits with maximum size of about 200 um were found on the 'Yin face, seen in Fig. 6b, exhibiting the familiar orphology of localized corrosions, especially pitting. ctually, after being cut off, the cross-sections of the "Yin' face further verified it was ascribed to the pitting corrosion in- deed. Compared with the smooth 'Yang' face( Fig. 7a). the corroded'Yin' face was provided with a variety of typical morphol ogies of pitting corrosion in theory [14, including narrow deep(fig. 7b), elliptical(Fig. 7c), wide shallow(Fig. 7d). subsurface(Fig. 7e). undercutting(Fig. 7f), and horizontal (Fig. 7g). What's more based on the eds results, aggressive bro- mide and chloride ions were detected on the corrosion products within such pits too seen in Fig 8 and Table 2. As mentioned above the bromine element was the remnant from the catalyst in the oxidation reaction, nevertheless the actual source of the chlorine element still needed to be determined. Anyway, all these facts had already sufficiently demonstrated that the peculiar "Yin-Yang, corrosion was led by the halide-ions-induced pitting corrosion, and its mechanisms would be discussed in detail as follows o Based on the analysis results above, it was pretty clear that localized corrosions, especially the halide-ions-inducett. stainless steel matrix material. Thus, it would firstly focus on the sources of the halide ions, especially the bromide and the chloride ions. As for the former one, it had been repeatedly mentioned that they were the remnant derived from the cat- alyst in oxidation unit. Then, how about the latter one? Since the process media in the shell side of the pta dryer were just the filter cakes, which were theoretically free of chloride ions. So what was the source? After investigating the maintenance management, it was learnt 3%(wt%) NaoH solution was used as the alkaline wash liquor for both the cta and the pta drye to eliminate the acidic scale deposits on the heat exchanger tubes surfaces during routine downtime, aiming to avoid crevice corrosion. However in industry, NaOH is always produced by electrolysis of saturated salt water, seen in Eq(2), thus the chlorine element will be inevitably intermingled into the NaOH products. As a result, if the 3% NaOH solution was not suf- ficiently purified before usage, chloride ions would be introduced, and then preferentially accumulate and attack the pre- isting defects like inclusions on the tubes' surfaces. Under this condition, localized corrosion, particularly pitting corrosion was initiated. In fact, this kind of chloride-ions-induced pitting corrosion due to inappropriate operations in maintenance management has been already discovered in our previous research on the failed Cta dryer [15] Fig. 6. SEM micrograph of the 'Yin-Yang corrosion on failed tubes (a) Boundary line between""Yang faces(b)corrosion pits onYin'face

2.4. Process media inspection Then, in order to find out the corrosive sources of this ‘Yin-Yang’ corrosion, the process media were inspected. Since the catalysts of the hydrogenation reaction were not so aggressive (Eq. (1)), attention was only paid to the process media before the refining unit, i.e. in the oxidation unit, particularly the CTA dryer. Fig. 5 showed the GC–MS result of the exhaust gas that evaporated from the CTA filter cake in the CTA dryer. In it, the peaks at 1.34, 1.53, 2.17 and 3.46 corresponded to CH3Br, CH3COOCH3, benzene (C6H6) and toluene (C6H5CH3) respectively, while the peaks at 3.15 and 3.62 represented C2H2Br2, and the peak at 6.25 was C2HBr3. In fact, all these substances were the by-products generated in the oxidation unit, especially because of the catalysts containing bromine element in the oxidation reaction [13]. However in terms of the refining unit, this GC–MS result positively demonstrated that most of the corrosive factors, i.e. the aggressive bromine element (ions) contained substances, had been ruled out. In other words, the ‘Yin-Yang’ corrosion inside the PTA dryer was not relevant to the process media in the service environment. 2.5. SEM & EDS At first, surface of the failed tube with ‘Yin-Yang’ corrosion was observed under SEM. As shown in Fig. 6a, a distinct boundary line, just like that in Fig. 3a, could be easily marked between the ‘Yin’ and the ‘Yang’ faces. After magnification, lots of corrosion pits with maximum size of about 200 lm were found on the ‘Yin’ face, seen in Fig. 6b, exhibiting the familiar morphology of localized corrosions, especially pitting. Actually, after being cut off, the cross-sections of the ‘Yin’ face further verified it was ascribed to the pitting corrosion indeed. Compared with the smooth ‘Yang’ face (Fig. 7a), the corroded ‘Yin’ face was provided with a variety of typical morphologies of pitting corrosion in theory [14], including narrow & deep (Fig. 7b), elliptical (Fig. 7c), wide & shallow (Fig. 7d), subsurface (Fig. 7e), undercutting (Fig. 7f), and horizontal (Fig. 7g). What’s more, based on the EDS results, aggressive bromide and chloride ions were detected on the corrosion products within such pits too, seen in Fig. 8 and Table 2. As mentioned above, the bromine element was the remnant from the catalyst in the oxidation reaction, nevertheless the actual source of the chlorine element still needed to be determined. Anyway, all these facts had already sufficiently demonstrated that the peculiar ‘Yin-Yang’ corrosion was led by the halide-ions-induced pitting corrosion, and its mechanisms would be discussed in detail as follows. 3. Discussion Based on the analysis results above, it was pretty clear that localized corrosions, especially the halide-ions-induced pitting corrosion was the main cause of this peculiar ‘Yin-Yang’ corrosion on the heat exchanger tubes with qualified 316L stainless steel matrix material. Thus, it would firstly focus on the sources of the halide ions, especially the bromide and the chloride ions. As for the former one, it had been repeatedly mentioned that they were the remnant derived from the catalyst in oxidation unit. Then, how about the latter one? Since the process media in the shell side of the PTA dryer were just the filter cakes, which were theoretically free of chloride ions. So what was the source? After investigating the maintenance management, it was learnt 3% (wt%) NaOH solution was used as the alkaline wash liquor for both the CTA and the PTA dryers to eliminate the acidic scale deposits on the heat exchanger tubes’ surfaces during routine downtime, aiming to avoid crevice corrosion. However in industry, NaOH is always produced by electrolysis of saturated salt water, seen in Eq. (2), thus the chlorine element will be inevitably intermingled into the NaOH products. As a result, if the 3% NaOH solution was not suf- ficiently purified before usage, chloride ions would be introduced, and then preferentially accumulate and attack the preexisting defects like inclusions on the tubes’ surfaces. Under this condition, localized corrosion, particularly pitting corrosion was initiated. In fact, this kind of chloride-ions-induced pitting corrosion due to inappropriate operations in maintenance management has been already discovered in our previous research on the failed CTA dryer [15]. Fig. 6. SEM micrograph of the ‘Yin-Yang’ corrosion on failed tubes. (a) Boundary line between ‘Yin’ and ‘Yang’ faces (b) corrosion pits on ‘Yin’ face. Y. Gong et al. / Engineering Failure Analysis 31 (2013) 203–210 207

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 return. 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 possibility 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’ corrosion 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

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