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) 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. 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 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. 3. Discussion Based on the analysis results above, it was pretty clear that localized corrosions, especially the halide-ions-induced pit￾ting 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 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 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 pre￾existing 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