S-M Hu et aL/Case Studies in Engineering Failure Analysis 3(2015)52-61 a Fig 8. Structure of corrosion pit at inner wall and depth measurement(a)the pit; (b)3-D synthesis image. Furthermore, SEM pictures of 10" tube exhibit the corroded defects caused by multiple corrosion ore clearly In Fig 9(a), micro cracking engendered on the corrosion layers and small pieces have scaled surface After cut apart and etched, the cross-section morphology of the pit in Fig 8 is imaged and it is easy the localized wall-thinning degree( Fig 9(b)). From EDS results of micro areas in Fig. 10. unusual presences of s and CI were verified there in deed (Table 4). In general, these strong discoveries of 10"tube are all in line with previous of 24 3.1. Sources of impurity elements On the basis of our analysis, abnormal existences of two impurity elements(Cl and s) were discovered at the defect zones(tables 3 and 4). It is generally known that chloride ion and sulfides can be regarded as corrosive media in certain situations. So in order to figure out what actually happened to the tubes, the sources of them turned to be the priority for us to identify. After referring to operation information, clues started to surface gradually. Although the medium ODB stable under current condition, it had flowed through other heat exchangers in the upstream before and was possible to bring in some foreign substances, which has been affirmed by the plant. Among them, phosgene(cocl2)is the one that we cannot ignore. It is worth noting that the chemical plant is situated in coastal area, and thus CoCl2 can hydrolyze into ydrochloric acid and carbon dioxide under this humid environment(Eq (1)), which precisely explained the source of chloride CoCl2+H20→2HCl+CO2 With regard to sulfur, it originated from high-pressure industrial cleaning water and mainly remained on the outer tube wall. In addition, for the connection between tubes and tubesheet is seal welding but without physical expansion(Fig. 2), tiny cracking took place at the welded joints during operation. Then media would penetrate those defect sites motivated byFurthermore, SEM pictures of 10# tube exhibit the corroded defects caused by multiple corrosion factors more clearly. In Fig. 9(a), micro cracking engendered on the corrosion layers and small pieces have scaled off the surface. After cut apart and etched, the cross-section morphology of the pit in Fig. 8 is imaged and it is easy to judge the localized wall-thinning degree (Fig. 9(b)). From EDS results of micro areas in Fig. 10, unusual presences of S and Cl were verified there in deed (Table 4). In general, these strong discoveries of 10# tube are all in line with previous discussion of 2# one. 3. Discussion 3.1. Sources of impurity elements On the basis of our analysis, abnormal existences of two impurity elements (Cl and S) were discovered at the defect zones (Tables 3 and 4). It is generally known that chloride ion and sulfides can be regarded as corrosive media in certain situations. So in order to figure out what actually happened to the tubes, the sources of them turned to be the priority for us to identify. After referring to operation information, clues started to surface gradually. Although the medium ODB is stable under current condition, it had flowed through other heat exchangers in the upstream before and was possible to bring in some foreign substances, which has been affirmed by the plant. Among them, phosgene (COCl2) is the one that we cannot ignore. It is worth noting that the chemical plant is situated in coastal area, and thus COCl2 can hydrolyze into hydrochloric acid and carbon dioxide under this humid environment (Eq. (1)), which precisely explained the source of chloride. COCl2 þ H2O ! 2HCl þ CO2 (1) With regard to sulfur, it originated from high-pressure industrial cleaning water and mainly remained on the outer tube wall. In addition, for the connection between tubes and tubesheet is seal welding but without physical expansion (Fig. 2), tiny cracking took place at the welded joints during operation. Then media would penetrate those defect sites motivated by Fig. 8. Structure of corrosion pit at inner wall and depth measurement (a) the pit; (b) 3-D synthesis image. 58 S.-M. Hu et al. / Case Studies in Engineering Failure Analysis 3 (2015) 52–61