S-M Hu et aL/Case Studies in Engineering Failure Analysis 3(2015)52-61 b of micro-areas on the 10 tube(a)site E: (b) site F. FeCl2+2H20- Fe(OH)2+2(H*CI-) Meanwhile, COz emerged according to Eq (1)could be introduced into unit. It is regarded that the attack of carbon dioxide solutions on steel is stronger than that of diluted mineral acids with the same pH value [ 17, 18. So its presence in ammonia evaporators would aggravate corrosion extent as well In our case, uniform corrosion and pitting occurred at the same time and both of them caused material degradation. Apart from what we, ve discussed above, the interaction behavior of them was also one ruling cause. On one hand, uniform corrosion led to metal dissolving to form Fe*, which was easy to hydrolyze and engender H*. When local pH dropped to 4.0 with concentration of H, it favored pitting at the weak sites of tubes, such as corrosion holes, crevices, and cracks On the other hand, since tubes experienced serious corrosion, brittle corrosion layers consequently formed on the tube surface. While subject to temperature fluctuation and slight media erosion during operation, self-detachment of the corrosion layers from matrix metal happened due to the difference of their CtEs(coefficients of thermal expansion). What's more, corrosion pits could act as stress concentration and promote an initial cracking growth( Fig. 6). EDS results of micro-areas at 10 tube(wt%). 1.35 8.23 1.05 Site FFeCl2 þ 2H2O ! FeðOHÞ2 þ 2ðHþClÞ (7) Meanwhile, CO2 emerged according to Eq.(1) could be introduced into unit. It is regarded thatthe attack of carbon dioxide solutions on steel is stronger than that of diluted mineral acids with the same pH value [17,18]. So its presence in ammonia evaporators would aggravate corrosion extent as well. In our case, uniform corrosion and pitting occurred at the same time and both of them caused material degradation. Apart from what we’ve discussed above, the interaction behavior of them was also one ruling cause. On one hand, uniform corrosion led to metal dissolving to form Fe2+, which was easy to hydrolyze and engender H+ . When local pH dropped to 4.0 with concentration of H+ , it favored pitting at the weak sites of tubes, such as corrosion holes, crevices, and cracks. On the other hand, since tubes experienced serious corrosion, brittle corrosion layers consequently formed on the tube surface. While subject to temperature fluctuation and slight media erosion during operation, self-detachment of the corrosion layers from matrix metal happened due to the difference of their CTEs (coefficients of thermal expansion). What’s more, corrosion pits could act as stress concentration and promote an initial cracking growth (Fig. 6). Fig. 10. Composition analysis of micro-areas on the 10# tube (a) site E; (b) site F. Table 4 EDS results of micro-areas at 10# tube (wt.%). Element C O Cl S Fe Site D 1.35 8.23 0.65 1.05 87.10 Site E 0.21 10.06 2.70 – 86.52 Site F 6.11 25.71 2.10 – 65.47 60 S.-M. Hu et al. / Case Studies in Engineering Failure Analysis 3 (2015) 52–61