S-M. Hu et al/Case Studies in Engineering Failure Analysis 3(2015)52-61 D Fig 9. Microscopic morphology of defects at 10# tube(a)corrosion products and cracking: (b)pit in view of cross section. pressure gradient from shell side(2.0 MPa) to tube side(1. 2 MPa ). Hence, sulfur was also found in site C at inner wall (Table 3). As is shown in Figs. 5 and 6, shallow holes and corrosion deposits ascribed to uniform corrosion distributed unevenly on he tube surface. Because the electrode potential of carbon steel is-0.6V[16. tubes could act as anode and endure anodic dissolution in humid environment. The half-cell and total reaction are presented here Anode:Fe→Fe2++ Cathode: H,O+1/202+2e- 20H Total reaction: 2Fe O2 +2H20- 2Fe(oH)2 4Fe(OH)2+O2+2H20- 4Fe(OH) (5) urthermore, CI in the medium would initiate pitting on the tube owing to its strong permeability, absorbability and migratory aptitude, and thus accelerated the rate of localized corrosion, seen in Fig. 8. This auto-catalyst cycle of degradation could repeat again and again. The mechanism is expressed as follows Fe2++2Cl-→FeClpressure gradient from shell side (2.0 MPa) to tube side (1.2 MPa). Hence, sulfur was also found in site C at inner wall (Table 3). 3.2. Multiple corrosion As is shown in Figs. 5 and 6, shallow holes and corrosion deposits ascribed to uniform corrosion distributed unevenly on the tube surface. Because the electrode potential of carbon steel is 0.6 V [16], tubes could act as anode and endure anodic dissolution in humid environment. The half-cell and total reaction are presented here: Anode : Fe ! Fe2þ þ 2e (2) Cathode : H2O þ 1=2O2 þ 2e ! 2OH (3) Total reaction : 2Fe þ O2 þ 2H2O ! 2FeðOHÞ2 (4) 4FeðOHÞ2 þ O2 þ 2H2O ! 4FeðOHÞ3 (5) Furthermore, Cl in the medium would initiate pitting on the tube owing to its strong permeability, absorbability and migratory aptitude, and thus accelerated the rate of localized corrosion, seen in Fig. 8. This auto-catalyst cycle of degradation could repeat again and again. The mechanism is expressed as follows: Fe2þ þ 2Cl ! FeCl2 (6) Fig. 9. Microscopic morphology of defects at 10# tube (a) corrosion products and cracking; (b) pit in view of cross section. S.-M. Hu et al. / Case Studies in Engineering Failure Analysis 3 (2015) 52–61 59