F- Chen et aL/ Engineering Failure Analysis 37(2014)29-41 al Fig. 11. Microscopic morphology of the furrow crack imaged by stereo microscope(a) furrow crack in the wear zone(b)measurement of furrow width. Anode. 2Fe-4e= 2Fe Cathode: 02+2H20+4e= 40H- Total reaction: 2Fe+O2 +2H20= 2Fe(OH)2 Then the ultra-active Fe(oHh formed soon turns into magnetite by the reaction 6Fe(OH)2+O2= 2Fe3O4+6H2O ut for those tubes of the outside bundles near the water inlet and outlet, enough oxygen brought in ric oxide by the reaction: 4Fe(OH)2+O2=2Fe203+4H2O 4Fe3 O4+02=6Fe20 That is why tubes of the outside bundles are covered by red corrosion products instead of black Fe3 O4 on the tubes of theAnode : 2Fe 4e ¼ 2Fe2þ Cathode : O2 þ 2H2O þ 4e ¼ 4OH Total reaction : 2Fe þ O2 þ 2H2O ¼ 2FeðOHÞ2 Then the ultra-active Fe(OH)2 formed soon turns into magnetite by the reaction: 6FeðOHÞ2 þ O2 ¼ 2Fe3O4 þ 6H2O But for those tubes of the outside bundles near the water inlet and outlet, enough oxygen brought in during inspection can turn the iron oxides into the red ferric oxide by the reaction: 4FeðOHÞ2 þ O2 ¼ 2Fe2O3 þ 4H2O and 4Fe3O4 þ O2 ¼ 6Fe2O3 That is why tubes of the outside bundles are covered by red corrosion products instead of black Fe3O4 on the tubes of the inside bundles. Fig. 11. Microscopic morphology of the furrow crack imaged by stereo microscope (a) furrow crack in the wear zone (b) measurement of furrow width. F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 29–41 37