Tooru Tsuru1, Tomohisa Zaitsu2, Katsumi Masamura3 and Shiro Haruyama1
1Department of Metallurgical Engineering, Faculty of Engineering, Tokyo Institute of Technology, Tokyo
Anodic behaviour of iron and iron oxides was studied in neutral and acidic solutions in order to determine the passivity of iron. The oxides exhibited passivity-like behaviour at the potentials corresponding to that of the passive state of iron. Cathodic chronopotentiograms of passivated oxides showed that the oxides were covered by a thin higher valent oxide of γ -Fe2O3, the thickness of which was comparable with that of passive iron. The impedance measurements of Fe, FeO, and Fe3O4, and the deposited γ -Fe2O3 in the passive potential region showed that the greater part of the applied potential appeared at the film-solution interface, and the faradic resistance at low ferequencies was of the order of 10-6 Ømega , corresponding to the fact that there were no appreciable transfer reactions across the interface. Because the formation of passive films on oxides does not involve a dissolution process, it is found that the precipitation process, which is often regarded as a pre-passivation process, is not a necessary for the formation of a passive film, but inevitable for the potential of iron to shift to the passive region. The open-circuit decay of passive iron was attributed to the coupled reaction of the anodic oxidation of iron substrate and the cathodic reduction of γ -Fe2O3, both forming Fe3O4. The mechanism was confirmed by the fact that the passive films on FeO and Fe3O4 did not show any open-circuit decay.
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