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Iron was anodically oxidized in a borate‐boric acid buffer solution of pH 8.4 in the potential range extending from the active region to oxygen evolution. In the active region there appeared to be some Fe3O4 on the surface. In the passive region the iron was covered with an oxide film 10–30Aå thick, depending on the potential. The structure of the film was studied by its cathodic behavior and electron diffraction and was found to consist of an inner “ Fe3O4 ” and an outer “ γ‐Fe2O3 ” layer. The outermost part of the film had a defect structure of the general formula Fex6+·Fe2−2x3+·□x·O3 . The thicknesses of the layers and the number of defects were found to be functions of the anodic potential. The chemical reactions involved in the anodic passivation and the subsequent cathodic reduction processes are discussed.

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