The anodic oxidation of platinum in 1N H2SO4 is studied by using opencircuit transients to determine Tafel slopes and by following film growth optically with an ellipsometer. The Tafel slope values are found to be proportional to the film thicknesses measured with the ellipsometer. This result is interpreted in terms of an ionic conduction model in which the overpotential appears across the anodic oxide film, and the logarithm of the ionic current density is proportional to the field in the oxide. The results are compared with the results of similar measurements on iron and tantalum. Although platinum, iron, and tantalum have quite different chemical properties, and their oxides span a thickness range from a monolayer to several thousand angstroms, it is concluded that the same basic process is responsible for the anodic oxidation of the three metals." Electrochemical Society Active Member.Electrochemical Society Student Member.

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The anodic oxidation of an iron electrode in a neutral borate electrolyte is studied using a following ellipsometer. The oxide film is found to consist of two layers with different refractive indices: an outer layer of Fe2O3 and a thicker inner layer of Fe3O4 . A thin layer of Fe3O4 is grown on the electrode in the active state, and the electrode passivates when a partial monolayer of Fe2O3 is grown at the Fe3O4‐normalelectrolyte interface. Passive state anodic oxidation causes simultaneous growth of the two layers, the inner layer growing approximately 80% faster than the outer layer. We conclude that the Fe2O3 layer inhibits anodic dissolution of the electrode, and that both the Fe2O3 layer and the portion of the Fe3O4 layer grown in the passive state act as the electrically limiting barrier across which the overpotential appears. The possibility that the two layers grow in a proportion dictated by the transport numbers of mobile defects (as has been proposed for the anodic oxide of tantalum) is discussed.

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