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Iron was anodically oxidized under both potentiostatic and galvanostatic conditions. It was found that the rate of film growth could be expressed by the over‐all equation where QT is the film thickness, E the potential, and κ', β, and B are constants. A model is proposed based on a "place‐ exchange" mechanism in which all the M‐O exchanges in a given row take place simultaneously. β is then related to the transfer of oxygen from the OH− in the solution to the oxide and B is related to the activation energy for an equivalent ion‐pair exchange. The activation energy for a single place exchange was calculated to be 3.5 kcal. This mechanism would lead to an increase of activation energy with thickness and a probable change of mechanism at some finite thickness.
Sato et al. (Wed,) studied this question.