The anodic passivation of Fe has been investigated in borate buffer, sulfate, and perchlorate solutions. In sulfate and perchlorate solutions of either pH 3.0 or 8.4, the passivation process is highly inefficient with very large amounts of anodic charge passing before a substantial decrease in current is observed. In these solutions, passivation appears to be associated with precipitation of a salt film and/or changes in the solution pH near the surface which permits the formation of a passive oxide. The efficiency for potential‐step passivation cannot be significantly increased by changing the concentration of the supporting electrolyte, the pH of the solution from 3.0 to 8.4, or the potential of anodization. However, the addition of <5% pH 8.4 borate buffer to the neutral sulfate or perchlorate solutions results in a dramatic increase in the efficiency, with the anodic passiyation charge decreasing by as much as two orders of magnitude. Similar trends are observed in pH 8.4 solutions with previously passivated Fe electrodes which were cathodically reduced to different extents prior to anodization. The more of the prior film that remains on the surface before the anodic potential step in sulfate, the more efficient is the passivation process. The marked similarity of these results suggests that the beneficial action of borate towards Fe passivation is due to its ability to assist in the formation of the surface oxide film. While the pH buffering capacity of the borate can be playing a role, it is possible that borate buffer contains strongly interacting anions which give it an inhibiting capability. The interaction of these anions with the Fe surface is believed to stimulate formation of a surface oxide film, and this could explain the results in the present work. Passivation of Fe in sulfate and perchlorate solutions would be inefficient because these anions, unlike those derived from borate, do not have inhibitive abilities, nor do their solutions have buffering capacity. The results indicate the important role played by prior oxide films, and the need for well‐defined starting surface conditions in passivation experiments.
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