Passivation of Iron in Sulfate, Perchlorate, and Borate Solutions: Role of Borate in the Passivation Process

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 associ-ated 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 concentra-tion of the supporting electrolyte, the pH of the solution from 3.0 to 8.4, or the potential of anodization. However, the addi-tion of <5 % pH 8.4 borate buffer to the neutral sulfate or perchlorate solutions results in a dramatic increase in the effi-ciency, with the anodic passivation charge decreasing by as much as two orders of magnitude. Similar trends are observed in pH 8.4 Na2SO4 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 bor-ate 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 sur