In order to estimate the effects of anions on passive film breakdown, the values of the time required for breakdown, tbd were determined for a passivated iron electrode in aqueous solutions of 0.1 M various anions. Relationships between log tbd and the hardness of anion base, H were discussed on the basis of the hard and soft acids and bases principle. According to the principle, anions were classified into four groups, hard base, oxidative hard base, soft and hard base, and soft base. For the former three groups, log tbd increased with H but in reverse, decreased with an increase in H for the latter. The former relationships could be interpreted by suppression of anion incorporation into the film due to stable hard acid-hard base interaction. However, the relationship could not be explained by this interaction for the group of soft base anions.
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