Thickness and structure of passive films on iron in acidic and basic solution

This paper describes the effect of solution pH on the passive film on iron which was investigated by ellipsometry and by cathodic reduction combined with chemical analysis. Anodic two-step passivation was employed to exclude anodic deposition of ferrous ion, resulting from anodic oxidation of iron. The film in acid solution is almost anhydrous oxide, whereas the film in neutral and alkaline solution has an outer hydrated oxide layer, whose thickness decreases with decreasing pH. The hydrated outer layer depends also on the anion present in solution. The transition layer model of the film is proposed to explain the results.