Anodic Passivation Films on Iron in Acidic and Basic Solutions

Abstract

The anodic passivation films formed on iron in sodium phosphate, borate and hydroxide solutions at various pH values have been investigated by means of ellipsometry, thermogravimetry, and electrochemical analysis.
Ellipsometric measurements reveal that the film consists of an inner layer of γ-Fe₂O₃ and an outer layer of hydrous ferric oxide.
The thickness of the inner layer increases linearly with rising passivation potential, and the potential at which the inner layer vanishes is found to agree well with the equilibrium potential of γ-Fe₂O₃ formation on iron in the pH range from 1 to 12. The outer layer appears to depend on the pH and anion of the solution but its thickness is almost independent of the potential at which the film is formed. These results indicate that the overvoltage of film formation occurs in the inner layer irrespective of the solution pH.
At constant overvoltage, therefore, the inner layer thickness is constant independently of the solution pH in neutral and basic solutions where no dissolution of the film occurs. In acidic solutions, however, the inner layer becomes thin as the solution acidity increases because of increase of the rate of film dissolution. The outer layer thickness decreases with decreasing solution pH in the basic and neutral pH range and is negligibly small in acidic solution.
It is also shown that the more basic solution results in the more amount of water diffusing into the outer layer.
A film model is proposed in which the outer layer is affected by the solution pH, while the inner layer is determined only by the overvoltage of film formation.