Abstract
Recent work on the structure and composition of the anodic oxide film on iron is reviewed. Experiments employing ellipsometry and catholic reduction techniques show that the passive oxide film formed in neutral solutions is composed of two different layers, an inner layer adjoining to the metal and an outer layer in contact with the solution. Ellipsometric analyses indicate that the refractive index is 3.0-0.5 i for the inner layer and 1.8-0.1 i for the outer layer. Chemical analyses conducted during cathodic reduction of the film reveals that there is no detectable amount of ferrous ion, Fe(II) in the inner layer as well as in the outer layer. Thermogravimetric experiments show that the anodic oxide film contains some amount of water probably concentrated in the outer layer. It is also shown that the inner layer thickness increases linearly with the anodic potential, whereas the outer layer thickness is almost independent of the potential. A practical model of the film is proposed in which the inner layer is a ferric oxide, probably γ-Fe2O3, containing a very little amount of water and the outer layer is a ferric hydroxide. The solution environment has direct effects on the outer layer which disappears in acidic solutions, but has almost no effect on the inner layer which depends only on the overpotential of anodic oxide formation.
