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.

View full abstract

Equilibrium diagrams for iron(III)-and iron(II)-orthophosphate systems were developed for the purpose of clarifying the effect of pH and electrode potential on the corrosion and surface treatment of steels in a solution containing phosphate ion. It has been confirmed from both the diagrams and experiments that iron(III)ion is precipitated in the form of FePO₄ in an acidic phosphate solution, and that the precipitate changes to Fe(OH)₃ with the increase of pH value. A titration experiment was performed to measure the formation constant, K, of iron(II) phosphate colloid. The value of K for the reaction, 3Fe²⁺+2H₂PO₄^-_??_Fe₃(PO₄)₂+4H⁺, was found to be 10^-7.8. The electrochemical conditions for the formation of an Fe₃(PO₄)₂ film on iron were investigated with the aid of an E-pH diagram developed for the iron-phosphate system. The anodic Fe₃(PO₄)₂ film was formed on iron in a neutral phosphate solution at a potential of active dissolution. The film growth was measured by in situ ellipsometry.

View full abstract

Corrosion of aluminum and its alloys in boiling CCl₄ is pitting at least in the early stages. In the electropolished, flat specimen-surface, a single pit could be developed from an indentation made by using a diamond needle. By measuring the growth rates of the single pits, corrosion rates of 1080 Al, Al-Mn and Al-Mg alloys have been determined in boiling CCl₄ containing pre-determined amount of water. A conical deposit of the corrosion product protruded in the solvent from the pit. The growth rate of pit depth is smaller than that of pit periphery for all of the alloys. As for 1080 Al, the effect of water on the single-pit growth has been studied. Presence of water in the bulk accelerates the growth of pit periphery but not of pit bottom.

View full abstract