Effects of Formation Procedure and Anion on Passive Films on Iron in Neutral Solution

Abstract

The composition, structure and thickness of passive films formed on iron by anodic oxidation in two steps with a preliminary oxidation at pH 11.50 followed by one hour oxidation in borate or phosphate solution at pH 8.42 have been investigated by the use of ellipsometric and electrochemical techniques, and the results are compared with those of passive films formed by a direct oxidation in the same solution. Chemical analyses of iron ion dissolved from the film and ellipsometric measurements of the film during galvanostatic catholic film reduction reveal that, irrespective of its formation procedure, the passive film is a two-layer film consisting of a deposit layer next to the solution and a barrier layer in contact with the metal. In borate solution, the compositions and the optical constants of the deposit and barrier layers exhibit almost no difference with different oxidation procedures, except for a minor difference in the barrier layer formed at less noble potential. In phosphate solution, the deposit layer composition is scarcely affected by oxidation procedure, but the barrier layer composition differs in different oxidation procedures with an Fe(II) ion-rich oxide in direct oxidation and a magnetite-like oxide in two step oxidation. The optical constants of the deposit and barrier layers and the ion enrichment at the deposit/barrier interface are found to depend on the oxidation procedure. The thickness of the deposit layer formed by direct oxidation is always thicker than that of the deposit layer formed by two step oxidation, but the barrier layer thickness is determined solely by the overpotential applied regardless of the oxidation procedure. The results are explained in terms of the ion selective property of the film, which differs in different oxidation procedures.