Abstract
The composition, layer structure and thickness of the passive films formed on iron in borate and phosphate solutions have been examined as functions of pH, potential and film formation procedure (one-step and two-step oxidation) by using electrochemical and ellipsometric techniques.
The ellipsometric measurements of δP-δA curve during galvanostatic-cathodic film reduction show that the layer structure of the passive film changes with pH from an outer barrier layer/an inner barrier layer in acid to a deposit layer/an outer barrier layer/an inner barrier layer in weak acid and to a deposit layer/a barrier layer in neutral and alkaline solutions. The barrier layer consists of two layers in the pH region below pH 5.5, but turns to be a single layer above this pH. The barrier layer thickness increases nearly linearly with potential, but once the overpotential is fixed, it remains constant regardless of pH, anion species and film formation procedure. The deposit layer thickness, which is almost independent of potential, tends to increase with increasing pH up to about pH 8.
Chemical analyses of iron ion dissolved from the film during cathodic reduction show no significant effect on the deposit layer composition (ZFe\fallingdotseq3.00) of pH, anion species, potential and film formation procedure except for a decrease in ZFe at low potentials. The mean oxidation valency of the barrier layer, however, varies from ZFe\fallingdotseq2.33 to ZFe=3.00, depending on pH, anion species, potential and film formation procedure. The results are explained in terms of the ion selective property of the passive film, and the layer structure and composition are summarized in the potential-pH diagrams.
