Grain Dependency of a Passive Film Formed on Polycrystalline Iron in pH 8.4 Borate Solution

Abstract

Passivating behavior of polycrystalline iron in pH 8.4 borate buffer solution was investigated by using two-dimensional (2D) ellipsometry and a micro-capillary cell (MCC) technique. In situ 2D ellipsometry of the passivating process of polycrystalline iron clearly demonstrated formation and growth of a passive film, though differences in growth rate and thickness of the film among substrate grains was observed less in ellipsometric images. Potentiostatic polarization and electrochemical impedance spectroscopy with the MCC showed that charge transfer resistance Rct and donor density ND of the passive film depended on the substrate orientation, though a clear correlation between these parameters was not observed. It was thought that several factors such as anisotropic growth, dissolution, and aging of oxides gave the grain-dependent defect structure of the passive film, depending on the surface crystallographic orientation of the substrate iron.