Heterogeneity of passive film formed on polycrystalline iron observed by in situ 2D-ellipsometry and micro-capillary cell technique

摘要

A passive film formed on metal surface plays an important role in preventing corrosion of metallic materials. The heterogeneity in the film based on metallographic texture may lead to non-uniform reaction such as pitting and local corrosion. Hence, it is necessary to consider how the metal texture of a metal surface affects the heterogeneity in the film. In this paper, the authors investigated the heterogeneity of an anodic oxide film formed on polycrystalline iron by using in situ 2D-ellipsometry as well as micro-electrochemical methods. The sample specimen was an annealed polycrystalline iron plate. After mirror-like polishing, the sample was mounted in an electrochemical cell combined with a 2D-ellipsometer. After a native oxide was removed, anodic polarisation was conducted in borate buffer solution to form a passive oxide film. Analyses of thickness distribution of the passive film were performed before and after the film formation. The local surfaces were also polarised by using micro-capillary cell. The 2D-ellipsometer combined with an electrochemical cell enabled to image the thickness distribution of the oxide film formed on polycrystalline iron. The average oxide thickness increased after the anodic polarisation and decreased after the reduction of the film. These results indicated that the in situ 2D-ellipsometry successfully revealed the growth of the oxide film. The detailed consideration of the grain-dependency on the oxide film is discussed on site.