Grain-Dependent Reduction of Thermally-Oxidized Iron Surface Observed By In Situ 2D Ellipsometry

Abstract

Thermal oxides grown on pure iron surface in high temperature atmosphere have characteristic composition, structure, and thickness depending on the surface condition of substrate iron as well as the condition of oxidation atmosphere. The crystallographic orientation of the surface is one of key factors that influence the formation rate of thermal oxides. Boggs et al. reported the orientation-dependent oxide formation of iron single crystal1. However, the heterogeneity of the thermally-grown oxides on polycrystalline iron consisted of numerous single grains with various orientations has been overlooked. In order to understand well the inhomogeneity in thermally-oxidized polycrystalline iron surface, it is important to elucidate the precise characteristics of the oxidized surface of each single grain from the viewpoint of crystallographic orientation. In this study, we conducted two-dimensional (2D) ellipsometry and micro-electrochemical measurements with a micro-capillary-cell (MCC) method2, and discussed the heterogeneity of the thermally-oxidized polycrystalline iron surface. Polycrystalline iron plates annealed in vacuum were used as specimens. After mirror-like polishing by using colloidal silica particles, the crystallographic orientation of the specimen surface was identified by an electron backscattering diffraction patterning. Then, the specimen was thermally oxidized at 573 K in air for 3.6 ks. The thickness distribution of the oxide formed on the polycrystalline iron specimen was measured with a 2D ellipsometer in air. Subsequently, the specimen was galvanostatically polarized in pH 8.4 borate buffer (0.3 mol dm–3 boric acid and 0.075 mol dm–3 sodium borate solution) in an electrochemical cell in situ, and the changes in ellipsometric parameters were recorded simultaneously. The characteristics of the oxidized surface of each single grain were investigated using an MCC. After the thermal oxidation, the specimen surface showed a patch pattern with various colors that related to metallographic texture. It indicated that the surface oxide film grew at different rate depending on the surface orientation of single grains. Micro-Raman spectroscopy revealed that the oxide film composed of α-Fe2O3 and Fe3O4. Bi-layered structure of the oxide film was observed in a cross-sectional SEM image. The thickness distribution of the surface oxide was successfully measured with the 2D ellipsometer. A relationship between the thicknesses of outer and inner oxide layers, and the surface orientation was summarized in an inversed pole figure map. Electrochemical impedance spectroscopy by means of the MCC successfully obtained the dependence of frequency response on the substrate orientation. The results of Mott-Schottky analysis implied that the orientation affected the defect structure of the thermal oxides. It is considered that anisotropic oxide growth results in grain-dependent dissolution of oxides and heterogeneous corrosion of polycrystalline iron surface. The detailed consideration of the grain-dependency on the thermally-grown oxide is discussed on site.