Heterogeneous Thermal Oxide Film Formed on Polycrystalline Pure Iron

The thermally-grown oxides formed on pure iron have characteristic composition, structure, and thickness, depending on the oxidation condition such as temperature and oxygen partial pressure. The crystallographic orientation of iron surface also affected the rate of thermal oxidation¹. However, the heterogeneity on the thermal oxide film formed on polycrystalline iron has not been discussed sufficiently. In order to well understand the non-uniform oxidation that progresses on polycrystalline iron, it is important to elucidate the precise characteristics of the thermal oxide film from the viewpoint of crystallographic orientation. In this study, we investigated the heterogeneity of thermal oxide film formed on polycrystalline pure iron by using a 2D-ellipsometer and a micro-capillary-cell (MCC) method².

The sample specimen was an annealed iron plate with a purity of 99.99%. After mirror-like polishing, the surface orientations of the sample were identified by using an electron backscatter diffraction patterning. Subsequently, the sample was kept at 573 K in air for 3.6 ks to form thermal oxide film. The thickness of the oxide film on the surface was mapped with the 2D-ellipsometer. The characteristics of the oxide film formed on each single grain were evaluated by electrochemical impedance spectroscopy (EIS) by means of an MCC with an inner diameter of 180 µm.

The sample surface as polished was smooth like a mirror showing metallic luster. After thermal oxidation, however, it showed various colors, depending on the surface metallographic structure due to interference of light. The colors of the sample surface indicated that the thermal oxides grew at different rate depending on the surface orientation of single grains on the substrate. The non-uniform oxide formation on the polycrystalline iron was also confirmed by two-dimensional thickness mapping with the ellipsometer. EIS measurements showed that the electric resistance and capacitive behavior of the oxide films were depended on the orientation of substrate surface. Furthermore, the results of Mott-Schottky analysis implied that the surface orientation affected the defect structure of the thermal oxides. It was considered that the grain-dependency on the thermal oxide film was originated in anisotropic growth of the oxide determined by the crystallographic orientation of the substrate iron surface. The detailed consideration of the grain-dependency on the oxide film is discussed on site.

References

[1] W. E. Boggs, R. H. Kachik, and G. E. Pellissier, J. Electrochem. Soc., 114, 32 (1967).

[2] Y. Takabatake, K. Fushimi, T. Nakanishi, and Y. Hasegawa, J. Electrochem. Soc., 164, C594 (2014).