Revealing the mechanism of oxide film defects governing the anisotropic corrosion rate of austenitic stainless steel in supercritical carbon dioxide using (100), (110), and (123) oriented single crystals

The dependence of corrosion rate on crystal orientation was studied using austenitic stainless steel single crystals ((100), (110) and (123)-oriented) in supercritical carbon dioxide, supported by SEM, XRD, TEM characterization and DFT calculations. The results show that a typical dual-layer structured oxide scale is formed on each sample, with the (110) and (123) orientations exhibiting significantly enhanced growth rates of 111.7 % and 137.4 %, respectively, relative to the (100) orientation. DFT analysis reveals that the (110) and (123) surfaces possess 4.5 % and 9.6 % higher surface energies, respectively, than the (100) surface, establishing a positive correlation between surface energy and oxide growth kinetics. The surface energy is positively correlated with the outward diffusion and oxidation rate of Fe, and determines the thickness of the outer magnetite layer. In the biphasic inner layer, the enhanced counter-diffusion of Fe and O in the spinel phase of high-surface-energy specimens facilitates the formation of elevated defect concentrations. This phenomenon is synergistically driven by accelerated outward Fe diffusion from the substrate matrix, thus providing additional pathways for inward oxygen diffusion. This continuously supplies oxygen to the oxidation front, thereby a thicker inner oxide layer with ultrafine duplex structure is formed.