Early-Stage Surface Oxidation of the Equiatomic Cocrfemnni High Entropy Alloy Studied in Situ by Xps

The mechanisms of initial oxide growth on the equiatomic CoCrFeMnNi high entropy alloy were studied in situ by X-ray photoelectron spectroscopy on clean, oxide-free surfaces exposed stepwise to low-pressure gaseous oxygen. Cr(III), Mn(III), Fe(III), Co(II) but no Ni(II) oxides were formed at 100 °C, and Mn(II) at 200 °C. Data quantification revealed a transition mechanism from selective initial oxidation to 3D kinetically-controlled growth, leading to stratification of the oxide film. Formation of the Co-rich outer layer above the Cr-rich inner layer first formed implies predominant cationic outward oxide growth supported by the overall stoichiometry of the film.