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 clean, oxide-free surfaces exposed stepwise to low-pressure gaseous oxygen. Cr(III), Mn(III), Fe(III), Co(II) but no Ni(II) oxides formed at 100 °C, and Mn(II) 200 °C. Data quantification revealed a transition mechanism from selective oxidation 3D kinetically-controlled growth, leading stratification film. Formation Co-rich outer layer above Cr-rich inner first implies predominant cationic outward supported overall stoichiometry