Oxidation Dynamics: Critical Differences in Early-Stage Oxidation of NiCr and NiCrMo Alloys

The oxidation of NiCrMo is studied in-operando with synchrotron-based X-ray photoelectron electron microscopy (XPEEM) and in-situ with X-ray photoelectron spectroscopy (XPS). XPEEM produces X-ray absorption spectroscopic (XAS) hyperspectral images and time-resolved information about the oxide growth and distribution. Oxide distribution in the XAS hyperspectral images is quantified with cosine similarity, and the time evolution of the oxide roughness is quantified in XPEEM timeseries. Low energy electron diffraction (LEED) indicates that the Cr ₂ O ₃ structure forms. In contrast to what has been observed for binary NiCr, the oxide of the ternary NiCrMo grows in a layer-by-layer growth mode for all five grain orientations observed, and the oxide structure observed with LEED is not dependent on the grain orientation. XPS indicates that only Cr and Mo oxides form, with Mo present predominantly as Mo(IV). Possible mechanisms for the role of Mo in changing the oxide growth mode are discussed.