Initial atomic-scale oxidation pathways on a Ni–15Cr(100) alloy surface

To understand the atomistic phenomenon behind initial oxidation processes, we have studied the nanoscale evolution of oxide growth prior to the formation of a complete layer on a Ni–15 wt%Cr(100) alloy surface using scanning tunneling microscopy/spectroscopy (STM/STS). At the onset of oxidation, a NiO superlattice forms oxide wedges across the step edges, eventually growing across the terraces. The completion of the NiO layer is followed by nucleation of the next layer, which always commences at the groove site of the superlattice. The Cr-oxide formation initiates as disk-shaped oxide particles early in the oxidation process, which Monte Carlo simulations reveal are likely caused by Cr clustering across the alloy surface. Upon further oxidation, a Cr(100)-p(2 × 2)O reconstructed surface is observed, indicating phase separation of Cr predicates the formation of the passive Cr-oxide film. The STS results vary across the oxide–alloy interface and between each oxide, providing greater insight into the origins of electronic heterogeneity and their effect on oxide growth. Using these data, we propose an oxidation model that highlights the growth of partial oxide layers on Ni–Cr(100) alloys within the pre-Cabrera–Mott regime.