Assessing Aqueous Stability of Nonequilibrium Nickel Chromium Oxides from First Principles

Ni-Cr alloys are used in high tempurature applications where corrosion resistance is critical to performance. It has generally been thought that this corrosion resistance comes from a passive film made of NiO with the rock salt crystal structure and Cr₂O₃ with the corundum crystal structure. Recently, however, new data shows that valence-precise compositions and bulk equilibrium structures do not necessarily form in the ultrathin limit[1]. Specifically, Ni-Cr alloys form nonequilibrium phases through a solute capture process, whereby Ni-Cr oxide in the rock salt structure with unexpectedly large solubility of Cr on the Ni lattice occurs (and likewise for Ni in the corundum lattice). In order to better understand the formation of this nonequilibrium oxide, we use ab-initio Density Function Theory calculations to parameterize a cluster expansion model of the Ni-Cr-O system as a function of Cr and Ni content. Next, we use energies of formation derived from the cluster-expansion for use in electrochemical Pourbaix diagrams to understand the impact of the nonequilibium compositions on the stability of the oxide in aqueous environments.
[1] Xiao-xiang Yu,et.al, Phys. Rev. Lett. 121 (2018), 145701.