Energetics and Diffusion Kinetics of Point Defects in Cr₂O₃ from First-Principles

High-temperature oxidation of a 21-2N steel alloy in a dry atmosphere produces a multilayered oxide film containing MnCr2O4, Cr2O3, and Mn3O4. To understand how this multilayered film evolves over time, it is crucial to identify the prevalent defect types and their respective high mobility diffusion pathways. In this work, the thermodynamics and kinetics of Mn and Cr point defects in the Cr2O3 system are evaluated with density functional theory calculations. High-temperature conditions consistent with the application domain are modeled by adjusting the ab initio chemical potential values to fit relevant thermochemical data. Mn is found to readily form substitutions on the Cr sublattice, thereby enabling rapid vacancy-mediated diffusion of MnCr defects. Furthermore, an interstitialcy mechanism is investigated as an alternative diffusion pathway for both Cr and Mn. Finally, formation energy and migration barrier heights from this work and others are synthesized to produce a mechanistic model of diffusion in the multilayered film. This model aims to predict changes in relative thickness at long exposure times and the specific interface at which oxide growth takes place.