Atomic-Scale Oxide Growth and Dissolution Kinetics on Ni-Cr Alloys

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Passivation oxide formation is the key for corrosion control of metal alloys. The kinetics of competing oxide formation and dissolution determines alloy corrosion behaviors in aqueous solution. Despite the important role of the multi-component oxide evolution, little has been known on the kinetics from the atomistic level. To this end, we have built a computational framework that enables simulations of competing kinetic processes in multi-component oxides from first principles. The effects of applied voltage, pH and temperature on oxide growth, dissolution and reprecipitation can all be captured in this model. Combining with our experimental measurements on alloy 22, we identified three voltage regimes with distinct oxide thicknesses and compositions. In the low voltage regime, oxide growth is the dominating process. The Cr/Ni ratio in the oxide is close to that in the alloy. In the medium voltage regime, Ni dissolves from the mixed oxide, NixCryO, and reprecipitates back as Ni oxide, NixO, to achieve phase separation. The resulted oxide film is significantly thickened with a duplex structure formed. In the high voltage regime, Ni dissolution dominates, and the Cr/Ni ratio increases. The temperature and pH dependencies have also been simulated. Higher temperature moves the dissolution-reprecipitation to the lower voltage; lower pH suppresses Ni reprecipitation and results in a thinner oxide film. These trends are also in agreement with the experiments.The oxide energetics of various stoichiometries are calculated by the density functional theory (DFT). Then the obtained data are used to train a surrogate lattice Hamiltonian with the cluster expansion (CE) method. Finally, kinetic Monte Carlo (KMC) simulations are run with the cation hopping barriers calculated on-the-fly based on the local environments from the combination of the above Hamiltonian and the linear Brønsted−Evans−Polanyi (BEP) relation.