Atomistic Simulations On Correlating Localized Corrosion With Near-Surface Oxide Layer Stoichiometry In Al Alloys And Its Control Through Electric Fields

Pitting corrosion, due to halide ion interaction with metal surfaces, is localized and is typically initiated at a lattice defect site such as an oxygen vacancy. Our atomistic modeling efforts are focused on studying the relationship of near surface oxygen concentration in ultra-thin passive oxide scale and their corrosion resistance. The main aim is to gain mechanistic understanding of the processes correlating the nanometer scale morphological changes in the passive oxide and the pit initiation. Three important factors that can impact the corrosion resistance are the oxide density, oxide stoichiometry and oxide composition in case of alloys. Molecular dynamics simulations have been performed to provide insights into the structure of the passive oxides at the atomic level and correlate with the experimental measurements on corrosion resistance. These simulations employ dynamic charge transfer between atoms and are used to investigate nanoscale oxides grown on Al and Al-Ni surfaces due to atomic and molecular oxygen. Furthermore, we have also investigated the effect of electric field such as those generated in photon-assisted synthesis on the microstructure and composition of oxides formed on metal and metal alloy surfaces. Our simulations indicate that the rate of oxygen incorporation into the near surface regions of the passive oxides can be dramatically enhanced with atomic oxygen and/or electric field compared to natural oxidation. Increasing the electric field (~ 10⁷ V/cm) drives the surface chemisorbed oxygen to the vacancy sites in the oxide interior leading to dramatic density and stoichiometry improvements of the grown ultra-thin oxide film. In case of Al-Ni alloy oxidation, our atomistic simulations suggest that photon-assisted synthesis overcomes the activation energy barrier for chemisorption through the creation of activated atomic oxygen and ionic migration due to electric field produced across an oxide film leading to significantly enhanced oxidation kinetics, enabling us to control the complex oxide composition at atomic length scales. The ability to tailor oxide microstructure and tune alloy oxide composition has tremendous implications for designing improved passivation layers for enhanced corrosion protection.