Strong correlations between structural order and passive state at water–copper oxide interfaces

A fundamental understanding of coupled electrochemical processes including metal dissolution, structural evolution and solvation dynamics at the atomic level is of interest to corrosion research and electrochemistry in general. Using molecular dynamics (MD) simulations based on a reactive force field (ReaxFF), we evaluate the impact of non-stoichiometry in a model system of copper oxide passive films on the local fluctuation of the chloride ion density and structure and dynamics of interfacial water layers. We investigate (a) the interplay of oxygen content in the passive oxide film and the solvation dynamics of halide ions in the aqueous interfacial layers during breakdown of the oxide film, and (b) their combined effects on the dissolution kinetics of copper and adsorption of chloride ions on the copper-oxide surface. We demonstrate that the solvation behavior, particularly near the oxide/aqueous medium interface, is strongly correlated with the interfacial chloride ion concentration, which in turn is influenced by the oxygen stoichiometry in the passive oxide. Residence probability and hydrogen-bond correlations show that water present in the aqueous media forms ordered layers on oxide films with high oxygen content; and as the oxygen content is reduced, this order gets disrupted due to increased chloride ion adsorption. Interfacial molecular order is, therefore, strongly correlated with stoichiometry of the passive oxide film.