Strong Correlations Between Structural Order and Passive State at Water-Copper Oxide Interfaces

Abstract

Fundamental understanding of coupled electrochemical processes at metal-oxide/aqueous media interfaces, e.g., interfacial structure evolution, metal dissolution, and solvation dynamics at the atomic level are of immense interest to corrosion research, surface chemistry, and electrochemistry in general. Using reactive force field (ReaxFF) molecular dynamics (MD) simulations of CuOx passive thin film on Cu substrate, we investigated the impact of non-stoichiometry of passive oxide film on its breakdown, and aqueous corrosion (in sea water) of the underlying copper. Upon exposure to aqueous media (halide concentration similar to seawater), we find that halide ions adsorb on to the passive oxide layer, and cause dissolution of Cu atoms via formation of water-soluble Cu-halide complexes; eventually, this results in the breakdown of the passive oxide layer, and consequently leads to corrosion of underlying Cu. Our MD simulations indicate that oxygen rich CuOx films show enhanced resistance to breakdown via impeding halide ion adsorption as well as metal dissolution; for instance, increasing oxygen stoichiometry from 0.2 to 0.79 causes by halide adsorption to drop by half. Thus, O-rich Cu-oxide films provide better corrosion resistance than O-deficient ones. Interestingly, we find that high O-content in the oxide film promotes ordering of water molecules at the water/oxide interface leading to 2D “ice-like” layers. These ordered water layers inhibit the adsorption of halide ions via a phenomenon similar to brine rejection. These findings clearly indicate the oxygen stoichiometry, dissolution kinetics, and solvation dynamics at the water/oxide interface are strongly correlated to each other [Narayanan et al. Electrochimica Acta 179, 386 (2015)]. To validate the predictions of theory, and to probe the initiation of pits during aqueous corrosion of Cu, we employ coherent x-ray diffraction imaging. This imaging technique coupled with large scale molecular dynamics simulations identify localized strain fields that arise in the Cu nanoparticles; this in turn, provides atomic scale insights into pit formation during aqueous corrosion.