Coupled outward and inward oxide growth during metal oxidation

Metal oxidation is traditionally described as a dichotomy: oxide scales grow either outward via cation diffusion or inward via anion diffusion, with the dominant pathway determined by the prevailing point defects in the oxide lattice. Here, we demonstrate that these two modes are not mutually exclusive but intrinsically coupled within a single oxidation process. Using in situ environmental transmission electron microscopy to monitor the oxidation of copper-a canonical cationic system-we directly visualize simultaneous oxide propagation at both the O₂/Cu₂O surface interface and the buried Cu₂O/Cu interface. Complementary simulations reveal the mechanistic origin of this behavior. Outward cation transport during surface oxide growth injects a sustained flux of vacancies into the underlying metal lattice, which facilitates inward oxygen incorporation and drives oxide growth at the buried interface. This vacancy-mediated feedback couples cationic and anionic transport process, providing a unified framework in which surface cationic growth directly enables inward anionic-driven growth. By identifying vacancy injection as a key mediator of interfacial mass transport, this work provides a mechanistic basis for predicting oxidation kinetics and defect-regulated solid-state transformation pathways in reactive environments.