Oxidation Protection with Amorphous Surface Oxides: Thermodynamic Insights from Ab Initio Simulations on Aluminum

Native surface films play a key role in the oxidation and corrosion protection of functional and structural materials. Here, we present a fully ab initio approach for understanding the thermodynamic driving force behind the initial phase selection among amorphous and crystalline structures for a surface film growing on a crystalline substrate. We apply the approach to elucidate the competition among corundum (α), spinel (γ), and amorphous (am.) Al₂O₃ films growing on aluminum metal. We show that the amorphous Al₂O₃ film becomes thermodynamically the most stable form below around ∼1 nm, that is, the relative energetic stabilities of thin polymorphic Al₂O films follow am. < γ < α. As the film thickness increases, the relative stability relation first changes to γ < α < am. and then to the bulk limit of α < γ < am. The nanoscale γ films distort substantially to form exclusively four- and fivefold-coordinated Al-O polyhedra, lose the close-packed O framework, and become "amorphous-like", that is, exhibit both short-range order and energetic characteristics that are commensurate with the amorphous form. Our results provide a quantitative, first-principles confirmation for the early hypotheses on the thermodynamic stability of amorphous surface films and provide insights for the critical role they play in oxidation protection. Handling the complexities associated with the initial film growth, including bulk, surface, interface, and strain energy effects in realistically complex ab initio simulations, we expect this approach to contribute to understanding of the mechanism behind effective passivation films for aluminum alloys and beyond.