Stability of trivalent and hexavalent chromium oxide layers on aluminum substrates from electronic structure calculations

Aluminum's light weight and high strength make it highly desirable as a structural material in the transportation sector. However, its relatively high corrosion susceptibility poses challenges for its wide application under exposure to the elements. Traditionally, hexavalent chromium (VI) oxide (${\mathrm{CrO}}_{3}$) coatings have been employed to provide corrosion protection, but their toxicity and environmental impact have motivated research to seek less harmful options. Trivalent chromium (III) oxide (${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$) suggests itself as a natural alternative due to its similar chemistry and lower toxicity. However, the stability of its bonding to pure Al metal substrates has not been thoroughly investigated. This study focuses on calculations of the adhesion work (${W}_{\mathrm{ad}}$) and interface energy (${\ensuremath{\gamma}}_{ij}$) of $\mathrm{Al}/{\mathrm{CrO}}_{3}$ and $\mathrm{Al}/{\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ interface structures with different Al crystallographic orientations ((100), (110), and (111)) using density functional theory calculations. Significant variations in the adhesion work ${W}_{\mathrm{ad}}$ and interface energy ${\ensuremath{\gamma}}_{ij}$ were observed for $\mathrm{Al}/\mathrm{CrO}{}_{3}$ interface structures across different Al planes, primarily attributed to the formation of interpenetrating Al-Cr oxide substructures and the formation of Al-O-Al bonds within the interface region. In contrast, the $\mathrm{Al}/{\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ interface energies exhibited a relatively weak dependence on the metal substrate orientation, with well-defined oxide and metallic layers and no oxygen penetration into the aluminum. Notably, $\mathrm{Al}/{\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ interfaces generally displayed higher adhesion work ${W}_{\mathrm{ad}}$ and lower interface energy ${\ensuremath{\gamma}}_{ij}$ compared to $\mathrm{Al}/{\mathrm{CrO}}_{3}$, suggesting that ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ forms more stable interfaces and adheres more strongly to Al. These findings contribute to the development of sustainable corrosion protection solutions in the aerospace and marine industries by highlighting the potential of ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ coatings as a viable environmentally friendly alternative to ${\mathrm{CrO}}_{3}$.