Structure and Behavior of Oxide-Coated Aluminum in Contact with Acidic and Alkaline Aqueous Solutions─A Reactive Molecular Dynamics Simulation Study

Aluminum oxide coating is inert to weak acidic and alkaline aqueous solutions and naturally provides a passivation layer on the coated substrate against corrosion. It exhibits good adhesion to the substrate. Reactive molecular dynamics simulations were performed to study the structure, topology, and behavior of the Al/oxide surface upon exposure to strong corrosive media at 363 K. A systematic study of OH– and H3O+ ion concentration on the behavior of Al/oxide surface was performed to describe structural, morphological, and topological differences associated with the mechanism of film growth in aqueous solutions. The difference found in the mechanism of film growth on the Al/oxide surface during corrosion influences the structure, surface topography, and interatomic bond topology of the formed films. Hydroxylation is a major reaction leading to the formation of a pseudoboehmite structure of film developed in alkaline solutions, whereas hydration is the reaction governing the film growth in acidic solutions. The mechanism of observed reactions during corrosion of the Al/oxide surface was confronted with available experimental data. The chemical composition of the developed films is pH-independent, and Al-enriched films form upon contact with both alkaline and acidic media. A higher content of OH group anchored to the Al/oxide surface is found in alkaline solutions, which increases the surface roughness and solvent-accessible surface area (SASA). Topological aspects of the grown films and Al/oxide interface were described performing Voronoi analysis supplemented with n-ring analysis. A dominance of lattice distortions of the Al substrate over crystal defects was observed, and a topologically complex amorphous nature of the films formed on Al/oxide surface.