First-Principles Surface Interaction Studies of Aluminum-Copper and Aluminum-Copper-Magnesium Secondary Phases in Aluminum Alloys

First-principles density functional theory-based calculations were performed to study θ-phase Al ₂ Cu, S-phase Al ₂ CuMg surface stability, as well as their interactions with water molecules and chloride (Cl ⁻ ) ions. These secondary phases are commonly found in aluminum-based alloys and are initiation points for localized corrosion. Density functional theory (DFT)-based simulations provide insight into the origins of localized (pitting) corrosion processes of aluminum-based alloys. For both phases studied, Cl ⁻ ions cause atomic distortions on the surface layers. The nature of the distortions could be a factor to weaken the interlayer bonds in the Al ₂ Cu and Al ₂ CuMg secondary phases, facilitating the corrosion process. Electronic structure calculations revealed not only electron charge transfer from Cl ⁻ ions to alloy surface but also electron sharing, suggesting ionic and covalent bonding features, respectively. The S-phase Al ₂ CuMg structure has a more active surface than the θ-phase Al ₂ Cu. We also found a higher tendency of formation of new species, such as Al ³⁺ , Al(OH) ²⁺ , HCl, AlCl ²⁺ , Al(OH)Cl ⁺ , and Cl ₂ on the S-phase Al ₂ CuMg surface. Surface chemical reactions and resultant species present contribute to establishment of local surface chemistry that influences the corrosion behavior of aluminum alloys.