Density functional theory study of influence of oxide thickness and surface alloying on Cl migration within α-Al₂O₃

Insertion and migration of chlorine atoms (Cl) in α-Al₂O₃ were studied by density functional theory calculations focusing on the influence of oxide thickness and doping by Mg, Cu and Si. Work function, electronic band gap, and insertion energy were calculated to explore thermodynamics of Cl migration. Partial density of state (PDOS) calculations revealed the role of dopants in the electronic character of metal–O and Al–Cl bonds. Work function data showed the effect of Cl insertion into the oxide film on the corrosion resistance. Cl can locate at an O vacancy (V_O) with a large exothermic insertion energy, independent of oxide thickness, but can only locate at superficial Al vacancy (V_Al) exothermically. The energy barrier for Cl migration via neighboring V_O increases with oxide thickness, and is 2 ∼2.5 eV for thicker oxides. Cl insertion causes a work function reduction exceeding 2 eV, implying a decreased corrosion resistance. The inhibition of Cl migration by Si-doping can be explained by a more intense hybridization peak of Si–O over Al–O in the PDOS profile, while the reduction of energy barrier by Mg–/Cu-doping probably is due to the deviation of metal-s state from the Fermi energy, thus facilitating Cl movement within the oxide.