Density-functional theory investigation of Al pitting corrosion in electrolyte containing chloride ions

The behavior of chloride ions (Cl−) and oxygen species (the oxygen atom, O or molecular oxygen, O2) on Al(1 1 1) surface has been studied by density functional theory calculations in order to deepen the molecular understanding of fundamental processes leading to pitting of aluminum (Al). The adsorption behavior of individual species, Cl−, O atom and O2 was determined first. Subsequently, three possible scenarios in different pitting stages were modeled exploring the repassivation and dissolution of Al in neutral electrolyte containing Cl−. In scenario i, it was found that Cl− can hardly destroy even an O-monolayer on Al(1 1 1) surface, however may lead to the elongation of AlO bond and the weakened binding between the first Al layer and subsequent Al layers. Both O2 and Cl− were simultaneously introduced onto Al(1 1 1) in scenario ii. The result showed a weakened AlO interaction and an intensive hybridization peak at −0.18 Ha between Al-3p with Cl-3p suggesting insufficient repassivation behavior of Al under this condition. Finally, scenario iii mimicked different local environmental conditions in pits formed on Al. At low coverage of Cl−, chloride ions had little effect on surface relaxation. The interaction among chloride ions and Al surface became stronger as Cl− coverage increased. Surface Al atoms dissolved gradually and substructures such as AlCl3 and Al2Cl5 formed when the coverage was larger than 2/3 ML of a monolayer.