Electrolyte Ion Effects on the Formation of Nanoscale Structure in Aluminum's Passive Oxide Prior to Pit Initiation

Abstract

The impact of pH, chloride concentration, and the presence of an alternate, adsorbing anion on interfacial void formation in the passive oxide of aluminum are explored. Constant current, anodic polarization of a model passive oxide on Al(111) oriented thin films exhibits a characteristic signature that can be used to track void formation. Void formation is shown to be highly sensitive to pH, consistent with competition between vacancy pairing reaction that forms voids and a vacancy filling reaction that leads to oxide growth. A lack of sensitivity is observed for chloride concentration range of 50 to 200 mM, arguing for a concerted proton-chloride interaction where a critical chloride concentration may be sufficient to drive void formation. Borate, as an alternate adsorbing anion, is shown to displace chloride at the outer:inner layer interface and provides a path for estimating this critical chloride concentration.