Chloride Ion Interactions with Oxide-Covered Aluminum Leading to Pitting Corrosion: A Review

Metals and alloys such as aluminum (Al), stainless steels, and nickel-based alloys exhibit corrosion resistance in a wide range of environments due to the presence of protective, passive oxides. However, in environments that contain aggressive anions such as chloride, Cl−, the passive film becomes unstable and degrades locally causing film breakdown and pitting corrosion. A number of theories describing the initiation of pitting corrosion have been postulated and discussed but to date there is no consensus on the mechanism of breakdown. Since all current mechanisms require Cl− interactions for oxide film breakdown in Cl− containing environments, the question is what is the nature of the interaction of aggressive anions such as Cl− with the passive film, adsorption and/or absorption, leading to pitting? This communication focuses on the interaction of Cl− with the passive oxide film on pure aluminum by reviewing and summarizing the available experimental data concerning Cl− interactions. It should also be noted that the observations for Cl− interactions with Al reviewed and summarized herein might not be applicable to all metals and alloys.