Localized Breakdown of the Natural Oxide Film on Aluminum by Chloride Ions and the Formation of Oxide Blisters

A sequence of events is discussed for the initiation and propagation of corrosion pits on aluminum in chloride solutions. Corrosion pit initiation takes into account adsorption of chloride ions on the oxide surface, penetration of chloride ions through the oxide film, and localized dissolution of aluminum at the metal/oxide interface. Corrosion pit propagation involves formation of blisters beneath the oxide film due to localized reactions which produce an acidic localized environment. The blisters subsequently rupture due to the formation of hydrogen gas in the occluded corrosion cell. Calculation of the local pH within a blister from the computed hydrogen pressure within the blister gives pH values in the range 0.85 to 2.3.