The Specific Effects of Chloride and Sulfate Ions on Oxide Covered Aluminum

The distribution of a‐c resistance throughout the thickness of oxide film on aluminum specimens, conditioned by pre‐exposure to 1.0 N solutions of sodium chloride and sulfate, has been determined by an electrical method. This method involves measurement of series capacitance and dielectric loss of the oxide covered aluminum as the surface films thin slowly in a slightly alkaline, passivating, sodium chromate solution. Results show that chloride ions become included at certain selected locations in the oxide film, create additional current carriers, and lower the ionic resistance of the film. Sulfate ions do not appear to become included in aluminum oxide films under these conditions and do not lower the film resistance. Aluminum oxide films containing chloride ions dissolve more rapidly in the chromate solution than similar untreated films or films that have been treated in sodium sulfate solution. These observations are held to throw new light on the well defined tendency of oxide covered aluminum to pit severely in nearly neutral sodium chloride solutions.