Effect of chromate on open-circuit pit growth in aluminum thin films

The pitting behavior of Al thin films at open circuit and the influence of the nature of the oxide film on this Phenomenon were Studied. At open circuit, small additions of dichromate or chromate induced metastable pitting but protected the rest of the sample surface for an extended period of time, even in the presence of persulfate, a strong oxidizer. The surface oxide on 3-year-old 100-nm Al thin films was sputter-etched away so that fresh oxide could form upon exposure to air. Galvanic coupling experiments in 1.0 M sodium chloride (NaCl) showed that the etched sample acted as the anode when coupled with a sample having the original oxide. However, when immersed in chromate-containing chloride solutions, the etched sample was the cathode. On some samples, the old oxide was sputter-etched away on only part of the sample surface, while the rest of the sample surface was left intact, Immersion of those samples in a chromate-containing chloride solution at open circuit resulted in the formation of metastable pits at both the etched side and unetched sides, but the etched side was cathodically protected. Cr was incorporated into the passive film on the etched side but was not found on the unetched side. It Was concluded that the etched sample formed a much better protective passive film and became more corrosion resistant since it Could more easily interact with dichromate in solution.