The Chemical Nature of the Anion Dependency of the Corrosion of Aluminum Alloy 7075-T6

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Abstract
The objective of this study was to elucidate the role of anions in reactions on an oxide-free aluminum alloy surface. The scrape potential technique was employed to measure three parameters, at open circuit, that are pertinent to the corrosion reaction. These are: (1) VSC, the potential of the metal scraped free of the passive film; (2) VSS, the steady state potential following passivation, and (3) dV/dt, the decay in potential during the formation of the passive reaction product. Experiments were conducted in sodium halide solutions of 0.1 to 4.0N concentration and of various pH's. All parameters were found to be anion, pH, and concentration dependent up to approximately 1.5M concentration. In this concentration range and at neutral pH, the anodic reaction rate on the fresh metal surface was observed to be anion dependent. This rate following the order F− > Br− > CI−. High negative values of VSS were observed in F− and OH− solutions. After scraping was stopped, and while the reaction product film was being formed, the potential decayed in the positive direction, linearly, in the millisecond range. This time is of the same order as the relaxation times of similar metal-anion complexation reactions. The linear decay rate (dV/dt1) was associated with the formation of metal-anion complexes following metal oxidation and preceeding the formation of the reaction product film. It is concluded that the role of the anion in metal dissolution lies in the formation of metal anion species at the fresh metal surface. The nature of these species directs the rate of formation and characteristics of the reaction product film.

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