The Chemical Nature of Aluminum Corrosion: III . The Dissolution Mechanism of Aluminum Oxide and Aluminum Powder in Various Electrolytes

Attention was focused on the first steps in the pitting process, the adsorption of aggressive anions and the dissolution of aluminum oxide. The adsorption of chloride on alumina and aluminum powder are quite similar and show a saturation plateau at high chloride concentration; the amount of chloride adsorbed is proportional to the weight of alumina. The amount of "free" aluminum ions dissolved from alumina is inversely proportional to the chloride concentration while the amount dissolved from aluminum powder appears to be independent of chloride concentration. Nitrate ion is reduced by aluminum metal but not by alumina. Alumina will greatly reduce the amount of "free" aluminum in solutions of , , and . These results support the idea that the first steps in aluminum pitting in aggressive solutions involve adsorption of the anion on the oxide film followed by chemical interaction to form a soluble species resulting in a thinning of the film and direct contact with the metal surface.