Aluminum + water reaction

Abstract

During the aluminum + water reaction, aluminum ions and electrons are removed in separate steps at different sites on the surface. Ions are removed nearly uniform over the entire surface, which is covered at all times with a thin amorphous oxide film. The outer surface of this oxide is first hydrolyzed and then dissolves to yield soluble species which either remain in solution or, at intermediate values of pH, precipitate as a porous hydroxide of extremely small particle size. The hydroxide appears to be identical to pseudoboehmite. The overall rate of the corrosion reaction is controlled by this dissolution of the film and by the disposition of the soluble products. The corrosion rate is nearly independent of specimen potential, solution pH below 10, and of the presence in solution of many salts at concentrations as high as 1 mole/1. The rate is strongly dependent on temperature, on the presence of specific inhibitive salts, and increases rapidly at high pH. In environments which preclude oxide dissolution, e.g., water + dioxane mixtures or water vapour, the corrosion rate is drastically reduced. The corrosion rate is constant when no solid hydroxide is formed, and is otherwise a strong function of the amount of reaction. At high temperatures the rate decreases with time as the precipitated hydroxide hinders transport. At lower temperatures the rate may first increase with time as nucleation of hydroxide provides sinks for soluble species close to the interface, and then at long times the rate decreases as the hydroxide layer thickens. Electrons are removed more readily at special sites, and in our specimens these sites were primarily at the grain boundaries. Electron removal results in increased hydroxyl ion concentration, and this in turn results in more rapid attack on the protective oxide. At such cathodes the oxide film is maintained at a small constant thickness at which there is a balance between the rates of its dissolution by the basic solution and growth because of the great affinity of aluminum for oxygen. At cathodic sites there is thus anodic activity as well, but electron removal predominates. The metal is corroded more rapidly at cathodes, producing pronounced grain boundary attack in our specimens. Application of anodic potentials eliminates grain boundary attack.