Electrode reactions on oxide covered aluminum

Abstract

AbstractA model of an aluminum electrode covered with a thin continuous oxide film is suggested. Anions and cations adsorbed on the oxide generate strong electrostatic fields across it. Anion adsorption produces electric fields attracting aluminum ions across the oxide whereas cation adsorption attracts electrons. The anion field strength depends on the anion diameter, valency and is inversely proportional to film thickness. The rate of aluminum ion migration under the influence of the adsorption field is an exponential function of the field strength and also depends on the defect structure of the oxide.The model has been applied to the corrosion of aluminum in sodium halide solutions where corrosion rate progressively increases with decreasing atomic number and anion radius. The anion radius also determines the value of constants in the inverse logarithmic growth law obeyed during oxidation of initially film‐free aluminum in these solutions.The model assists in explaining inhibition of the corrosion of aluminum by lead soaps and by neutral chromate solutions. Fatty acid anions have large limiting areas per ion and generate only weak fields accelerating aluminum ions through any pre‐existing film. Furthermore, very high energy is required for transport of aluminum ions through an adsorbed layer of fatty acid anions. Therefore in the absence of small anions such as chloride they inhibit corrosion. On initially film‐free aluminum thin oxide films are formed by the anodic discharge of fatty acid anions.Small divalent chromate ions produce strong electric fields across any preexisting oxide film. Since chromate ions and aluminum ions react at an oxide‐solution interface to form γAl2O3, these films thicken to greater values than those observed in dry air. The limiting film thickness is strongly pH dependant, being greatest at pH = 6.0, and is not significantly affected by small concentrations of chloride.