On the Formation of Voids in Anodic Oxide Films on Aluminum

A mechanism is proposed for the formation of voids at triple points between grains in the anodic oxide formed on aluminum and for the growth of porous films. The proposed mechanism involves the condensation of cation and/or metal vacancies below regions in the barrier film (triple points) that are characterized by high cation vacancy diffusivity, due to the high degree of lattice disorder. Vacancy condensation causes local decohesion of the barrier layer from the substrate, thereby inhibiting the growth of the layer into the metal in these regions. The lower film growth rate, relative to the surrounding areas, accounts for the observation that the voids subtend protrusions of the metal into the film. Periodic detachment of the voids, followed by nucleation and growth of new voids at the apex of the protrusions, accounts for decoration of the barrier layer with voids as well as for the formation of the hexagonal array of pores, when aluminum is anodized in some electrolytes.