Growth of Passive Films on Valve Metals and Their Alloys

Abstract

In this article, growth of thin oxide films formed anodically on aluminum, titanium, zirconium, magnesium, and their alloys is described with particular attention to the structure of oxides, ionic migrations in growing oxide films under the high electric field, and interfacial enrichment of alloying elements. Aluminum forms usually amorphous, compact oxide films in neutral aqueous solutions by anodic polarization, while zirconium forms crystalline oxide films with the phase depending upon the pretreatments, formation potential, and alloy concentration. Titanium shows an amorphous-to-crystalline transition, with the growth efficiency being reduced by the crystallization because of oxygen gas generation within the growing oxide films. Alloying elements incorporated into the oxide films modify the structure of the oxide films and also utilize as a tracer element to examine the ionic migration process. In the growing amorphous oxide films, the metal–oxygen bond strength controls the relative migration rates of cation species.