The anodic oxidation of titanium and its alloys

Titanium is naturally covered with a nanometric oxide film (TiO2) that accounts for most of its industrial applications, where corrosion resistance is the key property. In functional applications, like biomedical devices, photocatalytic reactors, water splitting, and more, focus is set on the TiO2 film characteristics. Anodic oxidation allows obtaining a variety of oxide features-amorphous or crystalline, nanometers micrometers thick, doped with other elements or ions, with surface texturing at the micro or nanoscale. Controlling TiO2 surface properties can not only drastically improve the material efficiency in existing applications but also broaden its fields of use; therefore, an increasing control on production methods is necessary. This article is intended to provide an overview of the anodic oxidation techniques with specific reference to titanium anodizing, and to describe the mechanisms of widely diffused anodic processes, focusing on the resulting oxide features.