In the case of aluminium, the anodizing process forms a layer of aluminium oxide - Al203, which is very hard, relatively inert, electrically insulating and can absorb dyes to colour the film.

The anodic film itself grows at the aluminium / aluminium oxide interface by the continuous formation and dissolution of a layer of oxide, this is the so-called barrier layer and its thickness is a function of the process starting voltage. A porous, more structured layer forms on top of the barrier layer making up the rest of the coating.