The anodic oxide film is formed on the surface of aluminum when it is oxidized electrochemically as an anode. The properties and growth mechanism of anodized films have been studied for many years. Anodic oxide film of aluminum has two types of structure, namely non-growing (barrier film) and growing (multi-pass film). Coloration is possible because of the porous structure (the holes are very regular and uniformly arranged). The following is the growth sequence of multi-pass anodized film.

(1) When anodic oxidation begins, a very thin oxide film is formed on the surface of the metal aluminum.

(2) Due to the insulating property of anodic oxide film, the high voltage on the oxide film causes the dielectric rupture of the oxide film, making the oxide film generate micropores.

(3) The electrolyte penetrates into the micropore, and the porous structure is formed due to the further growth of the electrified oxide film.

(4) Continuous growth of oxide film occurs at the bottom of micropore.

(5) This process continues to occur to form a complete multi-pass anodic oxide film.

The formation of the initial anodic oxide film micropores and the subsequent anodic oxide film micropores growth process.