The anodized film is composed of two layers, and the porous outer layer is grown on a dense inner layer having dielectric properties, and the latter is called a barrier layer (also called an active layer). Observation by electron microscopy revealed that the longitudinal and lateral faces of the film layer almost all showed tubular holes perpendicular to the metal surface, which penetrated the outer layer of the film up to the barrier layer between the oxide film and the metal interface. The barrier layer is composed of anhydrous aluminum oxide, which is thin and dense, has a high hardness and prevents the passage of electric current. The barrier layer has a thickness of about 0.03-0.05 um, which is 0.5% to 2.0% of the total film thickness.



The Anodized aluminum is surrounded by dense to form a honeycomb hexagonal body called a unit cell, and the entire film layer is composed of a myriad of such unit cells. When the electrolyte is sulfuric acid, the sulfate content in the film layer is normally from 13% to 17%. The porous outer layer of the oxide film is mainly composed of amorphous alumina and a small amount of hydrated alumina, and further contains a cation of the electrolyte. Most of the excellent properties of the oxide film are determined by the thickness and porosity of the porous outer layer, which are all closely related to the anodization conditions.