Fig.1 shows a cross section schematic illustration of an anodizing layer.

(1) Barrier Layer

An electro-conductive coating film that forms during the early stage of the electrolytic process. This layer supports the growth of a porous layer that forms on the top. This is where a porous film is formed by a reaction of electrolytically dissolved aluminum and oxygen at the bottom of the barrier layer. As the porous layer film grows, the bottom portion of the barrier layer migrates lower while the thickness remaining the same. The thickness is approximately 20nm.

(2) Porous Layer

The anodizing layer is an aggregation of this porous layer. It is a collection of hexagonal layer cells with a centrally located micropore, viewed from the top. The diameter of the cells is 100~300nm and the diameter of the micropores is 10~20nm, depending on the condition of the electrolytic process.
Dye Colored Anodizing is accomplished by applying the coloring dye at the top portion of the micropores, and Electrolytically Colored Anodizing is accomplished by depositing metal or metal oxides at the bottom portion of the micropores.