If the aluminum anodized film formed under one condition is anodized in the same electrolyte or different electrolyte under another condition, a new oxide film is formed between the aluminum matrix and the original oxide film. This reaction can, of course, be repeated many times to create multiple layers of oxide, each with a specific property, and a functional anodized film can be made using this process.

The double layer anodic oxide film is formed by placing aluminum matrix in sulfuric acid solution first and then oxalic acid solution. The micropore structure between each layer of the film is shown in FIG. 1, because of the difference in anodic oxidation voltage. Due to the different anodic oxidation conditions, it presents a special micropore geometry. The interconnecting appearance of the micropores is similar to the bifurcation of a branch, so it is sometimes called "bifurcation hole".

The change of micropore morphology reflects the change of porosity, which in turn affects the properties of anodic oxide film. If in the process of electrolytic coloring, metal electrolytically precipitates in the double-hole structure, the amount of metal precipitation is related to the porosity of each layer, and eventually a "sandwich" structure appears. This provides an important means to control the shade and tone of the color and adjust the performance of the functional film.