Structure, properties and application of aluminum anodized film

Aluminum or aluminum alloy products as the anode, placed in the electrolyte solution for power treatment, the use of electrolytic effect to form aluminum oxide film on the surface of the process, known as aluminum and aluminum alloy anodizing treatment.

Structure of anodic oxide film

Anodic oxide film consists of two layers, the outer layer is called porous layer, thicker, loose porous, low resistance. The inner layer is called the barrier layer (also known as the active layer), which is thin, dense and has high resistance. The porous outer layer grows on top of a dense inner layer with dielectric properties. In general, the anodic oxide film is an array of hexagonal cylinders, each of which has a star-shaped hole filled with solution, a honeycomb structure, and the thickness of the hole wall is twice the diameter of the pore.

(1) Barrier layer The barrier layer is composed of anhydrous A12O3, thin and dense, with high hardness and the effect of preventing current through.

(2) The porous outer layer of oxide film The porous outer layer is mainly composed of amorphous A12O3 and a small amount of r-A12O3.H2O also contains anions of the electrolyte.

The pore size of the oxide film is between 100nm and 200nm, the thickness of the oxide film is about 10 microns, the porosity is about 20%, and the hole distance is between 300 and 500nm. The cross-section of the oxide film shows that the oxide film hole is basically tubular structure, and the dissolution reaction of the oxide film is basically at the bottom of the hole. And the general sulfuric acid DC anodic oxide film aperture is about 20nm, if it is 12 micron oxide film, it is how deep the thin tubular structure ah! Assuming this is a 1m diameter well, it will be 600m deep.

Most of the excellent characteristics of the oxide film, such as corrosion resistance, wear resistance, adsorption, insulation and other properties are determined by the thickness and porosity of the porous outer layer, but the two are closely related to the anodizing conditions, so the anodizing conditions can be changed to meet the different requirements of the use of the film.

Film thickness is a very important performance indicator of anodized products, and its value directly affects the corrosion resistance, wear resistance, insulation and chemical coloring ability of the film. In a conventional anodizing process, the film thickens over time. After reaching the maximum thickness, it gradually becomes thinner with the extension of processing time, and some alloys such as A1-Mg and A1-MG-Zn alloys are particularly obvious. Therefore, the oxidation time is generally controlled within the maximum film thickness time.

Properties and applications of anodic oxide films

The anodic oxide film has high hardness and wear resistance, strong adhesion ability, strong adsorption capacity, good corrosion resistance and electrical insulation and high thermal insulation. Because of these special properties, it has been widely used in various aspects.

The main uses are:

(1) Improve the wear resistance, corrosion resistance and weather corrosion resistance of parts.

(2) The transparent film generated by oxidation can be colored into a variety of color films.

(3) As a capacitor dielectric film.

(4) Improve the binding force with organic coating. As a coating base.

(5) The bottom layer for electroplating and enamel.

(6) Other applications being developed, such as solar absorption panels, ultra-high hardness films, dry lubrication films, catalytic films, nanowires, and deposition of magnetic alloys in porous films as memory elements.