It is well known that aluminum and its alloys oxidize in the air, and the naturally formed oxide film on the surface of machined aluminum parts is amorphous, which will cause the surface of machined aluminum parts to lose its original luster, although this natural oxide film will protect the surface of machined aluminum parts but it is very thin, about 4 to 5 nm, and has a large porosity and poor mechanical properties. It cannot effectively prevent further corrosion of machined aluminum parts by various media in the atmosphere.
After anodizing, the surface of machined aluminum parts can get a layer of dense film (from tens of micrometers to even hundreds of micrometers) that is much thicker than natural oxide film. This artificial oxide film is then subjected to a sealing treatment. The amorphous oxide film is transformed into a crystalline oxide film, and the pores are also closed. Therefore, the gloss of the metal surface remains unchanged, and the corrosion resistance and mechanical strength are improved. After dyeing, a decorative appearance can also be obtained. Since the machined aluminum parts have many characteristics after anodization, the aluminum anodizing process is widely used in the surface treatment of machined aluminum parts. Applications in industry can be roughly classified into the following types.
(1) Prevention of corrosion of machined aluminum parts: The film obtained by anodization is subjected to proper sealing treatment and has good stability in the atmosphere. Regardless of the oxide film obtained from the sulfuric acid solution, the oxalic acid solution or the chromic acid solution in the normal process, the corrosion resistance is very good, such as a daily-use aluminum pot, a pot, a washing machine liner, and the like.
(2) Protection-Decoration: On the oxide film with high transparency, the oxide film has the characteristics of adsorbing various organic dyes or inorganic pigments. Various bright and bright colors and patterns can be obtained on the oxide film. The appearance of a few new processes such as multiple oxidation of an oxidation, fireworks patterns, wood grain patterns, transfer printing of oxidized offset printing, oxidation of porcelain, etc. makes the appearance of machined aluminum parts even more beautiful. This layer of color film is both a decorative layer and an anti-corrosion layer.
(3) As a hard wear layer: In a sulfuric acid or oxalic acid solution, a thick and hard film layer can be obtained by adjusting the anodizing process conditions, and the selective oil can be stored by using the pore and absorption properties of the film layer. It is effectively applied to the conditions of working in the friction state and has the characteristics of lubrication and wear resistance, such as engine cylinders and pistons of automobiles and tractors.
(4) As an electrical insulating layer: The oxide layer of machined aluminum parts has the characteristics of large electrical resistance, and the thickness of the layer is proportional to the electrical resistance. This feature has certain practical significance as electrical insulation and can be used as a capacitor. This feature is utilized can also be used in other electrical appliances.
(5) As the bottom layer for painting and electroplating: As the oxide layer has porosity and good adsorption capacity, it has good adhesion with the paint film and organic film, and can be used as the bottom layer for painting. The anodized layer of phosphoric acid can be used as the underlying layer on the aluminum plating.
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