It is well known that aluminum and its alloys oxidize in the air, and the naturally formed oxide film on the aluminum surface is amorphous, which will cause the aluminum metal surface to lose its original luster, although this layer of natural oxidation The film will slightly passivate the aluminum metal surface, but it is very thin, about 4 ~ 5nm, and porosity, mechanical properties are also poor, it can not effectively prevent the various media in the atmosphere of aluminum metal further corrosion.
After anodizing, the surface of aluminum and its alloy 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 aluminum and its alloy products have many characteristics after anodization, the aluminum anodizing process is widely used in the surface treatment of aluminum products. Applications in industry can be roughly classified into the following types.
(1) Prevention of corrosion of products: 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. The chromic acid oxidation method is particularly suitable for aluminum welded parts and riveted parts.
(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 aluminum even more beautiful. This layer of color film is both a decorative layer and an anti-corrosion layer. , such as lighters, gold pens and crafts.
(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 aluminum and its alloys 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. The dielectric, can also be used as a cable jacket of aluminum oxide skin, the surface of the insulating layer instead of plastic wrapper and plastic wrapper is more popular in foreign countries, the film thickness of 27.5μm, the penetration voltage of 441V. If the phenolic resin is used as the film hole filling, the breakdown voltage can be increased by a factor of 2. In the oxalic acid solution, when the film thickness is increased, a high-quality insulation layer having a resistance of 200 Ω and a breakdown voltage of 980 V can be obtained. Of course, this feature is utilized In addition to wire, it 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.
(6) For modern architecture: Due to the introduction of electrolytic coloring of aluminum alloys, domestic electrolytic coloring has been increasingly applied to architectural aluminum profiles in recent years. It not only has bronze color, black color, and red color suitable for architectural coloration, but also has excellent wear resistance. In general conventional oxidation, it is particularly worth mentioning that the lightfastness is very good, and no decoloration is observed in the atmosphere of daytime and nighttime over 20 years, which is incomparable with the oxidative coloring method. The electrolytic coloring of aluminum profiles can be used not only for building windows and doors, but also for shop counters and shelves.
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