Coating and Surface Treatment

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Surface Treatment

Aluminum naturally has an attractive surface and superior corrosion resistance. Various surface treatments can enhance these characteristics or provide additional characteristics.

Typical aluminum surface treatment methods include anodic oxide coating, coloration, coating, mechanical surface treatment, chemical film coating, bright anodic oxide coating (gloss treatment), enamel coating, and plating. In addition, new technologies have been developed including ion plating and sputtering.

Picture of Surface Treatment

Coating

The typical examples of coated aluminum include an aluminum coated plate (called colored aluminum) that is manufactured by continuously baking an aluminum coil coated with polyester, acrylic, epoxy, fluoric, or other coating material; aluminum cans that are coated before or after molding; aluminum sashes, blinds, and building materials with coated alumite priming. Of these coatings, colored aluminum is finding increasing application in building materials for roofs and outer walls, in vehicles, and in various other types of equipment.

Fig. Coating

Surface Functional Treatment

The surface treatment for aluminum was conventionally performed mainly for improving fanciness and corrosion resistance. Recently, however, a surface functional technology has been developed to providing materials with superior functionalities in compliance with the needs of electronics and other advanced technological fields. This technology is intended to produce functional film by applying various technologies for anodic oxide coating, chemical film coating, plating, etc.
The functionalities provided by functional coating include superior characteristics such as wear and corrosion resistance, and a lubricating property, magnetization, electric conductivity, luminescence, and other properties that facilitate moving machine parts.

Anodic Oxide Coating

The anodic oxide coating of aluminum, which is also known as Alumite, is one of our strongest technologies.

Aluminum is an active metal that naturally forms a thin film of aluminum oxide on its surface as it reacts with oxide in the atmosphere. Since this aluminum oxide, once formed, does not change, it protects the internal aluminum. This explains why aluminum typically corrodes very little. The oxide film that is naturally produced is very thin. In contrast, thick, strong oxide film is artificially produced by an electrochemical treatment, which is called the anodic oxide coating. In practice, an aluminum product is put in an electrolytic solution (acid solution). When low DC, AC, or AC/DC is applied with the aluminum product used as an anode, oxide film forms on its surface. If different types, temperatures, and current densities of the electrolytic solution, different types of aluminum alloys, and other conditions are combined here, various colors such as silver, gold, umber, and black can be produced (coloring/coloration) and harder film can be made available.

The surface of oxide film has a large number of micropores (0.01 to 0.05 µm in diameter; 60 to 800 micropores per µm2). If aluminum coated with oxide film is put in a pressure vessel and pressurized by blowing steam or if it is put in boiling water to have aluminum hydroxide formed in the superficial area of its micropores, the micropores are closed, making the surface smooth. This treatment is called sealing. In performing this treatment, the micropores may be colored by letting dyestuff penetrate into them.

Alumite coloration methods include the AC electrolytic coloring method and a method that leaves a naturally colored appearance.

Mechanical Surface Finishing

Mechanical surface finishes include embossing, dull surface finishing, shot blasting, buffing and barrel polishing. Embossing is particularly prevalent for finishing building and decorative materials. Dull surface finishes produced using lasers are used for such applications as automobile body panels.