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Application of Aluminum Anodizing Process in Industry
Aluminum and its alloys are prone to oxidation when exposed to air. The natural oxide layer that forms on the surface is amorphous, which can dull the metal's original shine. Although this thin layer (about 4–5 nm thick) offers some degree of passivation, it lacks density, has high porosity, and poor mechanical properties. As a result, it cannot effectively prevent further corrosion from atmospheric elements.
After anodizing, aluminum and its alloys develop a much thicker, more compact oxide layer—ranging from tens to hundreds of micrometers. This artificial oxide film is then sealed, transforming the amorphous structure into a crystalline one while closing the pores. This process maintains the metal’s gloss while significantly improving its corrosion resistance and mechanical strength. Additionally, the oxide layer can be dyed to create attractive, decorative finishes. Due to these beneficial properties, anodizing is widely used in industrial applications, covering several key areas.
First, anodizing is commonly used for corrosion protection. Sealed oxide layers formed through sulfuric, oxalic, or chromic acid processes provide excellent stability in atmospheric conditions. For example, household items like aluminum pots, washing machine linings, and similar products benefit from this treatment. Chromic acid oxidation is especially effective for welded or riveted aluminum parts.
Second, anodizing serves both protective and decorative purposes. Transparent oxide films can absorb dyes and pigments, allowing for vibrant colors and intricate patterns. Innovations such as multi-layer oxidation, fireworks effects, wood grain textures, and transfer printing have made aluminum surfaces even more visually appealing. These colored layers act as both decoration and corrosion protection, often seen in items like lighters, pens, and handicrafts.
Third, anodizing can create a hard, wear-resistant layer. By adjusting the anodizing parameters in sulfuric or oxalic acid solutions, a thick and durable oxide film can be produced. The porous nature of the film allows for oil absorption, making it ideal for frictional environments. It is commonly used in engine components like cylinders and pistons in vehicles.
Fourth, anodized layers can function as electrical insulators. Aluminum oxide has high electrical resistance, and the resistance increases with film thickness. This makes it suitable for use in capacitors or as insulation for cables. In some cases, phenolic resin filling enhances the dielectric properties. Oxalic acid-anodized layers can achieve high insulation values, such as 200 Ω resistance and 980 V breakdown voltage.
Fifth, anodizing provides a good base for painting and electroplating. The porous oxide layer has strong adhesion properties, making it ideal for coatings. Phosphoric acid anodizing, in particular, is used as a pre-treatment for electroplating on aluminum surfaces.
Lastly, anodizing plays a significant role in modern architecture. Electrolytic coloring techniques have become popular for architectural aluminum profiles. These treatments offer a range of colors, including bronze, black, and red, and provide excellent wear resistance. Unlike traditional coloring methods, electrolytically colored aluminum retains its color for over 20 years without fading. It is used in windows, doors, shop displays, and other architectural elements.