Anodizing of aluminum is an electrolytic oxidation process in which the surface of aluminum and aluminum alloys is typically transformed into an oxide film. This oxide film possesses protective, decorative, and other functional properties. Based on this definition, aluminum anodizing only includes the process of forming the anodic oxide film.
A metal or alloy workpiece is used as the anode, and an oxide film is formed on its surface through electrolysis. The metal oxide film alters the surface state and properties, such as surface coloring, improved corrosion resistance, enhanced wear resistance and hardness, and protection of the metal surface. For example, in aluminum anodizing, aluminum and its alloys are placed in a suitable electrolyte (such as sulfuric acid, chromic acid, oxalic acid, etc.) as the anode, and electrolysis is performed under specific conditions and an applied current. The aluminum or its alloy at the anode oxidizes, forming a thin layer of aluminum oxide on the surface, with a thickness of 5–30 micrometers; hard anodic oxide films can reach 25–150 micrometers. Anodizing aluminum or its alloys improves their hardness and wear resistance, reaching 250–500 kg/mm², enhances heat resistance (hard anodized films have a melting point as high as 2320 K), provides excellent insulation with a breakdown voltage up to 2000 V, and strengthens corrosion resistance, remaining uncorroded for thousands of hours in ω=0.03 NaCl salt spray. The thin oxide film contains numerous micropores that can adsorb various lubricants, making it suitable for manufacturing engine cylinders or other wear-resistant parts. The strong adsorption capacity of the micropores allows for coloring in various beautiful and vibrant colors. Non-ferrous metals or their alloys (such as aluminum, magnesium, and their alloys) can all undergo anodizing. This method is widely used in mechanical parts, aircraft and automotive components, precision instruments and radio equipment, daily necessities, and architectural decoration.
