Anodizing is an electrolytic oxidation process where the surface of aluminum and its alloys is typically converted into an oxide layer, which possesses protective, decorative, and certain other functional properties. Based on this definition, anodizing of aluminum only includes the process of forming the anodized oxide layer.

Anodizing processing

Anodic oxidation is a process in which a metal or alloy component is used as an anode, and electrolysis is employed to form an oxide film on its surface. The metal oxide film alters the surface state and properties, such as surface coloring, enhancing corrosion resistance, wear resistance, and hardness, and protecting the metal surface. For instance, in aluminum anodizing, aluminum and its alloys are placed in a corresponding electrolyte (such as sulfuric acid, chromic acid, oxalic acid, etc.) as the anode. Under specific conditions and with the application of an external current, electrolysis is conducted. The anodized aluminum or its alloy forms an aluminum oxide layer on the surface, with a thickness of 5 to 30 micrometers, and a hard anodized film can reach 25 to 150 micrometers. The anodized aluminum or its alloy increases hardness and wear resistance to 250 to 500 kg/mm², possesses excellent thermal resistance, with a melting point of up to 2320K, superior insulating properties, and breakdown voltage up to 2000V, enhancing its corrosion resistance. It remains uncorroded for thousands of hours in a 0.03NaCl salt mist environment. The oxide film has a large number of micropores that can adsorb various lubricants, making it suitable for manufacturing engine cylinders and other wear-resistant parts. The strong adsorption capacity of the micropores allows for coloring into various beautiful and vibrant colors. Anodic oxidation treatments can be applied to non-ferrous metals or their alloys (such as aluminum, magnesium, and their alloys), and this method is widely used in mechanical parts, aircraft and automotive components, instruments, radio equipment, household goods, and architectural decoration, among others. [1]

In general, anodes are typically made of aluminum or aluminum alloys, while cathodes are chosen to be lead plates. These are placed in a water solution containing sulfuric acid, oxalic acid, and chromic acid, among others, to undergo electrolysis, which forms an oxide layer on the surfaces of the aluminum and lead plates. Among these acids, the anodizing process is carried out using sulfuric acid.