Anodizing is a surface treatment process that modifies the surface chemistry of metals and other substrates. It helps protect against corrosion, improves appearance, resists scratching, and is one of the most durable finishing options available. For precision machined parts, three common anodizing processes are chromic acid anodizing, sulfuric acid anodizing, and hard anodizing.

Chromic acid anodizing is an electrochemical process that thickens the natural oxide layer on aluminum surfaces in a chromic acid solution. This improves the scratch resistance and corrosion resistance of aluminum parts.

The anodic oxide film produced by chromic acid anodizing is typically 2 to 5 μm thick. Because the oxide layer is relatively thin, parts can maintain their original precision and surface roughness, making this process especially suitable for precision components.

Aluminum and its alloys can be anodized in a diluted sulfuric acid electrolyte using direct or alternating current. This process produces a colorless, transparent oxide film with a typical thickness of 5 to 20 μm and good adsorption properties.

When the sulfuric acid concentration is higher, chemical dissolution of the film is accelerated, resulting in a thinner and softer oxide layer with more pores, stronger adsorption, and better dyeing performance. When the sulfuric acid concentration is reduced, the oxide film grows faster with fewer pores, producing higher hardness, better wear resistance, and improved reflectivity.

Hard anodizing is mainly used to improve the performance of aluminum alloys, including corrosion resistance, wear resistance, weather resistance, electrical insulation, and adsorption. By anodizing in a cooled, diluted sulfuric acid solution, a thick, high-hardness oxide film can be formed, making the process suitable for both wrought aluminum alloys and precision aluminum alloy parts.

The thickness of hard anodic oxide films is generally 25 to 150 μm, with most hard anodized films falling between 50 and 80 μm. Films under 25 μm are often used for parts such as toothed keys and screws, while oxide films around 50 μm are typically used when wear resistance or insulation is required.