anodizing

Anodizing is an electrochemical process that converts a metal surface into a durable oxide layer. It is most commonly applied to aluminum, but can also be used on magnesium, titanium, and other light metals. The oxide layer grows from the metal itself and is integral to the surface, not a coating deposited on top.

The workpiece is made the anode in an electrolytic bath. Under controlled voltage and temperature, oxygen is released at the surface and reacts with the metal to form a crystalline oxide. The oxide layer is typically porous at the outer region, which allows dyeing, sealing, and enhanced adhesion.

Common electrolytes include sulfuric acid for Type II (and hardcoat Type III with higher thickness), oxalic acid, phosphoric acid, and chromic acid (less common today due to environmental concerns). Titanium can form color via controlled oxide growth; aluminum alloys yield varying thicknesses and properties depending on process.

The resulting oxide is harder and more corrosion-resistant than the base metal. Porosity can be exploited for coloring with dyes or pigments; sealing—hot water, steam, or chemical sealants—reduces porosity and improves durability.

Applications span architectural components, automotive parts, cookware, electrical connectors, aerospace components, and medical implants. The process can alter surface reflectivity, wear resistance, and paint adhesion. Environmental and safety considerations include handling of acids and disposal of waste.