Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts. It increases resistance to corrosion and wear, and provides better adhesion for paint primers and glues than does bare metal.
Anodizing can be broken down into two broad sub-categories: decorative and hard anodizing. The main differences between the two is how thick and durable the coating is, and the exact process used to create it.
Chromic acid anodizing (Type I)
The oldest anodizing process uses chromic acid. It is widely known as the Bengough-Stuart process. In North America it is known as Type I because it is so designated by the MIL-A-8625 standard. Chromic acid produces thinner, 0.5 μm to 18 μm (0.00002" to 0.0007") more opaque films that are softer, ductile, and to a degree self-healing. They are harder to dye and may be applied as a pretreatment before painting. The method of film formation is different from using sulfuric acid in that the voltage is ramped up through the process cycle.
Sulfuric acid anodizing (Type II & III)
Sulfuric acid is the most widely used solution to produce anodized coating. Coatings of moderate thickness 1.8 μm to 25 μm (0.00007" to 0.001") are known as Type II, as named by MIL-A-8625, while coatings thicker than 25 μm (0.001") are known as Type III, hardcoat, hard anodizing, or engineered anodizing.
Hard anodizing, also known as Type III anodizing, is a process used to create a hard wearing, corrosion resistant coating on a variety of metals.