What are the kinds of anodizing aluminum？

There are four types of anodizing processes based on the acid bath type and thickness capabilities.

Chromic Acid Anodizing (Type I)

Type I is the ideal if you need a thin layer, especially for decorative and some functional uses. However, it can mimic the performance of Type II or hard coating after sealing. Meanwhile, the thickness of the layer ranges from 0.00002"-0.0001".   

Sulfuric Acid Anodizing (Type II)

This is the most common type and uses sulfuric acid as an electrochemical medium to form the oxide layer. Sulfuric acid anodizing uses a solution with a concentration of 15-20%. It forms a thicker oxide layer than Type I and is used for a wide range of applications. The thickness ranges from 0.0001"-0.001". In addition, Type II anodizing offers high corrosion and wear resistance and is available in a variety of color options.

Hard Anodizing (Type III)

Type III is the densest and strongest type and is used for thicker oxide layers on the surface. Therefore, it is well suited for harsh and chemical environments. Thickness can range from 0.0005" to 0.006". Hard anodizing is mainly used for high performance and low friction parts. The hard anodizing method can use chromic acid, sulfuric acid or oxalic acid as the electrolyte.

Phosphoric Acid Anodizing

It is primarily a surface treatment rather than a full-blown corrosion or wear treatment. Phosphoric acid anodizing uses a 15-30% phosphoric acid solution. Unlike the other types, it forms a very thin and porous oxide layer (< 0.0001 inches). It is well suited for the application of further adhesives or primers.