Anodizing is an electro-chemical process by which the surface of a metal material, commonly aluminum, is oxidized to form a porous and durable surface coating. The aluminum part, which becomes the anode, is submerged in a solution, commonly sulfuric acid, while a current is applied. The tank lining is commonly the cathode.
This is the opposite of electro-plating where the parts become the cathode (negatively charged) and positively charged metal ions in solution are deposited on to the metal surface to form a coating.
During the anodize process, the surface aluminum becomes oxidized (loss of electrons) and carries a positive charge. The aluminum quickly reacts with water to form aluminum oxide, Al2O3, and hydrogen gas. The aluminum oxide is the anodize coating. It is porous and may accept penetration by various color dyes. Apx. 1/2 of the coating thickness results in dimensional build up while 1/2 is penetration into the base material.
After dyeing, the color is fixed by sealing in a hot dilute nickel acetate or other equivalent solution. Titanium anodized color is controlled by coating thickness, not by dyeing.
ANODIZE REFERENCE Aluminum Anodizers CouncilSulfuric Anodize
Aluminun Anodizers Council