Anodizing

Anodizing is a chemical process that accelerates and controls the formation of an oxide coating on an aluminum substrate or machine part. This highly technical process consists of making the aluminum the anode (+) in a suitable acid and water electrolyte where the walls of the chemical container act as a cathode (-). When electrical DC or AC/DC pulse current is applied to the electrolyte, oxygen atoms (-) are aggressively attracted to the anode (aluminum part) to form an aluminum oxide film at the surface. The coating formed is an integral part of the substrate and rises both above and below the original surface.

Aluminum oxide has a hexagonal structure with an open pore at the center. The closing of this pore with a chemical such as nickel acetate is referred to as sealing; whereas the filling of this pore with organic dye or metal to reflect the light is called Coloring.

A sulfuric acid electrolyte is the dominant form of anodizing for commercial and industrial products. A common commercial coating would be applied at 72o F for 25 minutes to achieve .0003/5 inch thickness of aluminum oxide, making the substrate electrically non-conductive (all aluminum ladders are anodized as a safety precaution against electric shock). The coating is a light opaque gray.

Aluminum oxide is the 3rd hardest element known to mankind, but its characteristics can be influenced by the choice of the electrolyte as well as temperature.