Anodizing is the general name applied to methods of treating metals, where the work piece is made the anode in an electrolytic cell, usually to form oxide coatings for the purpose of increasing the performance of the surface. In the case of aluminum, the anodizing process forms a layer of aluminium oxide - Al203 - or corundum, which isvery hard, relatively inert, electrically insulating and can absorb dyes to colour the film. The anodic film itself grows at the aluminium / aluminium oxide interface by the continuous formation and dissolution of a layer of oxide, this is the so-called barrier layer and its thickness is a function of the process starting voltage. A porous, more structured layer forms on top of the barrier layer making up the rest of the coating.
Sulphuric Acid Anodizing
Embraces anything from heavy duty black dyed coatings for high-tech instruments to cheap coloured ashtrays, expected to last in a pub for a few weeks. It also includes architectural anodising primarily for protectingaluminium window frames etc from the elements. The natural colour of these films is light grey; other colours are achieved by dyeing the film.