In the case of aluminium, the anodising process forms a layer of aluminium oxide - Al203 - or corundum, which is very 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.
EXAMPLE OF THE ANODISING PROCESS
The film thickness is usually determined by a known relationship between current per unit area and process time. With most aluminium alloys as the anodic film grows its electrical resistance increases requiring the process voltage to be increased. This is especially marked with alloys containing certain elements like silicon, for example. The electrolyte is usually based on sulphuric acid but other acids are used i.e. chromic, phosphoric, boric or organic acids like oxalic, to achieve different properties.
As all anodised coatings are derived from oxidation of the metal surface, the alloy composition and heat treatment condition have a large effect upon the resultant coating. The anodic film has excellent adhesion to the substrate, as it is an integral part of the structure, in contrast to, say, a painted or electroplated component. Anodised coatings provide an enhancement in properties over the base material particularly in respect to wear, corrosion, temperature resistance and electrical insulation. Generally speaking, the surface finish of the original work piece is reproduced, with slight roughening; there is no effect. Any defects such as corrosion or polishing 'burns' will be emphasised rather than hidden.
Anodising conveniently divides into 3 major areas:
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 protecting aluminium window frames etc from the elements. The natural colour of these films is light grey; other colours are achieved by dyeing the film.
Is a branch of sulphuric acid anodising where process conditions have been pushed in a certain direction to achieve significantly harder, thicker, denser films. Applications involve resistance to wear, corrosion, temperature effects etc.
Produces coatings of exceptional corrosion and chemical resistance for its film thickness in an electrolyte, which is non-corrosive towards aluminium. These coatings are thin, and relatively soft and generally only used in specialised applications.
Please see this page to get an overview on practical information relevant to all of the anodising processes described above