Anodising 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 aluminium, the anodising process forms a layer of aluminium oxide which is 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.
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. 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.
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. It is commonly used for components in the military, aviation and automotive industries, also for pneumatic cylinder tubes, valve spools, hydraulic parts. Hard anodising gives increased service life and provides greater corrosion resistance.