An electrolytic process used to provide a hard, smooth, and corrosion-resistant surface to aluminium and some other metals. The piece for coating is connected to the anode of a DC circuit and is immersed in an acid solution. The flow of current liberates oxygen at the surface which reacts with the aluminium to form aluminium oxide. Chromic, oxalic, and sulphuric acids are commonly used. The anodized surface may typically have a thickness of between 0.005 mm and 0.018 mm.