Anodising (anodizing) covers a range of techniques, with different electrolytes. But each is an electrolytic process, primarily for aluminium alloys, transforming the surface layer into a hard aluminium oxide. The part is made the anode in an acid bath, with lead cathodes, and a closely controlled DC voltage is applied across them.
Hard anodising provides thicker and particularly harder layers in comparison to sulphuric anodising. The coating tint is darker that in the case of ‘normal’ anodising. This depends, among other things, on layer thickness, various process parameters, alloying and heat treatment.Consequently, hard anodising with a uniform colour is extremely difficult. In view of the relatively high layer thickness, critical dimensions for hard anodising should be corrected.
Hard anodizing is typically 25 to 50µ thick, used for providing wear resistance where surface contact loads are light, for instance in the low stress abrasive wear caused by food or pharmaceutical products. It is also used for corrosion protection and thermal or electrical insulation. It can be combined with polymers for a wider range of low friction and non-stick applications