Anodising is a process for increasing the thickness of the protective oxide layer on aluminium. The component to be anodised is immersed in an electrically conductive liquid medium (an electrolyte) and then connected to a source of direct current. In the resultant voltage field, oxygen-containing anions migrate to the aluminium surface, where they react with the material and form aluminium oxide. Hydrogen forms at the cathode and escapes in gaseous form. The resultant oxide layer comprises an extremely thin layer, the so-called ‘barrier layer’, which is almost free from pores, extremely dense and electrically insulating, and a much thicker layer, which is a slightly porous and electrically conducting covering layer, which forms as a result of a chemical reaction between the barrier layer and the electrolyte.

Further information on anodising