Here, they react with the material, forming aluminium oxide. Its larger volume increases the thickness of the metal surface by up to a third, and it is firmly bonded to the aluminium. Hydrogen forms at the cathode and escapes in the form of gas.
Sulfuric acid and sulfuric acid-oxalic acid are the most widely used electrolytes for anodisation. The two most important processes for the production of oxide layers are named for these substances: the direct current sulfuric acid (GS) process and direct current sulfuric acid oxalic acid (GSX) process. Other processes exist that lead to the formation of a coloured oxide layer.
Types of oxide layers
The resulting oxide layer consists of an extremely thin barrier layer, which is almost non-porous, very strongly bonded and electrically insulating, and a much thicker, slightly porous and electrically conductive cover layer, which is the result of a chemical reaction of the barrier layer with the electrolyte forms.