The anodising process is based on an oxidisation process. When aluminium is anodically polarised in an electrolyte, e.g., sulphuric acid, oxygen is released near the anode and reacts with the aluminium. The result is the formation of aluminium oxide. The surface of the aluminium anode is converted into an oxide layer which bonds and firmly adheres to the metal surface. The bond with the substrate is so strong that the oxide layer cannot be removed from its metal base material even by mechanical means – not even in fragments.
This compact primary layer forms a resistance to the passage of current and is punched through by the applied bath voltage. The electrolyte penetrates the resultant punch pores, causing the oxidation process to progress. As countless pores are formed, the surface layer "grows" deeper into the metal. To close the pores and then further processed in additional stages.
The coatings produced during this process are stronger and more resistant than naturally formed oxide layers.