Mechanism of anodic layer formation
Anodizing of aluminium and its alloys

Electrolysis in an aqueous medium with direct current normally produces off-gassing at both electrodes (hydrogen at the cathode, oxygen at the anode). However, if aluminum is used as the anode, no off-gassing is observed at the anode, while hydrogen is always visible at the cathode.

To a first approximation, this phenomenon can be described as a combination of dissolved aluminium and nascent oxygen, according to the reaction :