Solution

Aluminium develops a thin oxide layer when exposed to air. Thix aluminium oxide coat makes it resistant to further corrosion. The resistance can be improved further by making the oxide layer thicker. This process is called anodising.
During anodising, a clean aluminium article is made the anodee and is elecrolysed with dilute sulphuric acid. The oxygen gas evolved at the anode reacts with aluminium articles an attractive finish.