How is coloured anodized aluminium produced

Saucepans and other household implements made of aluminium often have a brightly coloured, shiny coating. This outer layer comprises aluminium oxide incorporating a small amount of dye. 

The layer is deposited with the saucepan immersed in a vat of dye solution (usually acidihed to pH 1 or 2), and made the positive terminal of a cell. As the electrolysis proceeds, so the aluminium on the surface of the saucepan is oxidized  

The aluminium is white and shiny before applying the potential. A critical potential exists below which no electro-oxidation will commence. At more extreme potentials, the surface atoms of the aluminium oxidize to form Al3+ ions, which combine with oxide ions from the water to form AI2O3. This electro-precipitation of solid aluminium oxide is so rapid that molecules of dye get trapped within it, and hence its coloured aspect. 

The dye resides inside the layer of alumina. Its colour persists because it is protected from harmful UV light, as well as mechanical abrasion and chemical attack. 

But the chemical reaction forming this coloured layer of oxide represents only one part of the cell. A cell contains a minimum of two electrodes, so a cell comprises two reactions - we call them half-reactions one describes the chemical changes at the positive electrode (the anode) and the other describes the changes that occur at the negative cathode. 

---