aluminium anodization

In the process of aluminium anodization, the electrolyte is dilute (1-2 M) sulphuric acid, the anode is the aluminium to be anodized, and any common cathode can be used (usually aluminium is also used as the cathode in laboratory).

圖片參考：http://www.artmetal.com/files/imported/project/TOC/finishes/00image/anod_bth.gif

Source of diagram :
http://www.artmetal. com/files/imported/p roject/TOC/finishes/ 00image/anod_bth.gif

H+(aq) ions migrate to the cathode and are then discharged to give hydrogen gas.
2H+(aq) + 2e- → H2(g)

Anodization occurs at the anode. OH-(aq) ions and SO42-(aq) ions migrate to the anode, but aluminium is oxidized more readily than they are. The aluminium anode is oxidized to give aluminium ions :
Al(s) → Al3+(aq) + 3e-
Each aluminium ion carries 3 positive charges and is small in size, and thus has a high polarizing power. Aluminium ions can thus react with water to give a layer of aluminium oxide :
2Al3+(aq) + 3H2­O(s) → Al2O3(s) + 6H+(aq)

因為AI係gp 3, 所以咪放e-

有Al係度, OH-同H+呢d 相比下弱d的ion,都只係用黎做balance of e-架者