ANODIZATION ce

At the anode, the aluminium electrode is oxidized to give aluminium ions.
Al → Al^3+ + 3e-

The aluminium ion is very small and carries 3 positive charges. The aluminium ion thus has a very high charge density, and reacts readily with water to give aluminium oxide.
2Al3+ + 3H2O → Al2O3 + 6H+
Al2O3 is formed continuously, and this make the aluminium oxide layer become thicker and thicker.

2009-12-31 01:58:15 補充：
Al is more reactive than Zn, and its position in electrochemical series is higher than Zn. Therefore, Al has a high tendency to be oxidized.

Electrolyte used in anodization is H2SO4. In the electrolyte, the concentration of OH^- is very low. Therefore, the OH^- has a low tendency to be oxidized.

2009-12-31 02:01:18 補充：
Therefore, the aluminium anode is oxidized instead of OH^- in anodization.

This is a tragedy that some textbooks in Hong kong state the OH^- is oxidized, although this is incorrect.

2009-12-31 13:15:45 補充：
Aluminium sheet is used as the cathode. The sheet can be made cylindrical so that the anode can be surrounded by the cathode in order to increase the efficient of anodization.

anodization係邊課教?

anodization 其實係electrolysis
electrolyte 係 sulphuric acid
positive, negative pole 都係 aluminium

During electrolysis,
在anode: H+ is reduced, equation H+ -> H2+ + 2e-
在cathode: OH- is oxidized, equation 4OH- -> O2 + 2H2O + 4e-

O2 produced react with Al to form Al2O3, hence the oxide layer increase in thickness

2009-12-30 00:06:33 補充：
H+ -> H2+ + 2e-係錯
應該係 H+ -> H+ + 2e-