Electroylsis

Question

In Electrolysis of very dilute NaCl (aq) , when copper or nickel is used as anode, copper/nickel anode oxidizes instead of hydroxide ions because Cu/Ni have a higher position in the ECS than OH minus. However, in anodization, Al is the anode. Al has a higher position in the ECS than Cu/Ni/OH minus. Why Al does not oxidize and become Al 3+???????Why can hydroxide ion still be discharged???

Answer 1

In anodization of aluminium, Al anode is actually oxidized to Al(III) in the form of Al2O3. Therefore, a thin layer of Al2O3 is coated on the surface of the Al anode after the anodization.

In the process, Al anode is firstly oxidized to Al3+ ion :
Al → Al3+ + 3e-

Since Al3+ is a very small ion carrying 3 positive charge, it has a great polarization power to react with water to give Al2O3.
2Al3+ + 3H2O → Al2O3 + 6H+