What is anodising? (Topic: 14003)

Anodising is an artificial thickening of the oxide layer on the surface. The aluminium profile is made the anode in the electrochemical process, giving the name of the process. The reactions in the anodising process are explained as follows:

The anode reaction:

There is an oxidation process taking place on the anode surface(aluminium material):

2 Al (metal) + 3H2O ñ> Al2O3 (oxide coating) + 6H+ + 6eñ

Aluminium react with water to produce the anodic oxide coating.

The cathode reaction:

There is a reduction process taking place on the cathode surface:

6H+ + 6eñ ñ> 3H2 (gas)

Hydrogen gas is evolved at the cathode and appears as bubbles during the anodising process. The anodic and the cathodic reaction can be added to make the following total reaction:

2 Al (metal) + 3H2O ñ> Al2O3 (oxide coating) + 3H2 (gas)

Aluminium metal is oxidised (anodised), and hydrogen gas is evolved at the cathode.

In the anodising tank there is a chemical solution called an electrolyte which makes the current flow more easily. Usually the electrolyte consists of sulphuric acid often with small amounts of oxalic acid. The electrolyte attacks the anodic oxide coating creating pores in the layer.