A thicker shield
Anodising aluminium.
A tougher coat
Aluminium already has a thin oxide layer, which makes it resistant to further corrosion.
How could the layer be made thicker?
What this lesson covers
The idea
Anodising forms a thick protective oxide layer on aluminium: a clean aluminium article is made the anode and electrolysed with dilute sulphuric acid, and oxygen evolved at the anode reacts with the aluminium.
A tougher coat
Aluminium already has a thin oxide layer, which makes it resistant to further corrosion.
How could the layer be made thicker?
- By using electricity in a special set-up
- By painting it black
- By leaving it wet
The anodising cell
Press Next step. This is only a picture of what is done in a factory or laboratory.
Thickening the layer
In anodising, a clean aluminium article is the anode, and dilute sulphuric acid is electrolysed. The oxygen given off at the anode reacts with the aluminium. This forms a thicker protective oxide layer.
Anodising forms a thick protective oxide layer on aluminium: a clean aluminium article is made the anode and electrolysed with dilute sulphuric acid, and oxygen evolved at the anode reacts with the aluminium.
Notes
Anodising: a clean aluminium article is the anode in dilute sulphuric acid; oxygen evolved at the anode reacts with aluminium to make a thick protective oxide layer.
Check yourself
In anodising, the aluminium article is made the ...
What acid is used in anodising?
What reacts with the aluminium to make the thicker oxide layer?
Anodising makes the oxide layer on aluminium ...
- cathode. The cathode is the negative electrode; the article is made the anode.
- anode — correct. Yes!
- electrolyte. The electrolyte is the dilute sulphuric acid, not the article.
- Concentrated nitric acid. The book names dilute sulphuric acid for anodising.
- Aqua regia. Aqua regia is a mixture of hydrochloric and nitric acids; anodising uses dilute sulphuric acid.
- Dilute sulphuric acid — correct. Yes!
- Oxygen gas evolved at the anode — correct. Yes!
- Hydrogen gas from the acid. The book says the oxygen gas evolved at the anode reacts with the aluminium.
- Sulphur from the acid. The book says the oxygen gas evolved at the anode reacts with the aluminium.
- thinner. The aim is a thicker protective layer.
- thicker — correct. Yes!
- disappear. Aluminium keeps a protective oxide layer; anodising makes it thicker.