Corrosion and its prevention
Corrosion and rusting
is the destruction of a material by chemical reactions with substances in its environment, such as oxygen and water.
is the corrosion of iron and steel. Iron rusts only when both oxygen (air) and water are present:
iron + oxygen + water → hydrated iron(III) oxide (rust)
An experiment shows this. Iron nails are put in three tubes:
| Tube | Contains | Rusts? |
|---|---|---|
| 1 | Air and water | Yes |
| 2 | Boiled water with oil on top (no air) | No |
| 3 | Air dried by calcium chloride (no water) | No |
Key terms
Preventing rusting
Barrier methods stop oxygen and water reaching the metal:
- painting, oiling or greasing, or coating with plastic
- electroplating with a less easily corroded metal
attaches a more reactive metal, such as zinc or magnesium, to the iron. The more reactive metal reacts with oxygen and water instead of the iron. It is used on ships' hulls and underground pipes.
coats iron with a layer of zinc. It works both ways: the zinc is a barrier, and if it gets scratched, the zinc still corrodes instead of the iron.
Aluminium doesn't need protecting: it has a thin layer of aluminium oxide that stops further corrosion.