Corrosion

Metals can oxidise in air. They react with oxygen and form metal oxides. For example, sodium is a very reactive metal. When sodium is cut or scratched, its freshly exposed shiny surface rapidly turns dull as a thin layer of sodium oxide forms:

Sodium + oxygen → sodium oxide

4Na(s) + O2(g) → 2Na2O(s)

Other metals may oxidise more slowly. Gold and other very unreactive metals do not oxidise in air at all.

Corrosion happens when a metal continues to oxidise. The metal becomes weaker over time, and eventually all of it may become metal oxide.

Rusting

Rusting occurs when iron or steel reacts with oxygen and water:

Iron + oxygen + water → hydrated iron(III) oxide

Hydrated iron(III) oxide (rust) is the orange-brown substance seen on the surface of rusty objects.

A rusting experiment

The experiment in the diagram shows that both oxygen and water are needed for rusting to happen.

Three sealed test tubes, iron nail. 1st: nail half-submerged in water. Air. Nail rusty. 2nd: nail submerged in boiled water, layer of oil. No air. Nail shiny. 3rd: nail on silica gel. Nail shiny.Calcium chloride (in the right-hand test tube) absorbs water

The nail only rusts in the left-hand test tube. It does not rust:

  • in the middle test tube, where there was water but no oxygen (because there was no air in the water)
  • in the right-hand test tube, where there was oxygen (air) but no water

Worked example

Explain whether iron is oxidised or reduced when it forms rust.

Iron is oxidised because it gains oxygen during rusting.

curriculum-key-fact
An iron or steel object only rusts if oxygen and water are present.