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 is a specific example of corrosion, which occurs when iron or steel reacts with oxygen and water:
iron + oxygen + water → hydrated iron(III) oxide
Hydrated iron(III) oxide 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.
The nail only rusts in the left-hand test tube. It does not rust:
Explain whether iron is oxidised or reduced when it forms rust.
Iron is oxidised because it gains oxygen during rusting.