Corrosion, alloys, glass, ceramics, polymers and composites

Corrosion and how to stop it

Corrosion is the destruction of a material by chemical reactions with its surroundings. Rusting needs both oxygen and water; salt speeds it up. Rust is hydrated iron(III) oxide, and rusting is an oxidation, so a rusting nail gets heavier.

1. A nail goes from 2.40 g to 2.52 g in salty water
2. increase = 2.52 − 2.40 = 0.12 g
3. percentage increase = 0.12 ÷ 2.40 × 100 = 5.00%

• Barriers keep oxygen and water out: paint, oil, grease, plastic, electroplating (for example with tin). Once scratched, they stop working.
• Sacrificial protection: a more reactive metal (zinc, magnesium) is oxidised instead of the iron, even through a scratch. Galvanising is a coating of zinc. Magnesium blocks on ships are slowly used up and replaced.

Aluminium is more reactive than iron but does not corrode away: a thin oxide layer forms and protects it.