What is corrosion?
Corrosion is the slow reaction of a metal with substances around it. For iron and steel this is called rusting. Rust is hydrated iron(III) oxide. Rusting weakens structures such as bridges, cars and ships.
Key idea
Iron rusts only when both water and oxygen are present. Removing either one stops rusting. Dissolved salt speeds rusting up, which is why cars rust faster near the sea.
Proving what is needed
A classic experiment uses three test tubes with iron nails: (1) water and air → rusts; (2) boiled water (no air) under oil → no rust; (3) dry air with drying agent → no rust. This shows both water and oxygen are needed.
Preventing rusting
- Barrier methods: paint, oil, grease or plastic keep out air and water.
- Galvanising: coating with zinc. Zinc is more reactive than iron, so even if scratched the zinc corrodes first and protects the iron.
- Sacrificial protection: blocks of a more reactive metal (zinc or magnesium) are attached; they corrode instead of the iron.
Remember
- Rust = hydrated iron(III) oxide; needs water + oxygen.
- Sacrificial metal must be MORE reactive than iron.
- Tin-plating protects only while unbroken; if scratched, iron rusts faster.
- Aluminium resists corrosion because a tough oxide layer protects it.