Rusting and corrosion
What air and water do to iron, and how we stop it
1. Rusting and corrosion: from zero
2. Nail vs ring
An iron nail in a wet garden turns red-brown in weeks. A gold ring can last for centuries. What happens to the nail?
| Object | What you see |
|---|---|
| Iron nail in a wet garden | turns red-brown in weeks |
| Gold ring | can last for centuries |
3. Corrosion
Corrosion
The slow reaction of a metal with substances in the air, like oxygen and moisture, that wears the metal away.
Rusting is the corrosion of iron. Rust is a hydrated iron (III) oxide. Some metals corrode quickly, like iron; gold hardly at all.
- Iron → Rust: hydrated iron (III) oxide (slow reaction with oxygen and moisture)
4. Which nail rusts?
Pick the right answer
Predict: after a week, which nail rusts?
- A. A: dry air
- B. B: boiled water under oil
- C. C: air and water
Explanation
Nail C, in air and water, is the one that rusts. Rusting needs oxygen and moisture together: tube A has dry air, and in tube B boiling removed the air from the water and the oil keeps air out.
Reveal
Only nail C rusts. Rusting needs oxygen and moisture together.
Rust is porous (full of tiny holes) and flakes off. So air and water reach fresh iron below, and rusting continues.
5. How to stop rusting
Paint, oil, grease
A barrier layer on bridges, car bodies and chains.
Galvanizing
Coating iron with a layer of zinc.
Plating
Covering iron with a thin layer of another metal that resists corrosion.
Each coating keeps oxygen and moisture away from the iron.
- Coating on the iron → Oxygen and moisture kept away
- Oxygen and moisture kept away → Rusting is slowed
6. Apply it
Pick the right answer
A bicycle chain is kept oiled. Why does it rust slowly?
- A. The oil keeps oxygen and moisture away from the iron
- B. The oil reacts with the rust and removes it
- C. Oil makes iron a less active metal
- D. A moving chain is too fast to rust
Explanation
The oil coats the chain and keeps oxygen and moisture away from the iron surface, so rusting is slow. The oil does not remove rust or change the iron itself.
Sort into groups
Will the iron rust quickly?
A galvanized iron bucket left in the rainA painted car door with a deep scratch, in humid airAn iron key kept in a dry, closed box with a drying agentAn old iron gate in a wet garden, with flaking rust
7. Summary
- Corrosion: a metal slowly reacts with oxygen and moisture in air. Rusting is the corrosion of iron.
- Rust is porous and flakes off, so rusting continues.
- Paint, oil, galvanizing (zinc) and plating with another metal keep air and moisture away.
Next in Chapter 1: chromium is a very active metal, yet it resists the air. Keep 'porous' in mind.
Can you explain why a scratch in the paint lets a car door rust?
Reveal
At the scratch the paint is gone, so air and moisture reach the bare iron. Rusting needs oxygen and moisture together, so the iron rusts there.