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Rusting and how to prevent it
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Iron rusts when it meets both water and oxygen. There are three main ways to stop it.
Conditions for rusting
Iron rusts only when both water and oxygen are present. Rust is hydrated iron(III) oxide. Rusting is an oxidation reaction.
Experiment: iron nails in three tubes. Water and air: rusts. Boiled water with a layer of oil (no oxygen): no rust. Dry air with a drying agent such as calcium chloride (no water): no rust.
Experiment: iron nails in three tubes. Water and air: rusts. Boiled water with a layer of oil (no oxygen): no rust. Dry air with a drying agent such as calcium chloride (no water): no rust.
Barrier methods
Coat the iron so water and oxygen cannot reach it: paint, oil or grease, or a plastic coating.
If the barrier is scratched, the iron underneath can rust.
If the barrier is scratched, the iron underneath can rust.
Galvanising and sacrificial protection
Galvanising: coating iron with zinc. The zinc acts as a barrier, and if scratched, zinc still protects the iron because it is more reactive and corrodes instead.
Sacrificial protection: attaching a more reactive metal, such as zinc or magnesium blocks on a ship's hull. The more reactive metal loses electrons (is oxidised) instead of the iron.
Sacrificial protection: attaching a more reactive metal, such as zinc or magnesium blocks on a ship's hull. The more reactive metal loses electrons (is oxidised) instead of the iron.
Why does a scratched galvanised bucket still not rust?
- The zinc coating is more reactive than iron.
- Zinc is oxidised in preference to iron.
- So the zinc corrodes instead of the iron (sacrificial protection).
Answer: Zinc is more reactive, so it corrodes instead of the iron.
Iron rusts when both water and oxygen are present. Prevent it with barriers (paint, oil, plastic), galvanising (zinc coating) or sacrificial protection (a more reactive metal corrodes instead).
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