Corrosion of metals
Rusting
- Rusting is the corrosion of iron and steel.
- Two things are needed: oxygen (from air) and water.
- The rust formed is hydrated iron(III) oxide.
Practice
Rusting of iron needs which two things? (Choose both.)
Iron rusts only when both oxygen and water are present; the rust is hydrated iron(III) oxide.
Stopping rust
- Barrier methods keep out air and water: painting, greasing (oil), plastic coating.
- Galvanising = coating iron with zinc. It works two ways:
- a barrier, and
- sacrificial protection — zinc is more reactive, so it corrodes instead of the iron (even if scratched).
Practice
Galvanising protects iron by coating it with:
Zinc acts as a barrier and, being more reactive, gives sacrificial protection.
Practice
In sacrificial protection, the more reactive zinc corrodes instead of the iron.
Zinc is more reactive, so it loses electrons and corrodes first, protecting the iron even if scratched.
You've got it
Key idea
- rusting needs oxygen AND water; rust = hydrated iron(III) oxide
- barrier methods: paint, grease, plastic
- galvanising (zinc) gives a barrier and sacrificial protection