Corrosion and Its Prevention — AQA GCSE Chemistry (Separate)
Corrosion damages metals by reaction with the environment. Rusting of iron is a common example.
What is corrosion?
Corrosion is the destruction of materials by chemical reactions with substances in the environment. The corrosion of iron and steel is called rusting.
Rusting needs both water and oxygen. If either is absent, iron will not rust. (You can show this with iron nails in tubes containing: water + air; boiled water + oil layer to keep out air; and a drying agent with air — only the first rusts.)
The word equation: iron + oxygen + water → hydrated iron(III) oxide (rust).
Preventing corrosion
Corrosion can be prevented by stopping the metal from contacting air and/or water:
Barrier methods:
- Painting, oiling/greasing, or coating with plastic.
- Electroplating with another metal.
Sacrificial protection:
- Coating the iron with a more reactive metal (e.g. zinc in galvanising). The more reactive metal corrodes instead of the iron, "sacrificing" itself. This works even if the coating is scratched.
Exam tips
- Rusting needs both water and oxygen — be able to describe the demonstration.
- Barrier methods work by keeping out air and water.
- Sacrificial protection uses a more reactive metal (e.g. zinc) that corrodes instead of the iron.
- Galvanising = coating iron with zinc.