Corrosion: metals destroyed by reaction with their surroundings
Definition
Corrosion
Corrosion is the gradual destruction of a metal through reactions with substances in its surroundings.
- Corrosion is the destruction of a material by chemical reactions with substances in its environment.
- Rusting is the corrosion of iron and steel.
- Rust is hydrated iron(III) oxide, formed when iron reacts with oxygen and water.
- The word equation is iron+oxygen+water→hydrated iron(III) oxide\text{iron} + \text{oxygen} + \text{water} \rightarrow \text{hydrated iron(III) oxide}iron+oxygen+water→hydrated iron(III) oxide.
Key Idea
- Corrosion happens at the surface, where the metal meets air and water.
- Rusting needs both oxygen and water, so removing either one stops it.
Both air and water are needed for iron to rust
- An iron nail left in water that is open to the air rusts.
- A nail in water that has been boiled and sealed under oil does not rust, because the air has been removed.
- A nail in dry air with a drying agent does not rust, because the water has been removed.
- This shows that iron only rusts when oxygen and water are both present.
Note
- Boiling the water drives out dissolved air, and the oil layer stops more air dissolving back in.
- A drying agent keeps the air free of water vapour, so the nail stays dry.
Barrier methods keep out air and water
Definition
Electroplating
The use of electrolysis to cover an object with a thin layer of metal.
- A barrier coating stops oxygen and water from reaching the metal surface.
- Oiling, greasing and painting all form barriers that must be renewed if they wear away.
- Electroplating uses electrolysis to coat the object with a thin, protective layer of another metal.
- Aluminium protects itself, because it forms a natural oxide layer that stops further corrosion.
Example
- A bicycle chain is oiled so that water cannot reach the steel.
- Steel car bodies are painted to seal the surface from air and water.
Sacrificial protection uses a more reactive metal
Definition
Galvanising
Coating iron or steel with zinc to prevent rusting by barrier and sacrificial protection.
- A more reactive metal can be attached to or coated onto iron to protect it.
- The more reactive metal corrodes in preference to the iron, so it is described as sacrificial.
- Galvanising coats iron or steel with zinc, which is more reactive than iron.
- The zinc acts as a barrier and, even if it is scratched, still protects the iron by sacrificial protection.
Exam technique
- To explain sacrificial protection, name a more reactive metal such as zinc and say it corrodes, or is oxidised, in preference to the iron.
- Make clear that this still works even when the coating is scratched, unlike a plain barrier.
Choosing a method to stop rust
- A plain barrier method works only while the coating stays intact.
- If a barrier is scratched, air and water reach the iron and it rusts.
- A sacrificial coating keeps protecting the iron even after it is scratched.
- The best method depends on cost, appearance and how likely the surface is to be damaged.
Self review
- What two substances must be present for iron to rust?
- How does painting protect iron from rusting?
- Why does aluminium not corrode away in air?
- Explain how galvanising protects iron even when the zinc layer is scratched.
- What is the difference between barrier protection and sacrificial protection?
