Why does iron rust?
Short answer
The red-brown rust we see forms when iron is exposed to both water and oxygen. A chemical change at the surface makes new substances, so the material is no longer just iron.
How rust forms
- A film of water forms: A thin layer of water covers the iron. Some oxygen from the air dissolves in it.
- Iron changes: Some iron atoms give up electrons (tiny particles with electric charge) and become iron ions in the water. Oxygen takes those electrons.
- Rust forms: The iron ions react further with oxygen and water. Rust is a mixture, not one single substance. It includes iron oxides (compounds of iron and oxygen) that contain water, and the exact mix can vary.
- Water and oxygen reach iron Oxygen dissolves in a thin film of water.
- Some iron enters the water Iron gives up electrons and becomes iron ions.
- Rust forms on the surface Iron ions react with oxygen and water.
Why salt speeds up rusting
When salt dissolves, it forms charged particles called ions. These ions help electricity move through water, which can make iron rust faster. Salt water, such as seawater or water on a salted road, can make rusting faster. Salt speeds the process; it is not required for iron to rust.
Rust does not protect iron
Rust does not make a tight, strong layer over the iron. It can crumble and flake away, exposing fresh iron to water and oxygen so rusting can continue.
Paint helps by keeping water and oxygen away from iron. A scratch can expose the metal. Zinc coatings cover iron too, and nearby zinc can react before the iron if the coating is scratched, helping protect it while zinc remains.