Bitesize has changed! We're updating subjects as fast as we can. Visit our new site to find Bitesize guides and clips - and tell us what you think!

Did you know?

We also have Bitesize study guides covering many subjects at National 4 and National 5 on our Knowledge & Learning BETA website.

Home > Chemistry > Metals > Corrosion




When metals are exposed to the open air and bad weather, you might have noticed that they rust. Chemists call rusting corrosion. In this topic we will look at what happens to metal atoms over this time to change them from a “shiny metal” appearance to a dirty, brown powder.

What causes rust?

This video shows some examples of objects which have become rusty.

Video: Rust

In order to see this content you need to have both Javascript enabled and Flash installed.


The following experiment can be conducted to demonstrate that water and oxygen are required for rust to occur with iron.


4 test tubes, each containing an iron nail. Test tube A is filled with boiled water and a layer of oil on top. Test tube B is filled with salt water. Test tube C contains air. Test tube D contains air and calcium chloride.


Test tubeConditionsResult
ABoiled water + oil layerNo rust. Boiled water has no oxygen and oil stops new oxygen entering.
BSalt waterSevere rust. Salt water is an electrolyte which conducts ions, speeding up rusting.
CAirRust. Air and moisture cause normal rusting.
DAir + calcium chlorideNo rust. Calcium chloride dries out the air.


Salt solution acts as an electrolyte, allowing electrons to flow and hence speeding up electron loss.

Therefore we can see that the three conditions required for rust to occur are:

  • oxygen
  • water
  • electrolyte (to allow conduction to take place)


A picture of Mia Cadaver

Tombstone Timeout

Test out your knowledge of chemistry with Mia Cadaver's chemistry game - it's serious fun!

BBC © 2014 The BBC is not responsible for the content of external sites. Read more.

This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.