Corrosion of iron
A metal object corrodes when it is exposed to the open air and bad weather. If the metal is iron, we call this change rusting, and the weaker, flaky brown compound that is formed is rust.
What causes rust?
Watch this video to see an experiment to investigate what is required for iron to corrode.
| Test tube | Conditions | Result |
| A | Boiled water and oil layer | No rust. Boiled water has no oxygen and oil stops new oxygen entering. |
| B | Salt water | Severe rust. Salt water is an electrolyte which conducts ions, speeding up rusting. |
| C | Air | Rust. Air and moisture cause normal rusting |
| D | Air and calcium chloride | No rust. Calcium chloride dries out the air. |
| Test tube | A |
|---|---|
| Conditions | Boiled water and oil layer |
| Result | No rust. Boiled water has no oxygen and oil stops new oxygen entering. |
| Test tube | B |
|---|---|
| Conditions | Salt water |
| Result | Severe rust. Salt water is an electrolyte which conducts ions, speeding up rusting. |
| Test tube | C |
|---|---|
| Conditions | Air |
| Result | Rust. Air and moisture cause normal rusting |
| Test tube | D |
|---|---|
| Conditions | Air and calcium chloride |
| Result | No rust. Calcium chloride dries out the air. |
Salt solution acts as an electrolyte (any substance containing free ions that allows the substance to conduct electricity) allowing iron to lose electrons more easily and so speeds up the rusting process.
Ferroxyl indicator
Ferroxyl indicator can be used to show the process of rusting. When iron atoms begin to rust, they lose electrons to form iron ions. Ferroxyl indicator turns blue in the presence of iron ions. This shows that rusting has begun, even if there is no reddish brown rust showing on the surface of the iron.
A pink colour is also produced by the ferroxyl indicator. This shows that the ions being lost by iron are being gained by the water and oxygen that are also involved in rusting.
