Rusting is a redox process: iron is oxidised where it loses electrons, and oxygen is reduced where it gains them. Every protection method works by blocking this process or by giving the electrons a cheaper place to come from.
This lesson is part of SPM Chemistry redox equilibrium. It applies the ideas in explaining simple chemical cells.
What happens when iron rusts?
On one region of the iron surface, iron loses electrons: Fe → Fe²⁺ + 2e⁻. This is the negative terminal, where oxidation takes place.
The electrons travel through the iron to another region, where dissolved oxygen is reduced: O₂ + 2H₂O + 4e⁻ → 4OH⁻. The Fe²⁺ ions are further oxidised and form hydrated iron(III) oxide, which is rust.
Water that contains dissolved ions acts as an electrolyte, so the two regions behave like a tiny chemical cell.
How do protection methods work?
Two ideas cover most methods.
Barrier protection. Paint, oil, grease or plastic keeps oxygen and water off the iron. The coating does its job only while it is intact.
Sacrificial protection. A more reactive metal is connected to the iron, for example magnesium blocks attached to a pipeline. Magnesium is higher in the series than iron, so magnesium is oxidised first: Mg → Mg²⁺ + 2e⁻. The iron stays protected.
Worked example: scratched zinc and scratched tin
An original comparison. Two iron sheets, one coated in zinc and one coated in tin, are both scratched to expose the iron underneath.
| Coating | Position compared with iron | What happens at the scratch |
|---|---|---|
| Zinc | Higher in the series | Zinc is oxidised, so the iron is protected |
| Tin | Lower in the series | Iron is oxidised, so the iron rusts |
Zinc protects iron even with a scratch because zinc gives up its electrons more readily. Tin is only a barrier, and once the barrier breaks the iron becomes the negative terminal.
The mistake that costs marks
The common slip is saying that a metal coating always protects. Coatings of a metal lower than iron in the series protect only while they are unbroken.
Another slip is explaining sacrificial protection as “magnesium is stronger”. Use the redox reason: magnesium is oxidised in place of iron because it loses electrons more readily.
Check yourself
Iron nails are wrapped separately with copper wire and with magnesium ribbon and left in salt water. Explain which nail rusts faster and which is protected.
Answer
Iron is higher than copper in the series, so iron is the negative terminal when in contact with copper. Iron is oxidised faster, so the nail wrapped in copper rusts faster.
Magnesium is higher than iron, so magnesium is oxidised instead: Mg → Mg²⁺ + 2e⁻. The iron nail wrapped in magnesium is protected.
What to study next
Put all six skills together with the redox practice set. For a wider view of how the chapter fits, see the redox equilibrium overview.
The mistake log and paper-error review tool helps you record explanation slips. If you want a teacher to check your written explanations, see online one-to-one Chemistry tuition.