Rusting and Protecting Iron
Use controlled rusting evidence to explain barriers, galvanising and sacrificial protection with magnesium.
On this page
A bicycle chain can rust even though iron does not react with cold water to release hydrogen. Rusting is a different reaction: iron reacts with oxygen in the presence of water. It forms a reddish-brown material, mainly hydrated iron(III) oxide. Unlike a protective surface layer, rust is flaky and exposes more iron beneath it.
What conditions are needed?
Iron rusting requires both oxygen and water. Compare three prepared tubes:
| Prepared tube | Conditions at the nail | Prediction |
|---|---|---|
| dry air with a drying agent, sealed to keep moisture out | oxygen present; water removed | no rust |
| boiled water, nail fully submerged, oil layer over the water | water present; dissolved oxygen removed by boiling and kept out by the oil | no rust if oxygen is effectively excluded |
| air and water | both oxygen and water present | rust forms |
The oil layer helps prevent oxygen from dissolving back into the boiled water. The relevant condition is oxygen at the nail; there may still be air above the oil. Compare similar clean nails over the same time, with the conditions maintained throughout.
Salt is not a required reactant for rusting. Salt water can speed corrosion, but clean water and oxygen are enough for iron to rust.
Barriers keep reactants away
Paint, oil, grease and plastic coatings form a physical barrier between iron and its surroundings. An intact barrier stops oxygen and water from contacting the iron. Oil or grease is useful where moving parts need lubrication.
If a coating is scratched, oxygen and water can reach exposed iron and rusting can begin. A barrier protects only while it remains effective.
Examiner practice 1
Explain a rusting control
Examination question
Explain why painting an iron gate prevents rusting while the coating remains intact. [3 marks]
Name both rusting conditions and the barrier action
View solution step by step
State the rusting conditions
2 marksMethod
Name both oxygen and water.Reason
Iron rusting requires both substances rather than either one alone.
Working
Required conditions: O₂ and H₂O.
Explain the coating
1 markMethod
Use paint as a physical barrier.Reason
An intact coating prevents oxygen and water from contacting the iron surface.
Working
No contact with both required reactants → rusting prevented.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark oxygen, water and barrier action separately.
Galvanising gives two kinds of protection
Galvanising means coating iron or steel with zinc.
- An intact zinc coating forms a barrier.
- Zinc is above iron in the reactivity series, so it loses electrons more readily. In wet conditions, remaining zinc in electrical contact with the iron can oxidise instead of iron, even near a scratch.
This second effect is sacrificial protection: the more reactive metal is used up while protecting the iron. It is different from simply sealing the surface.
Zn → Zn²⁺ + 2e⁻
Check: Two iron sheets are scratched, one painted and one galvanised. Both are exposed to air and rain. Explain why their exposed regions can behave differently.
Show answer
The paint barrier is broken, so the exposed iron can rust. The galvanised sheet can remain protected because zinc is more reactive than iron and oxidises in preference to it. This requires enough zinc remaining in electrical contact with the iron under the wet conditions; the zinc is gradually consumed.
An attached metal can protect iron
A block of magnesium can be connected to an iron structure, such as a steel water tank. Magnesium is more reactive than iron and oxidises in preference to it. Electrical contact, directly or through a conducting connection, and the wet environment allow protection to operate.
Mg → Mg²⁺ + 2e⁻
The magnesium is consumed and must eventually be replaced. A magnesium block placed nearby without an electrical connection does not provide this sacrificial protection. Copper is below iron in the series and cannot protect it by being sacrificed.
Mind stretcher 1: Damaged zinc coatingExtension
Question: A galvanised iron object is scratched. Explain why nearby iron can remain protected.
Show Answer
Zinc is more reactive than iron, so zinc oxidises in preference to the exposed iron. This sacrificial action continues to protect the scratched region while sufficient zinc remains in electrical contact with the iron under wet conditions.
Try independently: An iron pipe is exposed to oxygenated water. A magnesium block is securely connected to it. Later, the connection breaks, although plenty of magnesium remains. Predict how the protection changes and explain why.
Show answer and reasoning
While connected, magnesium oxidises in preference to iron and protects it. After electrical contact is lost, that sacrificial protection stops. Oxygen and water are still present at the iron, so the pipe can rust even though magnesium remains nearby.
Remember and continue
Rusting needs oxygen and water. Barriers exclude the reactants; zinc coatings also offer sacrificial protection. An attached magnesium block protects iron because magnesium is more reactive, but it needs electrical contact and is used up.
Use the Periodic Table topic check to practise and check your understanding.
Syllabus and review details
- SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24
Last reviewed: