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Learn0620Papers 1, 3 · CoreMetalsCorrosion of metals
Metals · Subtopic 3 of 4

Corrosion of Metals

Rusting is the corrosion of iron and steel. It needs both oxygen and water, and forms hydrated iron(III) oxide. Salt speeds it up. Barrier methods (paint, oil, grease, plastic, tin plating) keep oxygen and water away. Galvanising coats iron with zinc; zinc is more reactive, so even if scratched it corrodes first (sacrificial protection).

Cambridge 5070 Paper 1Cambridge 5070 Paper 2Cambridge 0620 Papers 2, 4Cambridge 0620 Papers 1, 3Edexcel 4CH1 Paper 1CEdexcel 4CH1 Paper 2C≈ 30 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • State the conditions needed for rusting.
  • Describe barrier methods of preventing rust.
  • Explain galvanising and sacrificial protection.
Pause and think

Why do cars rust faster in countries where roads are salted in winter?

Section 2 of 14

Core idea

Start hereThe cost of rust

Corrosion costs the world economy roughly 3% of its total output every year: bridges, ships, pipes and cars slowly turning back into the ore they came from.

The big idea

Iron rusts when both water and oxygen are present. Stop either reaching the iron, or give it a more reactive partner to corrode instead.

1. The evidence

The three-tube experiment

Only the nail with both water and air rusts.

2. Prevention

Worked exampleChoosing a method

Suggest how to protect (a) a garden gate, (b) a ship's hull, (c) a bicycle chain.

Goal
Suitable methods.
Strategy
Consider movement, size and scratching.
  1. (a) paint or galvanise\text{(a) paint or galvanise}
    Cheap barrier; zinc protects scratches.
  2. (b) zinc or magnesium blocks\text{(b) zinc or magnesium blocks}
    Sacrificial protection; paint gets scratched at sea.
  3. (c) oil\text{(c) oil}
    A barrier that also lubricates moving parts.
AnswerPaint/galvanise; sacrificial blocks; oil.
✓ CheckEach method suits the object ✓.
Key takeawaySacrificial protection A more reactive metal loses electrons in preference to iron: Zn→Zn2++2e−\mathrm{Zn\rightarrow Zn^{2+}+2e^-}. The electrons flow to the iron, keeping it as metal.
Pause and think

Why must sacrificial blocks on ships be replaced every few years?

Summary

  • Water + oxygen → rust.
  • Barriers keep them out.
  • Zinc/magnesium corrode first.
Key words
Rusting
Corrosion of iron forming hydrated iron(III) oxide.
Galvanising
Coating iron with zinc.
Sacrificial protection
A more reactive metal corrodes instead of iron.
HistoryThe Iron Pillar of Delhi. This 1600-year-old iron pillar has barely rusted. Its high phosphorus content and Delhi's dry air formed a protective layer on its surface.
Fun factRepainting the Forth Bridge. Scotland's Forth Bridge was famously "always being repainted". A modern glass-flake coating should now protect it for 25 years.
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