Explain why zinc is often used to protect iron structures, such as oil pipelines, from corrosion.

Edexcel GCSE Chemistry (1CH0) — 5.1 Transition metals, alloys and corrosion (Chem only) · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Iron structures buried underground or submerged in water are prone to corrosion. One method used to protect these structures involves attaching blocks of zinc directly to the iron.

Model answer (4 marks)

Zinc is used because it is more reactive than iron, so it corrodes preferentially. When zinc is attached to iron, the zinc acts as a sacrificial anode: it oxidises first, supplying electrons to the iron. The iron therefore does not corrode while the zinc remains. This sacrificial protection keeps the iron/steel safe for as long as the zinc is present.

Examiner tips

Use the term ‘sacrificial protection’ or ‘galvanic protection’.
State that zinc is more reactive (higher reduction potential) than iron.
Explain that zinc corrodes first, supplying electrons to iron.
Mention that the iron is protected until the zinc is consumed.

Common mistakes

Confusing zinc as a protective coating instead of a sacrificial anode.
Lacking the phrase ‘sacrificial protection’ or ‘galvanic protection’.
Forgetting to mention that zinc must remain to keep protection effective.

Mark scheme (4 marks)

The process described is sacrificial protection
Zinc is more reactive than iron
Zinc corrodes/reacts preferentially instead of the iron
The iron/steel is therefore protected from corrosion (for as long as the zinc remains)

Key terms in this question

corrosion

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