A farmer stores iron machinery outdoors. Some machinery is painted and some is left unpainted. Explain why painting iron machinery slows down rusting, and state what would happen to the rate of rusting if the paint became scratched.

Cambridge International IGCSE Chemistry (0620) — 9.5 Corrosion of metals · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Paint forms a physical barrier that prevents water and oxygen from reaching the iron surface. Both water and oxygen are required for the electrochemical reaction that produces rust, so excluding either one stops the reaction. If the paint becomes scratched, the protective layer is removed and the iron underneath is exposed to water and oxygen. The rate of rusting then increases at the scratched area and rust forms on the exposed iron.

Examiner tips

  • Use the word "barrier" or "protective layer" to show understanding of the role of paint.
  • Explain that both water and oxygen are needed for rusting – this shows knowledge of the reaction.
  • Mention that scratching removes the barrier and exposes iron – this directly answers the second part.
  • Use the phrase "rate of rusting increases" to match the mark scheme wording.

Common mistakes

  • Saying paint only stops oxygen but ignoring water, or vice‑versa.
  • Claiming that paint prevents rust entirely without mentioning the need for both water and oxygen.
  • Not stating that scratching exposes iron and increases the rate of rusting.

Mark scheme (4 marks)

  1. Paint acts as a barrier / prevents contact between the iron and water (and oxygen/air)
  2. Both water and oxygen are required for rusting, so excluding either prevents rusting
  3. If the paint is scratched, the iron beneath is exposed (to water and oxygen)
  4. The rate of rusting increases (at the scratched area) / rusting occurs at the exposed iron

Key terms in this question

rusting

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