Iron rusts when both oxygen and water are present. Comparing iron in dry air, in water without available oxygen, and in moist air helps establish what rusting needs.
Paint, grease and metal plating can form a barrier between iron and its surroundings. A scratched barrier can allow rusting at the exposed iron.
Zinc is more reactive than iron. When zinc remains in contact with iron under suitable conditions, it can be oxidised instead. This is sacrificial protection, so a scratch need not immediately expose iron to rusting.
Aluminium reacts with oxygen to form a protective oxide layer. Edexcel also asks how electroplating can improve a metal object’s appearance and resistance to corrosion.
Worked example
Three identical iron nails are placed in different conditions. The nail in moist air rusts. The dry-air nail and the nail in deoxygenated water show no rust over the same period.
This comparison supports the need for both water and oxygen. It does not predict the lifetime of a coating in everyday use.
Specification references
AQA 8462 section 4.10.3.1; Edexcel 1CH0 points 5.2C–5.4C. Both tiers.
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A scratch exposes the iron beneath paint and the iron beneath a zinc coating. Explain why the consequences can differ.
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Explain how an intact barrier helps.
Use relative reactivity to explain zinc protection.
State the condition needed for zinc to protect exposed iron.
Example answer
Paint protects by stopping water and oxygen reaching iron. A scratch breaks the barrier. Zinc can still be oxidised in preference to iron when it remains in contact in suitable conditions.
An answer to improve
Zinc stops oxygen existing, so scratched galvanising always works indefinitely.
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