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AQA GCSE · Question 06.2 · Using resources
Explain why aluminium alloy bicycle frames do not need protection from corrosion.
Explain why aluminium alloy bicycle frames do not need protection from corrosion.
How to approach this question
This question is about the corrosion of aluminium.
1. Recall that aluminium is actually a reactive metal (high in the reactivity series).
2. How does it protect itself from further reaction? Think about what it reacts with in the air.
3. Describe the properties of the layer that is formed and explain how it provides protection.
Full Answer
Aluminium is a reactive metal, but it reacts with oxygen in the air to form a very thin, tough, and unreactive layer of aluminium oxide on its surface. This layer is strongly bonded to the aluminium underneath and acts as a protective barrier, preventing any further corrosion.
Although aluminium is more reactive than iron, it does not readily corrode in the same way iron rusts. This is due to a phenomenon called passivation. When aluminium is exposed to air, its surface rapidly reacts with oxygen to form a very thin, but very strong and durable, layer of aluminium oxide (Al₂O₃). Unlike iron oxide (rust), which is flaky and porous, this aluminium oxide layer adheres tightly to the metal surface. It is also transparent and non-porous, so it forms a protective barrier that seals the aluminium from further contact with oxygen and water, thus preventing corrosion.
Common mistakes
✗ Stating that aluminium is unreactive. This is incorrect; it is a reactive metal.
✗ Confusing the protective layer with paint or another coating.
✗ Not naming the substance that forms the protective layer (aluminium oxide).
Practice the full AQA GCSE Chemistry Higher Tier Paper 2
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