Uses of aluminium
4 marks · Normal| Sample | Surface treatment | Observations after 1 month outside |
|---|---|---|
| A | None (bare aluminium) | No visible change |
| B | Surface scratched | White, powdery spots appear |
| C | Coated with oil | No visible change |
Study the table above, which shows different treatments applied to aluminium samples left outside.
Suggest why the untreated (bare) sample did not show corrosion after a month.
Explain why scratching the surface resulted in white, powdery spots.
What does the result for the oil-coated sample show about the use of oil as a protective measure?
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) A stable oxide layer formed which prevented corrosion.
(b) Scratching removed the oxide layer so aluminium underneath reacted with air and water to form aluminium oxide, seen as a white powder.
(c) Oil acts as a barrier, stopping exposure to air and water, so no corrosion occurs.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Identifies the protective oxide layer as the reason for no corrosion.
M1 (b) · 1 mark · Mentions scratching removed/disrupted the oxide layer exposing metal.
A1 (b) · 1 mark · Explains corrosion occurred because newly exposed aluminium reacted with air/water and formed aluminium oxide.
B2 (c) · 1 mark · States oil blocked air/water so protected aluminium from corrosion.
Worked solution
a) The untreated sample formed a thin, strong oxide layer that protected it from reacting further, so no corrosion was visible.
b) Scratching the surface removed this protective oxide layer. The exposed aluminium reacted with air and water, forming aluminium oxide as a white, powdery substance.
c) The oil stopped air and moisture from reaching the metal, so no corrosion took place.