Preventing rusting by using sacrificial protection
4 marks · NormalA scientist is choosing a metal to protect an iron water tank from rusting. She tests three metals (A, B, and C), each attached to iron and left in moist air. The results are shown below.
| Metal attached | Observation after 1 month |
|---|---|
| A | Iron is rusted; attached metal is not corroded |
| B | Iron is shiny; attached metal is heavily corroded |
| C | Iron is rusted; attached metal is also rusted |
Which metal, A, B, or C, acts as a sacrificial anode for iron? Explain your answer.
Explain, using the observations, why the chosen metal must be more reactive than iron.
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.
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(a) B; it acts as a sacrificial anode because iron stays shiny and B corrodes.
(b) The attached metal (B) must be more reactive than iron so it corrodes in place of the iron, protecting the iron from rusting.
Common mistakes: Confusing which metal is more reactive or not using correct reasoning.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly identifies metal B as acting as a sacrificial anode.
M1 (a) · 1 mark · Explains B works because it corrodes instead of iron (reference to iron remaining shiny).
M2 (b) · 1 mark · Explains the attached metal must be more reactive than iron to corrode first.
A1 (b) · 1 mark · Links greater reactivity to protection of iron (corrosion or rusting prevented).
Worked solution
a) Metal B acts as a sacrificial anode. The iron stays shiny (does not rust), while metal B corrodes heavily.
b) Metal B must be more reactive than iron, so it corrodes in preference to iron. Its greater reactivity ensures it is oxidised first, stopping the iron from rusting.