Preventing rusting by using sacrificial protection
4 marks · NormalA section of an underwater steel bridge has several aluminium blocks attached to it.
Explain why aluminium is able to protect the steel from rusting by sacrificial protection.
Describe what happens to the aluminium blocks over time and why they are eventually replaced.
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) Aluminium is more reactive than iron so it corrodes (is oxidised) in preference to the steel, protecting it from rusting.
(b) The aluminium blocks gradually corrode and lose mass, so they are eventually used up and have to be replaced.
Common mistakes: Ignoring that aluminium is more reactive than iron.
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · States that aluminium is more reactive than iron/steel.
M2 (a) · 1 mark · Explains that aluminium is oxidised/corrodes instead of iron, so the steel is protected.
M3 (b) · 1 mark · Describes that aluminium gradually corrodes/is used up/loses mass.
A1 (b) · 1 mark · States that aluminium blocks have to be replaced once all aluminium is used up.
Worked solution
a) Aluminium is more reactive than iron. It acts as a sacrificial anode, oxidising and losing electrons in preference to the steel, so the steel does not rust.
b) Over time, the aluminium blocks corrode and their mass decreases. Once all the aluminium has been oxidised, they must be replaced to continue protecting the steel.