Preventing rusting by using sacrificial protection
5 marks · HardAn engineer is tasked with protecting an iron pipeline buried underground using the method of sacrificial protection.
Explain why sacrificial anodes must be made of a metal more reactive than iron.
The engineer is considering aluminium, tin, and silver as possible metals for sacrificial anodes. Aluminium is above iron in the reactivity series, tin is just below iron, and silver is far below iron. Which of these metals could be used as sacrificial anodes for the pipeline? Justify your choice.
Describe what happens to the chosen sacrificial anode metal over time.
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) Because the sacrificial anode needs to oxidise (corrode) in preference to the iron, only a metal more reactive than iron will do this.
(b) Only aluminium can be used as a sacrificial anode because it is more reactive than iron. Tin and silver are less reactive than iron, so they cannot provide sacrificial protection as they will not corrode in preference to the iron.
(c) The aluminium anode slowly corrodes/oxidises over time and will eventually need to be replaced.
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Marking points
M1 (a) · 1 mark · States that the sacrificial anode must be more reactive than iron.
A1 (a) · 1 mark · Explains the more reactive metal oxidises/corrodes in preference to iron (or equivalent electron transfer explanation).
M2 (b) · 1 mark · Identifies aluminium as the only suitable choice, with justification referencing reactivity relative to iron.
A2 (b) · 1 mark · Correctly rejects tin and silver, justifying with their reactivity below iron (so will not corrode first).
I1 (c) · 1 mark · Describes that the sacrificial anode (aluminium) will corrode/oxidise and need replacing over time.
Worked solution
a) Sacrificial anodes must be made of metals more reactive than iron because only these metals will oxidise (corrode) in preference to the iron, preventing rusting of the iron.
b) Only aluminium could be used as a sacrificial anode, because it is above iron in the reactivity series and will corrode first. Tin and silver are below iron, so they will not corrode first; therefore, they cannot act as sacrificial anodes.
c) The aluminium sacrificial anode will react/corrode gradually over time. It will eventually be used up and need replacing.