Preventing rusting by using sacrificial protection
4 marks · NormalThe diagram shows an iron pipeline buried underground with blocks of magnesium attached at intervals.
Explain why the magnesium blocks must be more reactive than iron in order to protect the pipeline from rusting.
Explain what happens to the magnesium during this process.
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(a) The magnesium must be more reactive so it is preferentially oxidised instead of iron, protecting the iron from corrosion.
(b) Magnesium gradually corrodes/oxidises, turning into magnesium ions and losing mass, and may eventually need replacing.
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Marking points
M1 (a) · 1 mark · States magnesium must be more reactive than iron.
M2 (a) · 1 mark · Explains this is so magnesium oxidises/corrodes instead of iron (reference to rust prevented or electrons supplied is creditworthy).
M3 (b) · 1 mark · States magnesium corrodes/oxidises during the process (forms Mg2+ ions or loses electrons).
M4 (b) · 1 mark · Mentions magnesium is gradually used up/loses mass/needs replacing.
Worked solution
a) Magnesium must be more reactive than iron so that it oxidises in preference to iron. This means magnesium reacts and loses electrons, protecting the iron from rusting.
b) Magnesium is gradually oxidised to form magnesium ions, losing mass. Eventually, the block of magnesium is used up and must be replaced.