Sacrificial Protection of Metals
Cambridge IGCSE Chemistry 0620 · 15 Questions · 34 Total Marks
⚗️ The Core Concept
A more reactive metal loses electrons more readily than iron. When placed in electrical contact, the reactive metal corrodes preferentially — sacrificing itself so the iron survives. This is sacrificial protection.
Section A
Galvanising
5 questions
Section B
Sacrificial Protection
5 questions
Section C
The Electron Essay
5 questions
Section A: Galvanising
Iron coated with zinc — but what happens when the coating fails?
Name the metal used to galvanise iron.
Explain how galvanising protects iron even when the zinc coating is scratched and the iron is exposed.
State what is meant by the term 'galvanising'.
Compare galvanising with painting as methods of preventing iron from rusting. State one advantage of galvanising over painting.
The diagram below represents a scratched galvanised iron sheet exposed to rain water. Label which metal corrodes and which metal is protected, and state the direction of electron flow.
Section B: Sacrificial Protection
Magnesium blocks and buried zinc — real-world protection mechanisms.
Blocks of magnesium are attached to the steel hull of a ship below the waterline. Explain how this prevents the steel hull from rusting.
Suggest why magnesium is used rather than copper for sacrificial protection of an iron ship hull.
Underground steel pipelines can be protected from corrosion by connecting them to blocks of zinc buried nearby. (a) Name this method of protection. (b) Explain why the zinc must be replaced periodically.
State two conditions that are necessary for iron to rust, and explain why iron rusts more quickly in sea water than in fresh water.
A student says: 'Any metal can be used for sacrificial protection of iron.' Evaluate this statement.
Section C: The Electron Essay
Paper 4 extended writing — explain in terms of electron transfer.
Explain how sacrificial protection prevents iron from rusting. Your answer should include reference to electron loss and the reactivity series. [3 marks]
Write the half-equation for the oxidation of zinc during sacrificial protection.
A ship hull is made of steel. Blocks of magnesium are welded to the outside of the hull. Using ideas about oxidation and electron transfer, explain fully why the magnesium blocks must be replaced but the steel hull does not rust. [3 marks]
Explain why zinc provides sacrificial protection to iron but tin does NOT, even though tin can also be used to coat iron (as in tin cans).
[Extended Response] A student is designing a cathodic protection system for a new steel bridge. They have access to zinc, magnesium, copper, and tin. (a) Which TWO metals could be used? (b) Explain your choices using electron theory. (c) State one practical disadvantage of using magnesium rather than zinc. [4 marks]
⚠️ Examiner's Top Traps — Don't Lose These Marks
• Never say 'zinc protects iron because it is more reactive' alone — you MUST explain electron loss.
• The phrase 'greater tendency to lose electrons' is worth 1 mark on its own in many mark schemes.
• Rusting requires BOTH oxygen AND water — missing one condition = 0 marks.
• Tin provides PHYSICAL barrier protection only — tin is LESS reactive than iron, so it is NOT sacrificial.
• Sacrificial metals corrode and must be REPLACED — always state this in extended answers.
🏆 The 3-Mark Golden Template
[Mark 1] [Metal X] is more reactive than iron.
[Mark 2] [Metal X] has a greater tendency to lose electrons / is preferentially oxidised.
[Mark 3] [Metal X] corrodes instead of the iron, so the iron is protected from oxidation / rusting.
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