Galvanising and sacrificial protection
Explain zinc as both a barrier and the more readily oxidised metal, including what a scratch changes.
Exam scope and assessment notes
Extended only (Supplement) · Chemistry 0971, 2026–2028. Cambridge states that IGCSE 0971 has the same subject content as IGCSE 0620, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Locate zinc above iron in the required series.
- Write electron-loss half-equations and explain ordinary barrier failure at a scratch.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which ordering supports zinc sacrificial protection of iron?
What you will learn
- Describe zinc galvanising as barrier and sacrificial protection.
- Explain sacrificial protection through series position, electron loss and suitable electrical contact.
Zinc can do two different jobs
Galvanising coats iron or steel with zinc. An intact coating provides a physical barrier, limiting oxygen or water access. Zinc is also above iron in the reactivity series and loses electrons more readily in the suitable corrosion system. It can therefore be oxidised preferentially, providing sacrificial protection to the iron while usable zinc remains and the required conductive contact is present.
At a scratch, the simple barrier is locally broken, but electrically connected zinc can still act sacrificially under appropriate conditions. Zn→Zn²⁺+2e⁻ is the electron-loss half-equation. The zinc is consumed as it protects the iron; do not call it a catalyst or claim protection continues after all useful zinc or electrical contact is lost.
The protecting metal must have the right role
A suitable sacrificial metal is more reactive than iron in the stated model, so it is the one preferentially oxidised. Merely attaching any metal does not establish protection. A less reactive metal coating may provide an intact barrier, but it does not supply the same more-reactive sacrificial role; a scratch therefore cannot be assumed to receive zinc-like protection.
Explain both the structural barrier and electron-transfer role when asked about galvanising. Protection depends on the material, contact and conditions; placing a zinc object nearby without an electrical connection is not equivalent to a suitable sacrificial arrangement. Detailed electrochemical circuit design is beyond this objective, but the necessary contact limitation prevents a misleading anywhere-nearby-zinc claim.
Pause and explain
What happens to zinc during its sacrificial role?
Worked example
Explain why galvanised iron can remain protected at a small scratch while intact, electrically connected zinc remains under suitable conditions.
Show the worked solution
- The scratch locally exposes iron, so the barrier alone is no longer complete there.
- Zinc is more reactive than iron and preferentially loses electrons through Zn→Zn²⁺+2e⁻.
- Its sacrificial role can protect the exposed iron while usable zinc and conductive contact remain.
Answer Zinc is preferentially oxidised and consumed, giving sacrificial protection beyond the intact-barrier role.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain galvanising's two roles and write the zinc electron-loss equation.
Give me a hint
Separate the intact coating from preferential oxidation.
Compare my reasoning
- The zinc coating is a barrier against oxygen or water.
- Zinc's higher reactivity supports sacrificial oxidation.
- Zn→Zn²⁺+2e⁻ shows the electron loss and zinc consumption.
Barrier plus sacrificial oxidation; Zn→Zn²⁺+2e⁻.
Look for these in your work
- Both mechanisms are stated.
- The equation identifies zinc's own change.
Compare a scratched paint coating with a scratched zinc coating while usable zinc remains electrically connected under suitable conditions.
Give me a hint
Only one coating has the stated sacrificial effect.
Compare my reasoning
- The paint scratch permits reactant access and has no zinc-like sacrificial role.
- The zinc scratch breaks the local barrier, but connected zinc can still preferentially oxidise.
- This added protection depends on remaining useful zinc and suitable contact.
Paint loses local barrier protection; connected remaining zinc can add sacrificial protection at the scratch.
Look for these in your work
- The scratch affects both barriers.
- The additional zinc role and its limits are explained.
A plan places an unconnected zinc piece near iron and promises permanent protection even after the zinc disappears. Evaluate both claims.
Give me a hint
The model requires a suitable conductive arrangement and sacrificial material to consume.
Compare my reasoning
- Nearby zinc without electrical contact is not the specified sacrificial arrangement.
- Useful zinc must remain available for preferential oxidation.
- Permanent protection after its exhaustion is unsupported; both contact and remaining material matter.
Neither proximity alone nor exhausted zinc supports permanent sacrificial protection.
Look for these in your work
- Both failure conditions are identified.
- The explanation uses the mechanism rather than a generic reassurance.
Common mistakes
- Calling zinc unchanged overall like a catalyst.
- Assuming any metal or an unconnected nearby zinc piece provides the same protection.
- Claiming protection continues after the sacrificial metal is exhausted.
- Explaining only a barrier when a scratch and sacrificial protection are asked about.
What can you explain now?
Why is zinc the sacrificial metal rather than iron in this protection model?
Compare with the explanation
Zinc is more reactive than iron and loses electrons preferentially.
Its oxidation supplies the sacrificial role while the suitable contact and conditions protect iron.
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Make it stick
Explain an intact and scratched galvanised surface, write zinc oxidation and name two conditions that limit sacrificial protection.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.