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Galvanising coats iron or steel with zinc, and sacrificial protection attaches a more reactive metal that corrodes instead. 15 questions answered.
Sacrificial Protection and Galvanising is part of Using materials (chemistry only) in GCSE AQA Higher Chemistry.
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Galvanising coats iron or steel with zinc, and sacrificial protection attaches a more reactive metal that corrodes instead. 15 questions answered.
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Use this lesson to stay inside Using materials (chemistry only) while connecting the detail back to the wider Topic 10: Using resources topic.
Read the core explanations from Sacrificial Protection and Galvanising before testing yourself from memory.
Picture a car left outside for years - the shiny metal slowly turns into flaky orange rust. This costs the UK economy billions of pounds every year in repairs and replacements.
Rust forms when iron reacts with oxygen and water in the air. The chemical equation shows what happens: $4Fe + 3O_2 + 6H_2O \rightarrow 4Fe(OH)_3$
But there's a clever trick engineers use. Instead of trying to stop this reaction completely, they make something else rust instead. This leads us to one of chemistry's most useful applications.
Think of the reactivity series as a queue for a chemical fight. The metals at the top are the most eager to react - they'll push their way to the front and react first.
Here's the key section we need: Zinc > Iron > Copper. Zinc is more reactive than iron, which means zinc will always react with oxygen before iron does. This is the secret behind sacrificial protection.
Imagine iron as a celebrity and zinc as their bodyguard. When trouble (oxygen) arrives, the bodyguard (zinc) takes the hit instead of the celebrity (iron). The zinc gets damaged, but the iron stays safe.
In chemical terms: zinc is higher in the reactivity series than iron. When both metals are present with oxygen and water, the zinc reacts preferentially. The iron remains unreacted and doesn't rust.
You see this on ships, where zinc blocks are bolted to the hull. The zinc slowly dissolves away, but the steel hull stays protected. When the zinc is used up, they bolt on new blocks.
Galvanising gives double protection. First, the zinc coating acts like a waterproof jacket, keeping oxygen and water away from the iron underneath. But here's the clever part - even if the coating gets scratched, the zinc still protects the iron.
When the coating is damaged, you have zinc and iron both exposed to air. Since zinc is more reactive than iron, the zinc continues to corrode preferentially. The iron stays protected even through the scratch.
Worked example: How Galvanising Protects Iron
Explain what happens when galvanised iron gets scratched, exposing both zinc and iron to air and water.
Answer: The zinc corrodes preferentially, continuing to protect the iron even when the coating is damaged.
You'll find galvanised steel everywhere: garden fences, car bodies, roofing sheets, and playground equipment. The dull grey appearance is the zinc coating doing its job.
Not all protective coatings work the same way. Paint and plastic coatings just create a barrier - if they get scratched, the metal underneath starts rusting immediately. But sacrificial coatings like zinc keep working even when damaged.
Tin-plated steel (like food cans) uses a different approach. Tin is less reactive than iron, so it only works as a barrier coating. If a tin can gets scratched, the iron rusts faster than normal because tin doesn't provide sacrificial protection.
Exam questions on sacrificial protection often test your understanding of the reactivity series. The key is always to identify which metal is more reactive - that's the one that will corrode first.
Common trap: Students often think the less reactive metal protects the more reactive one. Remember the bodyguard analogy - the tougher one (more reactive) protects the celebrity (less reactive).
For 6-mark questions on corrosion prevention, structure your answer: define sacrificial protection, explain using reactivity series, give a specific example like galvanising, compare with barrier methods, and evaluate effectiveness.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-19.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Galvanising is coating iron or steel with a thin layer of zinc to prevent rusting. The zinc gives double protection: it acts as a physical barrier that keeps oxygen and water away, and it also provides sacrificial protection.
Sacrificial protection is a method of preventing corrosion where a more reactive metal is attached to a less reactive metal, so the more reactive metal corrodes instead. For example, zinc blocks are bolted to steel ship hulls and slowly dissolve away while the hull stays protected.
Zinc is more reactive than iron, and iron is more reactive than copper (Zinc > Iron > Copper). This means zinc will always react with oxygen before iron does, which is the basis of sacrificial protection and galvanising.
Galvanising is coating iron or steel with a thin layer of zinc, while sacrificial protection is any method of attaching a more reactive metal that corrodes instead. Galvanising gives double protection: the zinc is both a physical barrier and a sacrificial metal.
Galvanised steel is used for garden fences, car bodies, roofing sheets and playground equipment. The dull grey appearance is the zinc coating, which protects the steel both as a barrier and through sacrificial protection.
Galvanised iron does not rust when scratched because zinc is more reactive than iron, so the exposed zinc reacts with oxygen and water first. The zinc corrodes preferentially, continuing to protect the iron even when the coating is damaged.
A scratched tin can rusts faster than normal because tin is less reactive than iron, so it only works as a barrier coating. Once the coating is scratched, tin does not provide sacrificial protection, so the iron underneath rusts.
Only a metal more reactive than iron, such as zinc, can protect iron by sacrificial protection, because the more reactive metal reacts with oxygen first. A common trap is thinking the less reactive metal protects the more reactive one.
Zinc is used to protect iron because zinc is more reactive than iron, so it reacts with oxygen and water first. The zinc corrodes instead of the iron, sacrificing itself, which is called sacrificial protection.
Zinc blocks are bolted to ship hulls to give sacrificial protection. Zinc is more reactive than iron, so the zinc slowly dissolves away while the steel hull stays protected, and new blocks are bolted on when the zinc is used up.
Barrier methods such as paint and plastic coatings only block oxygen and water, so if they are scratched the metal underneath starts rusting immediately. Sacrificial protection keeps working even when damaged, because the more reactive metal corrodes preferentially instead of the iron.
A less reactive metal cannot give iron sacrificial protection; only a more reactive metal can, because it corrodes first. A common mistake is thinking the less reactive metal protects the more reactive one, and tin, which is less reactive than iron, only works as a barrier coating.
Sacrificial protection works because a metal higher in the reactivity series reacts preferentially when both metals are present with oxygen and water. Zinc is higher than iron, so the zinc reacts and the iron remains unreacted and does not rust.
In sacrificial protection, the zinc corrodes and slowly dissolves away instead of the iron. The zinc gets damaged but the iron stays safe, so zinc blocks on ships must be replaced with new blocks when the zinc is used up.
A 6-mark answer on corrosion prevention should define sacrificial protection, explain it using the reactivity series, give a specific example like galvanising, compare it with barrier methods and evaluate effectiveness. The key point is always identifying which metal is more reactive, because that one corrodes first.