Zinc can still protect exposed iron at a scratch if the zinc remains electrically connected and the surrounding conditions allow corrosion reactions. Zinc is more reactive than iron and oxidises preferentially. A less reactive coating such as tin offers a barrier while intact but does not provide the same sacrificial protection.
The 2027 SEC G3 Chemistry K324 syllabus includes sacrificial protection of iron using a more reactive metal. This guide compares intact and scratched coatings so you can explain which protection mechanism applies.
- Check whether the coating is intact.
- Compare the coating metal with iron in the reactivity series.
- Name the metal that oxidises preferentially.
- State the role of contact and moisture in the scratched-coating example.
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Distinguish two ways a coating protects
An intact coating can act as a physical barrier between iron and the water and oxygen involved in rusting. A scratch exposes some of the iron and breaks that barrier locally.
Zinc is more reactive than iron. In the scratched-coating example, zinc in electrical contact with the iron can oxidise preferentially when moisture allows ions to move.
The oxidation half-equation for zinc is Zn(s) → Zn²⁺(aq) + 2e⁻. Zinc is called sacrificial because it is gradually used up as it protects the iron.
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Worked example | Compare zinc and tin coatings
Two identical iron sheets have different coatings. One is coated with zinc and the other with tin. Each is scratched through to the iron and placed in a comparable moist, oxygen-containing environment.
| Coating | Reactivity compared with iron | Protection at exposed iron |
|---|---|---|
| Zinc | More reactive | Can provide sacrificial protection while connected |
| Tin | Less reactive | No sacrificial protection from the tin |
At the scratch in the zinc-coated sheet, zinc can oxidise preferentially. In the tin-coated sheet, the exposed iron can rust. The tin does not sacrifice itself in place of the iron.
A complete exam explanation names the metals, gives their relative reactivities and says which one oxidises.
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Practise when the barrier is intact
If the same two sheets are unscratched and their coatings fully cover the iron, both coatings can act as barriers. The question about sacrificial protection becomes especially relevant once iron is exposed.
If the zinc becomes disconnected from the iron or is used up, it can no longer protect the exposed iron by this mechanism.
Check which question you are answering. A barrier explanation discusses access of water and oxygen. A sacrificial protection explanation discusses which metal oxidises preferentially.
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Frequently asked questions
Does zinc stop all corrosion of the coated sheet?
Zinc itself can corrode while protecting the iron. The protection lasts only while sufficient connected zinc remains.
Does the zinc have to cover the scratch itself?
In the stated conditions, nearby connected zinc can protect exposed iron through sacrificial action.
What if the exam names a different coating?
Use the given reactivity information to decide whether it can oxidise in place of the iron.
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