Galvanising & Sacrificial Protection
Section: 9. Metals | Syllabus: Cambridge IGCSE Chemistry (0620)
Galvanising: Barrier AND Sacrificial Protection
Galvanising is coating iron or steel with a layer of zinc (typically by dipping the object in molten zinc). It is used for buckets, fences, roofing sheets, nails and car body panels.
Galvanising is special because it protects the iron in two different ways at once:
- Barrier protection - while the zinc coating is intact, it simply keeps oxygen and water away from the iron underneath, just like paint or plastic.
- Sacrificial protection - even if the coating is scratched and the iron is exposed, the iron still does not rust.
Why Sacrificial Protection Works
Zinc is more reactive than iron in the reactivity series. When the coating is scratched, the exposed zinc and iron are both in contact with oxygen and water - but since zinc is more reactive, it loses electrons more readily than iron:
Zn(s) → Zn²⁺(aq) + 2e⁻
Zinc is oxidised in place of the iron - it is "sacrificed" so the iron is not. The electrons released by the zinc effectively stop the iron atoms from being oxidised. As long as some zinc remains nearby, the iron stays protected, even at the exposed scratch.
Worked Example
- Question: coatings of copper, paint, plastic and zinc are compared for protecting iron. Which coating will continue to protect the iron even when the coating is damaged? [1]
- Answer: zinc. Paint and plastic are barrier methods only - once damaged, the exposed iron rusts immediately. Copper is less reactive than iron, so if anything it would make the iron rust faster at the exposed area, not slower. Only zinc, being more reactive than iron, keeps protecting it by corroding preferentially.
Common Mistakes
- ❌ Assuming any metal coating gives sacrificial protection
- ✓ the coating metal must be more reactive than iron. A less reactive coating (e.g. copper or tin) gives barrier protection only - once scratched, the iron rusts, and may even rust faster than if uncoated
Other Examples of Sacrificial Protection
The same principle - attaching a more reactive metal so it corrodes instead of the metal being protected - is used elsewhere too. Blocks of magnesium (even more reactive than zinc) are bolted onto the steel hulls of ships and onto underground steel pipelines. The magnesium corrodes away over time and is periodically replaced, while the steel structure itself stays protected.
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