Corrosion of metals
Rusting of iron, the conditions required, and methods of prevention.
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Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 9.5.1State the conditions required for iron to rust and describe barrier and sacrificial methods of prevention.
6 minute read
Corrosion of metals
Rusting is the corrosion of iron, forming hydrated iron(III) oxide (rust). It requires both oxygen (air) and water — if either is absent, iron does not rust. This can be shown experimentally: iron in dry air (no water) does not rust; iron in boiled, oxygen-free water (no oxygen) does not rust; only iron exposed to both air and water rusts.
Preventing rust: barrier methods
A barrier method simply keeps air and water away from the iron's surface. Examples: painting, oiling or greasing, coating with plastic, or electroplating with a less reactive metal like tin or chromium. As long as the barrier stays intact, the iron underneath is protected.
Preventing rust: sacrificial protection
Sacrificial protection uses a more reactive metal in contact with the iron. Because the more reactive metal (commonly zinc or magnesium) loses electrons more readily than iron, it corrodes in preference to the iron — "sacrificing" itself. This works even if the coating is scratched, as long as some of the more reactive metal remains in contact with the iron.
Galvanising is the specific process of coating iron or steel with a layer of zinc, which acts as both a barrier AND provides sacrificial protection — giving it an advantage over barrier-only methods like paint, which fail completely once scratched.
Think of it like this
Sacrificial protection is like a bodyguard who steps in front of an attack meant for someone else — the zinc "takes the hit" (loses electrons and corrodes) so that the iron behind it does not have to, and this protection keeps working even if the bodyguard is wounded (the coating scratched), as long as they are still there.
Common misconceptions
- Thinking rusting only needs oxygen, or only needs water. Both must be present together for iron to rust.
- Believing all coatings work the same way. A pure barrier method (like paint) fails completely once scratched, exposing bare iron to air and water; a sacrificial method (like galvanising) keeps protecting the iron even after being scratched, because it is the more reactive metal itself that is being consumed.
- Assuming any metal coating gives sacrificial protection. Only a coating metal MORE reactive than iron (such as zinc or magnesium) provides sacrificial protection — a less reactive coating like tin only acts as a barrier, and can actually make rusting worse locally if scratched, since it would make iron the more reactive metal in that pair.
In the exam
- To prove both oxygen and water are needed, describe (or recognise) the classic three-tube experiment: iron in normal air and water (rusts), iron in dry air only (no rust), iron in boiled water with oil on top and no air (no rust).
- For "explain how galvanising protects iron, even when scratched", the sacrificial explanation (zinc more reactive, corrodes instead of iron) is specifically required — a barrier-only answer will not gain full marks.