Rust costs a fortune in repairs every year, and it needs just two things: oxygen and water. Take away either one and iron stays shiny. Every prevention method either puts up a barrier or gives the oxygen something more reactive to attack.
Iron and steel rust only when both water and oxygen are present; rust is hydrated iron(III) oxide. Barrier methods (painting, greasing, coating with plastic) stop rusting by keeping out water and oxygen. Galvanising coats iron with zinc. Extended: zinc, or a block of magnesium, gives sacrificial protection: being more reactive, it loses electrons and corrodes instead of the iron, even if the surface is scratched.
Three iron nails, three test tubes, one week: only one nail rusts.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Which two substances are needed for iron to rust?
What is rust?
Why is a bicycle chain greased rather than painted?
What is galvanising?
Match the method to the object: paint for a bridge, grease for a bicycle chain, galvanising for a dustbin or a fence.
| Method | How it works | Example |
|---|---|---|
| painting | barrier to water and oxygen | bridges, cars |
| greasing or oiling | barrier; suitable for moving parts | bicycle chains, tools |
| plastic coating | barrier | garden fencing, dish racks |
| galvanising (zinc coating) | barrier, and sacrificial protection if scratched | dustbins, roofing, nails |
| sacrificial blocks (Extended) | more reactive metal corrodes instead | zinc or magnesium blocks on ship hulls |
A barrier only works while it is unbroken: once paint is scratched, iron underneath rusts. Galvanising protects even when scratched because zinc is more reactive than iron and corrodes in its place. Tin-plating does the opposite: tin is less reactive, so a scratched tin can rusts faster.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
In the rusting experiment, why is the water in one tube boiled and covered with oil?
(Extended) Why does galvanised iron not rust even when the zinc coating is scratched?
(Extended) Blocks of which metal could be attached to a steel ship's hull for sacrificial protection?
Which condition would make an iron nail rust fastest?
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