Rusting, Corrosion and Alloys
Rusting costs industry enormous sums, and the syllabus wants two things: the conditions required, and the methods of prevention with their reasons.
1. What rusting is
Rusting is the CORROSION OF IRON (and steel), forming hydrated iron(III) oxide — rust.
iron + oxygen + water → hydrated iron(III) oxide
Rusting is an OXIDATION reaction — iron loses electrons: Fe → Fe³⁺ + 3e⁻.
Only IRON and STEEL rust. Other metals corrode, but the word “rust” applies specifically to iron. Aluminium, for instance, forms a protective oxide layer rather than flaking away.
2. The conditions required
BOTH oxygen (air) AND water must be present.
Rusting is faster in the presence of salt (which is why cars rust quickly near the sea and on salted roads) and in acidic conditions.
The classic experiment — three test tubes with iron nails:
| Tube | Contents | Result |
|---|---|---|
| 1 | nail + water and air | RUSTS |
| 2 | nail + boiled water (air removed) + oil layer | no rust |
| 3 | nail + dry air (with a drying agent such as calcium chloride) | no rust |
The water is boiled to remove dissolved OXYGEN, and a layer of oil stops air redissolving. The drying agent removes water vapour.
This experiment proves BOTH conditions are necessary — that is what the control tubes are for. Explaining why each tube fails to rust is the standard exam question.
3. Preventing rust
Two strategies: keep out the air and water, or use another metal to corrode instead.
Barrier methods
Painting, greasing/oiling, plastic coating, or electroplating with a less reactive metal (e.g. chromium, tin).
All of these work by forming a physical barrier that keeps out oxygen and water.
Barrier methods fail if the coating is SCRATCHED — the exposed iron then rusts, and with tin plating it rusts faster, because tin is less reactive than iron.
Sacrificial protection
A MORE REACTIVE metal (usually ZINC or magnesium) is attached to the iron. The more reactive metal LOSES ELECTRONS and CORRODES INSTEAD of the iron — it is “sacrificed”.
Used on ships’ hulls (blocks of zinc bolted to the steel) and on underground pipelines.
Say the reason: the more reactive metal is oxidised in preference to the iron. Simply naming zinc earns little.
Galvanising
GALVANISING is coating iron with ZINC — and it works in BOTH ways: 1. As a barrier, keeping out air and water 2. As sacrificial protection if the coating is scratched, because zinc is more reactive than iron
This dual action is why galvanising is better than tin plating. If tin is scratched the iron rusts faster; if zinc is scratched the zinc still protects the iron. This comparison is examined regularly.
4. Alloys
An ALLOY is a mixture of a metal with one or more other elements.
Why alloys are harder
In a pure metal, the layers of atoms are regular and can slide over each other. In an alloy, the added atoms are a DIFFERENT SIZE, which DISRUPTS the regular layers so they cannot slide easily — making the alloy harder and stronger.
The disrupted-layers explanation is the mark-earning one. “It’s a mixture so it’s stronger” earns nothing.
The common alloys
| Alloy | Composition | Properties and uses |
|---|---|---|
| Steel | iron + carbon | strong — construction, tools |
| Stainless steel | iron + chromium + nickel | corrosion-resistant — cutlery, sinks |
| Brass | copper + zinc | hard, attractive — instruments, fittings |
| Bronze | copper + tin | hard — statues, bearings |
Brass is copper and zinc. Confusing which metal is in brass was recorded.
Stainless steel resists corrosion, which is why it is used where rusting would be a problem — the chromium forms a protective oxide layer.
5. Mistakes that cost marks
Giving only one condition for rusting.
Not explaining why the water is boiled or the drying agent used.
Saying all metals rust — only iron and steel do.
Naming a protection method without the reason.
Saying zinc protects because it’s a barrier only — it is also sacrificial.
Saying tin plating protects when scratched — it doesn’t.
Explaining alloy hardness without “disrupted layers”.
Confusing the compositions of brass and bronze.
Frequently asked questions
What is rusting? The corrosion of iron to form hydrated iron(III) oxide.
What conditions are needed? Both oxygen (air) and water.
What speeds it up? Salt and acidic conditions.
Why is the water boiled in the experiment? To remove dissolved oxygen.
Do all metals rust? No — only iron and steel. Others corrode differently.
How do barrier methods work? They keep out oxygen and water.
What is sacrificial protection? Attaching a more reactive metal that corrodes instead of the iron.
What is galvanising? Coating iron with zinc — acting as both a barrier and sacrificial protection.
Why is galvanising better than tin plating? If scratched, zinc still protects the iron; tin does not, and the iron rusts faster.
Why are alloys harder than pure metals? Different-sized atoms disrupt the layers, so they cannot slide.
Quick revision checklist
- I know rusting needs both oxygen and water
- I know rust is hydrated iron(III) oxide
- I know salt speeds it up
- I can describe the three-tube experiment and explain each tube
- I know why the water is boiled and a drying agent used
- I can give barrier methods and how they work
- I can explain sacrificial protection with the reason
- I know galvanising works both ways
- I can compare zinc with tin plating
- I can define an alloy
- I can explain hardness via disrupted layers
- I know the compositions of steel, stainless steel, brass and bronze
These notes cover rusting, corrosion and alloys in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons. These were among the less-covered subtopics in that set, so they are combined here and follow the syllabus closely rather than being padded. Always check the current syllabus for your own exam series.
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