Rusting and its Prevention

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 2.18 — The conditions under which iron rusts
• 2.19 — Preventing rusting by barrier methods, galvanising and sacrificial protection

Specification section 2(d) Reactivity series — also taught in The Gases in the Air, Combustion and Carbonates, The Reactivity Series, Redox.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

RUSTING is the corrosion of iron, and it needs BOTH WATER AND OXYGEN. Neither alone will do it. Iron in boiled water with a layer of oil on top, sealing out the air, does not rust; iron in a tube of dry air over a drying agent does not rust; iron in a tube open to ordinary damp air rusts within days.

That three-tube experiment is the standard demonstration and it is worth seeing what makes it convincing. Each tube removes exactly ONE of the two candidates, and the tube with both present is the only one that rusts. Testing them one at a time is what shows that both are necessary rather than that one of them happens to be enough.

The rust itself is hydrated iron(III) oxide, so the word equation is iron + oxygen + water → hydrated iron(III) oxide. All three appear on the left, which is the equation stating in symbols that both conditions are required.

Salt does not cause rusting and is not required for it, but it speeds it up a great deal, which is why cars near the coast and roads treated with grit in winter suffer badly. Acidic conditions accelerate it too.

Now prevention, statement 2.19, and there are two entirely different strategies. The first is obvious: KEEP THE WATER AND OXYGEN AWAY. The second is not obvious at all: let them reach the iron and arrange for something else to be attacked instead.

Strategy one — barrier methods

1. PAINTING — cheap and available in any colour, and used for bridges, railings and car bodies.
2. OILING or GREASING — used for moving parts such as chains and machinery, where a hard coating would seize them up.
3. COATING WITH PLASTIC — used for garden furniture, dish racks and street railings.
4. COATING WITH A LESS REACTIVE METAL, such as tin on a steel food can.
5. The weakness they all share: a barrier only works while it is INTACT. Scratch it and water and oxygen reach the iron at that point, and rusting begins there.

Think of it like an umbrella against a bodyguard

An umbrella keeps the rain off you and works perfectly until it has a hole in it, at which point you get wet exactly where the hole is. A bodyguard works differently: the danger still reaches your party, but it reaches somebody else first, and they go on protecting you as long as they last. Barrier methods are the umbrella and sacrificial protection is the bodyguard. This is why a scratch is fatal to paint and merely inconvenient to galvanising — the zinc does not care that the water has got through, because its job was never to keep the water out.

SACRIFICIAL PROTECTION is that second strategy. A more reactive metal — usually ZINC or MAGNESIUM — is attached to the iron. Because it is higher in the reactivity series, it reacts in preference to the iron and corrodes away instead, and the iron is untouched for as long as the sacrificial metal lasts. Blocks of zinc are bolted to ships' hulls and to underground pipelines, and replaced when they have corroded away.

GALVANISING is where the two strategies meet, and it is the reason the specification names it separately. Galvanised iron is iron coated with ZINC, and the zinc does BOTH jobs at once. Intact, it is a barrier keeping water and oxygen off the iron. Scratched, the exposed iron is still protected, because the zinc around the scratch is more reactive and corrodes in preference. That double action is what makes galvanising so much better than tin plating.

Why a scratched tin can rusts faster than an unprotected one

A steel food can is coated with tin. A galvanised bucket is coated with zinc. Both are scratched down to the iron. Explain what happens to each, and why the tin can is a special case.

1. GALVANISED BUCKET: zinc is ABOVE iron in the reactivity series, so at the scratch the zinc corrodes in preference and the iron is protected. The scratch does little harm.
2. TIN CAN: tin is BELOW iron in the reactivity series, so it cannot protect the iron sacrificially — it only ever worked as a barrier.
3. Worse, at the scratch the iron is now in contact with a less reactive metal, which makes the IRON corrode in preference.
4. So the scratched tin can rusts FASTER than bare steel would, because the tin is effectively a sacrificial protector working in reverse.
5. This is why tin-plated cans are lined and why any damage to the coating matters so much.

Answer: The galvanised bucket survives the scratch; the tinned can rusts faster than bare steel, because tin is below iron.

That comparison is the most useful thing on the page, because it shows the reactivity series doing real work. Whether a coating protects a scratch or accelerates the damage depends entirely on whether the coating metal is ABOVE or BELOW iron in the series — and that is a prediction you can make for any metal without having seen the experiment.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

The vocabulary of this page

Rusting

The corrosion of iron to form hydrated iron(III) oxide. Both water AND oxygen are required.

Barrier method

Preventing rusting by keeping water and oxygen away — painting, oiling, plastic coating, tin plating.

Sacrificial protection

Attaching a MORE reactive metal, which corrodes in preference and so protects the iron.

Galvanising

Coating iron with zinc, which acts as a barrier AND as sacrificial protection if scratched.

Statement 2.18 — the conditions for rusting

• BOTH water AND oxygen must be present — neither alone will do it
• The proof is three tubes: iron in boiled water under oil does NOT rust; iron in dry air does NOT rust; iron in damp air DOES
• Each tube removes exactly ONE condition, which is what shows both are necessary
• SALT speeds rusting up considerably but is not required for it
• Rust is hydrated iron(III) oxide

Statement 2.19 — the two strategies

Deciding whether a metal coating will protect a scratch

1. Find the coating metal in the reactivity series and compare it with iron.
2. ABOVE iron — it will corrode in preference, so the iron is protected even at a scratch. Zinc is the standard case.
3. BELOW iron — it protects only as an intact barrier, and at a scratch the IRON corrodes in preference.
4. So a scratched tin can rusts FASTER than bare steel would.
5. The rule applies to any coating metal, including ones you have never met.

Model answer [4 marks]

Describe an experiment to show that both water and oxygen are needed for iron to rust. [4]

Set up three test tubes, each containing an iron nail. In the first, place the nail in water that has been boiled to remove dissolved air, and cover the surface with a layer of oil to keep air out, so water is present but oxygen is not. In the second, place the nail in dry air with a drying agent such as anhydrous calcium chloride and seal the tube, so oxygen is present but water is not. In the third, leave the nail in ordinary damp air with both present. After several days only the nail in the third tube has rusted, showing that both water and oxygen are required.

Model answer [4 marks]

Explain why galvanising protects iron even when the zinc coating is scratched. [4]

Zinc is more reactive than iron, so it is higher in the reactivity series. While the coating is intact it acts as a barrier, keeping water and oxygen away from the iron beneath. If the coating is scratched so that the iron is exposed, the zinc around the scratch corrodes in preference to the iron because it is the more reactive metal. The iron is therefore still protected, and this sacrificial protection continues for as long as any zinc remains.

Not this: Any metal coating protects iron, so a scratched tin can is no worse than bare steel.

This: It is WORSE. Tin is BELOW iron in the reactivity series, so at a scratch the iron corrodes in preference to the tin — the coating works in reverse and the can rusts faster than unprotected steel would.

Mark-losing trap. BOTH water and oxygen are needed. Naming only one loses the mark.

Mark-losing trap. Salt SPEEDS UP rusting but is not required for it.

Mark-losing trap. A coating protects a scratch only if it is ABOVE iron in the series. Below it, the damage is worse.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [2 marks] — Which two substances must be present for iron to rust?
2. Grade 7 · Explain [2 marks] — Explain why painting a bridge prevents it from rusting.
3. Grade 8 · Explain [3 marks] — A block of magnesium is bolted to the steel hull of a ship. Explain how this protects the hull.
4. Grade 9 · Describe [6 marks] — Select every statement that belongs in a full-mark description of an experiment to show that both water and oxygen are needed for rusting.
5. 9+ · Analyse [6 marks] — A student argues that since tin plating and galvanising both coat steel with a metal, a scratched tin can and a scratched galvanised bucket will behave in the same way. Select every statement that belongs in a full-mark analysis.

The people behind this science

Two ways into the same idea — the one who invented sacrificial protection, and whose invention failed for an unexpected reason, and the one who identified what is actually attacking the iron. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Humphry Davy — the one who invented sacrificial protection, and whose invention failed for an unexpected reason

Sacrificial protection is Davy's invention. The Royal Navy asked him in 1823 to stop the copper sheathing of its hulls from corroding in sea water, and he reasoned from the reactivity series that attaching a more reactive metal would protect the copper. It worked exactly as predicted — and it was a disaster in practice, because the copper had been keeping barnacles and weed off the hulls precisely BY slowly dissolving, and protected copper fouled so badly that the ships were slower than before. The Navy abandoned it. It is one of the best examples in chemistry of a correct prediction producing an unwanted result.

• “How did you work out that a more reactive metal would protect the copper?”
• “Why did the Navy stop using your protectors?”
• “What was the copper sheathing doing that you had not accounted for?”
• “Does the method work better on iron than it did on copper?”
• “How do you judge an invention that works and still fails?”

Antoine Lavoisier — the one who identified what is actually attacking the iron

Rust is iron combining with oxygen, and that identification is Lavoisier's. Before him, rusting and burning were unrelated — one was slow and cold and produced a powder, the other fast and hot. He showed that both are the same chemistry, a metal combining with the component of air he named oxygen, differing only in rate. That is why rust is heavier than the iron it came from, and it is the reason a page on corrosion belongs in the same topic as a page on combustion.

• “What do rusting and burning have in common?”
• “Why is rust heavier than the iron it formed from?”
• “How did you show it was a component of the air doing it?”
• “Why does rusting take years and burning take seconds?”
• “What did people think rust was before you?”

Carry on through the course

• Previous lesson: The Reactivity Series
• Next lesson: Redox
• Group 1 — the Alkali Metals — Metals soft enough to cut with a knife, which set fire to water — and get worse as you go down.
• Group 7 — the Halogens — A green gas, a brown liquid and a grey solid — and a trend that runs the opposite way to Group 1.
• The Gases in the Air — Four gases, one of which is doing almost all the chemistry — and how to measure it without ever touching it.
• Combustion and Carbonates — Three things burning in oxygen, one carbonate breaking apart, and the 0.04% gas that changes the climate.
• The Reactivity Series — Putting eleven metals in order by watching what each one will and will not do.
• All of Chemistry · Chemistry key terms · required practicals · equations · Incandio Science · The 4CH1 specification