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9.5Metals

Corrosion of metals — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 8 parts Lesson 5 of 6 1 quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Lesson summary

9.5 Corrosion of metals — Notes — IGCSE Chemistry notes: Describe the rusting of iron, the conditions needed, and the methods of preventing it including sacrificial protection.Corrosion: the gradual destruction of a metal by reaction with its surroundings.Rusting: the corrosion o…

  • Galvanising
  • Barrier method
  • Rusting: the corrosion of iron and steel by water and oxygen, and both must be present
  • Iron + oxygen + water → hydrated iron(III) oxide (rust), which is reddish-brown, and the iron is oxidised
  • Rust is a flaky, porous orange-brown solid that falls off and exposes more iron
  • Salt water, salt, acid and acid rain speed up rusting

Part 1 of 6Rusting of iron

1
Corrosion: the gradual destruction of a metal by reaction with its surroundings.
2
Rusting: the corrosion of iron and steel by water and oxygen, and both must be present.
3
Iron + oxygen + water → hydrated iron(III) oxide (rust), which is reddish-brown, and the iron is oxidised.
4
Rust is a flaky, porous orange-brown solid that falls off and exposes more iron.
5
Salt water, salt, acid and acid rain speed up rusting.
6
Only iron and steel rust, and any other metal that breaks down in air is said to have corroded.

Part 2 of 6Investigating rusting

Investigating rusting
Set up three test tubes, each with an iron nail.
  • Tube Anail in tap water open to air (water and oxygen).
  • Tube Bnail in boiled water covered with a layer of oil (boiling removes dissolved air and the oil stops air re-dissolving, so no oxygen).
  • Tube Cnail in dry air with anhydrous calcium chloride and a stopper (no water).
Leave for several days and observe which nail develops orange-brown rust.
Only tube A rusts, so both oxygen and water are needed.

Part 3 of 6Preventing rust

MethodHow it worksExample use
PaintingBarrier keeps out O2 and H2OCars, bridges
Greasing or oilingBarrier; good for moving partsBicycle chains, tools
Plastic coatingBarrierGarden furniture, wire fences
Plating with tin or chromiumBarrierFood cans
Galvanising (zinc)Barrier plus sacrificial protectionDustbins, roofing, nails
Sacrificial blocks (Mg, Zn)More reactive metal is oxidised instead of ironShip hulls, pipelines
Preventing rust
Barrier methods stop oxygen and water reaching the iron, but rusting starts if the barrier is scratched.
If a tin coating is scratched, it no longer protects the iron.
Alloying: stainless steel contains chromium and nickel and resists rusting.

Part 4 of 6Galvanising

Galvanising
Galvanising is coating iron with zinc by electroplating or dipping in molten zinc.
Zinc reacts with oxygen and carbon dioxide in air to form ZnCO3, which acts as a barrier.
If the coating is scratched, zinc is more reactive and corrodes in preference to the iron, so it still protects the iron sacrificially.

Part 5 of 6Sacrificial protection (Supplement)

1
A more reactive metal (zinc or magnesium) is in electrical contact with the iron.
2
Zinc loses electrons more readily than iron, so zinc is oxidised, Zn → Zn2+ + 2e−, and corrodes.
3
Iron is not oxidised, so it does not rust.
4
Zinc or magnesium blocks on ships' hulls and pipelines corrode instead of the iron.
5
The blocks need not cover the whole hull, only be electrically connected, and are placed on edges and joints.
6
The blocks must be replaced before they corrode away completely.
7
Only a metal above iron in the series is suitable,
  • because with copper or tin the iron loses electrons more readily once the coating is scratched and rusts even faster.

Part 6 of 6More from the specification

1Acid-accelerated oxidation and electrochemical series
  • Acid-accelerated oxidation and electrochemical series contact with corrosive mineral acids in aqueous solution rapidly accelerates metal oxidation according to reactivity order.
2Dual Chemical Requirements for Rusting
  • The corrosion of iron to form hydrated iron(III) oxide (rust) requires the simultaneous presence of both oxygen and water: 4Fe + 3O2 + 2xH2O → 2Fe2O3·xH2O.
3Physical Barrier Protection Against Corrosion
  • Applying protective barrier coatings prevents water and atmospheric oxygen from contacting vulnerable metal surfaces.
    • (such as paint, oil, grease, electroplated chromium, or plastic coatings)
4Metal Reactivity with Dilute Hydrochloric Acid
  • Vigorous effervescence of hydrogen occurs when reactive metals like magnesium or zinc react with dilute hydrochloric acid,
  • whereas unreactive coinage metals exhibit no reaction.
    • (copper, silver, gold)
5Dilute
  • Mixed with water to be less concentrated.
6Sulfate
  • Salt of sulfuric acid.
7Hydroxide
  • A compound containing the OH− ion, formed by Group I metals with water.
8Vigorous
  • Fast reaction with rapid bubbling or heating

Key terms

TermMeaning
Corrosionthe gradual destruction of a metal by reaction with its surroundings.
Rustingthe corrosion of iron and steel by oxygen and water.
Galvanisingcoating iron with zinc to protect it.
Sacrificial protectionprotecting iron by attaching a more reactive metal that corrodes instead.
Barrier methoda coating that keeps oxygen and water away from a metal.

Avoid these mistakes

1
Saying rusting needs only air or only water: it needs both oxygen and water.
2
Saying rust is iron oxideit is hydrated iron(III) oxide.
3
Saying zinc protects iron because it is stronger: zinc is more reactive, so it loses electrons instead of iron.
4
Believing tin protects iron by sacrificeit only forms a barrier.

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