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9.5Metals

Corrosion of metals — Combined Science 0653 - Chemistry Revision Notes & Topical Questions

10 min read 18 parts Lesson 10 of 11 1 quiz

📚 Combined Science 0653 - Chemistry Hub / Topical Revision
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Lesson summary

9.5 Corrosion of metals — COMBINED SCIENCE (CHEMISTRY) notes: State the conditions needed for rusting and describe methods of preventing it.Rusting — = reaction of iron with water and oxygen, forming hydrated iron(III) oxide, a reddish-brown solid.Equation — iron + water + oxygen…

  • Both required
  • Salt and acid
  • Grease and oil
  • Paint and plastic
  • Choice of metal
  • Galvanising

Part 1 of 8Rusting

Rusting
  • = reaction of iron with water and oxygen, forming hydrated iron(III) oxide, a reddish-brown solid.
Equation
  • Equation iron + water + oxygen → hydrated iron(III) oxide.
Oxidation
  • Oxidation iron is oxidised during rusting.
Steel
  • Steel steel contains iron and rusts in the same way.

Part 2 of 8Conditions for rusting

Both required
  • Both required oxygen and water must both be present.
Salt and acid
  • Salt and acid speed up rusting.

Part 3 of 8Investigating the conditions

Test tubeConditionsResult
1Nail in air and waterRusts
2Nail in boiled water under oil, so no airNo rust
3Nail in dry air with calcium chloride, so no waterNo rust

Part 4 of 8Preventing rust with barriers

Grease and oil
  • Grease and oil keep out water and air, suited to moving parts.
Paint and plastic
  • Paint and plastic make a barrier on large structures.
Plating
  • Plating a coating of a less reactive metal such as tin acts only as a barrier.
Scratches
  • Scratches if the barrier is damaged the iron rusts again.

Part 5 of 8Sacrificial protection (supplement)

Method
  • Method a more reactive metal in contact with iron is oxidised in preference.
  • Zn → Zn2+ + 2e-.
Examples
  • Examples zinc or magnesium blocks on ship hulls and pipelines.
  • They must be electrically connected to the steel.
  • They are replaced before they corrode away.
Choice of metal
  • Choice of metal only metals above iron in the series protect it;
    • those below make rusting worse once scratched.

Part 6 of 8Galvanising (supplement)

Galvanising
  • = coating iron with zinc by electroplating or dipping in molten zinc.
Barrier
  • Barrier zinc reacts with oxygen and carbon dioxide to form zinc carbonate, which protects the iron.
If scratched
  • If scratched zinc still protects the iron by sacrificial protection.

Part 7 of 8Rusting and corrosion

Rusting
  • Rusting applies only to iron and steel.
Corrosion
  • Corrosion other metals break down by corroding, not rusting.

Part 8 of 8More from the specification

1
Electrochemical corrosion sequence order
  • Electrochemical corrosion sequence order oxidation of iron to iron(II) followed by hydrated iron(III) oxide precipitation proceeds in strict sequential order.
2
Metal reactions with dilute hydrochloric acid
  • Metals above hydrogen in the reactivity series react to produce metal chlorides and hydrogen gas.
    • (potassium, sodium, magnesium, zinc)
3
Unreactive metals with acids
  • Metals positioned below hydrogen do not react with dilute acids to evolve hydrogen.
    • (copper, silver, gold)
4
Displacement in sulfate solutions
  • A more reactive metal displaces a less reactive metal from its aqueous sulfate salt (e.g., Zn + CuSO4 → ZnSO4 + Cu).
5
Apparent unreactivity of aluminium
  • Rapid formation of an impermeable oxide layer initially shields aluminium from dilute acids unless disturbed.
6
Hydrogen
  • A gas that gives a squeaky pop with a lighted splint.
7
Dilute
  • Mixed with water to be less concentrated.
8
Sulfate
  • Salt of sulfuric acid.
9
Hydroxide
  • A compound containing the OH− ion, formed by Group I metals with water.
10
Vigorous
  • Fast reaction with rapid bubbling or heating

Key terms

TermMeaning
RustingCorrosion of iron to hydrated iron(III) oxide.
CorrosionBreakdown of a metal by reaction with its surroundings.
GalvanisingCoating iron with zinc.
Sacrificial protectionA more reactive metal corroding instead of iron.
Barrier methodCoating that keeps out air and water.

Avoid these mistakes

1
Saying rust forms without water.
2
Saying all metals rust — only iron and steel do.
3
Saying paint protects when scratched.
4
Using tin for sacrificial protection — tin is less reactive than iron.

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