Corrosion and its prevention (HT)
Corrosion is the unwanted gradual reaction of a metal with substances in its environment, weakening or destroying it. Most familiar: rust — the corrosion of iron and steel.
Rusting
Iron rusts only when both water and oxygen are present:
iron + oxygen + water → hydrated iron(III) oxide (rust)
Equation: 4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O.
Salt accelerates rusting — that's why coastal structures and roads salted in winter rust faster.
Three classic experiments
- Iron nail in boiled water with oil (no O₂): no rust.
- Iron nail in dry air with anhydrous CaCl₂ (no H₂O): no rust.
- Iron nail in air + water: rusts.
This proves both O₂ and H₂O are needed.
Methods to prevent corrosion
1. Barrier methods (block the air/water)
- Painting (cars, fences).
- Greasing (machine parts).
- Plastic coatings (garden chairs, cables).
If the barrier is broken, rust can start underneath.
2. Sacrificial protection
A more reactive metal is attached to (or coats) the iron. The reactive metal corrodes preferentially, "sacrificing" itself.
- Zinc blocks on ships' hulls and underground pipelines.
- Galvanising = coating steel with zinc (zinc layer corrodes first; even if scratched, zinc continues to protect).
Zinc is more reactive than iron, so zinc loses electrons preferentially: Zn → Zn²⁺ + 2e⁻.
3. Electroplating
A thin layer of another metal is electrodeposited on the surface (using electrolysis, C4.10).
- Tin-plating on steel cans (food).
- Chromium plating for decorative car parts.
- Silver plating for cutlery.
The object to be plated is the cathode; the plating metal is the anode in a salt solution of the plating metal.
If the coating is less reactive than iron (e.g. tin), it only protects as long as the coating is intact — once scratched, iron rusts fast.
Aluminium — special case
Aluminium is more reactive than iron, but it doesn't corrode away because it forms a thin, hard, transparent oxide layer Al₂O₃ that protects the metal underneath. This is why aluminium aircraft and window frames last decades.
⚠Common mistakes
- Saying iron rusts in dry air — rust requires both O₂ and H₂O.
- Confusing galvanising with electroplating — both put zinc on iron, but galvanising can be by dipping in molten zinc, while electroplating uses electrolysis.
- Saying aluminium doesn't corrode — it does, but the oxide layer protects.
- Forgetting that scratched tin-plate fails fast — tin is less reactive than iron, so doesn't protect once damaged.
Links
Builds on C4.1 (reactivity series), C4.10 (electrolysis). Connects to C10.8 (alloys), C2.4 (metallic bonding).
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