Corrosion and methods of preventing it
Corrosion is the slow eating away of a metal. Iron rusts only when air and water are both present. We prevent it by coating, alloying or sacrificial metals.
What corrosion is
Most metals slowly react with air, water and gases around them. This gradual damage is corrosion. When iron corrodes, it forms a reddish-brown, flaky layer called rust (Fe₂O₃·xH₂O). Rust falls off and exposes fresh iron, so the damage continues. Copper gets a green coating and silver turns black in the same way. Corrosion weakens bridges, ships, vehicles and pipelines.
Activity: what does iron need to rust?
- 1Take three test tubes, A, B and C, and put a clean iron nail in each.
- 2A: pour in tap water so the nail is partly covered. Leave the tube open to air.
- 3B: fill with boiled water (boiling removes dissolved air), add a layer of oil on top to keep air out, and cork it.
- 4C: add anhydrous calcium chloride (it absorbs moisture) and cork it, so the air inside is dry.
- 5Leave the tubes for a few days and observe.Only the nail in A rusts.
- 6★Conclusion: rusting needs both air (oxygen) and water.
★ key step
Ways to prevent corrosion
| Method | How it works | Example |
|---|---|---|
| Alloying | Mixing the metal with other metals that resist corrosion | Stainless steel (iron with chromium and nickel) |
| Galvanisation | Coating iron with a thin layer of zinc. Zinc is more active, so it corrodes first, even if the coat is scratched | Roofing sheets, buckets, water pipes |
| Electroplating | Coating one metal with another by electrolysis | Chromium or nickel plating on taps and cycle handles |
| Anodising | Making the natural oxide layer on aluminium thicker by electrolysis | Anodised aluminium cookware and window frames |
| Cathodic (sacrificial) protection | Joining the metal to a more active metal such as magnesium or zinc, which corrodes in its place | Ship hulls, underground pipelines |
| Painting, oiling, greasing | A coat that keeps air and water away from the surface | Iron gates, machine parts, tools |
Things that speed up corrosion
- Moisture and air together; dry air alone does not rust iron.
- Salt: sea air and salty water make iron rust faster, as on the coast at Chennai or Thoothukudi.
- Acidic gases such as sulphur dioxide and carbon dioxide in polluted air.
- Scratches in paint or coatings, which let air and water reach the metal.
Iron gates and bicycles in coastal towns like Chennai rust faster because sea air carries salt and moisture. That is why roofing sheets are galvanised and many new cooking vessels are hard-anodised aluminium.
2 marks: “What is galvanisation? Why does it protect iron even when scratched?” Coating iron with a thin layer of zinc. Zinc is more active than iron, so it corrodes first and protects the iron.
4 marks: “Describe an activity to show that both air and water are needed for rusting.” Three test tubes: nail in tap water open to air (rusts); in boiled water under oil (no rust); with anhydrous CaCl₂ in dry air (no rust). Conclusion: both are needed.
7 marks: “What is corrosion? Explain any four methods of preventing corrosion.” Definition and rust equation; then alloying, galvanisation, electroplating, anodising and cathodic protection, with one example each.
No air or no water: no rust. To stop it: Alloy, Galvanise, Electroplate, Anodise, Sacrifice, Paint.