Corrosion and rusting, their prevention, and alloys
Corrosion is the slow destruction of a metal by air and moisture. Iron rusts only when both oxygen and water are present. Painting, oiling, galvanisation and alloying prevent it.
What corrosion is
Corrosion is the slow eating away of a metal surface by air and moisture. The metal reacts with oxygen, so corrosion is a kind of oxidation. Old copper coins get a green layer of basic copper carbonate. For iron, corrosion is called rusting. Rust is hydrated ferric oxide: 4Fe(s) + 3O₂(g) + 2xH₂O(l) → 2Fe₂O₃·xH₂O(s), where x is a number that can vary. Your NIOS lesson prints xH₂O on the left; it must be 2xH₂O to balance. Rusting is fastest in the rainy season, when the air is moist.
Activity: conditions needed for rusting (lab)
- 1Take three clean, dry test tubes labelled A, B and C, and three clean iron nails.
- 2A: put in a nail, add distilled water to cover about half of it, and cork the tube.The nail meets both air and water
- 3B: put in a nail, cover it completely with boiled distilled water, and cork the tube.Water only. Boiling drives out dissolved air; a thin layer of oil on top keeps air out
- 4C: put anhydrous calcium chloride (which absorbs moisture) and a nail in a dry tube, and cork it.Dry air only, no moisture
- 5★Look after a few days.Only the nail in A rusts, with a flaky reddish-brown coat. Rusting needs both oxygen and water
★ key step
Ways to prevent rusting
| Method | How it works | Example |
|---|---|---|
| Painting | A coat of paint keeps air and water away from the iron | Iron gates, window grills |
| Oiling and greasing | A layer of oil or grease keeps air and water out and also reduces friction | Machine parts, bicycle chains |
| Galvanisation | A layer of zinc covers the iron. Even where it is scratched, the more reactive zinc corrodes first and protects the iron | Roof sheets, buckets, drums, trunks, manhole covers |
| Alloying | Mixing iron with other elements changes its properties | Stainless steel kitchenware |
Common alloys
| Alloy | Made of | Uses |
|---|---|---|
| Stainless steel | Iron with nickel and chromium (and a little carbon) | Utensils and kitchenware; it does not rust |
| Brass | Copper and zinc | Vessels, taps, musical instruments |
| Bronze | Copper and tin | Statues, medals, bells |
| Duralumin | Mainly aluminium, with copper and a little magnesium and manganese | Aircraft parts: light and strong |
| 22-carat gold | 22 parts gold with 2 parts copper or silver | Jewellery; harder than pure gold |
Why jewellers use 22-carat gold
Pure gold is 24 carat. It is so soft that it cannot hold its shape in fine wires and thin sheets. Jewellers mix 22 parts of gold with 2 parts of copper or silver to make 22-carat gold, which is harder. Look for 22K stamped on the back of gold jewellery.
How to draw the rusting experiment
- Draw three test tubes in a row, labelled A, B and C, each with one iron nail and a cork.
- A: water up to half the nail. B: boiled water covering the whole nail, with a thin oil layer on top. C: lumps of anhydrous calcium chloride at the bottom and a dry nail above.
- Shade the nail in A reddish-brown to show rust.
- Label: test tube, iron nail, distilled water, oil layer, cork, anhydrous calcium chloride.
Iron gates, grills and bicycle wheels are painted or greased before the monsoon, when moist air rusts iron fastest. Galvanised iron sheets are used for roofs and water drums because their zinc coat keeps rust away.
2 marks: “A roof of galvanised iron sheets stays free of rust for years, even where it is scratched. What is galvanisation, and why does the scratch not rust?” Galvanisation is coating iron with zinc. Zinc is more reactive than iron, so at a scratch the zinc reacts (corrodes) first and protects the iron.
3 marks: “Describe an experiment to show that both air and water are needed for rusting.” Tube A: nail half in water, corked (air and water). Tube B: nail fully in boiled water with oil on top (water only). Tube C: nail with anhydrous calcium chloride, corked (dry air only). After a few days only nail A rusts, so both air and water are needed.
1 marks: “Write the chemical formula of rust.” Fe₂O₃·xH₂O (hydrated ferric oxide).
Rust needs both O₂ and H₂O. Paint, oil, zinc or alloy keeps it away.