Corrosion, its prevention, and alloys
Iron rusts only when both air and water are present. Painting, oiling, galvanising and making alloys protect metals from corrosion.
Activity: what makes iron rust?
Put clean iron nails in three test tubes, and leave them for a few days.
- 1Tube 1: nails in ordinary tap water, open to the air.The nails rust: air and water are both present
- 2Tube 2: nails in boiled distilled water, with a layer of oil on top, corked.No rust: boiling removes the dissolved air, and the oil stops air dissolving again
- 3Tube 3: nails with anhydrous calcium chloride, corked.No rust: calcium chloride absorbs the moisture, so the air is dry
- 4★Compare the three tubes after a few days.Result: iron rusts only when both air and water are present
★ key step
Ways to prevent rusting
- Painting, oiling and greasing keep air and water away. Bicycle chains are oiled.
- Galvanisation: coat iron with zinc. Even if the zinc coat is broken, the iron does not rust, because the more reactive zinc protects it.
- Chrome plating: a shiny layer of chromium, as on taps and cycle handles.
- Anodising: making the natural oxide layer on aluminium thicker.
- Alloying: mixing iron with other metals, as in stainless steel.
Common alloys
| Alloy | Made of | Property or use |
|---|---|---|
| Steel | Iron + a little carbon (about 0.05%) | Hard and strong |
| Stainless steel | Iron + nickel + chromium | Hard and does not rust; used for kitchen vessels |
| Brass | Copper + zinc | Vessels, taps, musical instruments |
| Bronze | Copper + tin | Statues and medals |
| Solder | Lead + tin | Low melting point; used to weld electrical wires together |
| Amalgam | Mercury + another metal | Any alloy in which one metal is mercury |
| 22-carat gold | Gold + copper or silver | Harder than pure gold, so it is used for jewellery |
Why alloys are useful
Pure iron is soft and stretches easily when hot. Pure 24-carat gold is too soft for jewellery. Mixing in another element makes the metal harder and stronger. Alloys also have lower electrical conductivity and lower melting points than pure metals. So brass and bronze conduct electricity less well than copper, and solder melts easily. The iron pillar near the Qutub Minar in Delhi, more than 1600 years old, has hardly rusted. It shows the skill of ancient Indian iron workers.
How to draw the rusting experiment
- Draw three test tubes in a row, each with two or three iron nails.
- Tube 1: tap water partly covering the nails; leave the mouth open.
- Tube 2: boiled distilled water covering the nails, a thin layer of oil on top, and a cork.
- Tube 3: lumps of anhydrous calcium chloride at the bottom, dry nails above them, and a cork.
- Shade the nails in tube 1 reddish-brown to show rust. Label every part.
Stainless steel tiffin boxes and tumblers do not rust, which is why almost every Indian kitchen has them. Galvanised iron buckets and roof sheets have a zinc coat that stops rust.
2 marks: “What is galvanisation? Why does galvanised iron not rust even when the zinc coat is scratched?” Coating iron with zinc. Zinc is more reactive than iron, so it reacts first and protects the iron.
2 marks: “Pure iron is never used to make tools or vessels. Why? How does alloying change its properties?” Pure iron is very soft and stretches easily when hot. Mixed with a little carbon (about 0.05%), it becomes hard and strong. Mixed with nickel and chromium, it gives stainless steel, which does not rust.
3 marks: “Describe an activity to show that both air and water are needed for rusting.” Three tubes of nails. Tube 1, tap water and air: rusts. Tube 2, boiled distilled water under oil: no rust (no air). Tube 3, anhydrous calcium chloride: no rust (no moisture).
No air or no water, no rust. Alloys: Brass = Copper + Zinc, Bronze = Copper + Tin, Solder = Lead + Tin.