What is corrosion?
Corrosion is the gradual damage of a metal's surface when it reacts with air, moisture, acids or gases around it. Examples: silver articles turn black (a coat of silver sulphide from H2S in air), copper gets a green coat (basic copper carbonate from moist CO2), and iron gets a reddish-brown flaky coat called rust (hydrated iron(III) oxide, Fe2O3·xH2O). Corrosion costs a country huge sums every year in repairs of bridges, ships and pipelines.
Rusting of iron: the three-tube experiment
Take three test tubes with clean iron nails. A: nail in ordinary tap water (air + water). B: nail in boiled distilled water with a layer of oil on top (boiling removes dissolved air; oil stops new air). C: nail in dry air with anhydrous calcium chloride, which absorbs moisture. After a few days, only nail A rusts. Conclusion: both air and water are needed for rusting. Salt water speeds it up, which is why cars and ships near the sea rust faster.
Prevention of corrosion
- Painting, oiling, greasing: keep air and water away from the surface.
- Galvanising: coating iron or steel with a thin layer of zinc. Zinc is more reactive, so even if the coat is scratched it corrodes first and protects the iron.
- Chrome plating / electroplating: a thin shiny layer of chromium or tin (tin-plated food cans).
- Anodising: making the natural oxide layer on aluminium thicker by electrolysis, so it resists corrosion better (and can be dyed).
- Alloying: mixing iron with nickel and chromium gives stainless steel, which does not rust.
Alloys
An alloy is a homogeneous mixture of two or more metals, or a metal and a non-metal. It is made by melting the main metal and dissolving the others in it, then cooling. If one of the metals is mercury, the alloy is an amalgam.
- Steel: iron + a little carbon (about 0.05%) — hard and strong.
- Stainless steel: iron + nickel + chromium — does not rust.
- Brass: copper + zinc. Bronze: copper + tin.
- Solder: lead + tin — low melting point, used to join electrical wires.
- Gold jewellery: pure 24-carat gold is too soft, so 22-carat gold (22 parts gold, 2 parts copper or silver) is used.
Why alloys behave differently
Pure metals have neat rows of identical atoms that slide over each other, so they are soft. Different-sized atoms in an alloy block this sliding, making it harder. Alloys also usually conduct electricity less well and melt at lower temperatures than the pure metals (e.g. solder, and nichrome used in heater coils).
Key formulas and definitions
- Rusting: iron + oxygen + water → Fe₂O₃·xH₂O (rust)
- Rusting needs both air (O₂) and water
- Galvanising = zinc coating on iron; anodising = thicker oxide on aluminium
- Brass = Cu + Zn; bronze = Cu + Sn; solder = Pb + Sn; stainless steel = Fe + Ni + Cr
- Amalgam = alloy with mercury; 22 carat = 22/24 parts gold
Worked examples
1. Why does nail B in boiled water with oil not rust?
Boiling drives out dissolved air, and the oil layer stops fresh air from dissolving. There is water but no oxygen, so no rust.
2. A galvanised iron bucket gets scratched. Does it rust at the scratch? Why?
Not quickly. Zinc is more reactive than iron, so the exposed zinc around the scratch reacts first and protects the iron.
3. What percentage of 22-carat gold is pure gold?
22/24 × 100 = 91.67% gold. The rest (about 8.33%) is copper or silver to make it harder.
4. Why is solder used for joining electric wires?
Solder (lead + tin) melts at a low temperature, so it can be melted easily with a soldering iron without damaging the wires, and it sets to a firm, conducting joint.
Common mistakes
- Saying only water causes rust: air (oxygen) is needed too.
- Calling every corrosion 'rusting': only iron and its alloys rust; copper and silver tarnish.
- Thinking an alloy is a compound: it is a mixture with variable composition.
- Mixing up brass (Cu + Zn) and bronze (Cu + Sn).