Corrosion and alloys

Corrosion eats metals slowly. Silver turns black (silver sulphide), copper turns green (basic copper carbonate), and iron gets a brown, flaky coat of rust.

"What does iron need to rust?" The book's test uses three test tubes of iron nails:

So iron rusts only when both air and water are present. Take away either one and it stops.

That gives the ways to prevent rusting:

• painting, oiling or greasing, to keep air and water away;
• galvanising: coating iron with a thin layer of zinc;
• chrome plating and anodising;
• making alloys.

Galvanised iron stays protected even when the zinc coat is scratched. Zinc is more reactive than iron, so zinc reacts first and the iron is saved. Zinc fights first.

An alloy is an even mixture of two or more metals, or a metal and a non-metal. Pure iron is soft and stretches; a little carbon makes it hard and strong (steel). Add nickel and chromium and you get stainless steel, which does not rust.

An alloy with mercury in it is called an amalgam.

Pure gold, 24 carat, is very soft. Jewellery in India is usually 22 carat: 22 parts gold mixed with 2 parts copper or silver, which makes it harder.

Ancient India knew a lot about iron. The iron pillar near the Qutub Minar in Delhi was made more than 1600 years ago and has hardly rusted. It is about 8 m high and weighs about 6 tonnes.