The World of Metals and Non-metals
Rusting and corrosion
Investigate what conditions make iron rust.
Leave an iron thing out for a few days and it gets brown patches. Is it the air, the water, or both? Activity 4.5 finds out with three bottles and three clean iron nails, left for 8 to 10 days:
| Bottle | What is inside | Air? | Water? | Brown deposit? |
|---|---|---|---|---|
| A | nail + silica gel (keeps the air dry), capped | dry air | no | no |
| B | nail under boiled and cooled water with a layer of oil on top, capped | no | yes | no |
| C | nail partly in water, open | yes | yes | yes |
Boiling removes the air already dissolved in the water, and the oil stops more air getting in.
So iron needs both air and water, that is, moist air, to get the brown deposit. The deposit is rust, and the process is rusting. Rust eats into iron: gates, pipes and railings become weak and full of holes.
Other metals change colour too: copper gets a green coat and silver a black one. Any slow damage to a metal surface by air, water or other substances is called corrosion. (Rusting is the word for iron.)
Rusting is a serious problem in India; a lot of money is spent every year repairing or replacing rusted iron. It can be prevented by:
- painting (new park benches are painted; old ones whose paint has peeled are the rusty ones)
- oiling and greasing (like oiling a cycle chain)
- galvanisation: coating the iron with a thin layer of zinc
A wonder of ancient India. The Iron Pillar of Delhi was made in the time of Chandragupta II, more than 1600 years ago. It is about 8 metres high and weighs over 6000 kg. After all those years of wind and rain, it has barely any rust. It was made in a way that resists rusting, which shows how skilled Indian metalworkers were.
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