Page 10 - Curriculum Visions Dynamic Book

The nature of rust
When a material rusts it is because water and oxygen have combined with the metal iron. To understand what happens, chemists think about how the tiniest particles – the atoms – of each element behave when they react.
The unrusty iron is made of small heavy atoms that pack tightly together. The water is a molecule made of one oxygen atom and two hydrogen atoms. The oxygen gas is made up of pairs of oxygen atoms.
When rusting occurs on the surface of
a material, the atoms split up and regroup. The combination of several different sizes
of atoms is a bulkier and less dense material than pure iron, and it easily flakes off.
 Large structures like the Sydney Harbour bridge need to be painted constantly to prevent them from corroding away. Even so, corrosion in exposed places is difficult to prevent.
Rust and its prevention
Iron or mild steel, water and the oxygen in air combine to cause
rust. It is one of the most common chemical reactions around us. Rusting occurs even in damp air – no actual droplets of water are needed
– and even more quickly in air that has impurities in it. Impurities can include sea salt (which is one reason ships and iron objects near the sea rust so easily) and also pollution gases (such as may be found in cities and near factories or power stations). Sulphur dioxide (present in acid rain) is a particularly effective pollutant because it dissolves in water droplets to form sulphuric acid.
We use iron and steel in all manner of objects because it is cheap, strong and plentiful. To counteract rusting, iron is protected with paint or another metal such as zinc (when it is called galvanised iron).
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