Coating
Most steels when exposed to air will gradually rust. (This does not apply to all steels: stainless steel for example was invented - in Sheffield - specifically to resist rust.) Steel has therefore always been covered - frequently painted for example - by its users in order to protect it.
Nowadays the steelmaker can improve steel's corrosion resistance by coating it in the factory prior to delivery to the end-user. A wide range of different coatings is available, including:
- Zinc coating, normally called galvanising. The zinc can be applied either electrolytically (which gives a thinner coating) or by dipping the steel in a bath of molten zinc. Much of the sheet used to produce car bodies is zinc coated. This has enabled thinner steels to be used for car bodies, thus saving weight and improving fuel consumption. (Without this coating, the thinner steels would have rusted, shortening the car's life.) Wire is also frequently galvanised to extend the product's life.
- Organic coatings (plastic and paint) can be applied to extend the steel's life, while at the same time giving it an attractive appearance. The walls of many industrial and commercial buildings are made from pre-painted steel sheets. Frequently, a combination of galvanising and organic coatings are used.
- Tinplate is thin steel sheet with a minute coating of tin applied. This is used for producing food and drinks cans, where rust must be prevented at all costs.
- Other metals used are chromium, lead and aluminium. Electro-chromium coated sheet is used for the tops of steel drinks cans. Aluminium coated sheet provides a combination of corrosion and heat resistance ideal for car exhaust pipes.
Image 1 © Corus