IC6.6.5 Preventing rust © Oxford University Press Preventing rust

IC6.6.5 Preventing rust © Oxford University Press Rusting damages iron and steel objects, so various methods of preventing rusting have been developed.

IC6.6.5 Preventing rust © Oxford University Press Many methods of rust prevention rely on stopping air and water reaching the surface of the iron. These include:  coating the metal part with oil or grease  painting the surface of the metal part  plating the surface with zinc (galvanising)  plating the surface with tin. It is important to choose the correct method.

IC6.6.5 Preventing rust © Oxford University Press For example, it is sensible to oil a bicycle chain rather than painting it, but it is more sensible to paint car bodywork.

IC6.6.5 Preventing rust © Oxford University Press Some iron alloys are resistant to rusting. For example, this sink is made of stainless steel – an alloy of iron and chromium.

IC6.6.5 Preventing rust © Oxford University Press  The chromium in stainless steel oxidises to chromium oxide when exposed to the air.  The oxide forms a thin film on the surface of the steel.  The layer stops air and water reaching the metal below.

IC6.6.5 Preventing rust © Oxford University Press  Galvanising involves coating the surface of the iron or steel object with a layer of zinc.  The zinc coating stops air and water reaching the metal below.  Also, zinc is more reactive than iron, so it is more likely to be oxidised. It sacrifices itself to protect the iron below.  Galvanising is used to protect car body panels before painting, and to protect metalwork sited outdoors.

IC6.6.5 Preventing rust © Oxford University Press  Sacrificial protection also works with other metals that are more reactive than iron, such as magnesium.  The more reactive metal loses electrons more readily than iron does, so it is more readily oxidised.

IC6.6.5 Preventing rust © Oxford University Press For example, ships have zinc or magnesium blocks bolted onto their steel hulls under the waterline. These protect the hull but they gradually corrode away and have to be replaced.

IC6.6.5 Preventing rust © Oxford University Press photo of food can, eg OUP Capture web, ref Tin plating is used to protect the inside of steel food cans from rusting. The layer of tin stops air and water reaching the iron.

IC6.6.5 Preventing rust © Oxford University Press Unlike zinc, tin is less reactive than iron. This means that tin loses electrons less readily than iron does, so it is less readily oxidised. Unfortunately, the result is that the iron rusts even faster if the tin layer is broken or scratched.

IC6.6.5 Preventing rust © Oxford University Press Bicycle chains, cars, kitchen sinks, metal gates, ships, and steel food cans are all at risk of rusting. Can you explain the method that is used to prevent each one from being damaged?