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Redox reactions

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Rust prevention

Stainless steel is an iron alloy [alloy: An alloy is a compound of two or more elements, at least one of which is a metal. that does not rust. It is used to make kitchen sinks, for example.

There are several ways to prevent iron and steel rusting. Some of these work because they stop oxygen or water reaching the surface of the metal:

  • oiling - for example bicycle chains

  • greasing - for example nut and bolts

  • painting - for example car body panels

Iron and steel objects may also be covered with a layer of metal. Food cans, for example, are plated with a thin layer of tin.

Empty cans of food

‘Tin’ cans are made of steel coated with a thin layer of tin metal

Galvanising

Galvanising is a method of rust prevention. The iron or steel object is coated in a thin layer of zinc. This stops oxygen and water reaching the metal underneath - but the zinc also acts as a sacrificial metal [sacrificial metal: A metal, more reactive than iron, that loses electrons in preference to iron.. Zinc is more reactive than iron, so it oxidises in preference to the iron object.

Sacrificial protection – Higher tier

The reactivity series lists metals in order of how reactive they are.

Lists from most reactive to least reactive: potassium, sodium, calcium, magnesium, aluminium, zinc, iron, tin, lead, copper, silver, gold, platinum

The reactivity series for some metals

Magnesium and zinc are often used as sacrificial metals. They are more reactive than iron and lose their electrons [electron: A very small negatively-charged particle found in an atom in the space surrounding the nucleus. in preference to iron.

Although tin is used to coat steel cans, it does not act as a sacrificial metal. Instead, it acts only as a barrier to stop air and water reaching the surface of the iron or steel. The tin layer may actually make rusting happen faster if it gets scratched, because then iron loses electrons in preference to tin.

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