Air is a mixture of various types of gases and water vapours. The gases and water present in air react gradually with the surfaces of metals and form their oxides, sulphides, carbonates or sulphates. The formation of these compounds eats up metals slowly. This phenomenon is called corrosion of metals. It may be defined as the eating up of a metal by the action of air and moisture on its surface. Usually the process of corrosion takes place in reactive metals. Almost all the reactive metals corrode when they are kept exposed to damp air. For example, when iron is kept exposed in moist air for a long period of time, a brown coloured flaky coating called rust is formed over its surface. Chemically rust is hydrated iron oxide. The chemical formula of rust is Fe2O3. nH2O (where n is the number of water molecules which is variable). It should be noted that the corrosion of iron is called rusting.
In the same way, when copper is kept exposed in moist air for a long time, a green coloured layer deposits over its surface due to reaction of copper with carbon dioxide and water present in moist air. Due to formation of this green coloured layer over the surface of copper, things made up of copper lose their shine after some time. Gold and silver being least reactive do not corrode easily in atmosphere.
In this way we see that corrosion of metals is harmful for us as it destroys a large number of metals. But sometimes the corrosion of metal is useful. For example, when aluminium metal is kept exposed in air, the surface of aluminium is covered by a thin layer of aluminium oxide. This layer of aluminium oxide protects the metal underneath from further corrosion, and hence metal is protected.
Condition Necessary for Rusting of Iron
Following two conditions are necessary for rusting of iron.
1. Presence of air (oxygen)
2. Presence of water
Methods to Prevent Rusting of Iron
Rusting of iron can be prevented by cutting off the contact between the metal and air. Some methods which are used to prevent rusting of iron are
- Rusting of iron can be prevented by applying paints, oils and grease over the surface of iron.
- Rusting of iron can be prevented by galvanization. Galvanization is the process of depositing a thin layer of zinc metal on iron articles.
- Rusting of iron can be prevented by coating its surface with other metals like tin, nickel and chromium.
- Rusting of iron can be prevented by alloying it with chromium and nickel to make stainless steel.
Test your understanding and answer these questions:
- What is corrosion of metals?
- What is rusting of iron?
- What conditions are necessary for rusting of iron?
- How rusting of iron metal can be prevented?