Corrosion of metals is the chemical destruction of metals, when they come in contact with air and water (or moisture) present in the atmosphere over a period of time. Examples: Rusting of iron, greenish surface of copper and black surface of silver jewels.
If corrosion is neglected, it may lead to heavy loss; therefore, corrosion should be prevented. There are different methods available to prevent corrosion. They are:
1. Applying paint, grease or oil on metal surface- Applying paint, grease or oil over the surface of the metal after thoroughly cleaning the surface is the usual way to avoid corrosion. The thick layer of paint, grease or oil prevents oxidation of metal. But once this layer is scratched or broken, the metal starts to corrode at that place.
2. Covering a metal by another metal- Covering a metal with a layer of another metal resists corrosion to a good extent. Tin, chromium, nickel and zinc are generally used to cover other metals. This can be done by following methods:
* Galvanisation: It is a process where the cleaned metal is dipped into molten zinc to get a coating of zinc layer over the metal. For example, galvanised iron pipes are widely used for water pipes. Galvanised metals do not corrode even if there is scratch on the layer of zinc. This is because zinc gives sacrificial protection to iron by reacting it with the atmosphere.
* Electroplating: It is a process by which a layer of metal is coated (using electricity) on another metal to prevent corrosion. For example, iron is electroplated by tin. It is generally used to prepare tin cans (we cannot use zinc to store food because zinc is toxic), which are used to store food materials. However, the electroplated iron corrodes if there is a scratch on the tin layer. Chrome and nickel plating are widely used for electroplating in automobile industries to prevent rust. Chrome plating not only protects the surface of metals from corrosion but also gives attractive look.
3. Anodising- This process is specially used to protect aluminium from corrosion. Aluminium reacts with air to form a thin layer of aluminium oxide on the metal This layer of oxide prevents further reactions and protects the metal from corrosion. If the formation of this oxide layer is uniform, the metal below the layer is well protected. This is done by the anodising process. In this process, electric current is passed through sulphuric acid, with aluminium as anode. Oxygen, which is liberated at anode, reacts with aluminium and forms a uniform layer of aluminium oxide, protecting the metal from corrosion.
4. Alloying- Alloy is a mixture (or a solid solution) of two or more metals or one or more metals along with a non-metal.
Alloying usually changes the properties of metals. By alloying, corrosion of metals can be prevented. For example, stainless steel (an alloy of iron, chromium and nickel) is highly corrosion resistant.