• When a metal is attacked by substances around it such as moisture, acids, etc., it is said to be corroded and the process is known as corrosion.
  • When exposed to air, after some time silver articles become black, this is because of its reaction with Sulphur present in the air resulting in the formation of a coating of silver sulphide.
  • Copper reacts with moist carbon dioxide present in the air and because of this it slowly loses its shiny brown surface and gains a green coat. This green coat is copper carbonate.
  • If iron is exposed to the moist air for a very long time, then it acquires the brown flaky substance known as rust and the process is known as rusting of iron.

All the above three were examples of corrosion.

  • Corrosion of iron is such a serious problem that every year large amount of money has been spent to replace the damaged iron in this world.
  • Corrosion causes damage to car bodies, bridges, iron railings, ships, and all the objects made from metal, especially those of iron.



  • The rusting of iron can be prevented by oiling, chrome plating, painting, galvanizing, anodizing, greasing, and making alloys.
  • Protecting steel and iron from rusting by coating them out with a thin layer of zinc, this method is known as galvanization.
  • The very best method of improving the properties of metal is alloying. An alloy is a homogenous mixture of two or more metals, or a metal and a non-metal. By this process, we can get the desired properties in the metals.
  • For example, iron is the most widely used metal but it is never used in its pure form because it is very soft and stretches easily when heated.
  • But if we mix it with a small amount of carbon, it becomes hard and strong.
  • In the same way, when it is mixed with nickel and chromium, stainless steel is obtained which is hard and does not rust.
  • Thus, if a metal is mixed with any other substance, then its properties change and hence it can be prevented from the process of corrosion.


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