(a) Corrosion : Corrosion is the process in which metals are eaten up gradually by the action of air, moisture or a chemical on their surface.
(b) Rusting of iron metal is the most common form of corrosion.
(c) When silver objects are kept in air, they get tarnished and gradually turn black due to the formation of thin silver sulphide layer on their surface by the action of H2S of air.
(d) Corrosion weakens the iron and steel objects and structures such as railings, car bodies, bridges and ships, etc. and cut short their life, therefore, it is a serious problem.
(e) Methods to prevent corrosion are :
(i) Galvanisation is a method used to protect steel and iron from rusting.
(ii) Painting, greasing and oiling of metals is done to prevent corrosion.
(iii) Cleaning metals regularly and carefully.