The surface of some metals such as iron is corroded when they are exposed to
moist air for a long period of time. This is called corrosion.
(i) Silver becomes black when exposed to air as it reacts with air to form
a coating of silver sulphide.
(ii) Copper reacts with moist carbon dioxide in the air and gains a green coat
of copper carbonate.
(iii) Iron when exposed to moist air acquires a coating of a brown flaky
substance called rust.
Prevention of Corrosion
The rusting of iron can be prevented by painting, oiling, greasing, galvanizing,
chrome plating, anodizing or making alloys.
Galvanization : It is a method of protecting steel and iron from rusting by
coating them with a thin layer of zinc.
Alloy : An alloy is a homogenous mixture of two or more metals or a metal
and a non-metal.
Iron : Mixed with small amount of carbon becomes hard and strong.
Steel : Iron + Nickel and chromium
Brass : Copper + Zinc
Bronze : Copper + Tin (Sn)
Solder : Lead + tin
Amalgam : If one of the metal is mercury (Hg).