When some metals articles come in the contact of oxygen or moistures, this reacts with oxygen or water vapour and forms metal oxides which get rusted, resulting in fatal surface loss of metals. This surface loss of metal is known as corrsion.
Prevention of Corrosion:
Corrosion can be prevented by
(i) By painting, Oiling and by greasing.
(ii) By Galvanisation, chrome plating or by anodising.
(iii) By making alloys.
Galvanisation: A method of protecting steel and iron from rusting by coating them with a thin layer of zinc. This process is known as Galvanisation.
Alloying: Pure metals are not used to make articles. So there are mixed some other substances to make them hard and strong and causing changing in metal's properties. This process is called alloying.
- Alloying is a very good method of improving the properties of a metal.
- An alloy is a homogeneous mixture of two or more metals, or a metal and a nonmetal.
- It is prepared by first melting the primary metal, and then, dissolving the other elements in it in definite proportions. It is then cooled to room temperature.
Some examples of alloys:
(I) Iron is the most widely used metal. But it is never used in its pure state. This is because pure iron is very soft and stretches easily when hot. But, if it is mixed with a small amount of carbon (about 0.05 %), it becomes hard and strong steel.
(ii) Iron is mixed with nickel and chromium to make stainless steel.
(iii) Pure gold is of 24 carat gold, which is very shoft. so there can not be made jewellery by it. It is alloyed with either silver or copper to make it hard. 22 parts of pure gold is alloyed with 2 parts of either copper or silver such a gold is called 22 carat of gold.
||Copper + zinc
||Cu + Zn
||Copper + tin
||Cu + Sn
||Lead + tin
||Pb + Sn