Page 59 - NCERT Science Class 10 English Medium

3.5.1 Prevention of Corrosion

The rusting of iron can be prevented by painting, oiling, greasing,
galvanising, chrome plating, anodising or making alloys.
Galvanisation is a method of protecting steel and iron from rusting
by coating them with a thin layer of zinc. The galvanised article is
protected against rusting even if the zinc coating is broken. Can you
reason this out?
Alloying is a very good method of improving the properties of a metal.
We can get the desired properties by this method. For example, 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. When iron is mixed with nickel and chromium, we get
stainless steel, which is hard and does not rust. Thus, if iron is mixed
with some other substance, its properties change. In fact, the properties
of any metal can be changed if it is mixed with some other substance.
The substance added may be a metal or a non-metal. An alloy is a
homogeneous mixture of two or more metals, or a metal and a non-
metal. 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.
Do You Know? Pure gold, known as 24 carat gold, is very soft. It is, therefore, not suitable for making

jewellery. It is alloyed with either silver or copper to make it hard. Generally, in India,
22 carat gold is used for making ornaments. It means that 22 parts of pure gold is
alloyed with 2 parts of either copper or silver.

If one of the metals is mercury, then the alloy is known as an
amalgam. The electrical conductivity and melting point of an alloy is
less than that of pure metals. For example, brass, an alloy of copper and
zinc (Cu and Zn), and bronze, an alloy of copper and tin (Cu and Sn), are
not good conductors of electricity whereas copper is used for making
electrical circuits. Solder, an alloy of lead and tin (Pb and Sn), has a low
melting point and is used for welding electrical wires together.

More to Know! The iron pillar near the Qutub Minar in Delhi was built more
The wonder of ancient Indian metallurgy

than 1600 years ago by the iron workers of India. They had
developed a process which prevented iron from rusting. For
its quality of rust resistance it has been examined by scientists

weighs 6 tonnes (6000 kg).

Iron pillar at Delhi from all parts of the world. The iron pillar is 8 m high and

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