Page 44 - Science Class 08

We o en see that iron, nails, screws, pipes and layer on its surface to form silver sulphide
railings get rusted if exposed to the air for some which is black in colour.
me. The layer of rust is loosely a ached to the So, it is clear that air and water are necessary
surface of the metal. So, this layer can be easily for corrosion to take place. Let us make an.
removed from the main body of the object. experiment to explain this fact.

This wearing away of a metal layer by layer due
to forma on of compounds is called corrosion.
In case of iron, this corrosion is called rust. The Take three test-tubes with one shining iron
damage due to metals by corrosion is a big nail in each of them. Put a li le calcium

natural loss. So, we must try to follow the causes chloride in one of the test-tubes, well-boiled
behind corrosion and also try to prevent it. water in the second test-tube and normal
water in the third test-tube. Close the
How Corrosion Occurs
mouths of all the test-tubes and let them
In general, corrosion result from two processes
remain undisturbed for a few days. We o en
— oxida on and reduc on. see that iron, nails, screws, pipes and railings
v Oxida on results in loss of electrons by the get rusted if exposed to the air. A er few
atoms of a metal. days, we will observe that the nails in
calcium chloride and boiled water shall be as
v Reduc on causes these electrons to be
shining as before. But the nail in the normal
captured by the same metal or by another.
water will get rusted. So, it is evident that if a
v These moving electrons form an electric metal is kept away from both air and water,
current which causes corrosion. So, it will not corrode. Only one of these two
corrosion is an electro-chemical process. It cannot make them be corroded.
is of two kinds– local and uniform.

Uniform corrosion takes place over the en re
surface. So, it is less severe, i.e., mild. But, local
corrosion is severe and more harmful.
Causes of Metal Corrosion

Metals corrode faster in moist air. The oxygen
of the moist air acts upon most of them to
form their oxides or other compounds.
Calcuim Boil Normal
Let us have some examples: Chloride Water Water

v Iron combines with moisture and oxygen of
PREVAILING CORROSION
the air to form iron oxide (rust).
In order to protect a metal from corrosion, the
v Copper combines with moisture and following methods prove effec ve:
carbon dioxide of the air to form copper
Coa ng with a Paint
carbonate.
The commonest way of preven ng corrosion is
v Silver reacts with the moisture and oxygen
of the air to form silver oxide. It also reacts to coat a metal with a quality paint. You must
have no ced that grills of windows, bodies of
with hydrogen sulphide gas to form a thin

44 Science-8