Page 211 - Chemistry ICSE Class X
P. 211
Metallurgy 197
We have observed in Activity 1 that when iron is exposed to moist air
(air containing large quantity of water vapour), it gets covered with a layer
of brown powdery material.
The reaction which describes the rusting of iron is
4Fe(s) + 3O (g) + xH O o Fe O .xH O(s)
3
2
2
2
2
iron oxygen water hydrated ferric oxide
from air (rust)
Rust is soft, porous and powdery substance. It falls off from the surface
of iron of its own. This exposes the lower layers of iron to the atmosphere.
As a result, the rusting continues and over a period of time iron loses its
strength.
What are the necessary conditions for rusting of iron
Iron does not rust in dry air. For rusting to take place, the following &ŽƌŵĂƟŽŶ ŽĨ ďƌŽǁŶ ƉŽǁĚĞƌLJ
EQPFKVKQPU OWUV DG HWNſNNGF ŵĂƚĞƌŝĂů ŽŶ ƚŚĞ ƐƵƌĨĂĐĞ ŽĨ ŝƌŽŶ ŝŶ
ƚŚĞ ƉƌĞƐĞŶĐĞ ŽĨ ŵŽŝƐƚ Ăŝƌ ŝƐ ĐĂůůĞĚ
Presence of oxygen/air ƌƵƐƟŶŐ ŽĨ ŝƌŽŶ͘ ZƵƐƚ ŵĂŝŶůLJ ĐŽŶƐŝƐƚƐ
Presence of water/moisture ŽĨ ŚLJĚƌĂƚĞĚ ĨĞƌƌŝĐ ŽdžŝĚĞ
͘
;&Ğ O xH KͿ͘
Ϯ
Ϯ ϯ
Since the moist/wet air contains both oxygen as well as water, rusting
iron takes place when iron is exposed to moist air.
What accelerates the rate of rusting
The following factors accelerate the rate of rusting:
The presence of gases, such as carbon dioxide (CO ), oxides of sulphur
2
(SO , SO ) and oxides of nitrogen (NO, NO ) in the air increase the rate
2
2
3
of rusting.
The presence of electrolytes, such as sodium chloride, in water
increases the rate of rusting. It is because of this reason that iron rusts
faster in sea water than in distilled water.
Coating prevents corrosion
The presence of impurities of less electropositive metals in iron
increases the rate of rusting.
How can corrosion of iron (or rusting) be prevented
Rusting of iron takes place only in the presence of oxygen (air) and water
(moisture in the air). So, if iron is prevented from coming in contact with
air and water, its rusting can be prevented. Rusting of iron can be prevented
by the following methods:
By surface coating. Rusting of iron can be prevented by coating the
iron surface with oil, grease, paint and varnish.
By galvanisation of iron. Iron can be protected from corrosion by
coating it with a thin layer of zinc. The process of depositing a thin
layer of zinc on iron is called galvanisation. The iron coated with a
thin layer of zinc is called galvanised iron or simply as G.I.
Galvanisation is done by either of the following methods:
By spraying molten zinc on the iron surface
By dipping iron sheet/object into molten zinc
By electroplating iron with any corrosion-resistant metal. Certain
metals like nickel and chromium, do not corrode. Nickel and chromium Many bathroom fittings are
can be electroplated on iron. So, rusting of iron can be prevented chrome-plated
