XII Chemistry BIEK Topic - Corrosion (Short Note)

Corrosion (Short Note)
Definition
Corrosion is the deterioration of a metal, as a result of its reactions with the environment or any chemical agent.

It is an oxidation process that occurs at the surface of the metal.




Causes of Corrosion
Corrosion may be regarded as the natural tendency of metals to return to their oxidized state. The main causes of corrosion are as follows.

1. The atomosphere

2. Submersion in water

3. Underground Soil Attack

4. Emersion in chemicals

5. Corrosive gases

The most important of these is the atmosphere.




Examples of Corrosion
Some familiar examples of corrosion are as follows.

1. Rusting of Iron

2. Detarnishing of silver

3. Development of green coating on copper, brass and bronze.




Types of Corrosion
There are two types of corrosion.


1. Atmospheric Corrosion

When corrosion in metal is due to the action of atmosphere, it is known as atmospheric corrosion.


2. Corrosion in Liquid

When corrosion in metal is due to the reaction of a liquid on a metal, it is known as corrosion in liquid.




Rate of Corrosion
Corrosion is a chemical process. Different metals corrode at different rates. Gold does not corrode at all. Iron corrodes slowly. Tin, lead, copper and silver corrode very slowly.




Prevention From Corrosion
Corrosion causes great damages to metallic articles such as bridges, ships and vehicles. It has been found out that due to corrosion, one fifth of iron is lost annually. Therefore, any one of the following methods are adopted to prevent corrosion.


1. Protective Metallic Coating

In this method, the metal is coated with a thin layer of less oxidizing metal, which reduces the rate of corrosion. For example, corrosion of iron is prevented by coating it with zinc, tin or chromium.


2. Non-Metallic Material Coating

In this method, the metal is coated with a thin layer of non metallic material such as paints, oils, grease, plastic emulsion, enamels etc. For example, red lead (Pb3O4) and zinc chromate (ZnCrO4) are often used for this purpose.


3. Alloying of Metals

Metals can be made more reistant against corrosion by making their alloys. For example, stainless steal is an alloy of Fe, Cr and Ni.


4. Electro-Plating

Noble and bare metals are used for Electro-Plating on any desired metal.