CORROSION

 Metals when exposed in the atmosphere, it will react with substances around it such as moisture, air, acid etc. Such reactions led to loss of its shining appearance and become corrode. This process is known as corrosion.

Examples of corrosion 

1. Rusting of iron 

Iron when exposed to atmospheric oxygen and water for long periods resulted in formation of a brown coloured substance on it surface. That brown coloured substance is known as rust. This process is called rusting of iron. Rusting of iron is redox reaction 

Iron react with oxygen and water to produce oxides of iron

4Fe + 3O₂ + 2H₂O—> 2Fe₂O₃.H₂O

Rusting of iron can be prevented by coating iron with paint, grease or oil, galvanisation etc to prevent direct contact with oxygen and water.

2. Tarnishing of copper ( green coloured coating in copper )

When copper exposed to atmosphere for long period, it will react with water, carbon dioxide and oxygen to produce copper carbonate on it surface resulted in the formation of green coloured coating.

Reaction is:

Cu + H₂O +CO₂+O—> CuCO₃ . Cu(OH)₂

3. Tarnishing of silver ( Black coating of silver )

When silver exposed to atmosphere, it will react with sulfur or sulfur compounds and produce silver sulphide (Ag₂S)which is black in colour 

Reaction is:

4Ag + 2H₂S + O₂ —> 2Ag₂S + 2H₂0