5. Corrosion

CORROSION :-

The surface of metals is corroded when they are exposed to moist air for a long period of time. This is called corrosion

For example 

• Silver becomes black when exposed to air and form a coating of silver sulphide.
• Copper react with  moist Co_{2 and form a green coat of copper carbonate.}
• Iron acquires a coating of brown floky substance called rust.

 Prevention of corrosion. 

The rusting of iron can be prevented by painting , oiling, greasing, galvanizing, chrome plating anodizing and making alloys.

• Galvanisation:-  It is a method of protecting steel and iron from ressting by coating them a thin layer of zinc.
• Alloying is a very good method of improving the properties of a metal an alloy is a homogenous mixture of two or more metals or a metal and a non- metal
• Iron  →    it is mixed with small amount of carbon
• Steel →   iron + nickel and chromium.
• Brass  → copper  + tin
• Solder   →  lead  +  tin ( used for welding electric wire together )