Corrosion

The gradual destruction of pure metals by the action of air, moisture or a chemical (such as an acid) on their surface is called corrosion. 

• When an iron object is kept in damp air for a considerable time , then a red-brown substance called rust is formed on its surface.
• The Corrosion of Iron is called ‘Rusting’.
• Rust is soft and porous, and it gradually falls from the surface of iron objects, and then the iron below starts corroding.
• Thus, corrosion of iron is a continuous process which ultimately eats up the whole object.

Corrosion can be prevented by using the following methods :

1. Oil or Grease : We notice that machine parts or tools made of iron and steel are smeared with grease or oil. This is done to prevent them from rusting. 
2. Painting : When a coat of Paint is applied to the surface of an iron object, then air and moisture cannot come in contact with the object and no rusting takes place.
   But paint can be heated and washed , the paint can come off . A better option would be to galvanise the metal.
   
3. Galvanisation :
   • In galvanisation, a sacrificial coating of zinc is applied on iron and steel . 
   • Since Zn is more reactive than Iron, the Zn coating reacts with the surroundings, leaving the Iron untouched.
   • It prevents steel and iron from rusting .
     
4. Alloying: An alloy is a homogeneous mixture of 2 or more metals. In alloying ,a metal is mixed with some metal or nonmetal. This changes the properties of the metal .
   
   
   
   • Example: When iron is mixed with Nickel and chromium ,we get stainless steel .This prevents corrosion as iron rusts but stainless steel does not.
     
5. Tin plating and Chromium plating : Tin and Chromium are resistant to corrosion. When a thin layer of these metals is applied on an iron article by electroplating, then these items are prevented from rusting.