Corrosion is the rusting of an object by chemical reaction of the body with matter in atmosphere.
Rancidity is the oxidisation of food particles that contain fat in it. Detorioration os food is also called Rancidity.


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Corrosion is Rusting only in 90~95% of times.

Corrosion involves the slow oxidation of iron in presence of air and moisture. 
The reaction involved is: 
4Fe + 30₂ -------->    2Fe₂O₃ -------->   Fe₂O₃.xH₂O

Examples of corrosion are:
Nail kept in ordinary tap water rusts,
Iron sheets rust due to absorption of moisture

The decomposition of fats, oils & other bigger molecules is called RANCIDITY.