Question

what is the difference between corrosion and rancidity

Answer 1

There is no similarity between corrosion and rancidity as such because both are completely different processes, however, Corrosion is the oxidation/reduction reaction taking place on the surface of the metal whereas Rancidity is the oxidation of entire mass of substance.

Answer 2

Corrosion means damaging of metal internally like rusting of iron.
Rancidity means spoil of food like oxidised gram etc.