Corrosion occurs due to reactivity of metal, presence of air and moisture and strains present in metals.
Factors which are promoting corrosion of metal are:
Reactivity of the metal: Metals which are more active in the nature are readily corroded,
Air and moisture presence: Air and moisture present in the atmosphere promotes corrosion. Gases like $S{O}_2$ and $C{O}_2$ catalyze or increase the rate of corrosion.
Strains in the metal: Corrosion takes place easily at scratches, bends and cuts in the metal.