Following four factors are responsible for corrosion of Iron:

i) Moisture: Moisture reacts with iron to form rust(Mixture of Iron oxide and water)

ii) Oxygen: Oxygen reacts with iron to form Iron oxide which on combining with moisture becomes rust

iii) Poor maintenance: Poor maintenance leads to continuous exposure of iron to moisture and air which causes rust.

iv) Poorly built iron substances: If other substances like carbon, are not mixed while making iron substances in proper proportions, It gets rusted.