why doesn't the iron pillar near qutab minar rust ?
Dear student!
Actually, the iron pillar is made up of 98% wrought iron of pure quality, it is 23 feet 8 inches (7.21 m) high and has a diameter of 16 inches (0.41 m). Also, it was confirmed that the temperatures required to form such kind of pillars cannot be achieved by combustion of coal. The pillar is a testament to the high level of skill achieved by ancient Indian iron smiths in the extraction and processing of iron.
The corrosion resistant nature of pillar is due to a passive protective film at the iron-rust interface because of the high phosphorus content, which promotes the formation of a solid protective passivation layer of iron oxides and phosphates together with favorable local weather conditions, rather than the non-protective, cracked rust layer which are normally found in iron products.
I hope it clears!