The Iron Victory Pillar, Delhi that is not rusted for more than 1700 years

Iron Victory Pillar in Delhi (4th century).Image en.wikipedia.org

There is an Iron Victory Pillar in the Qutb complex, Delhi forged in the 4th century. It is 23.08 feet tall and its diameter at the base is 17 inches and the at the crest is 12 inches, weighing 6.3 tons. About 3 feet and 8 inches of the base of the column is below the ground. To give additional stability to the iron pillar, the base is set on a grid of a series of iron bars soldered into the upper layer of the stoned pavement. This Delhi Iron Pillar, with no cover over it, has withstood the vagaries of weather for 1700 years continuously with no sign of rusting so far. Scientists are at a loss and clueless about the chemical composition used for forging used then..

This centuries old pillar has attracted the attention of archaeologists and metallurgists and has been called "a testament to the skill of ancient Indian blacksmiths" because of its high resistance to corrosion. The corrosion resistance results from an even layer of crystalline iron hydrogen phosphate forming on the high phosphorus content iron, which serves to protect it from the effects of the local Delhi climate.


IThe Iron pillar Delhi, in the courtyard of Quwwat-ul-Islam Mosque, en.wikipedia.org

R. Balasubramaniam of the IIT, Kanpur explains how the pillar's resistance to corrosion is due to passive protective film at the iron-rust interface. The presence of second-phase particles (slag and un-reduced iron oxides) in the micro structure of the iron, that of high amounts of phosphorus in the metal, and the alternate wetting and drying existing under atmospheric conditions are the three main factors in the three-stage formation of that protective passive film.
Age of the later inscriptions dates to A.D. 1052 -Tomara king Anangpal II. This has suggested by some, without any substantial basis, that the pillar was installed in its current location by Vigraha Rāja, the ruling Tomar king. The pillar is thought to have originally been erected in what is now Udayagiri by one of the Gupta rulers. The suggested age of 402 CE is still a matter of serious discussion
Ref:
https://en.wikipedia.org/wiki/Iron_pillar_of_Delhi
On the Corrosion Resistance of the Delhi Iron Pillar, R. Balasubramaniam, Corrosion Science, Volume 42 (2000) pp. 2103–2129. "Corrosion Science" is a publication specialized in corrosion science and engineering.