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The iron pillar in India was manufactured by the forge welding of pieces of wrought iron. In a report published in the journal Current Science, R. Balasubramaniam of the IIT Kanpur explains how the pillar's resistance to corrosion is due to a passive protective film at the iron-rust interface.
The Iron Pillar of Delhi is one of the most unique metallic objects in the world. It stands amongst the ruins of the Quwwat-Ul-Mosque, which dates back to the 4th Century A.D. The pillar was forged 1,600 years ago (sometime in 300 AD) and moved to Delhi about 1,000 years ago before the mosque was built.