Ever wondered how the famous iron pillar of Delhi has withstood rusting and corrosion, in-spite of standing in open air for over 1,600 years? It is 7 meter high, 16 inches in diameter, and weights over 6 tons. The pillar was fashioned at the time of Chandragupta Vikramaditya (375–413). It is a living testament to the high level of skill achieved by ancient Indian blacksmiths.
The pillar has attracted the attention of both archaeologists and metallurgists. The pillar was manufactured by forge welding. The temperatures required to form such a pillar by forge welding could only have been achieved by the combustion of coal. It is believed that more than 30 similar pillars are still intact in various parts of Bharatvarsha.
Professor R. Balasubramaniam, of IIT Kanpur researched on how the pillar’s resistance to corrosion is due to a passive protective film at the iron-rust interface. According to him, the presence of second phase particles (slag and unreduced 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-stages formation of that protective passive film.