The pillar—almost seven meters (22 feet) high and weighing more than six tons—is said to have been fashioned at the time of Chandragupta II Vikramaditya (375–413), though other authorities give dates as early as 912 BCE. The pillar initially stood in the centre of a Jain temple complex housing twenty-seven temples that were destroyed by Qutb-ud-din Aybak, and their material was used in building the Qutub Minar and Quwwat-ul-Islam mosque. The pillar and ruins of the temple stand all around the Qutb complex today. The pillar is 98% pure wrought iron, and is a testament to the high level of skill achieved by ancient Indian ironsmiths. It has attracted the attention of both archaeologists and metallurgists, as it has withstood corrosion for over 1600 years in the open air.
The name of the city of Delhi is thought to be based on a legend associated with the pillar.
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 presence of second phase particles (slag and unreduced iron oxides) in the microstructure 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.
Lepidocrocite and goethite are the first amorphous iron oxyhydroxides that appear upon oxidation of iron. High corrosion rates are initially observed. Then an essential chemical reaction intervenes: slag and unreduced iron oxides (second phase particles) in the iron microstructure alter the polarization characteristics and enrich the metal–scale interface with P, thus indirectly promoting passivation of the iron (cessation of rusting activity). The second phase particles act as a cathode, and the metal itself serves as anode, for a mini-galvanic corrosion reaction during environment exposure. Part of the initial iron oxyhydroxides is also transformed into magnetite, which somewhat slows down the process of corrosion. But the ongoing reduction of lepidocrocite, and the diffusion of oxygen and complementary corrosion through the cracks and pores in the rust, still contribute to the corrosion mechanism from atmospheric conditions.
The next main agent to intervene in protection from oxidation is phosphorus, enhanced at the metal–scale interface by the same chemical interaction previously described between the slags and the metal. The ancient Indian smiths did not add lime to their furnaces. The use of limestone as in modern blast furnaces yields pig iron that is later converted into steel; in the process most phosphorus is carried away by the slag. The absence of lime in the slag, and the deliberate use of specific quantities of wood with high phosphorus content (for example Cassia auriculata) during the smelting, induces a higher P content (> 0.1%, average 0.25%) than in modern iron produced in blast furnaces (usually less than 0.05 per cent). There is also more phosphorus as solid solution throughout the metal than in the slags (one analysis gives 0.10% in the slags for 18% in the iron itself, for a total P content of 0.28% in the metal). This high P content and particular repartition are essential catalysts in the formation of a passive protective film of “misawite” (d-FeOOH), an amorphous iron oxyhydroxide that forms a barrier by adhering next to the interface between metal and rust. Misawite, the initial corrosion-resistance agent, was thus named because of the pioneering studies of Misawa and co-workers on the effects of P and Cu and those of alternating atmospheric conditions, in rust formation.
The most critical corrosion-resistance agent is iron hydrogen phosphate hydrate (FePO4-H3PO4-4H2O) under its crystalline form and building up as a thin layer next to the interface between metal and rust. Rust initially contains iron oxide/oxyhydroxides in their amorphous forms. Due to the initial corrosion of metal, there is more P at the metal–scale interface than in the bulk of the metal. Alternate environmental wetting and drying cycles provide the moisture for phosphoric acid formation. Over time the amorphous phosphate is precipitated into its crystalline form (the latter being therefore an indicator of old age, as this precipitation is a rather slow happening). The crystalline phosphate eventually forms a continuous layer next to the metal, which results in an excellent corrosion resistance layer. In 1,600 years the film has grown just one-twentieth of a millimetre thick.
Balasubramaniam states that the pillar is “a living testimony to the skill of metallurgists of ancient India”. An interview with Balasubramaniam and his work can be seen in the 2005 article by Veazy.
It was claimed in the 1920s that iron manufactured in Mirjati near Jamshedpur is similar to the iron of the Delhi pillar. Further work on Adivasi (tribal) iron by the National Metallurgical Laboratory in the 1960s did not verify this claim.
According to INTACH, further research has been proposed on the Iron Pillar to study the ancient metallurgy of India. The ASI is reported to have agreed to the proposed studies that would make comparisons by testing other ancient iron objects like the pillar at Dhar, the iron beams at Konarak, and so forth. The present research using non-intrusive technique as proposed by Dr.Baldev Raj who is the Director of the Indira Gandhi Centre for Atomic Research and a member of the panel of architects and scientists.
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