The pillar has attracted the attention of archaeologists and materials scientists because of its high resistance to corrosion and has been called a “testimony to the high level of skill achieved by the ancient Indian iron smiths in the extraction and processing of iron.” The corrosion resistance results from an even layer of crystalline iron hydrogen phosphate hydrate forming on the high-phosphorus-content iron, which serves to protect it from the effects of the Delhi climate.
The pillar weighs over 6,000 kg (13,000 lb) and is thought to have originally been erected in what is now Udayagiri by one of the Guptamonarchs in approximately 402 CE, though the precise date and location are a matter of dispute
Details of the top of iron pillar, Qutub Minar, Delhi.
The pillar 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 presence of second-phase particles (slag and unreduced iron oxides) in the microstructure of the iron, that of high amounts of phosphorusin 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.
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 polarisation characteristics and enrich the metal–scale interface with phosphorus, 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. 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