 |
The iron pillar is located near the Qutub Minar in Delhi. It is about 23 feet 8 inches tall or 7.2 metres high with diameter of 16 inches or 40.64 cm. The iron pillar was constructed by Chandragupta II (375-415 CE). The iron pillar enjoy the distinct fame of rust-resistant composition of the metals used in its construction. The weight of the pillar is over three tonnes or 6,614 pounds and is believed to have been erected elsewhere, possibly outside the Udayagiri Caves, and moved to its present location in Delhi.
The actual height of the pillar, from the top to the top of its base, is 7.21 m or 23 ft 8 in, of which 1.12 m or 3 ft 8 in, is below ground. Its bell pattern capital is about 306 mm or 12 in. It is believed to have an estimated weight of more than three tonnes or 6,614 lb. The pillar has been the centre of attraction for archaeologists and materials scientists, the reason being its high resistance to corrosion and has been rightly called a "testimony to the high level of skill achieved by the ancient Indian iron smiths in the extraction and processing of iron". The unbelievable corrosion resistance is a result of an even layer of crystalline iron(III) hydrogen phosphate hydrate forming on the high-phosphorus-content iron, which serves to protect it from the effects of the Delhi's harsh climate.
It was manufactured by the forge welding process whereby pieces of wrought iron were forged together. R. Balasubramaniam of IIT Kanpur explains in a journal Current Science, as to how the pillar's resistance to corrosion is due to a passive protective film at the iron-rust interface. Also 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-stage formation of that protective passive film.
Let us try to understand the rust resisting capability of the iron pillar. Lepidocrocite and goethite are the first amorphous iron oxyhydroxides that would appear upon oxidation of iron, so one can expect high corrosion rates are initially. Then, an important 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 thus leading to cessation of rusting activity.
The second-phase particles act as a cathode, and the metal itself behave as an anode, giving way to a mini-galvanic corrosion reaction during environment exposure. Part of the initial iron oxyhydroxides also get transformed into magnetite, which further slows down the process of corrosion. Meanwhile 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 which is further 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, however limestone is used in modern blast furnaces which yields pig iron that is later converted into steel but in the process, most phosphorus is carried away by the slag. The absence of lime in the slag and the use of specific quantities of wood with high phosphorus content during the smelting induces a higher phosphorus content than in modern iron produced in blast furnaces. This high phosphorus 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.
The iron pillar is a marvel of Indian metallurgical prowess and perhaps modern science should take notes from the same.
More