The Iron Pillar
The Iron Pillar in Delhi is a structure 23 feet 8 inches (7.2 metres) high with 16 inches (40.64 cm) diameter that was constructed by Chandragupta II, and now stands in the Qutab complex at Mehrauli in Delhi, India. The Iron pillar is a testimony of high-level skill achieved by the ancient Indian iron smiths in the extraction and processing of iron. It has attracted the attention of archaeologists and metallurgists, as it has withstood corrosion for nearly 1600 years.[i]
History of the Iron Pillar:
An iron pillar weighing over 6 tonnes, more than 7 metres tall is constructed in a single forge and is erected on top of the Vishnupada hill (somewhere in modern central India) with Sanskrit inscriptions on it in the Brahmi script about the great Gupta ruler Chandragupta Vikramaditya. Later about seven centuries ago it was installed in its current position by Tomar king Anangapala. [ii]
The solid iron pillar’s lower part is rough and pitted where it was once below ground, but the rest of the cylindrical column is smooth and tapers to a decorative bell at the top. An inscription commemorates the victory of King Chandragupta Vikramaditya over his enemies in the fourth century A.D. There is also damage that is believed to be caused by a cannon shot. The massive pillar was built in a single forge. In ancient times to forge such a huge and massive pillar in a single forge shows the advancement in technologies the metallurgists had achieved back then.
The composition of the pillar compared to typical modern steel of similar carbon content is shown in Table 1. [iii]
The solid iron pillar’s lower part is rough and pitted where it was once below ground, but the rest of the cylindrical column is smooth and tapers to a decorative bell at the top. An inscription commemorates the victory of King Chandragupta Vikramaditya over his enemies in the fourth century A.D. There is also damage that is believed to be caused by a cannon shot. The massive pillar was built in a single forge. In ancient times to forge such a huge and massive pillar in a single forge shows the advancement in technologies the metallurgists had achieved back then.
The composition of the pillar compared to a typical modern steel of similar carbon content is shown in Table 1. [iii]
According to the above table, this pillar contains above 98% of pure iron, still, it has not rusted yet. This is the biggest significance of the iron pillar from a metallurgist’s perspective. Even today it is difficult to build an iron pillar with this grade of corrosion resistance.
Why Is Iron Pillar Rust Free?
Following are the reasons why the iron pillar is still rust-free:
a) The purity of the iron that is used in the construction of the pillar
b) The phosphorus content is high
c) The sulphur content is less
d) There are no other metals present
e) The surface of the pillar is covered with cinder
f) Mass metal effect
The atmospheric conditions such as dry and uncontaminated Initially it was considered that the climate in Delhi is not particularly corrosive—at ~20 µm/y for mild steel (ISO category C2) could be the reason, but this did not state the perfect reason for the corrosion resistance of the iron pillar. Later, the scientists found that the excess phosphorus content has helped the iron pillar to form a passive layer which is corrosion-resistant.
The study conducted by scientists from the rust of the iron pillar gives us the reason for the corrosion resistance of it due to excess phosphorus. According to the study, rust characterization clearly established that the major constituents of the scale were crystalline iron hydrogen phosphate hydrate (FePO4·H3PO4·4H2O), α-, γ-, δ-FeOOH and magnetite. Experts at the Indian Institute of Technology have discovered that a thin layer of "misawite", a compound of iron, oxygen and hydrogen, has protected the iron pillar from rust. The protective film form within three years after the erection of the pillar and has been growing ever so slowly since then. After 1,600 years, the film has grown just one-twentieth of a millimetre thick, according to R. Balasubramanian of the IIT
Initially, the corrosion rate of the iron pillar is high due to the presence of slag particles. This results in the enhancement of surface P content. In the presence of P, the formation of a protective amorphous compact layer of δ-FeOOH, next to the metal surface, is catalysed and this confers the initial corrosion resistance. The critical factor contributing to the superior corrosion resistance, however, is the formation of iron hydrogen phosphate hydrate, as a thin layer next to the metal-oxide interface. The formation of the crystalline modification of this phosphate from the amorphous form is aided by alternate wetting and drying cycles (i.e., the environmental factor). The rate of corrosion is further lowered due to the low porosity content of the crystalline phosphate phase.[iv]
In a report published in the journal Current Science Balasubramanian says, the protective film was formed catalytically by the presence of high amounts of phosphorous in the iron—as much as one per cent against less than 0.05 per cent in today's iron.
If the iron pillar has good corrosion resistance, why is this steel composition not used today?
The reason behind this although iron pillar has good corrosion resistance due to phosphorus, it also gives it a disadvantage. The mechanical properties like ductility and impact toughness are significantly reduced due to high phosphorus content. Steel with such a high phosphorus content would be far too brittle and suffer from a problem that is known as “cold shortness”.
A high phosphorus content does not matter for the pillar that just stands there, but it is totally unacceptable for anything subjected to stresses, such as a bridge, building, rail, or any mechanical device.
Conclusion
The high phosphorous content in the iron pillar is a result of the unique iron-making process practised by ancient Indians, who reduced iron ore into steel in one step by mixing it with charcoal.
Modern blast furnaces, on the other hand, use limestone in place of charcoal yielding molten slag and pig iron that is later converted into steel. In the modern process most phosphorous is carried away by the slag. That is why we cannot see the ancient steel composition in today’s steels.
Thus, due to this advanced technological thinking of the ancient metallurgists, the Iron Pillar is one of mankind’s greatest technological accomplishments.
CREDITS & REFERENCES;-
- Team Meta Monday
- Research Papers of R. Balasubramainam sir
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