Why Qutub Minar's iron pillar has stayed rust-free for 1600 years

The historic Qutub Minar in New Delhi, has been a mystery to scientists for more than decades. But the monument houses something even more iconic!

The iron pillar located within the Qutub Minar complex, is one of India's most remarkable metallurgical achievements. It's 7.2 metres (23.6 feet) tall and weighs over six tonnes; the pillar is believed to have been erected during the reign of Chandragupta II in the 4th century CE.

It is renowned for its exceptional resistance to corrosion despite being exposed to the elements for over 1,600 years. Crafted from nearly pure wrought iron, the pillar bears a Sanskrit inscription praising Chandragupta II and remains a testament to the advanced ironworking skills of ancient Indian craftsmen. It was not until 2003 that a study by scientists from IIT Kanpur explained this phenomenon.

What makes the Qutub Minar's iron pillar resistant to rust after 1,600 years?

As per the findings of the Archaeological Survey of India, Qutub Minar's iron pillar is made of wrought iron and is known to have garnered much scientific attention due to the fact that iron structures corrode when they come into contact with water. According to reports, the structure carries inscriptions for Chandragupta Vikramaditya from the Gupta dynasty.

Qutub Minar's iron pillar has been studied by scientists since 1912. Murray Thompson and Percy of Roorkee Engineering College and the School of Mines analysed the chemical structure of the pillar. It revealed that the pillar was composed of wrought iron with a specific gravity of 7.66. But the cause for its high corrosion resistance was still unknown.

The breakthrough finally came in 2003. Scientists at the Indian Institute of Technology in Kanpur took a closer look at the pillar. The discovery showed that the iron pillar has a high content of phosphorus. This is believed to be the cause of its longevity.

Another finding from the study was the presence of a very thin protective coating known as 'misawite.' This substance is composed of iron, oxygen, and hydrogen. It helps create a passive guard to prevent further corrosion of the metal.

Another significant discovery talks about the making process of Qutub Minar. The ancient artisans employed the forge-welding method, whereby hot pieces of iron were hammered together. This technique preserved the high phosphorus content in the metal.

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