Mystery revealed: This enigmatic iron pillar has stood outdoors for 1,600 years without rusting - here’s why

For years, a rust-proof ancient monument in India was a source of bafflement and awe for scientists - until the mystery of its extreme resistance to corrosion was finally unlocked around the turn of the millennium.

A metal column known as the Iron Pillar of Delhi, the monument can be found at the UNESCO-listed Qutb Minar complex, in the Indian capital’s southern Mehrauli district.

Located in the central courtyard of the complex’s Quwat-ul-Islam mosque, the 24ft-tall, 16in-wide pillar was built in around 400 CE.

Yet despite its constant exposure to the elements, it has remained almost rust-free for the 1,600 years since then.

What is rust and how does it form?

Typically, rust forms on the surface of iron objects when the metal interacts with oxygen and moisture. This reaction creates a reddish-brown compound, whose scientific name is iron oxide.

Although the Iron Pillar of Delhi’s lack of rust might in part be put down to Delhi’s unusually low humidity, this does not account for the fact that the monument has not always stood in its current location.

It was originally installed in Udayagiri, in central India, before being moved to Delhi in the 13th century.

So how does the Iron Pillar of Delhi avoid rust?

The pillar’s durability is chiefly thought to be explained by a 2003 research paper published by a scientist at the Indian Institute of Technology (IIT).

Released in the journal Current Science, the paper points to the composition of the iron used to craft the centuries-old pillar.

According to the study’s author - R. Balasubramaniam, of the IIT’s Department of Materials and Metallurgical Engineering - it is the iron’s higher-than-usual concentration of phosphorus that is the “deciding factor” in the pillar’s resistance to corrosion.

Normally, Balasubramaniam notes, modern-day iron workers would attempt to remove phosphorus from iron, because the chemical element “leads to the embrittlement of the material”.

However, by skilfully implementing a technique known as ‘forge-welding’, the ancient craftsmen were able to shape the iron in a way that maintained a higher phosphorus content while avoiding brittleness.

Writing in CNN, the journalist Poonam Binayak explains that the forge-welding process essentially involved “heat[ing] and hammer[ing]” the iron. This is “a method uncommon in modern practices”, Binayak adds.

“Protective passive film mechanism”

Crucially, the phosphorus is credited with playing a key role in the formation of an anti-rust layer on the iron’s surface - a natural process referred to by Balasubramaniam as a “protective passive film mechanism”.

The protective shield combines crystalline iron hydrogen phosphate hydrate - a compound Balasubramaniam describes as “critical” to “superior corrosion resistance” - with the mineral magnetite and a compound of iron, hydrogen and oxygen dubbed ‘misawite’ by the IIT researcher.

According to Balasubramaniam, the choice of the name ‘misawite’ serves as a tribute to the “pioneering studies” carried out in this field by the Japanese scientist T. Misawa.

The Iron Pillar of Delhi is “a living testament to India’s ancient metallurgical prowess”, Balasubramaniam says.

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