An Rust-free Iron Pillar Yields its Secret at Last

04 August 2002
Posted by Chris Smith.

It has stood exposed to the elements for over 1600 years without showing any traces of corrosion and no one knew why. But now materials scientist Ramamurthy Balasubramaniam from the Indian Institute of Technology has solved the mystery of why the 7 metre-high, 6 tonne iron pillar, erected next to the famous Qutub Minaret in Delhi, seems so rust-resistant. Analysis of the metal in the pillar has revealed that it contains a very high level of phosphorus which, on the surface of the pillar, reacts with air and water to produce a protective layer of a compound called iron hydrogen phosphate hydrate. This surface coating prevents the iron beneath coming into contact with the air and going rusty. In modern furnaces limestone is added to remove the phosphorus, but 1600 years agothis wasn't the case. The ancient Indian Iron Smiths who cast the pillar would have deliberately picked an iron-ore rich they knew from experience would be resistant to corrosion - they are still being proved right 1600 years later.

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