The Iron Pillar of Delhi is known for its rust resistance despite being over 1,600 years old. The pillar is made of iron, with small amounts of phosphorus, carbon, manganese, nickel, silicon , copper, nitrogen, and sulfur. It is not completely rust-free—it has been found to have thin rust layers, one of which is composed of crystalline iron hydrogen phosphate hydrate, which, along with Delhi’s weather, gives the pillar its corrosion resistance. The high phosphorus in the iron allows these reactions to occur.
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