Iron pillar of Delhi : The mystery

The Iron Pillar located in Delhi, India, is a 7 m (23 ft) column in the Qutb complex, notable for the rust-resistant composition of the metals used in its construction.

The pillar has attracted the attention of archaeologists and metallurgists and has been called "a testament to the skill of ancient Indian blacksmiths" because of its high resistance to corrosion.The corrosion resistance results from an even layer of crystalline iron hydrogen phosphate forming on the high phosphorus content iron, which serves to protect it from the effects of the local Delhi climate. /Wikipedia/