The Iron Pillar of Delhi, also known as the Ashoka Pillar, is a remarkable historical artifact located in Delhi, India. The reason why the Iron Pillar does not rust to a significant extent is due to a protective layer of passive oxide that forms on its surface over time.
The Iron Pillar is primarily composed of iron, with trace amounts of other elements such as phosphorus, sulfur, and manganese. It is believed to have been constructed during the Gupta Empire, sometime between the 4th and 5th centuries AD.
When iron comes into contact with oxygen and moisture in the air, it typically undergoes a chemical reaction known as oxidation, resulting in the formation of iron oxide, commonly known as rust. However, in the case of the Iron Pillar of Delhi, a thin, protective layer of a specific type of iron oxide called "misawite" (FeO(OH)) forms on the surface of the pillar.
This misawite layer acts as a protective barrier, preventing further oxidation and inhibiting the spread of rust. Additionally, the presence of other elements, particularly phosphorus, is thought to contribute to the formation of this stable and protective oxide layer.
Over centuries of exposure to the elements, this passivation process has contributed to the preservation of the Iron Pillar, preventing significant rusting and corrosion. While the pillar does exhibit some signs of corrosion and weathering, the degree of rusting is minimal compared to what might be expected for ordinary iron structures exposed to similar environmental conditions. The Iron Pillar's resistance to rusting has fascinated scientists and historians for centuries and has made it a remarkable and historically significant monument.