Iron articles usually rust within few years. But since 400 A.D the Delhi iron piller Near Kutab Minar is existing without any rust or sign of decay. Explain how ?

Explanation

Iron rusts due to the formation of hydrated ferric oxide ` ( Fe _ 2 O _ 3 .x H _ 2 O ) ` which keeps on peeling off, exposing fresh surface to air and moisture for rusting. On the other hand, magnetite ` (Fe_ 3O _ 4 ) ` is another oxide of iron which sticks to the surface to metal of centuries. Thus, Delhi iron pillar does not rust since a thin film of magnetite has been applied on its surface by applying a mixture of different salts, heating and quenching.