A kinetic model for the evolution of atmospheric rust on the Delhi iron pillar (DIP) has been presented. The model is based on the known nature and structure of rusts on DIP and other corrosion-resistant ancient Indian irons. The initial fast rate of corrosion is aided by the entrapped slag inclusions and this results in the enrichment of P at the metal-scale interface. The presence of P at the metal-scale interface promotes protective film formation processes (catalytic formation of [delta]-FeOOH and phosphates), thereby reducing the corrosion rate. Growth rates have been roughly estimated for these two regions based on available DIP rust thickness measurements.