Mixed potential theory analysis of the corrosion resistance of Delhi iron pillar

The famous Delhi iron pillar has withstood corrosion for the last 1600 years. The theories that have been proposed to explain the corrosion resistance of the pillar have been critically reviewed. However, the theories fail to explain why the heterogeneous structure of the iron pillar (wrought iron with entrapped slag inclusions) is resistant to corrosion whereas it should be very prone to corrosion. This anomaly has been addressed in this paper which presents anew theory for the corrosion resistance of the Delhi iron pillar based on mixed potential theory. The analysis reveals that the enhanced cathodic reduction reactions due to the presence of slag particles aid the formation of a passive protective film, thereby providing corrosion resistance. Experimental support for the analysis is provided by electrochemical potentiodynamic polarization studies on a 700 year old ancient Indian iron, with and without slag inclusions. The possible mechanism of protective film formation on the iron pillar's surface has also been discussed.