The Delhi Iron Pillar: A study of the Corrosion Formed in Areas of Surface Deformation

Abstract

The Delhi iron pillar has long been considered a corrosion resistant structure. Recently, a fast growing deterioration in the form of yellow colored corrosion has been observed, especially in an area on the surface that suffered impact from a cannon ball in the 18th century. This type of corr-osion is causing accelerated corrosion of the structure. Close observation of the pillar surface shows that large areas below the impact site are developing iron oxides of different colors on the surface. This paper discusses the possible causes of this deterioration and suggests a plau-sible scenario to explain this phenomenon. Based on hist-orical information concerning the development of dents in the surface of the pillar, a laboratory-simulated dent was formed on a piece of iron with similar composition to that of the pillar. Electrochemical impedance spectroscopy (EIS), and cyclic polarization were performed on the sample. The results corroborate our proposed mechanism for the deterioration of the pillar. It is concluded that the dented area is subject to tensile stress and is simult-aneously exposed to a corrosive environment, and is there-fore corroding preferentially to the rest of the pillar.