In India's capital there stands the famous ancient Delhi Iron Pillar. It is a 7 m tall, 6.5 ton pure iron column from about 400 AD. It shows great resistance to atmospheric corrosion causing to much speculation over the years. The manufacturing process used was forge-welding of charcoalreduced iron powder. The iron contains a significant amount of phosphorous, which apparently helps form a protective passive film under atmospheric conditions which has protected the pillar over the past 1600 years. One small part of the column exhibits obvious rust, which most probably is the result of a wear-assisted corrosion phenomenon. It is located where visitors were able to touch the pillar. In the past it was customary for visitors to try to clasp their hands around the pillar in order to obtain good luck and fortune. A polished section corresponds very well with a region which would have been rubbed by the visitor's clothes. A section above this, located at the height of the column where visitors used to touch the pillar, shows a rough surface and material removal. Disturbance of the film by the rubbing fingers has enabled rust formation. Rubbing by subsequent visitors has removed part of the forming film, thus preventing passivation to fully develop. As a result some material was lost in each contact. It is possible to develop an extremely rough estimate of the rate of bio-tribo-chemical wear (2-3 micrometers/year over 1600 years, or possibly 1.5-2 nm/contact). The public can no longer reach the pillar and surface passivity is slowly reestablishing itself.
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