The site of the pillar is away from industrial areas and the climate of Delhi is fairly dry. It is known that serious corrosion of iron does not take place if critical value of relative humidity is less than 80%. At Delhi, the relative humidity exceeds 80% for only about 20 days in a year and exceeds 70% for only about 65 days in a year. Therefore, although the total rainfall annually is about 15-30 inches the atmosphere in Delhi is not very conducive to rusting of iron.
Besides a dry environment, there appears to be no doubt that the protective scale on the pillar has played a major role in protecting this monument. There are two major theories of formation of this protective scale. It is quite possible that both the factors have played equally important roles in formation of this scale.The impurity of iron which make it different from the other irons and also the it is said that a thin coating of a type of grease is used for making of it .
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