Question

Why isn't The Iron Pillar in Delhi rusting even though it's kept in the open without a coating of paint on it ?

Answer 1

Many theories have been put forward from time to time to explain the rust-free state of the pillar. For example, 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 great mass of metal might act as a temperature stabilizer, thus reducing condensation of moisture on it. It has already been mentioned that corrosion proceeds during those time when the effective relative humidity on the surface of the metal exceeds the critical value (e.g. 80%). In Delhi, this cannot normally occur during the day or early in the night because the air is very dry, except of course when it rains. During the remainder of the night the temperature slowly drops and because of its high heat capacity, the pillar remains warm and less liable to corrode than the relative humidity of the air would indicate. Just before day break the pillar is for a very short time cooler than air as dry, daytime conditions are quickly reestablished.

So, in brief, it can be concluded that the Corrosion Resistance property of the Delhi Pillar is due to: (i) the purity of its iron; (ii) high phosphorus; (iii) low sulphur; (iv) absence of any other metal; (v) cinder coating formed on the surface; (vi) better forge welding; (vii) drier and uncontaminated atmospheric condition; and (viii) mass metal effect