How has the iron pillar in Delhi not rusted inspite of being made of impure iron?

Question

How has the iron pillar in Delhi not rusted inspite of being made of impure iron?
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Answer

The Delhi Iron Pillar is a classical example of massive production of high class iron and is the biggest handforged block of iron from antiquity. It is a demonstration of the high degree of accomplishment in the art of iron making by ancient Indian iron and steel makers. It has been said that the Indians were the only nonEuropean people who manufactured heavy forged pieces of iron and the pieces were of the size that the European smiths did not learn to make more than one thousand years later.brbrMany theories have been put forward from time to time to explain the rustfree 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 . At Delhi, the relative humidity exceeds for only about days in a year and exceeds for only about days in a year. Therefore, although the total rainfall annually is about inches the atmosphere in Delhi is not very conducive to rusting of iron.brbrBesides 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.brbrProtective CoatingbrbrThe heterogeneous nature of the structure of the monument reveals two important points, namely, no heat treatment has been applied and the metal of the pillar has never been in the molten state, probably the last stage in the construction of so large a piece of iron at that date would almost certainly have consisted of the hammer forging together of balls of iron and thereafter repeated reheating and hammering process to create smooth surface. This must have taken a considerable time to complete. During this time an oxide film would have formed some of which could get hammered into the surface. Slag too would have oozed out and would have joined the scale. Owing to its high heat capacity and high ambient temperature the finished iron would have taken relatively long time to cool leading to a somewhat nonhomogenous normalization, the quality of the oxide layer produced by this sequence of operation would in all probability greatly promote the preservation of the pillar in pure and dry climate.brbrAccording to the second theory, the protective oxide could have formed from atmospheric exposure. Examination of small pieces of scale obtained from the iron pillar reveals that it consists of approximately of an oxide of iron having the properties of the solid solution phase of mixtures of FeO and FeO. About of this hydrated oxide of iron, approaching Limonite FeO.HO has also been reported. From the above reports it can be concluded that the scale was apparently formed under conditions of heating with significant extent of atmospheric oxidation occurring at the surface and penetrating along cracks running longitudinally in the scale.brbrThere have also been suggestions that in the past pillar was ceremonially anointed with purified butter. Tghee obtained from the milk of cow would have had a marked effect. A thin coating of linseed oil or lanoline or wool grease is well known to give good protection to steel for some months. If applied regularly and reinforced b the dust and sand which settle on it, it gives a good protective coating to the material underneath. However, the practice of ceremonial anointing would probably have been discontinued during Muslim occupation of the area in th century AD. brbrThe 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. . 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.brbrSo, 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.