Minerals and Metals in Ancient India v1.c27.s2.t2: The Composition and Properties of the Delhi Iron Pillar
Minerals and Metals in Ancient India v1.c27.s2.t2
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The Composition and Properties of the Delhi Iron Pillar
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The material of the pillar is wrought iron obtained by solid state reduction. Regarding the composition of the Delhi pillar, several analyses have been published, and some of these are summarised in Table 19.2.
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Table 19.2 Chemical Analysis of the Delhi Iron Pillar
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| | Ghosh (1963) | Hadfield (1925) | N.M.L. (1956) |
| C | 0.28 | 0.08 | 0.28 |
| Si | 0.026 | 0.016 | 0.056 |
| P | 0.18 | 0.114 | 0.155 |
| Mn | Nil | Nil | Nil |
| S | Trace | Trace | Trace |
| N | 0.0065 | 0.032 | — |
| Cu | — | 0.034 | — |
| | Total | Fe 99.72 | |
| | sp. gr. | 7.81 | |
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According to the latest analyses of the pillar, including the underground parts which assayed 0.44-0.48 per cent phosphorus, the average composition of the material in the pillar may be given as:
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0.15 per cent C, 0.25 P, 0.005 S, 0.05 Si, 0.02 N, 0.05 Mn, 0.03 Cu, 0.05 Ni balance Fe.
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The pillar was forge-welded from a great many sponge iron lumps of different composition, so that the elemental composition and the microstructure are variable from layer to layer. Micro-examination of unetched section of the pillar iron reveals slag particles distributed irregularly. In etched condition, polyhedral grains of ferrite with some slip bands in some grains near the surface are seen. Some ferrite grains have a small amount of pearlite at the grain boundaries. The slag content is high and this contains magnetite and FePO₄, seen as yellow specks on the surface.
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The hardness test for the different zones were related with the microstructures. The data are provided in Table 19.3.
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Table 19.3
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Hardness Test Results of the Delhi Iron Pillar
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| Description | Hardness (Brinell) |
| 1. Coarse ferrite grains-cold worked | 164, 155, 148 |
| 2. Coarse ferrite grains with slag | 84.8, 139 |
| 3. Coarse ferrite grains with nitride needles and slip band | 115 |
| 4. Ferrite with small amount of pearlite | 138 |
| 5. Ferrite with grain boundaries, pearlite and forge-welded zone | 136 |
| 6. Ferrite with slightly more pearlite | 140, 125 |
| 7. Pearlite with ferrite | 162 |
| 8. Increasing pearlite with ferrite | 162 |
| 9. Still more pearlite | 165 |
| 10. Maximum pearlite | 177 |
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The carbon content shows considerable variations, and its average value is not particularly low. The content of carbon and of pearlite are low in the surface layer (increase inwards) apparently as a result of surface decarburization during hammer-forging.
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The phosphorus content is very high and is less varying. While P in steel occurs as a solid solution in the ferrite (up to 1 %), a considerable part in wrought iron may appear as slag inclusions of FePO₄. Naturally, the strongly oxidized parts and the surface layer, depleted in carbon, tend to be rich in phosphorus. In accord with the above, Ghosh (1963) found that of the 0.28 per cent P in a sample from the pillar, 0.18 per cent occurred as elementary P, the remainder occurring as phosphate slag. In oxygen-consuming corrosion, as in water and in the atmosphere, high P-content actually exerts a beneficial influence, probably because phosphorus in the ferrite is oxidized to phosphate which, as an inhibitor, promotes the formation of protective oxide films.
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The sulphur content of the Delhi iron pillar is very low, according to all determinations, probably because charcoal was used in reducing the ore. Coupled with the fact that manganese-content of Delhi iron pillar is also very low, the low content of sulphur means that there are very few centers of iron-rich manganese sulphide in the pillar which could initiate pit corrosion by serving as effective local cathodes. The contents of Si, Cu and Ni in the pillar are also very low, these values being typical of the ancient Indian wrought irons.
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Wranglén (1970) has pointed out that in terms of the carbon and silicon contents, the pillar roughly corresponds to an ordianry construction steel of the type En 3 (AISI 1015) of semi-killed quality, and differs from the En 3 steel in the following ways:
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(i) high slag content and marked heterogeneity
(ii) higher P content, about 5 times as high in steel
(iii) lower Mn content, as compared to 0.5 per cent in steel
(iv) S content, an order of magnitude lower than in steel.
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The composition of the pillar is favourable for a good corrosion resistance in the atmosphere.
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Rusting of the Pillar
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The apparent rustlessness of the Delhi pillar has been investigated by several scholars. (Hadfield, 1912, 1922, 1925; Ghosh, 1963; Lahiri et al., 1963; Bardgett and Stanners, 1963; Wranglén, 1970) The part of the pillar which is below ground is covered by a rust layer, about 1 cm thick, and shows deep pitting. Above ground, the pillar is coated by a protective film, 50-500 µm thick, analysed by Ghosh (1963) as: 67.0 per cent Fe₃O₄, 13.1 per cent FeO, 14.8 per cent H₂O, 1.7 per cent FePO₄, 3.1 per cent SiO₂, 0.2 per cent MgO and 0.1 per cent CaO.
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The much larger proportion of the magnetic fraction in the rust of the pillar — both aboveground as well as underground — compared to the non-magnetic fraction is a unique feature. We find in the rust of the ordinary mild steel much greater proportion of the non-magnetic (Fe₂O₃, nH₂O) over the magnetic (Fe₃O₄). Evidently Fe₃O₄ in the Delhi pillar rust has not fully undergone further oxidation to the Fe₂O₃ stage.
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There is an enrichment of P in the rust (0.35 per cent P in Fe), as compared to the substrate. Wranglén (1970: 766) believes that phosphorus evenly distributed as a solid solution in the ferrite has probably contributed more to the formation of the protective surface film than the heterogeneously distributed inclusions of phosphate slags. The larger proportion of silicon on the surface (compared to that in the bulk) is explained through the proven presence of quartz which is the consequence of sandstorms common in the Delhi area. MgO and CaO on the surface might have arisen due to sandstorm or from the slag in the body of the pillar.
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The Causes for the Rustlessness of the Delhi Iron Pillar
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The remarkable freedom of the Delhi iron pillar from rusting has been attributed to various reasons which can be categorised under the following two headings:
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I. The compositional factors stressed by the Indian investigators, and
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II. The environmental factors stressed by the non-Indian investigators.
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The theorists, while trying to emphasize one category over the other, have tended to be partial, and we feel that the two categories of factors as stated above must have been augmentative rather than exclusive of each other. Let us now enumerate the various factors.
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Compositional Factors
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Ghosh (1963) has stressed that while the factors of drier and uncontaminated atmospheric condition, low sulphur, high cinder or slag coating in the pillar, better forge-welding etc. have played some role in lowering the rate of corrosion, the major factors seem to be (a) absence of manganese and (b) presence of high phosphorus in the iron of the Delhi pillar. We have discussed the mechanisms for high P and low Mn and S lowering the rate of corrosion, and these machanisms are universally accepted.
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Lahiri et al. (1963) have drawn analogy between the tribal wrought iron made by primitive method and the Delhi iron pillar, and their corrosion resistance, which, according to them, could be attributed to the mode of manufacture and slag or cinder content acting as the protective film. A primitive furnace iron sample was found to stand the SO₂ test better than a mild steel sample of similar carbon content. (Ghosh, 1963: 40). Prakash and Tripathi (1986: 575) have reported Igaki's polarisation studies of some ancient iron sample showing their superior corrosion resistance compared to a modern sample of ultra-pure iron.
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Ghosh (1963: 42-43) has argued that the atmosphere theory advocating primacy of the environmental factors cannot explain why Koṇārka (ancient) iron has not corroded faster in spite of being placed in the humid sea-side area, whereas a few modern iron beams lying in the temple courtyard have corroded so much. The rectangular pit at the top of the Delhi iron pillar must have collected rainwater from time to time and should have corroded mush faster, according to Ghosh, if the environmental factor exclusively determined the corrosion rate.
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Environmental Factors
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Countering Ghosh’s argument about the Delhi pillar, just now cited, Wranglén (1970: 763) pointed out that “the lowest part of the pillar, which is buried in the ground in a moister environment than the atmosphere, is covered by a rust layer more than 1 cm thick, and shows corrosion pits up to 10 cm deep; this indicates that climate rather than composition is the deciding factor in its corrosion-resistance”.
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A band of the circumference of the pillar at a height 1-1.5 m above the stone platform is very bright as if polished. This is due to the custom of visitors standing with their backs towards the pillar, and trying to clasp their hands around it ‘for luck’. As in the 2000-year old iron chain in a shrine on Adam’s peak in Sri Lanka, contact with human hands and consequent repeated polishing and greasing have practically prevented rusting.
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According to magnetic measurements (Bardgett and Stanners, 1963), the oxide film on the bright, polished section is less than 50 μm; immediately above this section it is 500-600 μm. If the oxide film is scraped off, the exposed iron starts to rust, and after a few years, the newly formed oxide cannot be distinguished from the main oxide of the pillar. This shows that the good state of preservation is mainly due to a protective film of corrosion products.
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According to Wranglén (1970), the deciding factor behind the rustlessness of the pillar is the dry and unpolluted climate of the Delhi area particularly during the 1500 years up to the beginning of this century. The atmosphere has low relative humidity which rarely exceeds the critical 70 per cent (the three monsoon months being exceptions), and is often as low as 20-40 per cent. The rainfall is relatively small. Before the twentieth century, the concentrations of the corroding CO₂ in the atmosphere were low. There has been more corrosion-passivating ammonia in the atmosphere on account of the biological waste products in the neighbourhood areas.
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The Delhi iron pillar has large mass, and absorbs lot of solar radiated heat. This counteracts dew precipitation during the night and results in rapid drying after rainfall.
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Lahiri *et al.* (1963) found a corrosion loss of 5 μm/year after exposure of a small smaple from the Delhi iron pillar for 3-1/2 years at Jamshedpur. The rate is more than that of a copper-containing, slow-rusting steel kept at Delhi and of a normal carbon steel (of the composition of the Delhi iron pillar) exposed in Sweden. (Wranglén, 1970: 766) When a protective film is formed, its thickness increases according to the parabolic law: y² = K.t; with y = 5µm for one year, Wranglén calculated that for 1600 years, y = 200 µm which is in excellent agreement with the measurements by Bardgett et al. (1963).
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Wranglén (1970: 767) concluded by saying that the rate of rusting of one and the same steel varies as 1 to 100, or more in different climates, different unalloyed or low-alloyed steels, exposed in the same location, show much smaller variations, perhaps as 1 to 5 or as 1 to 10 at the most. We are not sure whether the ratio in the second case (1:5 to 1:10) observed for short periods may or may not increase over a much longer period for corrosion.
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The compositional factors such as low Mn and S, high P, high slag content etc. were not denied by Wranglén (1970) altogether, and we have no hesitation in concluding that the unusually low corrosion rate of the Delhi iron pillar has been both due to Delhi (atmosphere) and the pillar (composition)!