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Corrosion resistance of Delhi iron pillar

Apropos to the article `Delhi iron pillar and nano powder' published in these pages on 29 August, R. Balasubramaniam, Department of Materials & Metallurgical Engineering, IIT, Kanpur, writes:

ALL ASPECTS of the 1600-year old Delhi iron pillar have been addressed in the book published by the author. New insights on the nature of the protective passive layer on the corrosion resistant Delhi iron pillar have now been obtained through modern state-of-art experimental studies.

Rust samples from the pillar were characterised by X-ray diffraction, Fourier transform infrared spectroscopy and Mossbauer spectroscopy.

The major constituents of the scale were crystalline iron hydrogen phosphate hydrate (FePO{-4}H{-3}PO{-4}4H{-2}-O) y-, y-, y-FeOOH and magnetite. The iron oxide/oxyhydroxides were present in the amorphous form. The process of protective rust formation on the Delhi pillar iron could be outlined on the basis of data from the rust analysis.

The iron contains a relatively high percentage of P compared to modern steels, because limestone was not added in the charge of ancient Indian bloomery furnaces.

Initially, the corrosion rate of iron is very high due to the presence of entrapped slag particles. This results in enhancement of surface P content.

In the presence of P, the formation of a protective amorphous compact layer of y-FeOOH (misawite), next to the metal surface, is catalysed and this confers the initial corrosion resistance.

The critical factor aiding the superior corrosion resistance of the Delhi iron pillar, however, is the formation of iron hydrogen phosphate hydrate, as a thin layer next to the metal-oxide interface.

The amorphous to crystalline transformation of the phosphate is aided by alternate wetting and drying cycles (i.e. the environmental factor). The rate of corrosion is further lowered due to the low porosity content of the crystalline phosphate phase.

When iron is exposed to the environment, the first oxyhydroxide to form is y-FeOOH. With time, a part of y-FeOOH transforms to another allotropic modification (y-FeOOH) and the rust at later times is composed of both these oxyhydroxides. Both forms of this oxyhydroxide are not protective against corrosion and readily crack allowing for ingress of oxygen and moisture to reach the metal surface and cause further corrosion.

However, with time, a part of the FeOOH formed transforms to magnetic oxides of iron, which are much more protective than these oxyhydroxides.

In addition to y- and y-FeOOH, an amorphous oxyhydroxide y-FeOOH can form on atmospheric exposure of iron.

In ordinary mild steels, this phase forms in a discontinuous manner and, therefore, is not protective in nature.

The formation of y-FeOOh as a continuous layer next to the metal surface is catalysed by the presence of P and Cu in the iron and it is this amorphous layer that confers superior corrosion resistance to the weathering steels.

`There has been no protective coating applied artificially on the pillar. This is verified by the fact that any freshly exposed part of the pillar surface attains the colour of the rest of the pillar in a relatively short period of time.

This conclusively indicates that the protective passive film on the Delhi iron pillar grows from the pillar material.

Moreover, the underlying heterogeneous structure of the pillar iron does not have any influence on the long-term atmospheric corrosion of the pillar, although its effect is felt only in the initial period of corrosion of the pillar iron.

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