The iron pillar in India was produced by the forge welding of pieces of wrought iron. In a report published in the journal Current Science, R. Balasubramaniam of the IIT Kanpur explains how the pillar's resistance to corrosion is due to a passive protective film at the iron-rust interface. The presence of second-phase particles (slag and unreduced iron oxides) in the microstructure of the iron, that of high amounts of phosphorus in the metal, and the alternate wetting and drying existing under atmospheric conditions are the three main factors in the three-stage formation of that protective passive film.
Lepidocrocite and goethite are the first amorphous iron oxyhydroxides that appear upon oxidation of iron. High corrosion rates are initially observed. Then, an essential chemical reaction intervenes slag and unreduced iron oxides (second phase particles) in the iron microstructure alter the polarisation characteristics and enrich the metal–scale interface with phosphorus, thus indirectly promoting passivation of the iron (cessation of rusting activity).
The second-phase particles act as a cathode, and the metal itself serves as anode, for a mini-galvanic corrosion reaction during environment exposure. Part of the initial iron oxyhydroxides is also transformed into magnetite, which somewhat slows down the process of corrosion. The ongoing reduction of lepidocrocite and the diffusion of oxygen and complementary corrosion through the cracks and pores in the rust still contribute to the corrosion mechanism from atmospheric conditions.
The next main agent to intervene in protection from oxidation is phosphorus, enhanced at the metal–scale interface by the same chemical interaction previously described between the slags and the metal. The ancient Indian smiths did not add lime to their furnaces. The use of limestone as in modern blast furnaces yields pig iron that is later converted into steel; in the process, most phosphorus is carried away by the slag.
The absence of lime in the slag and the use of specific quantities of wood with high phosphorus content (for example, Cassia auriculata) during the smelting induces a higher phosphorus content (> 0.1%, average 0.25%) than in modern iron produced in blast furnaces (usually less than 0.05%).
This high phosphorus content and particular repartition are essential catalysts in the formation of a passive protective film of misawite (d-FeOOH), an amorphous iron oxyhydroxide that forms a barrier by adhering next to the interface between metal and rust. Misawite, the initial corrosion-resistance agent, was thus named because of the pioneering studies of Misawa and co-workers on the effects of phosphorus and copper and those of alternating atmospheric conditions in rust formation.
The most critical corrosion-resistance agent is iron hydrogen phosphate hydrate (FePO-HPO-4HO) under its crystalline form and building up as a thin layer next to the interface between metal and rust. Rust initially contains iron oxide/oxyhydroxides in their amorphous forms. Due to the initial corrosion of metal, there is more phosphorus at the metal–scale interface than in the bulk of the metal. Alternate environmental wetting and drying cycles provide the moisture for phosphoric-acid formation. Over time, the amorphous phosphate is precipitated into its crystalline form (the latter being therefore an indicator of old age, as this precipitation is a rather slow happening). The crystalline phosphate eventually forms a continuous layer next to the metal, which results in an excellent corrosion resistance layer. In 1,600 years, the film has grown just one-twentieth of a millimetre thick.
In 1969, in his first book, Chariots of the Gods?, Erich von Däniken cited the absence of corrosion on the Delhi pillar and the unknown nature of its creation as evidence of extraterrestrial visitation. When informed by an interviewer, in 1974, that the column was not in fact rust-free, and that its method of construction was well-understood, von Däniken responded that he no longer considered the pillar or its creation to be a mystery.
Balasubramaniam states that the pillar is "a living testimony to the skill of metallurgists of ancient India". An interview with Balasubramaniam and his work can be seen in the 2005 article by the writer and editor Matthew Veazey. Further research published in 2009 showed that corrosion has developed evenly over the surface of the pillar.
It was claimed in the 1920s that iron manufactured in Mirjati near Jamshedpur is similar to the iron of the Delhi pillar. Further work on Adivasi (tribal) iron by the National Metallurgical Laboratory in the 1960s did not verify this claim.