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KN Note : When HHO is created by using steel rods. The black powder which drops down is unrustable iron. 
This might have something to do with the Iron pillar if it was iron made by the same method. 
I read this somewhere but have not confirmed that the iron which has other elements also is unrustable iron. Just that it was mentioned somewhere. 



[[https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.612.1357&rep=rep1&type=pdf|On the corrosion resistance of the Delhi iron pillar]]



[[https://link.springer.com/referenceworkentry/10.1007/978-1-4020-4425-0_8658|Iron Pillar at Delhi]]

The Delhi iron pillar (Fig. 1) is testimony to the high level of skill achieved by the ancient Indian ironsmiths in the extraction and processing of iron. Hadfield (1912) undertook the first systematic scientific study of the Delhi iron pillar. Results of scientific studies conducted in 1961 were summarized in a special issue of the NML Technical Journal (vol. 5, 1963). A review of its corrosion resistance appeared in 1970 (Wranglen 1970). While Anantharaman (1997) has reviewed the known scientific facts about the Delhi iron pillar, Balasubramaniam (2002, 2005) has compiled several new insights into the historical, scientific, and technical aspects of the Delhi iron pillar.


[[https://www.researchgate.net/publication/286992598_On_the_astronomical_significance_of_the_Delhi_iron_pillar|On the astronomical significance of the Delhi iron pillar]]

The astronomical significance of the Delhi iron pillar has been highlighted by addressing its probable original erection site at Udayagiri and the probable image that was atop the pillar's capital. Based on the astronomical significance of Udayagiri's location on the Tropic of Cancer, and earlier solar observations at Udayagiri, it has been shown that the iron pillar may have been aligned with the cardinal directions such that, on summer solstice day, the early-morning shadow of the pillar fell along a specially cut passageway in the direction of one of the important bas reliefs (in cave temple 15) at Udayagiri site. Evidences have been provided to corroborate the identification of a circular disc-shaped object (20″ diameter and 2″ thick) that was probably atop the Delhi iron pillar capital.

[[https://www.researchgate.net/publication/235791001_Nondestructive_evaluation_of_the_Delhiiron_pillar|Nondestructive evaluation of the Delhi iron pillar]]

We discuss results obtained on nondestructive evaluation of the Delhi iron pillar using various non-destructive techniques such as ultrasonics, impact echo, radiography, radiation gauging, X-ray fluorescence, in situ metallography and electrochemical analysis. The microstructural analysis of the ornamental portion of the pillar indicated the presence of forged structure in the main body of the pillar, whereas the top platform consists of microstructure similar to ‘as cast structure’. Further, the radiation gauging, radiography, ultrasonics and impact echo testing indicated the presence of voids at different locations in the pillar. The large aspect ratio (axial to radial) of the voids indicated that the pillar could have been forged in the radial direction rather than in the axial direction. The passivation current density for the ‘passive’ rust scales of the pillar formed on the surface clearly indicated the protective nature of the oxide structure developed and its stability against corrosion.

[[https://publications.iupac.org/ci/2005/2706/3_veazey.html|1 600 Years Young—Thanks to its Enduring Corrosion Resistance, an Indian Historical Artifact’s Appearance Belies its Age]]

A distance of approximately 7 200 miles (11 587 km) may separate Troy, New York, USA, and Delhi, India, but the two cities share an important bond for Professor R. Balasubramaniam. While earning his doctorate in materials engineering at Troy-based Rensselaer Polytechnic Institute (RPI) in the 1980s, Balasubramaniam attended a seminar given by the eminent corrosion scientist Helmut Kaesche that discussed one of Delhi’s most famous archeological landmarks. Having grown up in India, Balasubra-maniam was familiar with Delhi’s 24-ft (7.3-m)-tall pillar of iron that has remained largely free of rust since it was fabricated in approximately A.D. 400. However, he cites the Kaesche seminar as the source of his fascination with the artifact.
	
These photos show the upper and lower sections of the decorative bell capital atop the Delhi Iron Pillar. The different components were shrunk-fit around a hollow cylinder. Photos courtesy of R. Balasubrama-niam, IIT Kanpur.

Returning to his homeland in 1990 to accept a materials and metallurgical engineering assistant professorship at the Indian Institute of Technology, Kanpur (IIT Kanpur), Balasubramaniam began his investigation of the Delhi Iron Pillar in earnest—apart from his other research activities. Specifically, he sought to determine why the pillar exhibits such remarkable resistance to atmospheric corrosion. After more than a decade of independent, self-funded research, he concludes that the property stems from the formation of a protective passive film on the pillar’s surface.1

A Major Artifact
Originally fabricated and erected 1 600 years ago at Udayagiri near the present-day city of Bhopal in central India, the pillar was relocated to the Quwwat-ul-Islam mosque in Delhi’s Qutub Minar Complex approximately seven centuries ago. Constructed of rubble from earlier Hindu temples and now designated a World Heritage Site by the United Nations, the Quwwat-ul-Islam mosque is the first mosque built on the Indian subcontinent. The approximately six-tonne pillar was constructed during the Gupta Period (from A.D. 320 to 600), which is considered the golden age of Indian history.

“The Iron Pillar is considered a major artifact the world over,” says Balasubramaniam, now a full professor at IIT Kanpur. “Historians and archeologists consider the pillar to be a very important object of Indian history. The oldest Sanskrit inscription is famous, and its interpretation is still extensively discussed in academic circles.”

A Metallurgical Treasure
Balasubramaniam points out that scholarly fascination with the pillar is not limited to students of archeology and history. “Powder metallurgists claim [it] is a living example of an object manufactured by the powder metallurgical route,” he says. “Corrosion scientists are aware of the remarkable corrosion resistance of the pillar.”

Since the first such analysis in 1912,2 researchers have estimated that the pillar’s average composition is 0.15% carbon, 0.25% phosphorus, 0.005% sulfur, 0.05% silicon, 0.02% nitrogen, 0.05% manganese, 0.03% copper, 0.05% nickel, and the balance iron.3 “Interestingly, a sample of Delhi pillar iron was subjected to microprobe analysis in order to determine the composition of the elements manganese, chromium, copper, and nickel in the near-surface regions,” says Balasubramaniam. “It was found that the composition of copper [0.05%], nickel [0.05%], manganese [0.07%], and chromium [nil] was uniform through several millimeters into the sample from the surface.” 4

Balasubramaniam says that the pillar’s high phosphorus content has kept it from rusting on a widespread basis. “The presence of phosphorus is crucial to the corrosion resistance,” he notes, explaining that the phosphorus content is high because limestone was not used as a flux when the iron was extracted. “The absence of calcium oxide [CaO] in slags leads to a lower efficiency for removal of phosphorus from the metal, which invariably resulted in higher phosphorus content. Archeological evidence indicates that the ore for extracting the iron must have been carefully chosen so that a relatively high amount of phosphorus would result in the extracted metal.”

Typical of ancient Indian irons, the microstructure of the pillar shows a wide variety of structures, says Balasubramaniam. He adds that the structures also prove the iron was obtained by the direct reduction process rather than casting. “The pillar is a solid body with good mechanical strength,” notes Balasubra-maniam. He points out that the yield strength is 23.5 tons (21 319 kg)/in2 (645 mm2), the ultimate tensile strength 23.9 tons (21 682 kg)/in2, and elongation 5%.

Forge welding was the process used to manufacture the pillar. Balasubramaniam says that approximately 40- to 50-lb (18- to 23-kg) lumps of iron served as the raw materials. “Forge welding is an operation in which iron lumps were joined together by forging them in the hot state such that fusion is obtained between them,” he explains. “Research has indicated that the pillar was manufactured with the pillar in the horizontal position, and the addition of lumps was from the side,” he says. “The decorative bell capital is truly a marvelous example of blacksmithy and consists of seven distinct parts. These individual components were shrunk-fit around a hollow cylinder, which was joined to the main body by the aid of an insert.”
In this recent photo, materials engineer R. Balasubramaniam stands near the Delhi Iron Pillar. A new fence protects the base of the column from the large number of visitors to the site. Photo courtesy of R. Balasubramaniam, IIT Kanpur.

Environment or Materials?
Balasubramaniam says that two general schools of thought exist to explain why the pillar exhibits superior corrosion resistance: the environment and materials theories. “The proponents of the environment theory state that the mild climate of Delhi is responsible for the corrosion resistance,” he says, pointing out that the city’s relative humidity (RH) does not exceed 70% for significant periods of time in a given year. “It is known that atmospheric rusting of iron is not significant for humidity levels less than 70%.”

Advocates of the materials theory, to which Balasubramaniam subscribes, stress the construction material’s role in determining corrosion resistance. “The ideas proposed in this regard are the relatively pure composition of the iron used, presence of phosphorus, and absence of sulfur [and] manganese in the iron, its slag particles, and formation of a protective passive film,” he says. The passive film component of the theory stems from Balasubramaniam’s research. “The large mass of the pillar also plays a contributory role,” he adds.

Although the environment and materials camps comprise the two predominant sides of the debate, Balasubramaniam adds that the literature does feature other, less-widely held theories about the pillar’s corrosion resistance. These suppositions include: initial exposure to an alkaline and ammoniacal environment; residual stresses resulting from the surface finishing (hammering) operation; freedom from sulfur contamination both in the metal and in the air; the “cinder theory,” which holds that layers of cinder in the metal stop corrosion from advancing; and that surface treatments of steam and slag and coatings of clarified butter were applied to the pillar after manufacture and during use, respectively. “The use of surface coatings is readily discounted because a freshly exposed surface attains the color of the rest of the pillar in about three years’ time,” 5 says Balasubramaniam.

Balasubramaniam asserts that the low incidence of corrosion on ancient iron artifacts in more humid parts of India supports the materials theory. “That the material of construction may be the important factor in determining the corrosion resistance of ancient Indian iron is attested by the presence of ancient massive iron objects located in areas where the RH is high for significant periods of the year,” he says. The Surya temple at Konarak, located near the Bay of Bengal, and the Mookambika temple in the Kodachadri Hills, which rise near the Arabian Sea, reportedly are two such locations. Balasubramaniam says that ancient iron beams at the Surya temple and an iron pillar at the Mookambika temple all are in very good shape despite their proximity to coastlines.
The 1 600-year-old Delhi Iron Pillar. Photo courtesy of R. Balasubramaniam, IIT Kanpur.

Protective Passive Film
Balasubramaniam cautions that the Delhi Iron Pillar does rust, but he adds that the passive rust is so protective and thin that it keeps the occurrence—and appearance—of corrosion at a minimum. Because the region just below the decorative bell capital is inaccessible to the public, rust from this location is the oldest undisturbed rust on the pillar. Consequently, Balasubramaniam and a colleague collected rust from this region and characterized it by X-ray diffraction, Fourier Transform Infrared spectroscopy, and Mössbauer spectroscopy.6 Balasubramaniam says the inspections revealed that the rust contains amorphous iron oxyhydroxides (lepidocrocite [gamma-FeOOH], goethite [alpha-FeOOH], and delta-FeOOH) and magnetite. It also contains crystalline phosphates, including iron hydrogen phosphate hydrate (FePO4 · H3PO4 · 4H2O).

Balasubramaniam says that the rust layer becomes increasingly protective—and the rate of corrosion decreases—as its composition changes. “In the initial stages, the rust comprises lepidocrocite and goethite,” he says. “These forms of rust do not offer excellent protection and, therefore, the rate of corrosion is still maintained on the high side. Conversion of part of this rust to magnetite does result in lower corrosion rates.” However, he adds that cracks and pores in the rust allow oxygen to diffuse and complementary corrosion reactions to occur. “Moreover, reduction of lepidocrocite also contributes to the corrosion mechanism in atmospheric rusting,” he says.

According to Balasubramaniam, the catalytic formation of delta-FeOOH initiates the Delhi Iron Pillar’s enhanced corrosion resistance. “This phase is amorphous in nature and forms as an adherent compact layer next to the metal-scale interface,” he says. “Its formation is catalyzed by the presence of phosphorus in the iron. Upon its formation, the corrosion resistance enhances significantly because delta-FeOOH forms a barrier between the rust and the metal.” He says a similar mechanism is at work in weathering steels that contain copper and phosphorus.

“In the special case of Delhi pillar iron and in the general case of ancient Indian irons, the presence of significant amounts [greater than 0.1%] of phosphorus in the metal leads to further effects, which have a direct bearing on their corrosion resistance,” says Balasubramaniam. “Due to the initial corrosion of metal, there is enhancement of phosphorus at the metal-scale interface. This phosphorus reacts with moisture, and conditions are created in the rust that are ideal for formation of phosphoric acid [H3PO4], which eventually leads to the precipitation of phosphates in the long term.”

Balasubramaniam says that several phosphate formation reactions occur. Exposure conditions dictate the nature and type of these phosphates, which demonstrate an inhibitive nature and thus affect corrosion resistance. “Added benefits accrue when the phosphate forms as a continuous layer next to the metal,” the researcher says. “In the case of alternate wetting and drying cycles [such as those present in atmospheric corrosion], the amorphous phosphates can transform to crystalline modifications, and in this process there is a large reduction in porosity in the phosphate. This transformation results in further excellent corrosion resistance properties.”

Following the “Beacon”
When viewed from a nonscientific standpoint, the Delhi Iron Pillar’s ability to resist corrosion has often been called a “mystery.” Balasubramaniam is quick to dismiss this response. “There is nothing mysterious about the iron pillar,” he says. “The resistance to atmospheric corrosion is due to the presence of a relatively high amount of phosphorus in the pillar. The remarkable corrosion resistance can be understood by applying the basic principles of corrosion research.” He adds that the direct reduction technique used to produce the iron is no mystery, either. “The ancient Indian ironmaking technology is well-known,” he says. The established scientific facts notwithstanding, Balasubramaniam concedes that one feature of the pillar is difficult to explain. “There is one aspect that is not well-understood and this may be called a mystery, in one sense,” he says. “This is the method by which the iron lumps were forge-welded to produce the massive six-tonne structure.”

Mystery or not, the Delhi Iron Pillar serves as a guidepost for metallurgists in the 21st century and beyond, asserts Balasubramaniam. In fact, just as a seminar at RPI inspired him to study the pillar, he hopes that his research will motivate others to explore the potential uses of phosphorus-containing iron. “There are so many wonderful options available with phosphoric irons,” he concludes, adding that the Iron-Phosphorus phase diagram deserves as much attention as the more popular Iron-Carbon phase diagram. “There is an exciting future in developing phosphoric irons, particularly for corrosion scientists and engineers.7 The beacon of light showing the way to the future is the Delhi Iron Pillar, with its tested proof of corrosion resistance.”

References
1. R. Balasubramaniam, “On the Corrosion Resistance of the Delhi Iron Pillar,” Corros. Sci. 42 (2000): pp. 2103–2129
2. R. Hadfield, “Sinhalese Iron and Steel of Ancient Origin,” J. Iron St. Inst. 85 (1912): pp. 134–174.
3. G. Wranglen, “The Rustless Iron Pillar at Delhi,” Corros. Sci. 10 (1970): pp. 761–770.
4. W.E. Bardgett, J.F. Stanners, “Delhi Iron Pillar—A Study of the Corrosion Aspects,” J. Iron St. Inst. 210 (1963): pp. 3–10.
5. R. Balasubramaniam, “On the Growth Kinetics of the Protective Passive Film of the Delhi Iron Pillar,” Current Sci. 82 (2002): pp. 1357–1365.
6. R. Balasubramaniam, A.V. Ramesh Kumar, “Characteriz-ation of Delhi Iron Pillar Rust by X-Ray Diffraction, Fourier Infrared Spectroscopy and Mössbauer Spectroscopy,” Corros. Sci. 42 (2000): pp. 2085–2101.
7. R. Balasubramaniam, “Phosphoric Irons for Concrete Reinforcement Applications,” Current Sci. 85 (2003): p. 9.

Note: Balasubramaniam has compiled his entire body of research on different aspects of the Delhi Iron Pillar into a book titled Delhi Iron Pillar: New Insights (New Delhi, India: Aryan Books International, 2002). A separate paperback version written for a nontechnical audience, Delhi Iron Pillar: A Metallurgical Marvel (New Delhi, India: Foundation Books, 2005), will be published soon.

Matthew V. Veazey is the staff writer for Materials Performance (MP), a monthly magazine published by NACE International, Houston, Texas, USA.



[[https://www.nature.com/articles/s41598-021-81918-w|Uncovering the superior corrosion resistance of iron made via ancient Indian iron-making practice]]

Ancient Indian iron artefacts have always fascinated researchers due to their excellent corrosion resistance, but the scientific explanation of this feature remains to be elucidated. We have investigated corrosion resistance of iron manufactured according to traditional metallurgical processes by the Indian tribes called ‘Agaria’. Iron samples were recovered from central India (Aamadandh, Korba district, Chhattisgarh). Iron artefacts are investigated using a range of correlative microscopic, spectroscopic, diffraction and tomographic techniques to postulate the hidden mechanisms of superlative corrosion resistance. The importance of manufacturing steps, ingredients involved in Agaria’s iron making process, and post-metal treatment using metal-working operation called hot hammering (forging) is highlighted. This study also hypothesizes the probable protective mechanisms of corrosion resistance of iron. Findings are expected to have a broad impact across multiple disciplines such as archaeology, metallurgy and materials science.
Introduction

The Indian subcontinent has been famous among metallurgists, conservators and archaeologists for its ancient iron-making technological heritage1. Juleff et al. (1996) have highlighted the importance of South Asian iron and steel making technology, particularly in peninsular India during the first millennium AD2. Significant metallurgical achievements have been made in Indian subcontinent such as the discovery and application of wootz steel (also called “bulad” in Central Asia)3,4. Solid-state ore reduction was one of the important iron-smelting technologies applied in the iron making. This process generates agglomerated bloom as the end-product along with slag2. Iron-based archaeometallurgical artefacts are important in demonstrating the ancient technological developments that proceeded and dispersed in the ancient times5,6. The Indian blacksmiths were famous globally and the steel (wootz steel—produced from iron bloom manufactured by Indian tribes) produced by them was used in Damascus steelmaking, which was famous for its excellent mechanical properties7. Indian ancient metallurgical and iron making practices have attracted significant attention in the recent past but detailed investigations on the structural properties of the tribe’s made iron are yet to be performed8.

The ancient Indian iron-making technology has been famous for producing corrosion-resistant iron. The Delhi iron pillar, which was made around fifth century AD (~ 1600 years old) is famous for its immunity to rusting and a prominent example of the Indian iron making process9,10. The Delhi iron pillar is also an important example of tribal tradition (Indian blacksmiths) of iron making in India. Several theories have been postulated regarding corrosion resistance of the Delhi iron pillar. Some of those refer to the inherent nature of the construction material, such as the selection of pure iron, presence of slag particles and slag coatings, surface finishing using mechanical operation, phosphate film formation, or the Delhi’s climate11,12,13,14,15,16,17. The exact reason for the superior corrosion resistance phenomenon remains a mystery. It is not possible to examine the iron from the Delhi Iron pillar as it is an archaeologically protected monument.

We have examined iron that was manufactured by Indian blacksmiths (Agaria tribes) using an Indian traditional technique. Agaria tribes are regarded as important tribal community responsible for channelizing the traditional iron and steel making technological growth in various Indian states in the central region, such as Madhya Pradesh and Chhattisgarh, Orissa Eastern part of Uttar Pradesh and Bihar from the ancient times18. It is noteworthy that Agaria tribes were not allowed to reside near to the villages, hence they used to perform iron making practice in a deep forest and adopted this practice as their means of livelihood. They were famous for performing special rituals customs (i.e. worshipping their God called Lohasur) prior to the start of iron-making19. India’s status as a colony of the British Empire until 1947, affected the traditional iron-making practices that were banned by the British rulers. Therefore, the technological and historical knowledge of ancient iron-making by Agaria tribes was lost and only a few clues can be obtained from the travel documents written earlier. There are still a few old members of the same tribes who have seen iron making practices in their early childhood. These are the only remaining sources of the iron making knowledge other than the travel documents. Agaria tribes’ soundness and skills in iron making technology made them travel to Japan where one can view their crafts19. Igaki et al. (1986) compared the soundness of corrosion resistance of Japanese and Indian ancient iron and found superior corrosion resistance of the Indian iron20. Igaki along with Prakash travelled to Loharpara, Bastar Chhattisgarh, and were able to locate the ‘Mundia’ tribes (different tribes from Agarias) and studied their iron-making technology from furnace making and process implementation’s point of view. However, detailed structure–property evaluation of the iron made by these tribes, using their traditional furnaces have not been performed21.

In this study, we have applied a range of advanced microscopy, spectroscopy, diffraction, and tomography techniques to unveil the hidden corrosion resistance mechanism of iron made by ancient traditional techniques in India. The reasons for the corrosion resistance of ancient Indian iron is still not clear. Different theories such as the presence of phosphorus in iron, presence of inert slag inclusions in iron, intentionally coated slag layers in iron, use of mechanical forming techniques, etc. have been proposed in different studies performed on the different ancient Indian iron artefacts. However, the corrosion resistance of iron made by tribal traditional techniques has not been examined to date to the best of our knowledge.
Materials and methods
Material collection

Members of the ‘Agaria’ tribes living in central India (Aamadandh, Korba (District: Korba), Chhattisgarh) donated the iron samples made through ancient Indian metallurgical techniques and using bloom furnace to D.D. for this investigation. A field survey was also conducted, and iron ore samples and slags collected from the region where the iron was made. However, the furnaces have been destroyed by the weathering action, with only a few remaining signs evident.
Material characterization

In order to investigate the mechanism of superior corrosion resistance of iron made by tribes using ancient Indian metallurgical techniques, multiple characterization tools capable of characterizing the morphological and compositional features at various length scales (µm to nm) were used. Morphological features of the corrosion product layer formed on the top of the iron are analysed by field emission gun electron microscope (FESEM). FESEM image acquisition was conducted at 5 kV. Chemical composition of corrosion products was characterized using energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The EDS acquisition was done at 15 kV, with the EDS coupled to FESEM.
Laboratory-based surface characterization techniques
Grazing incidence X-Ray diffraction (GIXRD)

We have characterized corrosion products using the GIXRD to confirm the crystalline phases present in the corrosion products. Rietveld analysis was used for evaluating the phase fraction using GIXRD pattern. The penetration depth of the X-ray is limited inside the iron sample. Therefore, X-ray can reveal the corrosion film. However, the entire iron sample could not be analysed using GIXRD. GIXRD data were collected using a PANalytical Empyrean diffractometer using Cu Kα radiation. The incident beam had a parabolic mirror with 2.3° Soller slits, and was fixed at 2° to the specimen surface. The diffracted beam had 2.3° Soller slits, a 0.18° parallel plate collimator, and a point detector with a 0.05 mm receiving slit. Diffraction data were taken over the range 10°–100° 2θ in steps of 0.02°. The diffraction data were collected in this fixed-incident beam geometry in order to enhance the contribution from surface layers. Phase identification was carried out using the software package EVA14 with the ICDD PDF2 database. Crystal structures for the phases were taken from the ICDD PDF 4 + database. Fluorescence, which is considered as an unwanted phenomena in diffraction, depends on the type of X-ray radiation source used (Copper (Cu) Kα radiation in our case) and the type of analysed material (iron sample in our case). Iron causes the development of high fluorescence with Cu Kα radiation which leads to the poor signal to noise ratio in diffraction pattern. Therefore, we have chosen neutron diffraction, used at the Australia’s Nuclear Science and Technology Organisation (ANSTO), Sydney, to analyse iron samples thoroughly because neutrons penetrate deeper inside the iron sample and provide better signal to noise ratio22.
X-ray photoelectron spectroscopy (XPS)

It is also very important to perform chemical characterization of the top layers of corrosion product film that is exposed to the atmosphere directly. Therefore, XPS was used23. The XPS analysis was exercised using a Kratos Axis Ultra DLD spectrometer (spatial resolution˜5 µm) with an irradiation source (operated at 225 W) of monochromatic Al Kα (1486.6 eV). The analysis chamber was kept maintained at the pressure of 9 × 10–9 Torr until the analysis was completed. C1s spectrum (284.8 eV) was used for calibrating the electron binding energy scale and a survey scan was carried out with the pass energy of 160 eV. CasaXPS software was used for data processing.
Field emission scanning electron microscope (FESEM)-EDS

The microstructural characterization of corrosion products formed on iron made through traditional ancient iron-making practices by Agaria tribes was conducted using a Field emission gun electron microscope (SEM, Zeiss NEON 40EsB).
Scanning transmission electron microscope (STEM)-EDS

Slag compositional analysis was conducted using FEI Titan G2 80–200 TEM/STEM at 200KV and collected high angle annular dark-field (HAADF) images along with the scanning transmission electron microscopy (STEM).
Synchrotron and neutron based characterization techniques
Neutron diffraction

Neutron diffraction data were collected at the ANSTO (ECHIDNA—High resolution powder diffractometer, Sydney, Australia) over the range of 4–173° 2θ in steps of 0.5° using Ge monochromators with 0.15° mosaic spread and vertically oriented linear position sensitive detector 3He of 30 cm height (detector radius = 1.29 m)24.
Neutron tomography

Due to the deeper penetration depth of neutrons inside the iron sample, neutron tomography was used to extract the corrosion product layer formed on the iron as well as characterized pores’ volume inside the iron in three dimensions. Synchrotron tomography is also used for characterizing ancient iron, however, due to the development of the instrumental artefacts, we have selected the application of neutron tomography for three-dimensional characterization of iron made by tribes through ancient technology. The neutron tomographic analysis of the samples was carried out on DINGO25, the ANSTO imaging station (Sydney, Australia) fed by a thermal neutron beam coming from the 20 MW OPAL research reactor in Sydney. High resolution acquisition mode with an L/D ratio of 1000 was used and a field of view 55 × 55 mm2 was set to acquire images with a pixel size of 27 μm. A 6LiF/ZnS scintillation screen (thickness of 50 μm) was used. All the projections were acquired in the step of 0.25°over 360° and an exposure time of 50 s each. The tomographic reconstructions were obtained by using Octopus package26 while AVIZO27 was used for data visualization, compiling and quantification.
Synchrotron X-ray tomography

Synchrotron X-ray computed tomography (CT) experiment was done at Hutch 2B of the Imaging and Medical Beamline (IMBL) at the Australian Synchrotron using the ‘Ruby’ detector. It is a custom-designed detector based on a photo-sensitive device coupled by a bright lens to a suitable X-ray sensitive scintillator28. PCO edge sensor, which was controlled by the motorized system has been secured from direct and scatter beam radiation by a mirror. This is used to view the scintillator plate (placed orthogonally to the direction of the beam). For this experiment, the sensor was equipped with a Nikon Micro-Nikkor 105 mm/f 2.8 macro lens allowing the slide to be used as a zoom control. A terbium-doped gadolinium oxy-sulfide (Gadox, P43) scintillator 12 microns thick was used. During the experiment, the system was tuned to produce 2560 × {772}{763} pixels images giving a field of view of {33.5 × 10.1}{32.8 × 9.8}mm with a pixel size of {13.1}{12.8} microns. Data were collected using monochromatic X-rays of energies {30}{80}{120} keV, in order to ensure reasonable X-ray transmission. The pieces were positioned vertically and horizontally on the sample stage such that their centre of rotation kept the region of interest within the field of view of the detector. Each tomographic scan was collected over a 180 range in 0.10 steps, making 1800 views in total, with an exposure time of {0.07}{0.3}{1.5}s per view.
Small-angle neutron scattering (SANS)

SANS measurement was carried out using QUOKKA SANS instrument at the ANSTO (Sydney, Australia)29. Two detector positions (L = 1.3 m and L = 12 m) are used with a Q range of 0.0055715 to 0.714133 / Å. No significant neutron activation of the samples was found after the experiment30. SANS data were fitted using Guinier-Porod law and a combination of a Guinier-Porod law and these details can be found elsewhere31,32. The theory of SANS is provided in the Supplementary section.
Results and discussion

The novelty of our study lies in determining the corrosion resistance characteristics of iron made by traditional methods developed by Indian tribes called ‘Agaria’, by using state-of-the-art analytical methods. The ancient iron was collected from the Agaria tribe members, who no longer continue the iron-making tradition. The iron sample was manufactured using ancient technology using a typical bloom furnace. Construction of this type of furnace is well described in the article published by Juleff et al. (1996)2. Metal products from an ancient bloom furnace are very rare to find and pieces of evidence of furnaces have been destroyed due to weathering actions. It has been suggested in the literature that Agaria tribe’s ancient iron-making technology using bloom furnace was in place even before 1200 AD, but the exact date is not available33.

The initial step in this study involved collection of the iron samples from tribes (Suppl. Figure 1 (a) and (b)) and iron ores which were used in making ancient irons by tribes from the forest of Aamadandh, Korba, Chhattisgarh, India.
Figure 1
figure 1

(a) FESEM images of (a) the morphology of the corrosion product film formed on the surface of iron made through ancient Indian metallurgical methods; (b) flake formation in the film.
Full size image

We have collected and analysed ore samples from the same region (see Suppl. Figure 1 (c)). In the iron making process, these ores were placed inside the bloom furnace. Bowl-shape bloom furnaces were usually built below the ground level by digging a pit (typically of cylindrical shape) with dimensions of about 800 mm in height and 200 mm in diameter. The shaft of the furnace was constructed below the 600 mm mark. The construction design of the furnace was completed by a bowl-shaped hearth with a typical diameter of about 240 mm and a depth of about 100 mm. The bowl-shaped hearth had a hole for tapping out slag (i.e. removing slag from the furnace), which is the waste product of the iron making process. Images of final iron products and slags (by Agaria tribe’s traditional iron-making process) collected from the site are shown in Fig. 1 (a). The placement of the furnace below the ground level was designed to reduce the excess air inside the furnace. It must be highlighted that air blowing was used to maintain the temperature at around 1150 °C. Iron-making was designed to take around 5–6 h/kg iron production and semi-fused mass of sponge iron block came out at the end with a significant amount of slag. The slags that remained in the final iron block were removed using forging technique21.

Morphological analysis of corrosion product layer of the iron sample (see Fig. 1 (a)) was conducted using field-emission scanning electron microscope (FESEM). Figure 1 (a) reveals the formation of a thick corrosion product layer on the top of the iron with obvious cracks. It is evinced from Fig. 1a that cracks were in the order of micrometres in size and the majority of the cracks are in the range of ~ 4–5 µm, and a few large cracks were also observed. At the surface areas with a thicker surface film, less cracks occurred. Flake formation is noticed in the FESEM image (Fig. 1b) which is a common characteristic of atmospheric corrosion. It is evident from earlier studies that fine flakes are the characteristic for atmospheric corrosion process whereas coarse flakes resemble marine corrosion34, which is clearly evident from Fig. 1b.

The corrosion product film is known to be composed of small-size particles of different phases, as evident later in this study. We anticipate that the particle-precipitation phenomenon led to formation of the phases through nucleation and growth that are found in corrosion product film34. Nucleation of the initial phase (of corrosion product) is known to assist in the nucleation of another phase by acting as a seed. We believe that phenomenon would have led to the formation of a thick corrosion layer35. Cox et al. (1994) suggested the importance of pre-existing oxide layer for nucleation of secondary phase particles or new oxide phases during the formation of passivation layer35. Therefore, we hypothesize that the nucleation and growth of corrosion product layer are also supported by slag phase particles present in the iron. We found that it is difficult to predict the exact phenomenon of growth of corrosion product film formation in the uncontrolled corrosion experiments (like in this study) because the nucleation and growth is a complex phenomenon and depends on various factors such as substrate crystallographic nature, interface between the substrate and layer, specific free surface energy, adhesion energy, etc36. Corrosion product film is found composed of random shape particles as depicted in FESEM analysis (Fig. 2a,b). Mild heterogeneities in morphological features confirmed the less aggressive atmospheric corrosion34.
Figure 2
figure 2

(a) FESEM image of the corrosion product film found on the top of the iron made by Agaria tribe (b) FESEM image depicting the inhomogeneity in the shape and size of the corrosion phase particles that are found in corrosion product film (c) represent the small-angle neutron scattering (SANS) scattering data collected from the iron made through ancient Indian metallurgical methods, scattering data is fitted with the Guinier-Porod model.
Full size image

Further features of corrosion product film was characterised using small-angle neutron scattering (SANS) (Fig. 2c). Scattering data were fitted with a shape-independent model called Guinier-Porod model31 which revealed the formation of a rough surface with an interlinked structure on the top of the iron, with a Porod exponent of m = 3 (Fig. 2c). Additionally, shape-independent model fitting was performed to investigate the surface features of the corrosion product film using SANS. These results are well-corroborated with the FESEM analysis which exhibited the formation of a rough heterogeneous corrosion product film. The compositional analysis of the corrosion product film is performed by FESEM-energy dispersive spectroscopy (FESEM-EDS) which confirmed the presence of Fe, C, O, Si and Ca (Fig. 3a,b-EDS mapping).
Figure 3
figure 3

(a) FESEM images of corrosion product film formed on the surface of iron made through ancient Indian metallurgical methods (b) EDS mapping of Fe, O, C, Si, Ca without P (dots appeared in P mapping are due to instrument noise).
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Earlier proposed theories discard the utilization of limestone in ancient Indian iron-making furnaces because of the absence of CaO in slag which led to the high P retainment in iron37. On the contrary, the presence of P and Ca is confirmed by the STEM-EDS analysis of collected slags (Suppl. Figure 2). It is worth mentioning that the removal of P from the iron (to slag) depends on various factors, such as the presence of CaO and other basic compounds. This theory is well corroborated with the principles of slag thermodynamics and chemistry that suggest higher P removal with CaO than the FeO38. It is noteworthy that the P can lead to the formation of liquid phosphides at the grain boundaries and cause cracking during mechanical working such as forging39,40. Therefore, in order to clarify further that whether CaO was added intentionally during the iron making process to remove P from iron, we have characterized the iron ores, which were used in iron making practice and collected from the same site, using XRD. The XRD analysis (Suppl. Figure 3) confirms the presence of hematite, kaolinite, and anatase phases and excludes the presence of Ca in the ores. We speculate that the addition of Ca to the iron was through the clay used for preparing the bottom part of the furnace along with the fine coal dust. It is worth mentioning that a slanting platform made from clay-coated bamboo was used for sliding the charge into the furnace and therefore some Ca addition is also possible from this operation. However, the probability of introducing Ca into the melt from the bottom portion of the furnace is more likely compared to the slanting platform used for the charge sliding19. Figure 4 and Suppl. Figure 4 elucidate the outwards segregated diffusion of slag constituents such as Si and Ca. (Slag constituents are expected to come from the iron ore used in the iron-making by tribes as discussed later with the help of XRD of iron ore analysis, which confirms the presence of kaolinite.) This phenomenon led to the localized dissolution of Fe in the corrosion layer. Metal dissolution is considered as anodic reaction whereas oxygen reduction is known as a cathodic reaction (see the reaction (1)). Significant depletion of Fe is observed which affirmed the anodic dissolution. This depletion of Fe causes the diffusion of Si and Ca that were found segregated at few locations. It is also noteworthy that the diffusion of Si and Ca from the surface of iron into the corrosion layer (where particularly Fe was found depleted) is attributed to the raising oxygen potential due to the combined effect of formation of FeO layer and electrochemical erosion41. Buffered surface of iron discards the hydrogen reduction as prominent cathodic reaction such as:
2Fe + H2O +32O2=2FeOOH
(1)
Figure 4
figure 4

Cross-sectional FESEM images of corrosion product film formed on iron made through ancient Indian metallurgical methods depicting (a) interface formation between corrosion product film and substrate (iron) (b) interface formation between corrosion product film and substrate (iron) at different location and in different (view) orientation depicting the small holes on film.
Full size image

It is also noteworthy that initially formed lepidocrocite is further transformed to goethite and spinel iron oxide-hydroxide. This continuous transformation resulted in the formation of maghemite and hematite (as observed). Hematite is the most stable phase among these two phases due to highly negative Gibbs free energy42. FESEM-EDS mapping (see Fig. 3 and Suppl. Figure 4) showed the formation of a slag layer consisting of Ca and Si with Fe and O in the corrosion layer. It was well documented in earlier research that ion-exchange driven non-stoichiometric removal of cations drives the inward diffusion of hydrogen species (H3O+, H2O)43. Atmospheric corrosion can lead to this condition that offers outward diffusion of cations, while alkali ions in this study are either dissolved or diffused inwards. We, therefore, speculate that Fe acted as an anodic cell due to the formation of a localized galvanic cell composed of Fe as an anode, whereas slag phases acted as the cathode and appeared to be dissolved at certain areas of the corrosion layer. This localized chemical imbalance led to changes in the chemical potential which drove the diffusion of Si and Ca and assisted in their segregation43.

The presence of slag phases in the corrosion layer of iron was further confirmed from the EDS spectra from various locations at the surface (see Suppl. Figure 4). Ca, Si, Al were detected along with Fe, C, and O. Localized dissolution of Fe and non-homogenous distribution of slag elements were further noticed from the EDS data, confirming our earlier argument of localized cell formation that leads to diffusion of ions. Formation of bubbles in the iron was also noticed, which is a typical characteristic of iron made in bloom furnace44. EDS analysis confirms the presence of Fe, O, Si, Al, Ca, and Ti; without any presence of P. It is speculated that the surface segregation of Ti would have played an important role in making the Agaria tribe’s iron corrosion resistance. A recent study conducted with TiO2-clay composite also confirmed the hydrophobic nature of this composite45. Therefore, we hypothesize that the surface segregation of Ti would have supported in the formation of the protective passive-film on iron and helped Agaria tribe’s iron to be corrosion resistant. The source of Ti addition in iron was iron ores which were used in the iron-making process and contain 4 wt% of ilmenite (see Suppl. Fig. 3). Therefore, it is believed that Ti was not alloyed in iron intentionally. We have carried out STEM-EDS (elemental mapping) (see Suppl. Figure 2) of slag, which is a by-product of the iron-making process. Ti was not evidenced from the STEM-EDS mapping. Hence, it is believed that Ti was added in the iron material, namely from iron ore with anatase that was used in the iron-making process by tribes. The role of Ti in the prevention of metal corrosion is well described46.

In order to investigate the interface between the iron and the corrosion film, cross-sectional FESEM was performed (see Fig. 4a,b) and shows the formation of a coherent interface with a thick corrosion layer.

Cross-sectional FESEM images confirmed (within the resolution limit of FESEM) the absence of pores and pits at the interface and showed coherent interface as one of the paramount factors responsible for high corrosion resistance of the ancient iron. It is worth mentioning that defects such as holes were noticed on the top of the corrosion product layers. However, we could not identify from the FESEM data whether these holes extended to the iron surface. We have therefore employed neutron tomography as mentioned later.

Correlative microscopic and spectroscopic techniques are found useful for in-depth analysis of corrosion products formed on the ancient iron artefacts47,48. In this study, corrosion layer was analyzed using GIXRD. The formation of hematite was exhibited in the diffraction pattern obtained by GIXRD analysis (Suppl. Figure 5). GIXRD analysis of corrosion layer has confirmed the presence of hematite (Fe2O3), quartz (SiO2) and calcite (CaCO3), while Rietveld analysis proved the formation of Fe2O3 (70wt%) along with SiO2 (19wt%) and CaCO3 (11wt%) (Suppl. Figure 5). These protective corrosion products might have led to high corrosion resistance47. However, the signal to noise ratio was poor and the ancient iron sample was thus further characterized using neutron diffraction. In fact, neutrons are electrically neutral and can penetrate deeper inside the ancient iron. Neutron diffraction confirmed the presence of iron (Fe), cementite (Fe3C, cohenite in mineralogy) and maghemite (ϒ-Fe3O4), but a few diffraction peaks remained unclassified. Rietveld analysis of neutron diffraction pattern confirmed the association of ~ 92 wt% of Fe with 1.1 wt% of Fe3O4 and 1.7 wt% of Fe3C (Suppl. Figure 6). Unidentified phases were about 5 wt%. The unidentified peak positions (at 2θ°) were 40.62°, 42.38°, 64.49°, 76.86°, 96.73°, and 115.34°. Neutron diffraction data confirmed the formation of BCC α-Fe. Detection of BCC α-Fe made us believe that Agaria’s furnace was suitable to be operated below 1000 °C.
Figure 5
figure 5

(a) FESEM images of corrosion product film formed on the surface of iron made through ancient Indian metallurgical methods exhibits (b) EDS mapping of Fe, O, Si, P (dot points indicate the instrument noise), Ca, Al and, C.
Full size image
Figure 6
figure 6

Neutron tomographic images depicting the volume of the pores that are formed on the iron made through ancient Indian metallurgical methods in (a) un-hammered iron (b) hammered (forged) iron.
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The corrosion product film was found composed of hematite, maghemite and slag phases as confirmed by GIXRD and neutron diffraction. Maghemite is expected to be formed underneath these layers as detected in neutron diffraction analysis (Suppl. Figure 6), which is a technique used for bulk-material characterization. Among these iron oxide polymorphs, hematite is the most stable iron oxide with Gibbs free energy of formation of –744.4 ± 1.3 kJ mol-1 whereas maghemite is considered a less stable polymorph with Gibbs free energy of formation of –731.4 ± 2.0 kJ mol-1 at 298 K and 1 bar pressure49. It is believed that the formation of oxides could have occurred through internal oxidation mechanism50. Internal oxidation allows oxygen to enter into the material and this diffusion process leads to the sub-surface precipitation of oxides of alloying elements. The concentration of these solute elements is imperative and decides the transition between internal to external oxidation. We have noticed segregated Si in EDS mapping, which indicates the long-range diffusion of Si from the matrix (parent metal). It is also worth to note that this diffusion makes the chemical potential of Si (in the oxide zone) lower than the surrounding matrix and is considered the driving force for the Si diffusion. However, in EDS mapping analysis, we have noticed the widespread Si and Ca with O which confirms the diffusion of these elements from the grain/sub-grain of the matrix (iron) to the oxygen diffusion zone. We have confirmed the formation of several oxide zones (multilayer oxide formation), which is a typical characteristic of internal oxidation, by using various techniques (GIXRD, neutron diffraction, XPS, synchrotron and neutron tomography). Hematite is the most stable phase among the iron oxides49 and maghemite is therefore expected to be transformed into hematite thermodynamically. During the initial stage of oxidation of Fe, outward diffusion of Fe leads to the vacancy formation which later accumulates as cavities. Cavities are responsible for the formation of micro-channels that are known to pave the path for fast oxidation51. Corrosion film is found with cracks which seem to be formed during the oxide scale growth on Agaria tribe’s-iron as illustrated by Neff et al. (2005)47. These cracks are expected to provide a path for continuous oxidation of Fe till the passive calcite and quartz phases are formed. It is noteworthy that precipitate phase formation depends on the localized presence of ions (e.g. Ca2+, CO32-, etc.)47. However, the stability of these phases depends vastly on the atmosphere52.

Earlier studies have delineated the formation of crystalline iron hydrogen phosphate hydrate (FePO4·H3PO4·4H2O), α-, γ-, δ-FeOOH and magnetite in the case of Delhi iron pillar whereas Gupta dynasty’s Eran iron37,53 has shown the presence of γ -FeOOH and δ –FeOOH. It is worth mentioning that the researchers were unable to detect phosphate phase (formed due to the phenomenon called micro-segregation) using Mössbauer spectroscopy which was identified using micro-XRD54,55. In this study, the presence of hematite and maghemite were identified as scale phases on the top of the iron made by the Agaria tribe. P is found present in slag whereas the presence of P in iron was not detected within the limit of the analytical techniques used in this study. On the basis of this result, we speculate application of lime and other basic compounds during the iron making process which would have led to the transfer P to slag.

The presence of slag elements such as Si, Ca (along with the Fe and O) in the scale grown on iron made by Agaria tribes is confirmed by XPS (Suppl. Figure 7). Oxides of these elements (Fe, Si and Ca) are generally known to be cathodic in nature and do not allow current to flow through and help in protecting iron from corrosion. These observations depict cathodic prevention as one of the reasons for the high corrosion resistance of the iron made through ancient Indian traditional methods. Second phase particles (such as slags and unreduced oxides) act as cathodic sites whereas metals such as Cr and Mn in the pure iron act as anodic sites. Second phase particles are known to form passive layers16.
Figure 7
figure 7

Neutron tomographic images of corrosion product film thickness that are formed on the iron made through ancient Indian metallurgical methods in (a) un-hammered iron (b) hammered (forged) iron.
Full size image

The XPS survey scan does not show the presence of Al, which was noticed in FESEM-EDS (Suppl. Figure 4). This postulates that the Al-containing slag phase might have formed close to these layers and was beyond the resolution of the XPS instrument. This also means that the slag phases were formed at different depths within the corrosion scale formed on iron. It has been postulated earlier that mobile oxygen ions lead to the formation of a slag-oxide interface with the formation of large amounts of iron oxide products. In this study, iron corrosion products along with the slag phases are detected in the film formed on the top of the iron. We therefore speculate that atmospheric O played an important role in building up the inhibitive layers on the iron surface. It is observed from the FESEM-EDS mapping figures (Fig. 5a,b) that Si, Ca, and Al—slag phase elements are not segregated but randomly distributed and can be therefore considered as mobile elements at the surface of the corrosion layer. Also, Fe was found depleted in a few instances.

Similarly, these slag elements also remained non-depleted which indicates formation of microscale galvanic cells in the corrosion product layer. This highlights the slag-phases’ role in maintaining the cathodic passivity that provides corrosion protection to the underlying steel56.

We have also analysed the un-hammered and hammered iron with the help of neutron tomography (Fig. 6a,b) (see suppl. Figure 8 for the 3-D projection of different orientation views), which confirmed the presence of small volume pores. Hammering (forging) is known to remove the impurities (such as slag, pores, etc.) and to consolidate the pores available in materials. The consolidation of pores in hammered iron can also be seen (Fig. 6b). The values of the pores’ area (in 3-D) is found consolidated and most of them are found in the region of less than 1mm2 in hammered iron, whereas un-hammered iron has exhibited the formation of large size pores, mostly found concentred within the region of 1 to ~ 3.5mm2 (see Suppl. Figure 9 (a)). The majority of pores’ volume that is depicted in un-hammered and hammered iron are found below ~ 0.002 mm3 and below ~ 0.04 mm3 respectively (see Suppl. Figure 9(b)). The mathematical value of the pores’ volume in hammered iron is found higher than the un-hammered iron. On the other hand, the quantities of the pores inside the iron are also found higher in hammered iron. We speculated that this phenomenon could be attributed to the dislodgement of the inclusions (detachment of slag phases) from iron during the hammering (forging) operation. However, the superlative corrosion resistance of hammered iron could be associated with the thick passive corrosion product film formation on the top of the iron as delineated earlier in this article which made iron remained inert. It is also noteworthy that the porosity obtained in neutron tomography was present inside the iron. A comparison of histograms of equi-diameters and length of the pores are shown in Suppl. Figure 10 (a) and (b). We also analysed the pores present in iron made by Agaria tribes using synchrotron X-ray tomography. However, the low transmission and scattering through the sample material altered the quality of the data by introducing strong artefacts and prevented accurate porosities analysis. On the other hand, the neutron tomography images were free from these artefacts (See Suppl. Figure 11).

In order to further relate the high corrosion resistance to the presence of a protective surface film, we have carried out the analysis of the corrosion product film thickness at the hot hammered (forged) and un-hammered (un-forged) iron. It is noticed that the thicker corrosion product film was present on the hammered iron compared to the un-hammered iron. As illustrated earlier, metal-working process (mechanical metallurgical operations) called hot hammering (forging) is known for removing impurities such as slags and consolidating internal pores, we hypothesize that the metal-working operation carried out by Agaria tribes has a great impact on making the iron corrosion resistance. We hypothesize that thick corrosion product layer, which is known to protect the substrate from further corrosion, formed on hammered iron and consolidated pores formation can play a vital role in iron’s corrosion resistance (See Fig. 7 (a) and (b); and Suppl. Figure 12).
Conclusions

In summary, we have investigated the iron manufactured using the traditional iron making process followed by Indian tribes known as ‘Agaria’. Our results clearly demonstrated the importance of analytical techniques operating at various length scale, in proposing the mechanisms of corrosion resistance including other properties of archaeomaterials. We have proposed the hypothesis for the excellent corrosion resistance of Agaria-tribe’s iron. In this article, the role of iron ore’s composition in the formation of corrosion products film (protective film against corrosion) on the iron surface is highlighted. Hematite and maghemite along with slag phases were depicted by using GIXRD and neutron diffraction as the corrosion products on the iron surface. The investigations also highlighted the essential role of a multi-analytical techniques approach in the analysis of archaeomaterials. These findings were identified as the primary reasons for the high corrosion resistance of the iron. Unlike Delhi iron pillar, P (which was found in the iron of Gupta’s period) is not found in the corrosion product layer formed on the iron (irrespective of hammered/un-hammered) manufactured by Agaria tribes within the limit of the correlative analytical techniques that are used in this study. However, the scope of further investigation still prevails. The effect of metal-working operation (followed by Agaria tribes) in making the iron corrosion resistance is also demonstrated. We hypothesize that the metal working operation known as hot hammering, has led to the consolidation of internal pores (which were present in iron before pre-treatment) and removal of inclusions such as slag. Thicker passive corrosion product film, which is known to be protective on the Agaria tribes’ iron, is observed for hammered iron than un-hammered iron. Hence, it is postulated that consolidation of pores, expected removal of inclusions and thick passive corrosion product layer formation have supported the iron in attaining the excellent corrosion resistance. We anticipate that our findings will assist in uncovering the hidden knowledge pertaining to the archeomaterials’ (ferrous) degradation phenomenon, particularly in Japan and the South Asian region’s, with a history of close collaboration in ferrous metallurgy.
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Acknowledgements

We acknowledge technical assistance and facilities of the Curtin University Microscopy & Microanalysis Facility and the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy, Characterisation & Analysis (CMCA), The University of Western Australia (Prof. Martin Saunders), both of which are partially funded by the University, State, and Commonwealth Governments, the WA X-ray Surface Analysis Facility, funded by the Australian Research Council LIEF grant (LE120100026)). The authors are grateful for the support from the School of Molecular and Life Sciences (Mr. Peter Chapman), Curtin University. The authors thank Dr. Maxim Avdeev (Echidna, ANSTO) and Dr. Anton Maksimenko and Dr. Daniel Hausermann (IMBL, Australian Synchrotron) for technical assistance with neutron diffraction and synchrotron X-ray CT measurements, respectively. DD acknowledges Curtin University for a Curtin International Postgraduate Research Scholarship. DD thanks the Australian Institute of Nuclear Science and Engineering (AINSE) for their kind financial support under the AINSE Postgraduate Research Award (PGRA) scheme to enable this work. Authors acknowledge the Australian Nuclear Science and Technology Organization (ANSTO), Sydney (Quokka, Dingo (P6202) and Echidna (P6202) beamlines) and the Australian Synchrotron, Melbourne (IMBL beamline) (P 12293 and P 11768) for allowing us to use the respective beamlines. The authors, particularly DD, greatly acknowledge the support and materials received from local Agaria tribes for this research. D. D. would like to dedicate this article to Professor S. P. Mehrotra (Visiting Professor, Indian Institute of Technology, Gandhinagar), and late Professor R. Balasubramaniam (Professor of Indian Institute of Technology, Kanpur), who was one of the pioneers in the field of archaeometallurgy, and to his teachers.
Author information
Authors and Affiliations

    Curtin Corrosion Centre, WA School of Mines: Minerals, Energy and Chemical Engineering, Faculty of Science and Engineering, Curtin University, Perth, Australia

    Deepak Dwivedi & Kateřina Lepková

    Australian Centre for Neutron Scattering, Australian Nuclear Science and Technology Organisation (ANSTO), Lucas Heights, NSW, 2234, Australia

    Jitendra P. Mata & Filomena Salvemini

    John de Laeter Centre, Curtin University, Perth, Australia

    Matthew R. Rowles

    School of Molecular and Life Sciences (Chemistry), Curtin Institute for Functional Molecules and Interfaces, Faculty of Science and Engineering, Curtin University, Perth, Australia

    Thomas Becker

Contributions

D.D. collected the samples, proposed and designed the experiments. D.D. completed all experiments in this project, interpreted results, wrote the main manuscript and prepared supporting information materials including figures. D.D. and M.R. performed XRD and analyzed diffraction data. D.D. and F.S. performed neutron tomography and discussed the results. D.D. and J.M. performed SANS and discussed data. D.D., T.B. and K.L. discussed the results. D.D., T.B. and K.L. reviewed the final manuscript version. All authors participated in the discussion.

[[https://www.jstor.org/page-scan-delivery/get-page-scan/24110622/0]]

[[https://www.sciencedirect.com/science/article/abs/pii/S0010938X00000457|Characterization of Delhi iron pillar rust by X-ray diffraction, Fourier transform infrared spectroscopy and Mössbauer spectroscopy]]

Abstract

Rust samples obtained from the region just below the decorative bell capital of the Delhi iron pillar (DIP) have been analyzed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Mössbauer spectroscopy. The identification of iron hydrogen phosphate hydrate in the crystalline form by XRD was unambiguous. Very weak diffraction from the oxyhydroxides/oxides of iron was observed indicating that these phases are most likely to be present in the amorphous form in the rust. The present XRD analysis of rust obtained from an inaccessible area of the DIP has also been compared with earlier analyses of DIP rust obtained from regions accessible to the public. FTIR indicated that the constituents of the scale were γ-, α-, δ-FeOOH, Fe3−xO4 and phosphate, and that the scale was hydrated. The unambiguous identification of the iron oxides/oxyhydroxides in the FTIR spectrum implied that they are present in the amorphous state, as XRD did not reveal these phases. The FTIR results have also been compared with earlier FTIR spectroscopic results of atmospheric rust formation. Mössbauer spectroscopy indicated that the rusts contained γ-FeOOH, superparamagnetic α-FeOOH, δ-FeOOH and magnetite, all in the amorphous form. The Mössbauer spectrum also confirmed that iron in the crystalline iron hydrogen phosphate hydrate, whose presence was confirmed by XRD, was in the ferric state indicating that it was a stable end corrosion product.

[[https://en.wikipedia.org/wiki/Iron_pillar_of_Delhi|Iron pillar of Delhi]]


Iron pillar of Delhi
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Iron pillar of DelhiQtubIronPillar.JPG
The Iron pillar of Delhi
Iron pillar of Delhi is located in India
Iron pillar of Delhi
Location in India
Coordinates	28°31′28.76″N 77°11′6.25″ECoordinates: 28°31′28.76″N 77°11′6.25″E
Location	Qutb complex at Mehrauli in Delhi, India
Designer	Chandragupta II
Material	Rust-resistant Iron
Height	7.21 m (23 ft 8 in)
Completion date	5th century
Dedicated to	Vishnu

The iron pillar of Delhi is a structure 7.21 metres (23 feet 8 inches) high with a 41-centimetre (16 in) diameter that was constructed by Chandragupta II (reigned c. 375–415 CE), and now stands in the Qutb complex at Mehrauli in Delhi, India.[1][2] It is famous for the rust-resistant composition of the metals used in its construction. The pillar weighs more than 6 tonnes and is thought to have been erected elsewhere, perhaps outside the Udayagiri Caves,[3] and moved to its present location by Anangpal Tomar in 11th century.
Contents

    1 Physical description
    2 Inscriptions
        2.1 Inscription of King Chandra or Chandragupta II
            2.1.1 Inscription
            2.1.2 Studies
            2.1.3 Issuance
            2.1.4 Text
        2.2 Samvat 1109 inscription
    3 Original location
        3.1 Relocation
    4 Scientific analysis
    5 Evidence of a cannonball strike
    6 See also
    7 References
        7.1 Bibliography
    8 External links

Physical description
The iron pillar stands within the courtyard of Quwwat-ul-Islam Mosque

The height of the pillar, from the top to the bottom of its base, is 7.21 m (23 ft 8 in), 1.12 m (3 ft 8 in) of which is below ground. Its bell pattern capital is 306 mm (12 in). It is estimated to weigh more than six tonnes (13,228 lb).[4] The pillar has attracted the attention of archaeologists and materials scientists because of its high resistance to corrosion and has been called a "testimony to the high level of skill achieved by the ancient Indian iron smiths in the extraction and processing of iron".[5][6] The corrosion resistance results from an even layer of crystalline iron(III) hydrogen phosphate hydrate forming on the high-phosphorus-content iron, which serves to protect it from the effects of the Delhi climate.[5]
Inscriptions

The pillar carries a number of inscriptions of different dates, some of which have not been studied systematically despite the pillar's prominent location and easy access.[citation needed]
Inscription of King Chandra or Chandragupta II
Detail showing the inscription of King Chandragupta II

The oldest inscription on the pillar is that of a king named Chandra (IAST: Candra), generally identified as the Gupta emperor Chandragupta II.[7]
Inscription

The inscription covers an area of 2′9.5″ × 10.5″. The ancient writing is preserved well because of the corrosion-resistant iron on which it is engraved. However, during the engraving process, iron appears to have closed up over some of the strokes, making some of the letters imperfect.[8]

It contains verses composed in Sanskrit language, in shardulvikridita metre.[9] It is written in the eastern variety of the Gupta script. The letters vary from 0.3125″ to 0.5″ in size, and resemble closely to the letters on the Allahabad Pillar inscription of Samudragupta. However, it had distinctive mātrās (diacritics), similar to the ones in the Bilsad inscription of Kumaragupta I.[10] While the edges of the characters on the Allahabad inscription are more curved, the ones on the Delhi inscription have more straight edges. This can be attributed to the fact that the Allahabad inscription was inscribed on softer sandstone, while the Delhi inscription is engraved on the harder material (iron).[11] The text has some unusual deviations from the standard Sanskrit spelling, such as:[10]

    pranśu instead of praṃśu: the use of dental nasal instead of anusvāra
    mūrtyā instead of mūrttyā: omission of the second t
    kīrtyā instead of kīrttyā: omission of the second t
    śattru instead of śatru (enemy): an extra t

Studies

In 1831, the East India Company officer William Elliott made a facsimile of the inscription. Based on this facsimile, in 1834, James Prinsep published a lithograph in the Journal of the Royal Asiatic Society of Great Britain and Ireland. However, this lithograph did not represent every single word of the inscription correctly.[12] Some years later, British engineer T. S. Burt made an ink impression of the inscription. Based on this, in 1838, Prinsep published an improved lithograph in the same journal, with his reading of the script and translation of the text.[12][13]

Decades later, Bhagwan Lal Indraji made another copy of the inscription on a cloth. Based on this copy, Bhau Daji Lad published a revised text and translation in 1875, in Journal of the Bombay Branch of the Royal Asiatic Society. This reading was the first one to correctly mention the king's name as Chandra. In 1888, John Faithfull Fleet published a critical edition of the text in Corpus Inscriptionum Indicarum.[13]

In 1945, Govardhan Rai Sharma dated the inscription to the first half of the 5th century CE, on paleographic grounds.[14] He observed that its script was similar to the writing on other Gupta-Era inscriptions, including the ones discovered at Bilsad (415 CE), Baigram (449 CE), and Kahanum (449 CE).[11] R. Balasubramaniam (2005) noted that the characters of the Delhi inscription closely resembled the dated inscriptions of Chandragupta II, found at Udayagiri in Madhya Pradesh.[15]
Issuance
The name "Candra" (Gupta allahabad c.svgGupta allahabad ndr.jpg) on the iron pillar of Delhi, thought to represent Chandragupta II. Gupta script: letter "Ca" Gupta allahabad c.svg, followed by the conjunct consonant "ndra" formed of the vertical combination of the three letters n Gupta allahabad n.svg d Gupta allahabad d.svg and r Gupta ashoka r.svg.[16][17]

The inscription is undated, and contains a eulogy of a king named Candra, whose dynasty it does not mention.[10] The identity of this king, and thus the date of the pillar, has been the subject of much debate. The various viewpoints about the identity of the issuer were assembled and analyzed in a volume edited by M. C. Joshi and published in 1989.[18]

The king is now generally identified with the Gupta King Chandragupta II.[19] This identification is based on several points:

    The script and the poetic style of the inscription, which point to a date in the late fourth or early fifth century CE: the Gupta period.[18]
    The inscription describes the king as a devotee of the God Vishnu, and records the erection of a dhvaja ("standard", or pillar) of Vishnu, on a hill called Viṣṇupada ("hill of the footprint of Viṣṇu").[20] Other Gupta inscriptions also describe Chandragupta II as a Bhagavata (devotee of Vishnu).[10] The names of the places mentioned in the inscription are also characteristic of the Gupta Era. For example, Dakṣiṇa Jalanidhi (the Indian Ocean) and Vaṅga (the Bengal region).[20]
    The short name 'Candra' is inscribed on the archer-type gold coins of Chandragupta II, while his full name and titles appear in a separate, circular legend on the coin.[15]
    A royal seal of Chandragupta's wife Dhruvadevi contains the phrase Śrī Viṣṇupada-svāmī Nārāyaṇa ("Nārāyaṇa, the lord of the illustrious Viṣṇupada").[21]

As the inscription is a eulogy and states that the king has abandoned the earth, there has been some discussion as to whether it is posthumous, i.e. whether King Chandra was dead when the record was created. Dasharatha Sharma (1938) argued that it was non-posthumous.[22] According to B. Chhabra and G. S. Gai, the inscription states that the king's mind is "fixed upon Vishnu with devotion", and therefore, indicates that the king was alive at the time. They theorize that it may have been recorded when Chandragupta II abdicated his throne, and settled down as a vanaprastha (retiree) in Viṣṇupada.[10]
Text
Bankelal's 1903 tablets

Following is the Roman script transliteration of the text:[23]

    Yasy odvarttayah-pratīpamurasā śattrun sametyāgatan Vańgeshvāhava varttinosbhilikhitā khadgena kīrttir bhuje
    Tirtvā sapta mukhāni yena samare sindhor jjitā Vāhlikāyasyādya pyadhivāsyate jalanidhir vviryyānilair ddakshinah
    Khinnasy eva visrijya gām narapater ggāmāśritasyaetrām mūr(t)yā karmma-jitāvanim gatavatah kīrt(t)yā sthitasyakshitau
    Śāntasyeva mahāvane hutabhujo yasya pratāpo mahānnadhayā pyutsrijati pranāśista-ripor Yyatnasya śesahkshitim
    Prāptena sva bhuj ārjitan cha suchiran ch aikādhirājayam kshitau chandrāhvena samagra chandra sadriśīm vaktra-śriyam bibhratā
    Tenāyam pranidhāya bhūmipatinā bhāveva vishno (shnau) matim prānśurvisnupade girau bhagavato Vishnuordhidhvajah sthāpitah

J. F. Fleet's 1888 translation is as follows:[24]

    (Verse 1) He, on whose arm fame was inscribed by the sword, when, in battle in the Vanga countries (Bengal), he kneaded (and turned) back with (his) breast the enemies who, uniting together, came against (him); – he, by whom, having crossed in warfare the seven mouths of the (river) Sindhu, the Vahlikas were conquered; – he, by the breezes of whose prowess the southern ocean is even still perfumed; –
    (Verse 2) He, the remnant of the great zeal of whose energy, which utterly destroyed (his) enemies, like (the remnant of the great glowing heat) of a burned-out fire in a great forest, even now leaves not the earth; though he, the king, as if wearied, has quit this earth, and has gone to the other world, moving in (bodily) from to the land (of paradise) won by (the merit of his) actions, (but) remaining on (this) earth by (the memory of his) fame; –
    (Verse 3) By him, the king, attained sole supreme sovereignty in the world, acquired by his own arm and (enjoyed) for a very long time; (and) who, having the name of Chandra, carried a beauty of countenance like (the beauty of) the full-moon,-having in faith fixed his mind upon (the god) Vishnu, this lofty standard of the divine Vishnu was set up on the hill (called) Vishnupada.

Due to the tablets installed on the building in 1903 by Pandit Banke Rai, the reading provided by him enjoys wide currency. However, Bankelal's reading and interpretation have been challenged by more recent scholarship. The inscription has been revisited by Michael Willis in his book Archaeology of Hindu Ritual, his special concern being the nature of the king's spiritual identity after death. His reading and translation of verse 2 is as follows:[25]

    [khi]nnasyeva visṛjya gāṃ narapater ggām āśritasyetarāṃ mūrtyā karrmajitāvaniṃ gatavataḥ kīrtyā sthitasya kṣitau [*|]
    śāntasyeva mahāvane hutabhujo yasya pratāpo mahān nādyāpy utsṛjati praṇāśitaripor yyatnasya śeṣaḥ kṣitim [||*]

The Sanskrit portion given above can be translated as follows:[25]

    The residue of the king's effort – a burning splendour which utterly destroyed his enemies – leaves not the earth even now, just like (the residual heat of) a burned-out conflagration in a great forest.
    He, as if wearied, has abandoned this world, and resorted in actual form to the other world – a place won by the merit of his deeds – (and although) he has departed, he remains on earth through (the memory of his) fame (kīrti).

Willis concludes:

    Candragupta may have passed away but the legacy of his achievement is so great that he seems to remain on earth by virtue of his fame. Emphasis is placed on Candragupta's conquest of enemies and the merit of his deeds, ideas which are also found in coin legends: kṣitim avajitya sucaritair divaṃ jayati vikramādityaḥ, i.e. "Having conquered the earth with good conduct, Vikramāditya conquered heaven".[26] The king's conquest of heaven combined with the description of him resorting to the other world in bodily form (gām āśritasyetarāṃ mūrtyā), confirms our understanding of the worthy dead as autonomous theomorphic entities.[25]

Samvat 1109 inscription

One short inscription on the pillar is associated with the Tomara king Anangpal, although it is hard to decipher.. Alexander Cunningham (1862–63) read the inscription as follows:[27]

    Samvat Dihali 1109 Ang Pāl bahi [Translation:] In Samvat 1109 [1052 CE], Ang [Anang] Pāl peopled Dilli

Based on this reading, Cunningham theorized that Anangpal had moved the pillar to its current location while establishing the city of Delhi. However, his reading has been contested by the later scholars. Buddha Rashmi Mani (1997) read it as follows:[27]

    Samvat Kinllī 1109 Aṅgapāla bādi [Translation:] Anangpal tightened the nail [iron pillar] in Samvat 1109

Original location

The pillar was installed as a trophy in building the Quwwat-ul-Islam mosque and the Qutb complex by Sultan Iltutmish in the 13th century.[28] Its original location, whether on the site itself or from elsewhere, is debated.[29][30]

According to the inscription of king Chandra, the pillar was erected at Vishnupadagiri (Vishnupada). J. F. Fleet (1898) identified this place with Mathura, because of its proximity to Delhi (the find spot of the inscription) and the city's reputation as a Vaishnavite pilgrimage centre. However, archaeological evidence indicates that during the Gupta period, Mathura was a major centre of Buddhism, although Vaishnavism may have existed there. Moreover, Mathura lies in plains, and only contains some small hillocks and mounds: there is no true giri (hill) in Mathura.[31]

Based on paleographic similarity to the dated inscriptions from Udayagiri, the Gupta-era iconography, analysis of metallurgy and other evidence, Meera Dass and R. Balasubramaniam (2004) theorized that the iron pillar was originally erected at Udayagiri.[15][32] According to them, the pillar, with a wheel or discus at the top, was originally located at the Udayagiri Caves.[33] This conclusion was partly based on the fact that the inscription mentions Vishnupada-giri (IAST: Viṣṇupadagiri, meaning "hill with footprint of Viṣṇu"). This conclusion was endorsed and elaborated by Michael D. Willis in his The Archaeology of Hindu Ritual, published in 2009.[34]

The key point in favour of placing the iron pillar at Udayagiri is that this site was closely associated with Chandragupta and the worship of Vishnu in the Gupta period. In addition, there are well-established traditions of mining and working iron in central India, documented particularly by the iron pillar at Dhar and local place names like Lohapura and Lohangī Pīr (see Vidisha). The king of Delhi, Iltutmish, is known to have attacked and sacked Vidisha in the thirteenth century and this would have given him an opportunity to remove the pillar as a trophy to Delhi, just as the Tughluq rulers brought Asokan pillars to Delhi in the 1300s.
Relocation

It is not certain when the pillar was moved to Delhi from its original location. Alexander Cunningham attributed the relocation to the Tomara king Anangpal, based on the short pillar inscription ascribed to this king.[27] Pasanaha Chariu, an 1132 CE Jain Apabhramsha text composed by Vibudh Shridhar, states that "the weight of his pillar caused the Lord of the Snakes to tremble". The identification of this pillar with the iron pillar lends support to the theory that the pillar was already in Delhi during Anangpal's reign.[35]

Another theory is that the relocation happened during the Muslim rule in Delhi. Some scholars have assumed that it happened around 1200 CE, when Qutb al-Din Aibak commenced the construction of the Qutb complex as a general of Muhammad of Ghor.[36]

Finbarr Barry Flood (2009) theorizes that it was Qutb al-Din's successor Iltutmish (r. 1210–1236 CE), who moved the pillar to Delhi.[27] According to this theory, the pillar was originally erected in Vidisha and that the pillar was moved to the Qutb complex, by Iltutmish when he attacked and sacked Vidisha in the thirteenth century.[37]
Scientific analysis
Details of the top of iron pillar, Qutb Minar, Delhi.

The iron pillar in India was manufactured 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.[38]

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[39] (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 iron pillar in Qutb Minar, c. 1905

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.[40]

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.[41]

The most critical corrosion-resistance agent is iron hydrogen phosphate hydrate (FePO4-H3PO4-4H2O) 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.[5] In 1,600 years, the film has grown just one-twentieth of a millimetre thick.[39]

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.[42][43] 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.[44][45] 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.[46] Further research published in 2009 showed that corrosion has developed evenly over the surface of the pillar.[47]

It was claimed in the 1920s that iron manufactured in Mirjati near Jamshedpur is similar to the iron of the Delhi pillar.[48] Further work on Adivasi (tribal) iron by the National Metallurgical Laboratory in the 1960s did not verify this claim.[49]
Evidence of a cannonball strike
Upper half of pillar, demonstrating horizontal fissuring thought to be caused by cannonball strike

A significant indentation on the middle section of the pillar, approximately 4 m (13 ft) from the current courtyard ground level, has been shown to be the result of a cannonball fired at close range.[50] The impact caused horizontal fissuring of the column in the area diametrically opposite to the indentation site, but the column itself remained intact. While no contemporaneous records, inscriptions, or documents describing the event are known to exist, historians generally agree that Nadir Shah is likely to have ordered the pillar's destruction during his invasion of Delhi in 1739, as he would have considered a Hindu temple monument undesirable within an Islamic mosque complex.[51] Alternatively, he may have sought to dislodge the decorative top portion of the pillar in search of hidden precious stones or other items of value.[52]

No additional damage attributable to cannon fire has been found on the pillar, suggesting that no further shots were taken. Historians have speculated that ricocheting fragments of the cannonball may have damaged the nearby Quwwat-ul-Islam mosque, which suffered damage to its southwestern portion during the same period, and the assault on the pillar might have been abandoned as a result.[53]
See also

    Related topics
        Ancient iron production
        History of metallurgy in South Asia
        Parkerizing
        Serpent Column
        Wootz steel
    Other pillars of India
        Ashoka's Major Rock Edicts
        Dhar iron pillar
        List of Edicts of Ashoka
        Pillars of Ashoka
        Heliodorus pillar
        Stambha
    Other similar topics
        Early Indian epigraphy
        Hindu temple architecture
        History of India
        Indian copper plate inscriptions
        Indian rock-cut architecture
        List of rock-cut temples in India
        Outline of ancient India
        South Indian Inscriptions
        Tagundaing

References

Finbarr Barry Flood, 2003, "Pillar, palimpsets, and princely practices", Res, Xliii, New York University, pp97.
"IIT team solves the pillar mystery". The Times of India. 2005.
R. Balasubramaniam 2005, p. 1.
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Identity of Chandra and Vishnupadagiri of the Delhi Iron Pillar Inscription: Numismatic, Archaeological and Literary Evidence, R Balasubramaniam, Bulletin of Metals Museum, 32 (2000) 42–64.
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Bibliography

    B. Chhabra; G. S. Gai (2006). "Mehrauli Iron Pillar Inscription of Chandra". In Upinder Singh (ed.). Delhi: Ancient History. Berghahn Books. ISBN 978-81-87358-29-9.
    Rene Noorbergen (2001). Secrets of the Lost Races: New Discoveries of Advanced Technology in Ancient Civilizations. TEACH Services. p. 57. ISBN 978-1572581982.
    Cynthia Talbot (2015). The Last Hindu Emperor: Prithviraj Cauhan and the Indian Past, 1200–2000. Cambridge University Press. ISBN 9781107118560.
    R. Balasubramaniam (2005). Story of the Delhi Iron Pillar. Foundation Books. ISBN 978-81-7596-278-1.
    King Chandra and the Mehrauli Pillar, M.C. Joshi, S.K. Gupta and Shankar Goyal, Eds., Kusumanjali Publications, Meerut, 1989.
    The Rustless Wonder – A Study of the Iron Pillar at Delhi, T.R. Anantharaman, Vigyan Prasar New Delhi, 1996.
    Delhi Iron Pillar: New Insights. R. Balasubramaniam, Aryan Books International, Delhi, and Indian Institute of Advanced Study, Shimla, 2002, Hardbound, ISBN 81-7305-223-9. [4] [5]
    The Delhi Iron Pillar: Its Art, Metallurgy and Inscriptions, M.C. Joshi, S.K. Gupta and Shankar Goyal, Eds., Kusumanjali Publications, Meerut, 1996.
    The World Heritage Complex of the Qutub, R. Balasubramaniam, Aryan Books International, New Delhi, 2005, Hardbound, ISBN 81-7305-293-X.
    "Delhi Iron Pillar" (in two parts), R. Balasubramaniam, IIM Metal News Volume 7, No. 2, April 2004, pp. 11–17 and IIM Metal News Volume 7, No. 3, June 2004, pp. 5–13. [6]
    New Insights on the 1600-Year Old Corrosion Resistant Delhi Iron Pillar, R. Balasubramaniam, Indian Journal of History of Science 36 (2001) 1–49.
    The Early use of Iron in India, Dilip K. Chakrabarti, Oxford University Press, New Delhi, 1992, ISBN 0195629922.

External links
Wikimedia Commons has media related to iron pillar.
Wikiquote has quotations related to Iron pillar of Delhi.

    Detailed list of Publications on Delhi Iron Pillar by Balasubramaniam, IIT Kanpur
    IIT team solves the pillar mystery
    Corrosion resistance of Delhi iron pillar
    Nondestructive evaluation of the Delhi iron pillar Current Science, Indian Academy of Sciences, Vol. 88, No. 12, 25 June 2005 (PDF)
    The Delhi Iron Pillar
    IIT team solves the pillar mystery, 21 Mar 2005, Times of India (About Nondestructive evaluation of the Delhi iron pillar)
    "New Insights on the Corrosion Resistant Delhi Iron Pillar" by R. Balasubramaniam 

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