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Passivity, Passivation (see
terminology): None other than Michael Faraday (more on history ... ) has been accredited with the first "correct" account of passivation of iron [1] - he reportedly attributed it to prior oxide layer formation on the surface, in 1836. The term "passivity" was reportedly coined by Schonbein [2] and the passivation of iron had reportedly been observed (but not rationally explained) before Faraday's time. The Delhi Iron Pillar (in India, now more than 1600 years of age) is an historic landmark on which passive surface films are believed to have played a significant protective role. This remarkable wrought iron structure has reportedly been subject to formal scientific "passivation" scrutiny and coupon corrosion monitoring since 1953 [3]. Dr. Balasubramaniam has given a fascinating modern account of the protective, passivating surface films on this structure [3]. According to this work, at least three protection mechanisms, under the overall protective passive film growth concept, are at play on the surface of this imposing pillar:
The wrought iron in the pillar reportedly had a relatively high phosphorous content, much higher than typically found in modern steel making. Editor's note: A publication of the Real Wrought Iron Company ("The Wrought Iron Advisory Centre, WIAC") has also highlighted the higher degree of corrosion resistance of wrought irons, compared with modern carbon steels or "pure" irons. It was pointed out that the wrought iron work heritage, in many cases several centuries old, has survived due to this relatively high (atmospheric) corrosion resistance.
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