The nature of the protective passive layer on the corrosion resistant Delhi iron pillar (DIP) has been addressed based on the detailed characterization of its rust, obtained from the region just below the decorative bell capital. Rust samples were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) and Mossbauer spectroscopy. XRD patterns revealed the presence of iron hydrogen phosphate hydrate (FePO_4.H_GPO_4.4TH_CO) in the crystalline form. The results of FTIR and Mossbauer spectroscopy clearly established that the major constituents of the scale were #alpha#, #gamma#, #delta#-FeOOH, magnetite and hydrated phosphates. Mossbauer studies further indicated that the iron oxide/oxyhydroxides were present in the amorphous form. The role of slag particles in the matrix of the DIP iron in enhancing the passive film formation process is briefly addressed. The process of protective rust formation on iron is outlined based on the rust analysis. Initially, the corrosion rate of the iron is high due to the presence of the slag particles which results in enhancement of surface P content. In the presence of P, the formation of a protective amorphous compact layer of #delta#-FeOOH next to the metal surface is catalysed and this confers the initial corrosion resistance. The critical factor aiding the superior corrosion resistance of the Delhi iron pillar, however, is the formation of crystalline iron hydrogen phosphate hydrate, as a thin layer next the metal-metaloxide interface, which drastically lowers the rate of corrosion due to its low porosity content. The colour of the stable and growing rust on the pillar is also discussed. The passive film formation on the Delhi iron pillar has been contrasted with rusting of normal and weathering steels.Delhi iron pillar;;passive film;;mixed potential theory;;phosphate;;microstructure
展开▼
