Corrosion product analysis of corrosion resistant ancient indian iron

Abstract

The corrosion product formed on an ancient 1500-year old non-corroded iron clamp was studied by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Mössbauer spectroscopy. The iron possessed a non-uniform grain structure with dispersed second phase particles. Microscopy indicated that rust present on the surface was composed of an adherent and compact inner layer and a loose outer layer. Enrichment of P was observed at the metal-scale interface and in the inner rust layer. XRD indicated that the rust was amorphous in nature. The FTIR spectrum provided that the major constituents of the scale were α−, γ−, δ−FeOOH and FePO 4.2H 2O. Mösbauer studies further indicated that these constituents were present in a very fine form, confirming the results obtained by XRD. The presence of a magnetic oxide with a lower field strength than that of magnetite was also identified. The process of protective rust formation on the ancient Indian iron has been elucidated based on the results of the study. Initially, the corrosion rate of the iron is high due to the presence of the slag particles which results in enhancement of the P content in the surface. In the presence of P, the formation of a protective amorphous compact layer of δ−FeOOH is catalyzed and this confers corrosion resistance. The conversion of FeOOH to magnetite is also indicated and this would further improve the corrosion resistance. Another factor aiding corrosion resistance is the formation of iron phosphates which would lower the rate of corrosion.