Fe(II) hydroxycarbonate Fe2(OH)2CO3 (chukanovite) as iron corrosion product: Synthesis and study by Fourier Transform Infrared Spectroscopy

Fe(II) compounds were precipitated by mixing FeCl2 · 4H2O, NaOH and Na2CO3 · 10H2O solutions and the influence of the concentration ratios R = [FeCl2 · 4H2O]/[NaOH] and R′ = [Na2CO3 · 10H2O]/[NaOH] on the nature of the precipitates was studied. The conditions favouring the formation of chukanovite Fe2(OH)2CO3 at the detriment of Fe(II) hydroxide Fe(OH)2, often considered as the first solid forming from the Fe(II) dissolved species generated by the dissolution of steel, were determined. Chukanovite was the unique compound to form for R > 1 and R′ > 0.5. When one of these conditions was not met, that is when the amount of Fe(II) dissolved species or the amount of carbonate species was not sufficient, Fe(OH)2 was precipitated together with chukanovite. For the smallest values of R and R′, 0.6 and 0.1, respectively, chukanovite was not obtained. However, it was demonstrated that ageing at room temperature of a carbonate-containing suspension of Fe(OH)2 could lead to the total transformation of Fe(II) hydroxide into chukanovite. The Fe(II) compounds were analysed by ATR/FT-IR spectroscopy under inert atmosphere, since this method allowed us to distinguish unambiguously Fe2(OH)2CO3 from Fe(OH)2. As a result, chukanovite could also be identified by infrared micro-spectroscopy inside the rust layers of an archaeological iron nail extracted from a soil presenting anoxic conditions.

Fe(OH)2 was prepared by mixing FeCl2 · 4H2O and NaOH solutions. A NaHCO3 solution was then added to the suspension, so that [NaHCO3]/[NaOH] = 0.9. The system was aged at RT. ATR/FT-IR spectra after 24 h and 2 months show that the precipitate transformed progressively into chukanovite.Figure optionsDownload as PowerPoint slide