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.