Conversion electron Mossbauer spectroscopy and X-ray diffraction studies of the formation of carbonate-containing green rust one by corrosion metallic iron in NaHCO3 and (NaHCO3+NaC1) solutions

The corrosion of alpha-iron in 0.1 mol l(-1) NaHCO3 solutions, with or without additional NaCl, leads to the formation of a deep-green homogeneous layer which covers the metallic surface. It is analysed by X-ray diffraction (XRD) and conversion electron Mossbauer spectroscopy (CEMS) and proves to be made of carbonate-containing green rust one, GR1(CO32-), an Fe-(II)-Fe-(III) hydroxide-carbonate with chemical formula [Fe-4((II))Fe-2((III))(OH)(12)][CO3 . 2H(2)O]. If left in solution, the green rust layer oxidizes into alpha-FeOOH goethite. The corrosion process is: Fe --> GR1(CO32-) --> alpha-FeOOH, without previous formation of ferrous hydroxide, as expected from the Pourbaix diagram of iron in carbonate-containing aqueous media. If removed from the solution and oxidized in the air, the green rust layer oxidizes into a mixture of ferrihydrite or delta-FeOOH, i.e. poorly crystallized hydrated ferric oxide, and of a compound that could be called ferric green rust which keeps, in spite of the oxidation of the Fe-(II) ions, the original stacking sequence. Iron samples corroded in (0.1 mol l(-1) NaHCO3 + 4 mol l(-1) NaCl) solutions are also covered with carbonate-containing green rust one layers and the chloride-containing green rust one, [Fe-3((II))Fe-(III)(OH)(8)][Cl . nH(2)O] is not observed even though the Cl-HCO3-ratio of the solution is as large as 40:1.