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 homog eneous layer which covers the metallic surface. It is analysed by X-ra y diffraction (XRD) and conversion electron Mossbauer spectroscopy (CE MS) 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, th e green rust layer oxidizes into alpha-FeOOH goethite. The corrosion p rocess is: Fe --> GR1(CO32-) --> alpha-FeOOH, without previous formati on of ferrous hydroxide, as expected from the Pourbaix diagram of iron in carbonate-containing aqueous media. If removed from the solution a nd oxidized in the air, the green rust layer oxidizes into a mixture o f ferrihydrite or delta-FeOOH, i.e. poorly crystallized hydrated ferri c oxide, and of a compound that could be called ferric green rust whic h keeps, in spite of the oxidation of the Fe-(II) ions, the original s tacking sequence. Iron samples corroded in (0.1 mol l(-1) NaHCO3 + 4 m ol l(-1) NaCl) solutions are also covered with carbonate-containing gr een 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.