Change of Molecular Spectra and Magnetism in Synthetic Cr-substituted Goethite Rusts

Abstract

For a better understanding of the solid-state properties of the Cr-substituted goethite α-(Fe_1-x, Cr_x)OOH as the final protective rust layer on weathering steel bearing chromium, Raman and FT-IR spectroscopies and magnetic measurement have been carried out for the synthetic Cr-substituted goethite rusts containing various amount of Cr. It is found from the Raman spectroscopy that addition of Cr(III) ions to Fe(II) ion alkaline aqueous solution delays the precipitation of Cr-substituted goethite in the air oxidation process. Despite the increase of Cr content, the behavior of FT-IR spectra suggests that the Cr-substituted goethite keeps a goethite (α-FeOOH) type oxyhydroxide crystal structure. Ferromagneticity appears over about 2 mass% Cr content though the Cr-substituted goethites without and with lower Cr content less than 2mass% possess the antiferromagneticity. This occurrence of magnetization proves that Cr atoms in the lattice of Cr-substituted goethite are substituted at the sites of Fe atoms. The particular content of about 2mass% Cr in the antiferro/ferromagnetic change corresponds to that producing finer particles of Cr-substituted goethite by the action of Cr(III) ions.