For a better understanding of the solid-state properties of the Cr-substituted goethite #alpha#-(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 were carried out for the synthetic Cr-substituted goethite rusts containing various amount of Cr. We 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 (#alpha#-FeOOH) type oxyhydroxide crystal structure. Ferromagneticity appears over about 2 mass percent Cr content though the Cr-substituted goethites without and with lower Cr content less than 2 mass percent 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 2 mass percent Cr in the antiferro/ferromagnetic change corresponds to that producing finer particles of Cr-substituted goethite by the action of Cr(III) ions.
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