Argonne National Laboratory, Materials Science and Chemical Technology Divisions, 9700 South Cass Avenue, Argonne, Illinois 60439, U.S.A.
Abstract:
“In situ” laser Raman spectra of the corrosion films on iron have been observed in aerated 5 M KOH and 0.15 M NaCl solutions via surface enhancement by the electrodeposition of a silver overlayer. Essentially the same spectra are observed in the two solutions as a function of applied potential in spite of a breakdown of passivity on iron in the chloride solution. Fe(OH)2 and Fe3O4 are found in the prepassive potential region while FeOOH is present in the passive region. A film which is very difficult to reduce appears to be always present on the electrode surface even at hydrogen evolution potentials; this film is believed to be -FeOOH. Surface enhanced Raman spectra of the corrosion films on chromium have also been obtained in NaCl solution for the first time. The passive film has a composition that corresponds most closely to an amorphous form of Cr2O3, with some Cr(OH)3 also present. The film is converted in the transpassive region to a higher oxide form, presumably CrO2−4. Reversible reduction of this species to Cr2O3 is indicated.