Surface enhanced Raman spectroelectrochemical studies of the corrosion films on iron and chromium in aqueous solution environments

“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.