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 thr 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 alpha-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), also present. The film is converted in the transpassive region to a higher oxide form, presumably CrO42-. Reversible reduction of this species to Cr2O3 is indicated.