A SURFACE-ENHANCED RAMAN-SPECTROSCOPIC STUDY OF THE PASSIVE FILMS FORMED IN BORATE BUFFER ON IRON, NICKEL, CHROMIUM AND STAINLESS-STEEL

Surface enhanced Raman spectroscopy (SERS) was used to investigate the passive films formed on iron, nickel, chromium and 308 stainless stee l in berate buffer solution (pH = 8.4) at low potentials in the passiv e region. The decay of these passive films, as the potential was scann ed in the cathodic direction, was observed via SERS. The passive film on iron at - 100 mV(SCE) consisted of an amorphous Fe(OH)(2)-like spec ies and amorphous Fe3O4 or gamma-Fe2O3, which have similar Raman spect ra. The passive film on nickel at -100 mV consisted primarily of amorp hous beta-Ni(OH)(2) and some NiO. The passive him on chromium at -100 mV in berate buffer consisted of Cr(OH)(3) and another substance that may be similar to Cr(OH)(3). The passive film on 308 stainless steel c onsisted of amorphous Fe(OH)(2) and Fe3O4 or gamma-Fe2O3, Ni(OH)(2), N iO, Cr(OK)(3) and the Cr(OH)(2)-like species. For all metals, as the e lectrode was cathodically polarized the various constituents of the pa ssive film were concurrently reduced, a result that is inconsistent wi th passive films composed of discrete layers. In all SERS experiments carbon, a ubiquitous surface contaminant, was detected. The identity o f the carbonaceous species changed with potential from predominantly a morphous graphite and CO2 at high potentials to saturated hydrocarbons at low potentials.