A surface enhanced Raman spectroscopic study of the passive films formed in borate buffer on iron, nickel, chromium and stainless steel

Abstract

Surface enhanced Raman spectroscopy (SERS) was used to investigate the passive films formed on iron, nickel, chromium and 308 stainless steel in borate buffer solution (pH = 8.4) at low potentials in the passive region. The decay of these passive films, as the potential was scanned 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 species and amorphous Fe 3O 4 or γ-Fe 2O 3, which have similar Raman spectra. The passive film on nickel at −100 mV consisted primarily of amorphous β-Ni(OH) 2 and some NiO. The passive film on chromium at −100 mV in borate buffer consisted of Cr(OH) 3 and another substance that may be similar to Cr(OH) 2. The passive film on 308 stainless steel consisted of amorphous Fe(OH) 2 and Fe 3O 4 or γ-Fe 2O 3, Ni(OH) 2, NiO, Cr(OH) 3 and the Cr(OH) 2-like species. For all metals, as the electrode was cathodically polarized the various constituents of the passive film were concurrently reduced, a result that is inconsistent with passive films composed of discrete layers. In all SERS experiments carbon, a ubiquitous surface contaminant, was detected. The identity of the carbonaceous species changed with potential from predominantly amorphous graphite and CO 2 at high potentials to saturated hydrocarbons at low potentials.