IMPEDANCE STUDY OF THE PASSIVE FILM ON STAINLESS-STEEL-304 IN PH8 CARBONATE SOLUTION

The effects of applied d.c. potential and polarization time on the pas sivation of stainless steel 304 (SS304) were investigated in deaerated 1 M NaHCO3 aqueous solutions at pH 8. Electrochemical impedance spect roscopy was used in conjunction with a rotating disc electrode. The da ta were analysed by considering an equivalent circuit. The changes in impedance parameters at applied d.c. potential signal changes in the p roperties of passive films on SS304 and allow to distinguish the param eters at low potential (-0.6 to 0.3V vs SCE) from a different one at h igh potential (0.5 to 0.8V vs SCE). The oxidation reactions were contr olled by both charge transfer and mass transfer processes. Diffusional resistance was high for both passive films and was considered to repr esent the resistance to movement of ions or vacancies through the surf ace layer of oxide films. It is deduced that the passive film present in the low potential region is partially dissolved at 0.4V vs SCE and that a new passive film is formed in the higher potential region. The equivalent circuit used to obtain the best fit and the fitting paramet ers was dependent on the electrode potential and the polarization time . The reproducibility of the impedance spectra at constant potentials demonstrate that the passive film formation is highly irreversible pro cess. No traces of localized corrosion were detected but, for a high p otential and long polarization time, the electrode surface coloration to a uniform gold colour confirms the film thickening.