Modelling of the passivation mechanism of Fe-Cr binary alloys from ac impedance and frequency resolved rrde .2. Behaviour of Fe-Cr alloys in 0.5 M H2SO4 with an addition of chloride

In a previous investigation, three Fe-Cr alloys with various Cr contents were examined in 0.5 M H2SO4 with and without an addition of chloride. These studies were performed with simultaneous analysis of ac impedance and frequency resolved rrde (rotating ring disc electrode), determined simultaneously at different potentials in the active-to-passive transition range. A reaction model was developed based on the experimental results. The proposed model describes the main features of the characteristic behaviour of the electrode process in a simplified manner rather than giving a detailed description of the full reaction. The model was initially developed to describe the behaviour of Fe-Cr alloys in 0.5 M H2SO4 without an addition of chloride and has now been extended to 0.5 M H2SO4 With chloride. The simulated model is fitted to the experimental polarization curve and thereafter the impedance is calculated. Then the emission efficiency of iron species and the differential capacitance, that is the relaxation of the electrical charge stored at the electrode surface as adsorbed species under ac potential perturbation, are also simulated. (C) 1997 Elsevier Science Ltd.