MODELING OF THE PASSIVATION MECHANISM OF FE-CR BINARY-ALLOYS FROM AC-IMPEDANCE AND FREQUENCY-RESOLVED RRDE .1. BEHAVIOR OF FE-CR ALLOYS IN 0.5 M H2SO4

In a previous investigation, three Fe-Cr alloys with various Cr conten ts were examined in 0.5 M H2SO4. These studies were based upon the ana lysis of ac impedance and frequency resolved rrde (Rotating Ring-Disc Electrode), determined simultaneously at different potentials in the a ctive-to-passive transition range. The experiments provided quantitati ve information about the adsorption of reacting intermediates from the ac response of anodic current and electrical charge involved in the f ilm formation. Based on the experimental results, namely the electrode impedance and emission efficiency, it was calculated that ferrous spe cies were leaving the electrode surface with a current efficiency grea ter than unity at intermediate frequencies. Then, the differential cap acitance, ie, the potential dependence of the electrical charge stored at the electrode surface as reaction intermediates, was evaluated. Th e model proposed in this paper describes the main features of the char acteristic behaviour of the electrode process in a simplified manner r ather than giving a detailed description of the full reaction. The sim ulated model is fitted to the experimental polarisation curve and ther eafter to the impedance. Then the emission efficiency and the differen tial capacitance were calculated.