THE ELECTROCHEMICAL IMPEDANCE RESPONSE OF TRANSPASSIVE DISSOLUTION OF CHROMIUM IN NEUTRAL SOLUTIONS CONTAINING SODIUM-CHLORIDE AND SODIUM-FLUORIDE

The kinetics of transpassive dissolution of chromium have been investigated using electrochemical impedance spectroscopy (EIS). Nyquist plots of electrochemical impedance responses corresponding to transpassive dissolution of chromium in neutral solutions containing chloride and fluoride ions show capacitive and inductive behaviour at around 0.7V(SSE) and an apparent Warburg impedance at noble potentials. A theoretical model which takes into account the electrochemical processes at the electrode surface and the diffusion process of generated chromate ion is proposed. Experimental results of electrochemical impedances and polarization curves were simulated on the basis of the proposed model. They are in good agreement with the simulated results. Transpassive dissolution rate of chromium in the presence of fluoride ion is larger than in chloride ion. This discrepancy was explained using the reaction rate constants obtained by the simulation.