Study of relaxation current oscillations observed during the transition from the active to the passive state of a cobalt electrode in phosphoric acid solutions

Abstract

The behaviour of the electrochemical oscillator Co/ x M H 3PO 4 was studied in terms of the electrolyte composition, mass transport conditions and Co disc electrode potential. The dynamic response of the electrochemical oscillator Co/ x M H 3PO 4 exhibits the phenomena of bistability, hysteresis and current oscillations of the relaxation type which are observed during the transition between the active and passive state of the Co anode. Activity and passivity are the two stable steady states of the system, whereas an unstable state exists within the hysteresis loop. Hysteresis is observed between the potential scans towards positive and negative values. A hysteresis loop is formed by the jump transitions between the two stable steady states—the active and the passive. The current oscillations, which are of the relaxation type, are explained in terms of competition between the growth and the chemical dissolution of metal oxide films. During the active part of the periodic cycle the chemical dissolution rate of the passive film rises above the film growth rate. The part of the periodic cycle where the current decays is associated with more complete formation of a metal oxide layer and thus the growth of the passive layer is much faster than its dissolution. Finally, the passive part of the periodic cycle represents complete formation of the passive film where film dissolution again becomes dominant so that the active part of the cycle is repeated. Furthermore, the period and shape of the current relaxation oscillations were studied by varying the electrolyte composition, the rotation rate of the electrode and the value of an external resistance. These parameters control the form of the Z-shaped polarization curve within the hysteresis loop.