A TRANSITION IN THE KINETICS OF ELECTROREDUCTION OF ANODICALLY FORMEDCOBALT OXIDES - INFLUENCE OF POTENTIOSTATIC AGING

The kinetics of the electroreduction of oxide layers prepared through controlled anodisation of high purity polycrystalline cobalt electrode s in carbonate-bicarbonate buffers, has been analysed with special emp hasis laid on the influence of potentiostatic ageing during passive fi lm formation. The present results reveal the occurrence of an ageing p rocess related to the reaction product formed in the second oxidation stage. The ageing process of the anodically formed cobalt passive laye rs appears as an apparent increasing irreversibility of the electrored uction reaction. Longer times of ageing also result in the thinning of the oxide layer through partial chemical dissolution. The transition of the growth mechanism for the new phase from a 3-D to a 2-D electror eduction process is related to the magnitude of the rate constant orth ogonal to the surface and the time of potentiostatic ageing. Physical limit for the growing phase in the vertical direction can be reached a t relatively short times during the potentiostatic current transient e xperiments. The calculated values of the height of the growing phase d uring electroreduction conditions correlates well with the thickness o f the anodic film.