Periodic and aperiodic current oscillations induced by the presence of chloride ions during electrodissolution of a cobalt electrode in sulphuric acid solutions

Abstract

The current oscillations observed during the electrodissolution of a cobalt rotating disc electrode in 1.58 M H 2SO 4 containing chloride ions are studied. The oscillatory behaviour appears in a certain potential range within the limiting current plateau region. The conditions under which periodic and aperiodic current oscillations occur are investigated. In general, the electrochemical behaviour exhibits different features at low (0.03–0.09 M) and high (0.09–0.7 M) chloride ion concentrations. Detailed experimental results are given in the presence of 0.07 and 0.2 M Cl −. The initiation of oscillations is connected with pitting corrosion phenomena and occurs when the chloride ion concentration is higher than 0.03 M under certain rotational conditions of the Co disc electrode. Sustained current oscillations occur when active and passive sections of the electrode surface are allowed to coexist on the electrode surface. For the range of low Cl − ion concentrations, the recorded current-time oscillations, at a fixed potential, exhibit a variety of waveforms (relaxation, mono periodic, mixed mode and aperiodic or chaotic). The nature of the current oscillations is strongly dependent on both the potential and the rotation rate of the Co disc electrode. The electrode potential and rotation rate are the factors affecting the formation, the dissolution rate of the film on the electrode surface and consequently its thickness. For the range of high Cl − ion concentrations the effect of the rotation rate is also significant, but that of the potential is weak. The potential oscillatory region is correlated with the region of negative resistance of the polarization curve which is defined in the transition from the partially passive to the overpassive state of the Co electrode.