Aspects of the electrochemistry of lead in acid media Part III. Active dissolution and the active-passive transition in oxygenated and deoxygenated sulphuric acid solutions

Abstract

Steady-state current-potential, double-layer capacity and impedance measurements, together with some linear sweep and potentiostatic pulse measurements, have been made on a rotating lead disc electrode in oxygenated and deoxygenated 3.0 M and 0.10 M H 2 SO 4 solutions over a wide potential range. Further evidence is provided to support the view that the lead dissolution process is highly reversible even in dilute solution. An examination of the electrochemical parameters which pertain at the point of PbSO 4 nucleation in deoxygenated solutions leads to the conclusion that the process of passivation is not satisfactorily described by either the classical “solid-state” or “dissolution-precipitation” models, but rather by an intermediate mechanism which nevertheless contains some of the elements of the classical theories. Thus, although film growth proceeds directly on the electrode surface, there is strong evidence to suggest that nucleation occurs at a critical surface supersaturation of Pb(II) species and not at a critical potential or overpotential. In oxygenated 3.0 M H 2 SO 4 the passivation process occurs at a potential which is 30 mV more cathodic than that in the equivalent deoxygenated solution. It is demonstrated that oxygen enhances the active dissolution of lead at any constant anodic potential by over an order of magnitude. This considerably increases the degree of supersaturation at the electrode surface, thereby facilitating the nucleation of PbSO 4 . There is, however, an additional opposing influence of oxygen which appears to reduce the number of PbSO 4 nucleations and inhibit crystal growth. It is postulated that this is caused by interference from intermediates in the oxygen reduction reaction which may become preferentially adsorbed on to sites normally occupied by film nuclei and which may also become incorporated into the film lattice. Thus, despite the fact that nucleation of PbSO 4 occurs at a critical supersaturation, the characteristics of the active - passive transition are influenced by the physical state of the electrode surface in a way which cannot be explained by the classical dissolution-precipitation mechanism. This investigation, therefore, highlights the importance of supersaturation effects in the nucleation and growth of thick insoluble salt layers on metal surfaces.