Analysis of the of cobalt open-circuit-potential behavior in a slightly alkaline media

Abstract

The analysis of the transients of the Open Circuit Potential (OCP) constitutes a useful tool for the understanding of the dissolution processes of a metal. Nevertheless, its usefulness depends on a previous knowledge of the system metal/media in study. In this work, the evolution of the OCP of cobalt immersed in a carbonate/ bicarbonate solution was analyzed by means of a rotatory disc electrode (RDE); the effect of mass transport, and the reactions that can explain the dissolution of the metal in this media were considered. It was observed that the time necessary to reach the stationary value of the OCP is a function of the mass transport. Also, the OCP evolves towards more positive values as the rotation of the electrode is increased. It was confirmed that the corrosion process of the metal is controlled initially by oxygen reduction (cathodic reaction). However, after some time of exposure, i.e. the necessary for the first corrosion products to appear, the formation of a non-passive film of CoO on the surface makes the dissolution of the metal to switch to be anodically-controlled; this process is also controlled by the diffusion of the Co(CO3)22- complex from the metal surface to the bulk of the solution.