Current oscillations associated with pitting corrosion processes induced by iodide ions on the partially passive cobalt surface polarized in sulphuric acid solutions

Abstract

Various oscillation patterns were observed within the current plateau region of the IE polarization curves of a cobalt disc electrode in sulphuric acid solutions containing iodides. The oscillatory phenomena were induced by the presence of iodide ions and rotation of the cobalt disc electrode beyond a threshold concentration and rotation rate, respectively. Two separate potential regions were detected in which two different types of current oscillations were observed as the iodide concentration increases. The nature of the oscillatory response in current of the Co/1.58 M H 2SO 4 + x M I − system can be described as a function of iodide concentration, cobalt disc rotation rate and applied potential. Analysis of the experimental results has shown that the potentiostatic current oscillations were associated with pitting corrosion processes in the partially passive cobalt surface. The damping of the current oscillations observed under certain conditions is attributed to insufficient coupling between different regions of the passive surface and the pits created by iodides. This is further supported by the transitions occurring during the evolution of oscillations in time exhibiting new dynamical behaviour. The occurrence of oscillations can be rationalized in terms of a periodic sequence of dissolution of the passive layer by iodides and the reformation of a film due to the dissolution products.