A study of the electrochemical behaviour of cobalt in aqueous H2CO3/HCO3-/CO32- solutions at pH 7-9

The electrochemical behaviour of a cobalt rotating electrode in H2CO3/HCO3-/CO32- aqueous solutions was investigated in the pH region from 7 to 9. The effects of H2CO3/HCO3-/CO32- concentration, pH, and the presence of phosphates as inhibitors was explored using a rotating disc electrode at 1000 rpm. Some potentiodynamic experiments indicate that for pH 8.5 and higher, carbonate and bicarbonate species play a key role on the rate of electrooxidation of cobalt. For pH lower than 8.5, the electrochemical behaviour of cobalt changes drastically and very aggressive corrosion is observed. The involvement of carbonic acid must be considered in the corrosion process of cobalt in this pH region. The study of passive film potential decay under open circuit potential and galvanostatic reduction was performed on preanodized cobalt electrodes in solutions of various compositions. These experiments indicated the phenomena occurring at the electrode-solution interface during the corrosion process. A mechanism involving competitive adsorption of different species is suggested on the basis of the experimental evidence. (author)