Competitive Role of Chloride and Phosphate Ions in the Electrochemical Passivation of Co in NaOH Solutions

Abstract

The electrochemical oxidation of Co in NaOH solutions containing chloride, phosphate or mixtures of both ions has been studied. The experimental technique involved cyclic potentiodynamic, potentiostatic and open circuit potential decay measurements. The anodic peaks are interpreted in the light of previous reports in which surface analysis methods were employed for the identification of oxidation states. Co(II)-oxide is believed to form via an instantaneous nucleation mechanism. The formation of Co(III)-oxides follows a diffusion controlled mechanism, and phosphate additions inhibited the Co(II) peak. Additions of Cl⁻ had no effect on the Co(II) process, and it is proposed that the role of Cl⁻ in the breakdown of Co passivity resides in a post-oxide-crystallisation mechanism. In NaOH solution colltaining a mixture of Cl⁻ and phosphate, the role of the latter in promoting passivity protection overcomes completely the Cl⁻ breakdown effect. A mechanism explaining the competitive roles of cl⁻ and phosphate in the breakdown and promotionof Co passivation is discussed.