Kinetics of oxide film growth on cobalt in neutral aqueous solutions

The rate of oxide film thickening on Co was followed by measuring the open circuit potential in solutions of Cl^–, Br^–, SO^2– ₄, NO^– ₂, NO^– ₃, PO^3– ₄ and WO^2– ₄ of varying concentrations until steady values were reached. In all the solutions studied, the steady state potentials E _st. were reached from negative values denoting oxide film healing and growth. The rate of oxide film thickening was found to follow a direct logarithmic law as evident from the variation of the open circuit potential with the logarithm of the immersion time t. In solutions of the first five anions, the rate of oxide film growth decreased with increasing anions concentration. However, in solutions of PO^3– ₄ and WO^2– ₄, the rate of the oxide film thickening increased with increasing concentration. The steady state potentials varied with the anions concentration according to straight line relationships. The significant role of the anions in the process of oxide film growth could be identified.