Effect of chloride ions on the corrosion and passivation behaviours of cobalt in neutral solutions

The corrosion and passivation behaviours of cobalt in neutral solutions were investigated. Two different solutions, phosphate and phthalate buffers, at pH 7 were used. The effect of chloride ions on the corrosion parameters of cobalt in both solutions was studied by using open-circuit potential measurements, polarization experiments and electrochemical impedance spectroscopy (EIS). The results have shown that the rate of corrosion of the metal in phosphate containing solutions is relatively high. The presence of chloride ions in phosphate buffers decreases the corrosion resistance of the metal due to the destruction of the violet phosphate barrier layer [CO3(PO4)(2)] which is formed on the metal surface. Increase of concentration of chloride ions or long immersion of the electrode in the chloride containing solution leads to the formation of corrosion pits which are recorded by scanning electron microscopy (SEM). X-ray photoelectron spectroscopic investigations (XPS) have shown that the electrode surface contaminates phosphorous due to the presence of Co-3(PO4)(2). In phthalate buffer solutions the corrosion resistance of the metal increases due to the formation of the more stable Coo barrier film. The mechanism of the corrosion process and the formation of a passive film in neutral solutions beside the effect of chloride ions on these phenomena was discussed.