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 wer e investigated. Two different solutions, phosphate and phthalate buffers, a t pH 7 were used. The effect of chloride ions on the corrosion parameters o f cobalt in both solutions was studied by using open-circuit potential meas urements, polarization experiments and electrochemical impedance spectrosco py (EIS). The results have shown that the rate of corrosion of the metal in phosphate containing solutions is relatively high. The presence of chlorid e 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 chlorid e ions or long immersion of the electrode in the chloride containing soluti on 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 t o 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 o n these phenomena was discussed.