Anodic polarization characteristics and electrochemical properties of Fe₃C in chloride solutions

The anodic polarization behavior of cementite (Fe₃C), which was prepared by gas carburizing, was investigated in 10 mM NaClcontaining boric-borate buffers with pH values of 6.0, 7.0 and 8.0. The active dissolution current of the Fe₃C was found to be lower than that of the ferrite. This suggests that the corrosion resistance of the Fe₃C was clearly higher than that of the ferrite. From the potential-pH diagrams, carbon was predicted to be generated on the Fe₃C at lower potentials during the anodic polarization, and the existence of carbon was confirmed by XPS. The carbon layer seems to act as a protective film and suppress the active dissolution of the Fe₃C at lower potentials. AES depth profiles clarified that iron oxide layers existed not only on the ferrite but also on the Fe₃C. The Volta potential of the Fe₃C was approximately 40 mV higher than that of the ferrite. The higher Volta potential of the Fe₃C layer seemed to be associated with the nature of the oxide film formed on the Fe₃C, and there is a possibility that the passivation of the Fe₃C at higher potentials is achieved by an oxide film.