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 NaCl-containing 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.