Non-uniform distribution of passive film formed on carbon steel

Passive film of S45C carbon steel formed in pH 8.4 boric acid-borate buffer solution was investigated by using scanning electrochemical microscopy (SECM). Passivity-maintaining current of the carbon steel was about 3 times as large as that of pure iron. Current of electron-transfer reaction (ETR) of redox mediator through the passive film was strongly dependent on metallographic structure of the carbon steel. For example, the passive film formed on perlite eutectic phase in the carbon steel allowed larger ETR current than that formed on primary phase ferrite. Raman spectroscopy revealed that the surface of carbon steel was covered with non-uniform magnetite and amorphous carbon. It was suggested that cementite in the perlite affected not only the formation of oxide but also the semi-conductive property of the film.