Analysis of Film Generated on Various Stainless Steel Surfaces in Soy Sauce by EIS and XPS

This paper was developed the passivation phenomenon for various stainless steels in soy sauce by evaluation for the electrochemical impedance characteristic and the distribution of elements in the specimens measured by X-ray photoelectron spectroscopy. It is possible to show a passivation phenomenon using the electrical equivalent circuit that is assumed consisting of a series connection of two parallel RC circuits and a resistance of solution. By equivalent circuit analysis, the resistance of passive film and the corrosion resistance values of duplex stainless steels are 10 times higher than that of the austenitic stainless steels, the duplex stainless steels are superior in corrosion resistance to the austenitic stainless steels in soy sauce. In stainless steels with higher corrosion resistance, as for the element distribution in the passive film that had been generated to the stainless-steel surface, it was shown that Cr was concentrated to an outermost surface of the film by the X-ray photoelectron spectroscopy （XPS） analysis. And, the thickness of the passive film is thinner. When the high local Cr concentration is generated at the outermost surface of passive firm thus leading to the increase of the film resistance value, its corrosion resistance is enhanced, the corrosion current becomes smaller and corrosion resistance is heightened. The ratio of concentration of Cr is proportional to its film resistance value as well as its charge transfer resistance.