Electronic Properties of Passive Films Formed on Ferritic Stainless Steels by Using Capacitance Measurement and Cyclic Voltammetry Techniques

摘要

Effects of Cr content and the film formation potential on electronic properties of the passive film on Fe-Cr alloys were investigated in pH 9.2 borate buffer solution. The influence of pH on both Fe-Cr and Fe-18 percent Cr-4 percent Mo alloys was also examined. According to Mott-Schottky plot results, passive films of Fe-Cr alloys characterized p-type (Cr-oxide) at EE_(fb), respectively. The doping density decreased with the increase in Cr content and the film formation potential while the thickness of the space charge layer increased. Cyclic voltammetry results showed that as Cr content increased, the peak current densities of oxidation and reduction of Fe ion species decreased, but the peaks associated with reduction of Cr~(6+) to Cr~(3+) increased. The reduction peaks were observed only when the passive films were formed above 0.6 V_(sce). The donor concentration of Fe-18 percent Cr-4 percent Mo alloy tended to be similar to that of Fe-25 percent Cr alloy as the pH values decreased. AES results showed that as Cr content, the film formation potential and pH values increased, the highest peak of Cr moved from the outside to the inside within the film. AFM images for passive films formed on Fe-18 percent Cr and Fe-18 percent-4 percent Mo alloys showed that Mo affected the smoothness of the passive film.