Effects of Mn on the localized corrosion behavior of Fe–18Cr alloys

Effects of Mn on the localized corrosion, anodic dissolution behavior, and repassivation kinetics of Fe–18Cr–xMn (x = 0, 6, 12) alloys were examined using potentiodynamic tests with or without micro-electrochemical cell, electrochemical impedance spectroscopy (EIS), and rapid scratch electrode tests. The addition of Mn to Fe–18Cr alloy significantly degrades passivity by decreasing the resistance to localized corrosion, and also by expanding the active region in the noble direction. The decrease in the resistance to localized corrosion of the alloys is due primarily to an increase in the number and size of Mn-containing oxides, acting as initiation sites for pitting corrosion. It was demonstrated using a micro-electrochemical test that the inherent protectiveness of passive film is also considerably reduced by the Mn addition, even though the deleterious influences of Mn-containing oxides are completely excluded. Mn facilitates the metal dissolution reaction by enhancing the activity of iron adsorbed intermediate or by generating second intermediate species (possibly manganese adsorbed intermediate) acting as another dissolution path. Further, the Mn addition appears to suppress the passivation process by reducing the activity of Cr-adsorbed species in an acidic solution, and hence the repassivation rate is significantly decreased with Mn content of the alloys.