Corrosion resistance of molybdenum silicides in aqueous solutions

The corrosion performance of Mo-22Si and Mo-25Si alloys in 0.5 M sodium chloride (NaCl) and 0.5 M sodium hydroxide (NaOH) solutions, at room temperature, was evaluated using electrochemical techniques. In 0.5 M NaCl, additionally, the effect of solution pH (3, 7 and 10) and concentration (0.1, 0.5 and 1.0 M) was studied using techniques such as potentiodynamic polarization curves, linear polarization resistance and electrochemical noise in current. The alloy contained either -Mo or Mo₅Si₃ phases in a Mo₃Si matrix. Polarization results showed that only the alloys containing 22Si developed a passive film in 0.5 M NaOH solution, whereas the alloy containing 25Si was passivated only in 0.5 M NaCl, pH 10 solution. In 0.5 M NaCl, pH 7 and 0.5 M NaOH solutions, the alloy with 25Si was the one with the highest corrosion rate, whereas the one containing 22Si was the most corrosion resistant. In NaCl solutions, the alloys exhibited a localized type of corrosion, but not in NaOH solutions. Alkaline NaCl solutions increased the corrosion rate of the 75Mo-25Si alloy with respect to acidic or neutral solutions, whereas diluted (0.1 M) or concentrated (1.0 M) NaCl solutions produced lower corrosion rates than the 0.5 M NaCl solution. Some localized type of corrosion occurred in the NaCl solutions, due to a selective corrosion of the -Mo and Mo₅Si₃ phases with respect to the Mo₃Si matrix.