Effect of chromium on the corrosion behavior of iron aluminides in thiosulfate-chloride solution

Aqueous corrosion characteristics of iron aluminides in thiosulfate-chloride solution were studied as a function of chromium addition. Four kinds of iron aluminides, namely, FA-61, FA-77, FA-72 and FA-78, were prepared by arc melting followed by thermomechanical treatment. The corrosion behavior in thiosulfate-chloride solution for the prepared alloys were investigated by electrochemical tests (potentiodynamic test, potentiostatic test and electrochemical impedance spectroscopy (EIS) measurement) and surface analyses. The results of the potentiodynamic test indicated that the breakdown potential increased with increasing Cr content. Cr additions were found to prevent passive film from undergoing pitting corrosion. In EIS measurement, the depression angle was inversely related to pitting resistance, and decreased with increasing chromium content. The SEM observations of the sample surfaces reveal the different forms of pit as a function of chromium content. The AES results give evidence that the thiosulfate ions are reduced on the metallic surface, which inhibits the repassivation process.