Investigation of passivity and its breakdown on Fe₃Al-Cr-Mo intermetallics in thiosulfate-chloride solution

Aqueous corrosion characteristics of iron aluminides in thiosulfate-chloride solution were studied as a function of Mo addition. Four kinds of iron aluminides, namely. FA-72(Fe-28Al-4Cr), FA-138(Fe-28Al-4Cr-0.5Mo), FA-139(Fe-28Al-4Cr-1Mo) and FA-140(Fe-28Al-4Cr-2Mo), were prepared by arc-melting followed by thermomechanical treatment. An aerated thiosulfate-chloride solution was used to simulate a sulfur-bearing environment. The corrosion behavior in thiosulfate-chloride solution for the prepared alloys was investigated by electrochemical corrosion tests (cyclic polarization test, potentiostatic test and electrochemical impedance spectroscopy (EIS) measurement) and surface analysis (SEM and AES). The results of the cyclic polarization test indicated that the breakdown potential and the protection potential increased with increasing Mo content. EIS measurements showed that the semicircle was less depressed when the Mo content increased. Furthermore, the Mo-bearing alloys present higher R_et values than a Mo-free alloy at breakdown, repassivation and passivation states. The SEM observations of surface morphology after corrosion tests showed that the iron aluminides containing Cr and Mo had far fewer pits than those containing only Cr. The AES results gave evidence that the thiosulfate ions are reduced on the metallic surface, which inhibits the repassivation process. However, by enhancing the Al and Cr in oxides, Mo additions aid in the development and stability of a repassivating layer which further provided resistance against Cl and S₂O^2-₃ ions attack.