The effect of microstructure on corrosion of molybdenum-bearing titanium alloys in high chloride and acidic solution at high temperature

The effect of microstructure on corrosion of heat-treated Ti-15Mo alloys was investigated by Electrochemical impedance spectroscopy (EIS), Field emission scanning electron microscopy (FE-SEM), Transmission electron microscopy (TEM) and Energy dispersive X-ray analysis (EDAX). The sample subjected to solution heat treatment (ST) had a single β phase and samples subjected to aging heat treatment at 600°C had α phase precipitation in β phase. EIS results showed that the corrosion resistance of the aging heat-treated samples was lower than that of the ST sample, but much higher than that of pure Ti in 10% NaCl solution of pH 0.5 at 97°C. Laser micrographs and depth profile of the heat-treated samples indicated that a phase at the grain boundary and in the grain was selectively corroded and caused selective dissolution in NaCl solution. The results of TEM combined with EDAX showed that the Mo content was 18 mass% in the β phase and 0.8 mass% in α phase. Hence, less Mo α phase was selectively corroded in the NaCl solution. Moreover, the sample which had continuous precipitation of α phase had lower corrosion resistance than samples which had separated needle-shape α phases in the base β phase. Thus, it was also found that the form of precipitation of α phase affected the corrosion of these alloys. Finally, it was concluded that it is possible to maintain the high corrosion resistance of heat-treated Ti-Mo alloy by controlling the microstructure of α phase.