Corrosion resistance of aging heat-treated Ti-8Mo-5Fe alloy in highly acidic chloride solution

The corrosion resistance of an aging heat-treated Ti-8Mo-5Fe alloy 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) featured a single β-phase microstructure, and the samples subjected to aging heat treatments at 600-700°C showed a-phase precipitates in the α-phase matrix. EIS results showed that the corrosion resistance of the aging heattreated samples was lower than that of the ST sample, but much higher than that of pure Ti in a long-term immersion test in an acidic 10 mass% NaCl solution (pH 0.5) at 97°C. Laser micrographs of the aging heat-treated samples indicated susceptibility to selective corrosion of a-phase at the grain boundary and in the grains, causing selective dissolution in the NaCl solution. The results of TEM combined with EDAX analyses showed the presence of the βphase matrix composed of 5.3 mass% Mo and 4.8 mass% Fe, and a-phase of 0.7 mass% Mo and 0.1 mass% Fe in the sample aged at 600°C. Thus, the Mo-poor a-phase precipitates were selectively dissolved in the 10 mass% NaCl solution (pH 0.5) at 97°C. In the results, the ST sample of only β-phase microstructure showed the highest resistance, and the aging heat-treated samples containing a-phase precipitates (0.7 mass% Mo) showed higher values than that of pure Ti in the corrosion test. Addition of Fe did not decrease the corrosion resistance of the alloy under the ST condition. Moreover, as Fe was involved in the β phase with Mo which showed remarkable increase in corrosion resistance, the addition of Fe did not decrease the corrosion resistance of the aging heat-treated Mo-Fe-Ti alloy.