The Effect of Microstructure on Corrosion of Molybdenum-Bearing Titanium Alloys in High Chloride and Acidic Solution at High Temperature

Abstract

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 phase and samples subjected to aging heat treatment at 600 C had phase precipitation in 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 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 phase and 0.8 mass% in phase. Hence, less Mo phase was selectively corroded in the NaCl solution. Moreover, the sample which had continuous precipitation of phase had lower corrosion resistance than samples which had separated needle-shape phases in the base phase. Thus, it was also found that the form of precipitation of 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 phase.

Effect of Microstructure on the Electrochemical Behavior of Ti-10 Mass% Mn Alloys in High Chloride Solution

“…It has been documented that alloying Ti with sufficient quantities of molybdenum (Mo) provides high corrosion resistance (Ref 9,10). In our work, Ti-15 mass% Mo alloy was found to be a highly corrosion-resistant alloy instead of rare metal in high level acidic solution at elevated temperatures (Ref 11,12).…”

Corrosion Resistance of Aging Heat-Treated Ti-8Mo-5Fe Alloy in Highly Acidic Chloride Solution

“…In the case of the Ti-Mo binary alloy, microgalvanic coupling occurred between the Mo-poor phase and Mo-rich phase, and the active part of the phase was highly susceptible to corrosion. 13) Hence, as for the Ti-8Mo-5Fe alloy in Fig. 7, corrosion preferentially occurred at the phase in Sample HB2-2 in the same way as Ti-Mo alloy.…”

“…This is explained by the fact that the resistance in the low-frequency region is the passive film resistance, and the film capacitance appears in the EIS spectrum. 13) At 5 days, the EIS spectra showed another resistance component in the mid-frequency region. This resistance in the mid-frequency region was considered to be caused by -phase dissolution.…”

“…1. 13) In order to further elucidate the precipitated phase in the grains and at the grain boundaries, TEM with EDAX was used, which can identify the form and elemental composition of the microstructure. TEM-EDAX mapping analysis of heattreated sample HB2-2 was carried out as shown in Fig.…”

Optimization of Strength, Ductility and Corrosion Resistance in Ti-Mo Base Alloys by Controlling Mo Equivalency and Bond Order

“…[3][4][5] Meanwhile, it has been reported that titanium alloys with sufficient content of Mo provides the high corrosion resistance. [6][7][8][9] Nishimura 8) found that a Ti-15Mo alloy (mass%), which was developed in the 1950s as a corrosion resistance titanium alloy, 10) exhibits an extremely high corrosion resistance in highly acidic solution at 373 K, which is able to against the crevice corrosion in seawater at the high temperature. However, this alloy shows a low yield strength in the as-solution treated condition because of its deformation by f332gh113i twinning.…”