Corrosion resistance of Mo-Fe-Ti alloy for overpack in simulating underground environment

In order to examine the application of Mo-Fe-Ti alloy for overpak, the corrosion resistance of heat-treated its alloys was investigated by electrochemical impedance spectroscopy (EIS) and transmission electron microscopy (TEM). The sample subjected to solution heat treatment (ST) had a single beta phase and samples subjected to aging heat treatment at 600-700 degrees C had alpha phase precipitation in beta 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 degrees C which simulating the crevice solution. Laser micrographs of the aging heat-treated samples indicated that alpha phase was caused selective dissolution in test solution. The TEM combined with EDAX (energy dispersive X-ray) analyses showed that beta phase matrix composed of 2.7 wt.% Mo and 4.8 wt.% Fe, and alpha phase composed of 0.7 wt.% Mo and 0.1 wt.% Fe in sample aged at 600 degrees C. Thus, Mo-poor alpha phase was selectively dissolved in a test solution. In EIS, the ST sample of only beta phase showed the highest resistance, and aging heat-treated samples containing alpha phase (0.7 wt.% Mo) showed higher values than pure Ti in the corrosion test. As Fe was involved in beta phase with Mo which increased remarkably the corrosion resistance, the addition of Fe did not decrease the corrosion resistance of aging heat-treated Mo-Fe-Ti alloy in simulating underground environment. (C) 2011 Elsevier B.V. All rights reserved.