Abstract
Hydrogen embrittlement has been known to cause degradation in titanium alloys. The absorption of hydrogen into titanium can lead to the formation of brittle titanium hydrides, which decrease the fracture toughness of the metal. Weldments of dissimilar titanium alloys can be especially susceptible to hydride cracking. Titanium vessels, which include titanium welds, have been proposed for use in various nuclear waste repository systems. Experimental tests and field experiences have shown that hydrogen can diffuse to and accumulate on one side of the titanium weldment. It is possible for hydrogen to diffuse to one of the welded alloys, even against a concentration gradient. This process, known as uphill diffusion, can occur because the chemical activity of hydrogen in the two welding metals may be different. One cause of the different activity is a difference in aluminum concentration between the two welded titanium alloys. Because hydrogen can undergo uphill diffusion, it can concentrate on one side of the weld leading to the formation of titanium hydrides and decreased fracture toughness. It has been proposed that uphill diffusion may be minimized if a transitional filler metal with intermediate chemical composition is used between the two dissimilar titanium welds. The transitional titanium alloy, would contain an aluminum concentration between the two welding materials. The objective of this work is to gain an understanding of how different titanium weld configurations, corrosion rates, and temperatures affect the hydrogen uphill diffusion process. Initially, laboratory testing was conducted to measure the corrosion rates of representative titanium alloys with 6 wt% and 0 wt% aluminum. These rates were then used in an uphill diffusion model. Using the measured corrosion rates, it was determined that a transition titanium alloy successfully reduced hydrogen concentration, but only at high temperatures {>100 °C [212 °F]}. However, when the temperature was kept at 25 °C [77 °F], the transition titanium alloy led to a higher maximum hydrogen concentration in the weldment.
