Measurement and evaluation of hydrogen trapping in thoria dispersed nickel

The trapping of hydrogen in nickel-2 vol pct thoria has been measured by permeation techniques. The theory of McNabb and Foster was used to determine the density of trap sites, the rate constant for trapping, and the rate constant for untrapping. Annealed material has trap sites associated with the thoria-nickel interface with the density of sites equal to about a monolayer of hydrogen at the interface. Cold-rolling up to 59 pct reduction in thickness increases the trap site density, probably by opening more sites at the nickel-thoria interfaces. The trapping energy was determined to be about 7.1 kcal/mol H. The trapping rate constant has an activation energy similar to the activation energy for hydrogen diffusion. It was shown that trapping models assuming equilibrium between lattice hydrogen and trapped hydrogen do not apply to the nickel-thoria system. For this system trapping is relatively rapid and untrapping is slow, particularly at low temperatures.