Interaction of hydrogen with dislocations in palladium-I. Activity and diffusivity and their phenomenological interpretation

Abstract

Activity and diffusivity of hydrogen in deformed and annealed palladium has been measured by an electrochemical technique for different degrees of deformation (73, 50, 15, and 6%), at 295 and 322 K and over a wide range of hydrogen concentration from about 1 to 104 at.ppm. Deviations from the ideal solution behavior are drastic at the low concentration side where the solubility enhancement is as high as 1.2·106 for 73% deformation and 1 at.ppm. From a plot of the free excess enthalpy the ratio of the dislocation densities in samples of different deformation is obtained. The diffusivity decreases with decreasing hydrogen concentration but at very low concentrations of some at.ppm an enhancement of the diffusion is observed. The high concentration results can be calculated from the activity measurements assuming that the activity gradient is the driving force for diffusion, but the increase of diffusivity at low concentrations disagrees with these calculations and is explained by dislocation pipe diffusion. Evidence for this transport mechanism is also given by an unusual diffusion behaviour during current pulse measurements at low concentrations. At 295 K the dislocation pipe diffusivity is about ten times as much as the bulk value in annealed samples. © 1981.