Hydrogen Diffusion and Trapping in Electrodeposited Nickel

In an attempt to quantify grain boundary diffusion of hydrogen in nickel, permeation in electrodeposited foil has been investigated. Pinhole-free specimens were plated from a nickel sulfamate bath onto a reusable anodized titanium cathode. The microstructure was a mixture of regions of fine grains (diameters <0.1 tm) and individual grains up to 2 tm in diameter. The specimens were subjected to several heat treatments that resulted in grain growth; at lower temperatures, the growth was limited primarily to the fine-grained regions. At higher temperatures, second phase particles formed throughout the nickel. Electrochemical boundary conditions were used to produce permeation. The effective diffusion coefficient was determined from the transient in the permeation current density. With the electrodeposited nickel, this value was found to decrease with an increase in the initial concentration of hydrogen in the specimen. This behavior indicates the presence of hydrogen trap sites in the material. The effective diffusion coefficient measured with fully annealed specimens was in agreement with previously-reported values of the lattice diffusion coefficient (7.8 x 10-14 m2/s at 30 *C). This suggests that trapping has a negligible effect on diffusion in this material. Thus, the relationship between the input hydrogen concentration (CO) and the cathodic current density applied to the input surface (ic) could be determined through Fick's First Law. Assuming that the relationship between Co and ic is also valid for the electrodeposited nickel, the true diffusion coefficient (which is unaffected by trapping) can be determined from the