Abstract
The hydrogen permeation rate through both annealed and cold rolled Pd foil specimens under the conditions of various cathodic current densities which form the region of co-existence of α-Pd and β-Pd phase hydride was measured at 299±1 K by means of the electrochemical permeation method.
The linear relation of log(t1⁄2·Jt) vs 1⁄t in the permeation transients was valid at the beginning of the permeation process, and the moving rate parameter of the α⁄β phase boundary was estimated by using an αmax value previously known. The results were explained by such a model that the rate-determining step of the permeation is a hydrogen-diffusion process in the remaining α-phase Pd of the matrix, accompanying the α⁄β-Pd phase transformation.
The apparent diffusion coefficient of hydrogen determined from the permeation transient by the theoretical solution of diffusion equation where the specimen is assumed to be a homogeneous medium is equivalent to the true diffusion coefficient in the α-Pd phase, but the apparent hydrogen concentration directly beneath the cathodic surface corresponds to the phase equilibrium hydrogen concentration of α-Pd phase in the α⁄β phase boundary related to the moving rate parameter of the phase boundary.
Additions of H2SeO3 to the electrolyte for cathodic polarization strongly inhibited the discharge step of hydrogen ions for both Pd specimens but scarcely influenced the hydrogen permeation behaviors.
