The Hydrogen Trapping in Cold Rolled Palladium

From the diffusivity and solubility data of hydrogen in both annealed and as cold rolled Pd measured by the electrochemical permeation method, it could be discussed that the major source of the more reduced diffusivity and/or the more enhanced solubility of hydrogen in the cold rolled Pd than the annealed one is the hydrogen trapping at dislocations and the surrounding stress fields. The free energy change of the hydrogen trapping reaction evaluated by the metal-hydrogen solid solution model increased with increasing the cathodic current densities, i.e., the hydrogen concentration directly beneath the cathodic surface. The hydrogen concentration dependence of diffusivity in the cold rolled Pd is attributed to the variation in magnitude of the binding energy of the trapped hydrogen. The energy values in Pd are larger than the average hydrogen-dislocation binding energies in α-iron and/or nickel. The hydrogen concentration in a cylindrical volume around an edge dislocation increases the solubility of hydrogen in the metal at a dislocation density of 6.9 x 1014m-2 estimated from the measurement of X-ray line broadening by about 1.2 to 2.7