Diffusion coefficient of hydrogen in alpha-phase palladium and palladium-platinum alloy

Diffusion of hydrogen in the alpha -phase Pd- and Pd81Pt19-H systems was in vestigated at 298 K by electrochemical methods in an aqueous solution of H2 SO4. Smooth or Pd-black covered specimens were used. The concentration of h ydrogen in the metal matrix was gradually increased by control of the elect rode potential; equilibrium concentrations were attained. The diffusion coe fficient of hydrogen was evaluated by impedance spectroscopy close to equil ibrium inside the specimen, at various hydrogen concentrations. The hydroge n evolution parallel to its absorption and diffusion was taken into account in the analysis of the experimental data. At the largest concentrations, t he larger the concentration the smaller is the diffusion coefficient in bot h systems. This is especially evident for the Pd81Pt19 matrix. The diffusio n coefficient evaluated from the rate of hydrogen stripping from specimens saturated to various concentrations of hydrogen was of lower reliability. T he Pd-black coverage of the specimens decreased the reliability of estimate s of the hydrogen diffusion coefficient and of its concentration. The tempo rary formation of beta =PdHn directly under the specimen surface during Pd- black deposition, resulting in the creation of hydrogen traps, is postulate d.