Electrochemical methods for diffusion measurements

The hydrogen pressure at the surface of a metal sheet, e.g. a palladium foil immersed in an electrolyte, can be measured by means of the electrochemical potential. With proper activation of the metal surface the partition equilibrium between hydrogen dissolved in the metal and in the electrolyte prevails at all times. This yields the relationship between the atomic ratio H/Me within the metal and the electrochemical potential. On this basis various techniques have been developed for the measurement of diffusion coefficients of hydrogen in Pd. ‐ These methods are described in a short review. The great advantage of electrochemical methods compared to other techniques will become clear by a more detailed description of two electrochemical time‐lag methods, the step and the pulse technique, which have been used to investigate the diffusion of hydrogen and deuterium in Pd and Pd/Ag alloys up to 60 percent silver content and more recently in Ta. With simple equipment the dependence of the diffusion coefficient on hydrogen concentration from extremely low pressures up to one atmosphere could be measured very precisely. The values obtained with these convenient methods range from 10−6 cm2 s−1 in pure Pd to 10−10 cm2 s−1 in Pd/Ag 40/60. Furthermore the methods are sensitive enough to yield the differences in diffusion coefficients of H and D in these alloys. The isotope effect will not be discussed here. A more detailed interpretation is given in following papers.