In the present work, hydrogen transport through electrodeposited pure Pd, Pd-Ni and Pd-Au alloys on Pd substrate has been investigated by using ac-impedance method combined with an electrochemical hydrogen permeation technique in 0.1 M NaOH solution at room temperature. The electrodeposited pure Pd, Pd-Ni and Pd-Au alloys were obtained from ammoniacal and cyanide baths by applying pulse current. The ac-impedance measurements were carried out in the potential range of -0.75 to -0.95 $V_{SCE}$ applied to the cathodic side of the electrodeposits after the hydrogen permeation achieved a steady-state. The hydrogen diffusivities in the electrodeposits were obtained by using time lag method for the bilayer. The measured impedance spectra were analyzed as a function of applied potential in order to determine such kinetic parameters as hydrogen charge transfer resistance ($R_{ct}$), diffusivity and equilibrium absorption constant ($K_{eq}$) of hydrogen by using complex non-linear least squares (CNLS) curve fitting method on the basis of Faradaic admittance for hydrogen absorption and diffusion. The variation in $R_{ct}$ for hydrogen on the electrodeposits was explained in terms of the hydrogen overvoltage and surface roughness. From relatively low values of the hydrogen diffusivity in the electrodeposited Pd alloys as compared to those in the Pd membrane and electrodeposited pure Pd, it seems that the thin $Ni(OH)_2$ layer on the electrodeposited Pd-Ni alloy and the increased interaction between Pd and H in the presence of Au in the electrodeposited Pd-Au alloy act as a barrier for hydrogen diffusion. Also, from the applied potential dependence of $K_{eq}$ for the electrodeposits, it is suggested that the alloying elements of Ni and Au impede the formation of β-phase palladium hydride on the surfaces of the electrodeposited Pd alloys.
