The Randles–Sevčik relationship has been applied to evaluate atomic hydrogen diffusivity in massive LaNi 5 intermetallic compound. The electrode was cathodically hydrogenated in 6 M KOH solution (22 °C), and then voltammetry measurements were carried out at various, very slow potential scan rates ( υ = 0.01–0.1 mV · s −1 ). At potentials more noble than the equilibrium potential of the H 2 O/H 2 system, the anodic peaks were registered as a consequence of oxidation of hydrogen absorbed in cathodic range. The peak potentials linearly increase with the logarithm of the scan rate with a slope of 0.059 V. The slope testifies to a symmetric charge transfer process with symmetry factor α = ½. The peak currents linearly increase with the square root of the potential scan rate, and the straight line runs through the origin of the coordinate system. The slope of the I a (peak) = f( υ 1/2 ) straight line is a measure of the atomic hydrogen diffusion coefficient. Assuming the hydrogen concentration in the LaNi 5 material after cathodic exposure to be C 0,H = 0.071 mol · cm −3 (63 % of theoretical value), the hydrogen diffusion coefficient equals D H = 2.0 · 10 −9 cm 2 s −1 . Extrapolation of rectilinear segments of potentiodynamic polarization curves with Tafel slopes of 0.12 V and linear polarization dependencies from voltammetry tests allowed the exchange current densities of the H 2 O/H 2 system on the tested material to be determined. The exchange current densities on initially hydrogenated LaNi 5 alloy are close to 1 mA · cm −2 , irrespective of the electrode potential scan rate.
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