Study of the diffusion kinetics and mechanism of electrochemical hydriding of Mg–Ni–Mm alloys

Electrochemical hydriding in a 6 M KOH solution at 20 and 80 °C for 480 min was applied on a series of as-cast binary Mg–Ni and ternary Mg–Ni–Mm alloys (Mm = mischmetal containing 45% Ce, 38% La, 12% Nd and 4% Pr) containing 11–24 wt. % Ni and 0–6 wt. % Mm. The kinetics and mechanism of the hydriding process, as well as hydrogen release temperatures, were studied by glow discharge spectrometry hydrogen profiling, scanning electron microscopy, energy dispersion analysis, X-ray diffraction and mass spectrometry. A maximum hydrogen concentration of 1.1% was achieved in the eutectic MgNi24Mm5 alloy hydrided at 80 °C. In all cases, the main hydriding product was binary MgH2 hydride. Mass spectrometry revealed its destabilization due to Ni and Mm because its decomposition temperature was lowered by about 100 °C. Both nickel and mischmetal showed positive effects on hydriding and dehydriding kinetics. These effects are discussed in relation to the hydriding mechanism, electronic structure and atomic size of additives and structural variations of the alloys. Based on the H-concentration profiles, the diffusion coefficients of hydrogen were estimated. For the eutectic MgNi24Mm5 alloy, the H diffusion coefficient at 20 °C was 4 · 10−10 cm2 s−1.