Hydrogen permeability, thermal stability and hydrogen embrittlement of Ni–Nb–Zr and Ni–Nb–Ta–Zr amorphous alloy membranes

► Amorphous Ni–Nb–Zr–Ta membranes were tested for long-term hydrogen permeability. ► Permeability increased with Zr content, but so did hydrogen embrittlement. ► Membranes suffered from flux decrease and embrittlement at 400 and 450 °C. ► Ta increased resistance to hydrogen embrittlement but decreased permeability. ► An AES depth profile study showed metallic interdiffusion. Amorphous alloys are a promising alternative to Pd alloy membranes for hydrogen separation because of their lower cost and comparable hydrogen permeability. A series of amorphous alloy membranes consisting of Ni 60Nb 20Zr 20 (at%), (Ni 0.6Nb 0.4) 100− x Zr x and (Ni 0.6Nb 0.3Ta 0.1) 100− x Zr x (where x = 0, 10, 20 or 30) were prepared by melt spinning and then coating the foil surfaces with a thin (500 nm) layer of Pd using physical vapor deposition (PVD). A (Ni 0.6Nb 0.4) 70Zr 30 membrane exhibited the highest hydrogen permeability (1.4 × 10 −8 mol m −1 s −1 Pa −0.5) of any of the materials, measured in pure hydrogen at 450 °C. Membrane permeability increased with Zr content, but membranes higher in Zr were more susceptible to brittle failure and were more thermally unstable. Decreases in hydrogen permeability were almost always observed during long-term permeability tests at 400 and 450 °C. The addition of Ta slightly increased the thermal stability, but moderately lowered the hydrogen permeability. An AES depth profile of the membrane surface showed that metallic interdiffusion had taken place between the Pd coating and the bulk membrane, which probably accounts for the reduction in hydrogen permeability over time at 400–450 °C.