Alloying effects of Ru and W on the resistance to hydrogen embrittlement and hydrogen permeability of niobium

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Abstract

The alloying effects of ruthenium and tungsten on the hydrogen solubility, the resistance to hydrogen embrittlement and the hydrogen permeability are investigated for Nb–M (M = Ru and W) alloys. The hydrogen solubility decreases by alloying and also by increasing the temperature of the alloy. As a result, the resistance to hydrogen embrittlement is improved owing to the low hydrogen content in the alloy. On the other hand, the hydrogen flux increases with increasing hydrogen concentration difference, ΔC, between the inlet and outlet sides of the alloy membrane. It is found that Nb–5 mol%X (X = Ru and W) alloys possess excellent hydrogen permeability without showing any hydrogen embrittlement when used under the appropriate permeation conditions of the temperatures and hydrogen pressures.

Introduction

Palladium and its alloys are well-known hydrogen permeable membranes used for the separation and purification of hydrogen gas [1]. Recently, there has been a great demand for the development of new hydrogen permeable alloys to be substituted for currently used Pd-based alloys, in order to reduce the material cost and to improve the hydrogen permeability [2], [3], [4], [5]. Niobium is less expensive than palladium and exhibits the highest hydrogen permeability among metals [6], so it is one of the most promising metals for hydrogen permeable membrane. However, there is still a large barrier to the practical use due to its poor resistance to hydrogen embrittlement.

Recently, the mechanical properties of niobium metal in a hydrogen gas atmosphere have been investigated by using an in-situ small punch (SP) test apparatus [7]. The hydrogen solubility and hydrogen embrittlement of Nb–Pd alloys have also been studied [8]. From these results, a concept for alloy design of Nb-based hydrogen permeable membrane has been proposed [9]. Following this concept, the resistance to hydrogen embrittlement will be improved by reducing hydrogen concentration in niobium metal in some ways, for example by alloying. On the other hand, high hydrogen permeability will be expected when a large hydrogen concentration difference, ΔC, is set between inlet and outlet sides of the membrane.

In this study, the alloying effects on the hydrogen solubility, the resistance to hydrogen embrittlement and the hydrogen permeability are investigated quantitatively for Nb–Ru and Nb–W alloys.

Section snippets

Sample preparation

The purity of the raw materials used in this study is 99.96 mass% for niobium and 99.95 mass% for ruthenium and tungsten. Nb–Ru and Nb–W alloys are prepared by using tri-arc furnace in a purified argon gas atmosphere. The nominal compositions of the alloys prepared in this study are listed in Table 1. According to the Nb–Ru and Nb–W equilibrium phase diagrams, all the alloys are in a single solid solution phase with bcc crystal structure.

Hydrogen pressure-composition-isotherm (PCT) measurement

In order to examine the hydrogen solubility for Nb–Ru and

Alloying effects of Ru and W on the hydrogen solubility

The PCT curves measured at 673 and 773 K are shown in Fig. 1 for Nb–xmol%Ru (x = 5, 10, 15) alloys. For comparison, the result for pure niobium reported by Veleckis et al. [10] is also drawn in the figure. As is evident from this figure, the PCT curve shifts toward the left and upper side with increasing ruthenium content in the alloy, indicating that the amount of dissolved hydrogen in niobium at a given pressure decreases by the addition of ruthenium. The equilibrium hydrogen concentration

Conclusion

The alloying effects on the hydrogen solubility, the resistance to hydrogen embrittlement and the hydrogen permeability are investigated quantitatively for Nb-based hydrogen permeable alloys. It is shown that the hydrogen solubility decreases with the addition of alloying elements, ruthenium and tungsten, into niobium metal and also with increasing temperature of the alloy. As a result, the resistance to hydrogen embrittlement is improved owing to the low hydrogen concentration in the alloy. It

Acknowledgement

This research was supported in part by the Japan Society for the Promotion of Science (JSPS).

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