The first part of this paper, published in the July issue of this Journal, described the combination of physical metallurgical techniques and hydrogen solubilities used to help in characterising defects in palladium (Pd) and Pd alloys. In this second part, the solubilities of hydrogen
(H2) in internally oxidised Pd alloys are discussed. Internal oxidation, for example of a palladium-aluminium alloy, results in the formation of small alumina precipitates within the Pd matrix. Dissolved H in the alloy is strongly trapped at the metal/oxide interface. This can be
detected by deviations in H solubility from that expected for Pd. Hydrogen in Pd and its alloys has been modelled mathematically as the occupation by H atoms of interstitial sites within fixed metal sublattices. However, recently it has been realised that at moderately high temperatures and
H2 pressures some alloy lattices are not fixed, as the dissolved H promotes metal atom diffusion. This results in phase separation in some alloys, for example (Pd + Pt + H), according to a ternary equilibrium. The dissolved H can be removed from such alloys at low temperatures allowing
the metastable, phase-separated alloy lattices to be characterised via measurements of H2 solubilities and suitable physical metallurgical techniques.
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