Palladium embrittlement

The method is more useful with titanium, and the effect of alloying titanium with a small amount of palladium is described in Section 5.4. The use of platinum in the prevention of hydrogen embrittlement in tantalum. 

Hydrogen has a low solubility in solvents for example, at ambient conditions, only 0.018 and 0.078 mL of gaseous H2 dissolves into each milliliter of water and ethanol, respectively. However, the solubility is much more pronounced in metals. Palladium is particularly notable in this respect, which dissolves about 1000 times its volume of the gas. The adsorption of hydrogen in steel may cause "hydrogen embrittlement," which sometimes leads to the failure of chemical processing equipment [4],... 

Pure palladium becomes brittle in the presence of hydrogen during thermal cycling due to dimensional changes caused by a transformation between two phases (a and ft) of palladium hydride around 300°C. To avoid metal embrittlement and resulting membrane cracking or distortion, pure palladium membrane should not be exposed to hydrogen at temperatures below 300°C. To increase resistance to embrittlement, Pd is alloyed with... 

Some 100 years after Cavendish s discovery of hydrogen, and only 3 years after it was realized that hydrogen sorbed from chemical or electrochemical sources causes blistering and embrittlement to steel vessels, Graham [29] observed the ability of palladium to absorb hydrogen and wrote in Philosophical Transactions of the Royal Society of London ... 

The content of this chapter is closely related to permeation, which is the transport of a solute across a layer of solvent (or membrane) under the action of a difference in activity. For example, the permeation of hydrogen through a metal foil has been studied, particularly for palladium [F.A. Lewis (1967)] and iron [J. P. Hirth (1980) H. H. Johnson (1988)]. One reason for studying the permeation of hydrogen through iron is to understand the hydrogen embrittlement of steel. 

R.E. Buxbaum, R. Subramanian, J.H. Park, and D L. Smith, Hydrogen Transport and Embrittlement for Palladium Coated Vanadium-Chromium-Titanium Alloys, Journal of Nuclear Material, Part A, 233-237, 1996, pp.510-512. 

Pd alloys are preferred over pure Pd as the membrane materials because of several considerations. First of all, pure palladium can become embrittled after repeated cycles of hydrogen sorption and desorption. Second, the hydrogen permeabilities of certain Pd-alloys are higher than those of pure palladium. Third, the catalytic activities of the alloy membranes, in many cases, exceed that of palladium alone. Finally, palladium is very expensive. Alloying with other metals makes it more economically attractive for... 

Membrane embrittlement. StrucUiral changes or degradations in mechanical propeities of the membranes in response to the application environments can be a significant issue. Pd alloys are preferred over pure Pd as the material of choice for dense membranes in that pure Pd can suffer from embrittlement after repeated cycles of hydrogen sorption and desorption particularly at a temperature higher than approximately lOO C. It is believed that there are two hydride phases of pure palladium a phase at lower hydrogen partial pressures and P phase at higher pressures. The two phases can coexist under... 

Buxbaum, R. E, Subramanian, R, Park, J. H, Smith, D. L. Hydrogen transport and embrittlement for palladium coated vanadium-chromium-titanium alloys. J Nucl Mater. 1996 233-237 510-2. 

The hydrogen permeation process is influenced by the surface topography, the purity of the metal and its defect structure (e.g., grain boundaries and dislocations). Within the metal, the hydrogen occupies octahedral interstitial sites. At high hydrogen concentrations, above 20°C, the a phase of palladium hydride exists, and one of the problems associated with pure palladium as a membrane is hydrogen embrittlement and the distortion of the metal by repeated adsorption/desorption cycles of exposure. 

Binary addition elements, having in general Face-Centered-Cubic (FCC) crystal structures, stabilize the a-hydride phase against the P-hydride phase transition, reducing the problem of embrittlement, and also increase hydrogen permeability above that of pure palladium (see Table 14.2). 

It follows from the work of Darling and others that formation of the beta phase palladium hydride must be avoided to prevent embrittlement, cracking and delamination of thin catalytic films of unalloyed palladium used on composite membranes of Nb, Ta, Ti, V, and Zr. This also holds true for alloys of palladium susceptible to hydrogen embrittlement. Likewise, transformation of Nb, Ta, Ti, V and Zr substrates into brittle hydrides at low temperatures or at high hydrogen partial pressures must also be avoided. 

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