The use of palladium to obtain reproducible boundary conditions for permeability measurements using galvanostatic chargi
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INTRODUCTION
THE absorption of hydrogen in metals can introduce many well-known problems.~ One way of alleviating them is to reduce the absorption rate by adding surface coatings of oxide or a less permeable metal, as has been briefly reviewed by Chatterlee et al. 2 Several problems arise when comparing the effectiveness of these surface coatings. At high temperature the comparison is relatively easy, since the boundary conditions of hydrogen at the sample surfaces are well understood. However, near room temperature, the measurements are more difficult. For example, the flow rates are lower, especially if a permeability barrier is added, and the boundary conditions at the surfaces are more complex. Heidersbach et al.3 suggest that most reported work comparing the permeability of hydrogen through different materials is questionable due to lack of control of the surface conditions. An obvious solution is to coat all samples with a layer of high permeability metal, such as palladium, and then produce the same boundary condition for hydrogen at the palladium surface layer on all the samples. The main objective of these experiments was to develop a technique for doing this. Since permeability measurements using a gas cell at high temperatures are quite dependable, it is tempting to get room temperature values by extrapolation. This method works well provided the values are sufficiently accurate, the microstructure is stable at the higher temperatures, and the same permeation mechanism is dominant throughout the whole temperature range. One example of an unstable microstructure was the addition of tin as an impermeable layer on iron 4 and nickel. 5 The tin layer was so impermeable that relatively high temperatures were needed to achieve J. BOWKER, former Postdoctorate Fellow at McMaster University, now is at PMRL Canmet, 568 Booth Street, Ottawa, ON K I A 0GI, Canada. G.R. PIERCY is with the Department of Metallurgy and Materials Science, McMaster University, Hamilton, ON L8S 4M l, Canada. Manuscript submitted September 2, 1982. METALLURGICAL TRANSACTIONS A
sufficient accuracy using the gas permeability cell. Unfortunately, interdiffusion at these high temperatures decomposed the tin layer. A second objective of the present work was to compare the permeability values measured at room temperature with those extrapolated from measurements at higher temperatures. An accepted way to measure permeability values at room temperature is to coat the sample with palladium, charge the entrance surface with hydrogen gas, and electrochemically measure the diffusion current of hydrogen from the exit surface. 6 An alternate and more sensitive way since the flow rate is higher is to cathodically charge the hydrogen at the entrance surface. This has been used by Kumnick and Johnson to measure the effect of deformation on the permeability of hydrogen through iron. 7 It is very effective for measuring relative values on the same material, since the entrance boundary condition for the hydrogen need not be known, but must be the same in al
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