On the effective hydrogen permeability in metastable beta titanium alloy, niobium and 2.25CM Mo ferritic steel
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2086-- VOLUME 17A, NOVEMBER 1986
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On the Effective Hydrogen Permeability in Metastable Beta Titanium Alloy, Niobium and 2.25Cr-1Mo Ferritic Steel T. P. PERNG and C. J. ALTSTETTER The gas permeation constant in metals is a very useful quantity, since it is a measure of the total amount of solute per unit time that can penetrate into a metal from a gas phase. Thus, it often can be correlated with changes in material behavior due to exposure to gas atmospheres. It is well known that gaseous hydrogen permeates through a metal membrane by a series of processes, namely, physical and chemical adsorption, dissociation, solution, diffusion, recombination, and desorption. Any one of these can be the rate limiting process for hydrogen permeation. If the permeation rate is bulk diffusion-controlled, from Fick's laws the permeation constant (qS) can be directly related to the lattice diffusivity (D) and solubility constant (S) by the equation
b=D.S
[1]
Therefore, the hydrogen permeability in a metal can be calculated if both hydrogen diffusivity and solubility in that metal are known. To ensure a lattice diffusion-controlled permeation process, a clean surface of the metal is important. In laboratory experiments, it is a common practice to clean the specimen surface by electropolishing and/or ion sputtering, followed by Pd coating to facilitate the entry and evolution of hydrogen. The Pd coating also serves to protect the surface from oxidation possible at elevated temperatures. In most practical engineering applications, however, metallic materials are often used without any surface treatment. Hydrogen T.P. PERNG, formerly Research Associate, Department of Materials Science. University of Illinois at Urbana-Champaign, is Associate Professor, Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu. Taiwan. C.J. ALTSTETTER is Professor of Physical Metallurgy, Department of Materials Science, University of Illinois at Urbana-Champaign, Urbana, IL 61801. Manuscript submitted September 20, 1985.
METALLURGICAL TRANSACTIONS A
permeation in such materials can vary greatly, depending on their surface treatment. One of the potential problems is the presence of oxides or other metallic compounds which, acting as diffusion barriers, may significantl
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