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Hydrogen permeation through α-iron and the influence of trapping

Authors Wat, Iva Ching On
Issue Date 2001
Summary The present work includes the analysis of hydrogen permeation through slab samples containing hydrogen traps in an inland manner. The analysis is based on the flux continuity boundary conditions that predict the effects of sample thickness, absorption and desorption processes on the permeation behavior. And analytical solution in series is derived for traps with a low coverage fraction. The analytical results show that the reciprocal flux at steady state, l/J∞ is a linear function of the sample thickness, L. The exact relationship may be found in trap-free cases but with different k values, where k denotes the desorption rate. The time lag, tL, is derived in the form of theoretical expressions as tL = (1/D)F(L,k/D) for trap-free samples and tL = ((l+α)/D)F(L,k/D) for samples with traps where α is an integrated trap parameter and D is the diffusivity of hydrogen in a perfect lattice. The function F(L,k/D) is identical for samples with and without traps. The ratio k/D may be determined from 1/J∞ vs. L. We propose to characterize hydrogen traps in an inland manner using only hydrogen permeation tests on two groups of samples with each group with varying thicknesses. The one group without traps serves as a reference and yields the value of D, which is then used to determine the trap parameter from the group with traps. The effect of charging current densities on hydrogen permeation in high-purity iron is also examined. The hydrogen permeation model considers the absorption and desorption processes at the surface, in comparison to the time lag model. Permeation experiments were conducted on iron membranes of varying thicknesses at room temperature, using the electrochemical permeation method in an aqueous alkaline medium. It was found that the diffusivity and the desorption rate increased with charging current densities, and the time to steady state is longer for relatively smaller charging current densities. At low charging current densities, the two-step phenomenon is observed. It is speculated that surface effects are attributed to this behavior in permeation. Keywords: charging current density, permeation current density, constant concentration boundary conditions, flux continuity boundary conditions, hydrogen traps, trap density, trap binding enthalpy, capture rate and release rate, time lag
Note Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2001
Language English
Format Thesis
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