MODELING OF THERMAL-DESORPTION OF HYDROGEN FROM METALS

The thermal desorption technique can be used in principle to determine the trapping characteristics of different microstructural trap sites in metals provided there are adequate models to fit to the experimenta l data. A brief review of models of thermal desorption is presented wh ich indicates that there are limitations in the assumptions made or in the scope of existing models. A more rigorous mathematical model has now been developed which accounts for diffusion, detrapping, and retra pping at one or more type of trap site and which allows for varying tr ap occupancy. The effect of material and experimental variables on the thermal desorption spectrum has been evaluated and the validity of si mple models of desorption assessed. The simpler analytical models, suc h as the detrapping model of Lee and Lee, in which diffusion is neglec ted relative to detrapping, do not inspire confidence and are applicab le only under very limiting circumstances; for example, in low alloy s teels at very low hydrogen contents. It is recommended that thermal de sorption measurements be made at progressively decreasing values of in itial hydrogen content until the simple analysis yields a consistent v alue for the trapping parameters. This experimental approach is applic able also to models of thermal desorption which account for diffusion using an effective diffusivity, since trap occupancy is neglected in t hese. The more rigorous model described herein can be used to determin e the binding energy of the traps directly which, together with the de nsity of trap sites, is the most important parameter with respect to h ydrogen assisted cracking. The height of the energy barrier to trappin g, at constant value of the binding energy, is shown to have only a mo dest effect on the thermal desorption spectrum compared with the impac t of binding energy and of density of trap sites. Crown copyright (C) 1997 Published by Elsevier Science S.A.