Effects of surface processes on hydrogen outgassing from metal in desorption experiments

Abstract

Effects of surface processes on hydrogen outgassing during desorption experiments are investigated using a standard reaction–diffusion model describing hydrogen transport, retention and desorption in material. Three mechanisms for hydrogen migration and desorption on material surface are considered: hydrogen migration from material bulk to material surface (readsorption), hydrogen migration from material surface to material bulk (reabsorption) and hydrogen desorption from material surface by molecular recombination. Three hydrogen outgassing regimes are identified: (i) recombination-limited outgassing when hydrogen recombination and desorption is fast compared to hydrogen reabsorption and slow compared to hydrogen transport from material bulk onto material surface (ii) reabsorption-limited outgassing when hydrogen recombination and desorption is slow compared to hydrogen reabsorption and when the effective hydrogen recombination and desorption is slow compared to hydrogen transport from material bulk onto material surface (iii) bulk-limited outgassing otherwise. Regimes of hydrogen outgassing from tungsten and beryllium are estimated for various experimental conditions. Analytical expressions of the outgassing flux are then derived for each outgassing regime, and used to characterize TDS spectra obtained in thermal desorption spectroscopy (TDS) experiments. It is shown that TDS spectra in recombination-limited regime are skewed toward high temperature, while TDS spectra in reabsorption-limited and bulk-limited regimes are skewed toward low temperature. Furthermore, the temperature of the desorption peak in TDS spectra is shown to decrease in recombination-limited regime and to increase in reabsorption-limited and bulk-limited regimes as the total amount of hydrogen stored in material increases. Finally, it is observed that the effective hydrogen recombination rate measured in reabsorption-limited permeation experiments may be not be reliably used to model and predict hydrogen retention and recycling from tungsten during plasma operations in tokamak.