Absorption and desorption of H by NbMo alloys at high temperature

Absorption and desorption of hydrogen have been investigated in Nb95Mo5 and Nb80Mo20 alloys over wide temperature ranges. On continuous heating H desorption from Nb95Mo5 was found to take place between 800 and 1000 K and from Nb80Mo20 between 900 and 1100 K. The observed increase in the desorption temperature with increasing Mo content has been attributed to a higher stability of the Mo and NbMo oxides with respect to those of Nb. The solid–gas reaction during absorption was first order and the rate limiting process consisted in the penetration of H atoms through surface oxides. At high temperatures and in the presence of H the oxides are expected to become permeable to H due to the reduction of higher valence to lower valence oxides. The values of the activation energy for H diffusion within the oxide films were 0.82 ± 0.04 eV for Nb95Mo5 and 1.1 ± 0.1 eV for Nb80Mo20. The thicknesses of the oxide films estimated from the absorption data were of the order of 1 μm.