The kinetics of a moving metal hydride layer

The reaction of gaseous hydrogen with hydride-forming metals and alloys often involves a hydride layer formed on the metallic surface. Under proper steady state conditions, this layer is moving into the bulk metal, retaining constant thickness and velocity. In this work, the kinetics of the moving hydride layer is analyzed, using a model combining the four main sequential steps: adsorption (chemisorption), penetration, diffusion and reaction, in which the hydrogen is transferred from the gas phase into the reaction site. The model yields the rate of absorption during the steady state hydriding process (proportional to the hydride layer velocity) as a function of the pressure, the rate constants of the system (adsorption, desorption, penetration, decomposition, diffusion and interface emission) and the critical concentrations of hydrogen in the hydride, C-max, C-p and C-min (the last is associated with the equilibrium absorption pressure P-eq). According to the model, for sufficiently high pressures, the rate is pressure-independent. A simple expression for the pressure independent rate is derived. The conditions leading to rates limited by one of the four sequential steps are analyzed and demonstrated. Relatively simple expressions are derived for the rate's pressure dependence. It is shown that for the interface and diffusion controlled cases the general pressure dependence is of the form: Rate(-1) proportional to [(P/P-eq)(1/2) - 1](-1). For the adsorption controlled case the pressure dependence is Rate proportional to (P - P-eq). Based on the model, a numerical procedure is proposed for a system, removed from an initial steady state, describing the time-dependent approach to the new steady state determined by the applied change. The model is successfully tested for a real case, the uranium-hydrogen system, which is shown to obey the interface control rate equations. (C) 2000 Elsevier Science B.V. All rights reserved.