Hydrogen-diffusion-rate-limited hydriding and dehydriding kinetics

Abstract

Hydriding and dehydriding kinetics are derived within the framework of the Johnson‐Mehl‐Avrami equation. Hydriding is considered to be rate limited by hydrogen diffusion through a growing hydride layer, and dehydriding by hydrogen diffusion through a growing metal layer. Incubation time effects due to surface contamination are taken into account by a delayed nucleation function. The dominant composition‐dependent terms in the thermodynamics and in the mobility are taken into account to derive normative diffusivity for hydrogen. The temperature dependence of hydriding is defined by an activation energy (Q_D−ΔH̄_H+ΔH₁₂) where Q_D is the activation energy for diffusion of hydrogen in the hydride, ΔH̄_H is the relative partial molar enthalpy of hydrogen in the hydride, and ΔH₁₂ is the enthalpy of the hydrogen‐pressure plateau reaction. For dehydriding the activation energy is (Q_D+ΔH̄_H−ΔH₁₂), but now Q_D and ΔH̄_H refer to hydrogen in the metal. The solution thermodynamics contributions to the activation energy are shown to be very important. It is emphasized that in order to determine a meaningful activation energy, it is not the pressure that must be maintained constant but rather a factor T[1−(P_d/P)^1/2] in hydriding and a factor [T[1−(P/P_d)^1/2] in dehydriding, where P_d is the dissociation pressure of the hydride. Several hydriding studies from the literature are critically disccused in terms of these results.