On the importance of equilibrium in studies on the transport of hydrogen in metals

In this paper, specific properties of the metal–hydrogen systems, M–H's, and bulk metal–hydrogen solids, MHn's, are reviewed, with an emphasis of problems related to reaching the equilibrium and its confinements. In MHn's the chemical potential of interstitial H species is determined both by its concentration and the self-stress of the metal matrix, these being in a feedback relation. Consequently, the transport of hydrogen in metals is non-Fickian. This imposes special requirements on the studies of this transport. For determining its kinetic parameters, not only should the experiments be initiated at equilibrium in the system under study, as in the case of studies of Fickian transport, but the experiments themselves must also be performed close to equilibrium. Accordingly, the recently proposed transfer function spectroscopy at quasi-equilibrium has to be considered as the only technique making possible a reliable estimation of the kinetic parameters of this transport: not only the diffusion coefficient of hydrogen, but also the bulk elastic modulus of MHn. The properties of various transfer functions are compared and the usefulness of specimens of different shapes are discussed.