Solubility and diffusion of hydrogen in pure metals and alloys

Basic facts are presented of the absorption of hydrogen gas by metals and t he diffusion of hydrogen in metals. Specifically considered are crystalline metals without defects and lattice disorder (pure metals), low hydrogen co ncentrations and the possibility of high hydrogen gas pressures. The first introductory topic is a short presentation of typical phase diagrams of met al hydrogen systems. Then, hydrogen absorption is discussed and shown to be decisively determined by the enthalpy of solution, in particular by its si gn which specifies whether absorption is exothermic or endothermic. The for mation of high-pressure hydrogen gas bubbles in a metal, which can lead to blistering, is addressed. It is demonstrated that bubble formation will, un der realistic conditions, only occur in strongly endothermically hydrogen a bsorbing metals. The chief aspects of hydrogen diffusion in metals are disc ussed, especially the large size of the diffusion coefficient and its depen dence on lattice structure. It is shown that forces can act on hydrogen in metals, causing a directed hydrogen flux, Such forces arise, for instance, in the presence of stress and temperature gradients and can result in local hydrogen accumulation with potential material failure effects. The final a spect discussed is hydrogen permeation, where the absorption behavior of th e hydrogen is found to be in general more decisive on the permeation rate t han the value of the diffusion coefficient.