Gmelin: Handbook of Inorganic and Organometallic Chemistry: Fe - Iron. Supplement Vol B1: Compounds with Noble Gases and Hydrogen

The key property governing the behavior of noble gases towards iron is their extremely low solubility. No solid stoichiometric Fe-H compound is known. However, under high hydrogen pressure the solid phase does exist. In the gaseous state the molecules FeH and FeH2 have been shown to exist. Hydrogen, as an extraordinarily mobile solute, poses strong challenges to such topics as the theoretical understanding of solid solutions and the dynamics of diffusion processes. It is remarkable that hydrogen, with only a very small solubility in iron, can exert strongly deleterious effects on the mechanical properties, even on those of high strength steels. As a result, catastrophic failure of structures is possible. These phenomena are collectively referred to as hydrogen embrittlement. Dissolved hydrogen is not homogeneously distributed in the iron lattice, but is also segregated in atomic and microstructural imperfections. The two prominent effects of this "trapping" are to increase the apparent hydrogen solubility and to decrease the apparent diffusivity. Even small concentrations of impurities can strongly affect the properties of iron through the interaction with lattice defects. Hence the results of studies of H-charged iron using "commercially pure" or "technical grade" iron should be interpreted with caution.