Trapping of hydrogen in metals

Abstract

| 1 Hydrogen can be trapped in many kinds of defects in solids. The binding energy of trapping is of the same order as that for other interstitials (C, N, and O) to defects— few tenths of an eV. However, the high mobility of hydrogen in metals permits equilibrium to be established at much lower temperatures than is true of the other interstitials ; thus, the effects of trapping are more pronounced. 2) Hydrogen can be trapped in voids of macroscopic size. For small voids-tetrahedra observed microscopically—the nature of the trapping is less certain. For the smallest possible void—a lattice vacancy-the details of trapping are even less clear. 3) Dislocations trap hydrogen by forming atmospheres. Since the dilatational strain field about a dissolved hydrogen atom may have nearly cubic symmetry in cubic metals, the interaction may mainly be through the dilatational strain field of a dislocation. Thus, edge dislocations appear to have strong binding (of order 0.25 eV) but screw dislocations may have little interaction. 4) Strain field interactions must be very large, both externally produced strains and internal strains of boundaries and dislocations. 5) Enhancement of diffusion along dislocations cores seems uncertain, but moving dislocations transport hydrogen effectively. 6) Trapping of hydrogen by impurities is pronounced, especially by interstitials O, N, and C in bcc metals. Trapping energies are typically a few tenths of an eV. The geometry of the clusters is uncertain, and the relative binding energies of clusters of various geometries are not known.