FIG. 3.

Trap theory states that hydrogen amasses at microstructural defects such as voids, dislocations, grain boundaries, and precipitates, which in turn act as traps. In this figure, we show a schematic demonstrating the hydrogen accumulation at different microstructural features, including (a) normal interstitial lattice sites, (b) surface sites, (c) subsurface sites, (d) grain boundaries, (e) dislocations, and (f) vacancies. It is observable that, depending on the characteristics of the defects and traps, the effect on the diffusing hydrogen can vary significantly. Some traps are attractive, subjecting the hydrogen to an attractive force that influences the diffusion; others are physical in nature, with no long-range forces such that the hydrogen randomly falls into the trap. Reprinted from Pundt and Kirchheim, Annu. Rev. Mater. Res. 36, 555 (2006).⁵⁸ Copyright 2006 Annual Reviews.