Hydrogen affecting defect density and mobility in metals and vice versa

Defects like vacancies, dislocations and grain boundaries are discontinuities of the crystal lattice and may attract dissolved hydrogen atoms leading to a distribution of site energies and a higher solubility of hydrogen at a given partial pressure of hydrogen gas. On the other hand hydrogen atoms at discontinuities may either enhance or retard defect motion. Hydrogen also affects the generation of defects reducing their formation energies to zero or even negative values, which is the main reason for hydrogen embrittlement of metals. The basic physico-chemical laws describing this behavior of hydrogen are presented together with examples of experiments, where these laws play a major role.