By analyzing the experimental data on the influence of gaseous hydrogen on physicomechanical properties of steels, we consider the mechanism of reversible hydrogen embrittlement, focus our attention on the processes of surface interaction, and explain the surface-active properties of hydrogen. The low solubility, high mobility, and affinity to metals characterize hydrogen as the most efficient surface-active element with respect to metals. We propose to consider the ability of hydrogen to concentrate in certain microvolumes of metal as the main point for explanation of the mechanism of reversible hydrogen embrittlement. The actual behavior of the material is determined by hydrogen localized in defects of the structure, but its total concentration cannot characterize the degree of danger of hydrogen degradation. Depending on the deformation conditions, the interaction of a metal with hydrogen either promotes plastic flow or leads to selective fracture.Hydrogen degradation of metals is a result of the harmful influence of hydrogen on mechanical behavior and integrity of metals. This degradation leads to deterioration of the service characteristics of structures and increases the risk of accidents due to the unpredicted loss of integrity and resistance and a decrease in service life.The rather complicated phenomenon of hydrogen degradation includes three groups of effects which are inherent in individual materials or manifest themselves under certain conditions of the interaction of a metal with hydrogen.The first group is associated with reversible hydrogen embrittlement, i.e., a deterioration of the characteristics responsible for strength and durability of metals deformed in the presence of hydrogen present in the metal or in the environment. Fracture occurs without any signs of chemical reactions, microstructural changes, transformations, and damages, which is not connected with deformation. The characteristics of ultimate strength and strain are most sensitive to the action of hydrogen. After removal of hydrogen, the properties of metal are completely restored. Reversible hydrogen embrittlement is inherent in the majority of iron-based alloys and other alloys that do not form hydrides under conditions of low-temperature interaction (up to 473 K). The second group presents a phenomenon of chemical or physicochemical degradation: the formation of new phases of hydrogen-containing compounds in alloys, or phase and structural transformations which are possible only under the action of hydrogen. This group involves hydride embrittlement of titanium and zirconium alloys and alloys of other metals, hydrogen corrosion of carbon steels, "hydrogen disease" of copper, martensite transformations in austenitic steels induced by hydrogen, etc. The effects are not directly caused by deformation but, very often, deformation can stimulate these effects. The third group is presented by hydrogen destruction, which means the appearance of defects not connected directly with the action of loading or chemical reactions and transform...
