What best describes hydrogen embrittlement?

Hydrogen embrittlement specifically refers to the process whereby the presence of hydrogen in a metal reduces its ductility, making it more susceptible to fracture. This phenomenon occurs as hydrogen diffuses into the material, leading to the weakening of the metallic bonds and disrupting the normal interatomic forces that maintain the metal's structure. As a result, the metal becomes brittle and may fail under stress, even when the applied loads are below the material's normal yield strength.

In contrast, the other options describe different phenomena that do not specifically capture the essence of hydrogen embrittlement. For instance, the formation of internal pressure in the metal is more aligned with gas evolution or other types of embrittlement but does not directly address the ductility loss caused by hydrogen. Cracks along grain boundaries can be a consequence of various types of embrittlement but do not specifically define hydrogen embrittlement itself. Lastly, the accumulation of moisture in metals is related to corrosion and oxidation processes rather than the specific mechanisms that lead to hydrogen embrittlement. Thus, the correct characterization of hydrogen embrittlement is indeed about the reduction of metal's ductility due to hydrogen diffusion.