Hydrogen embrittlement is the name given to the process in which metals ( e.g steel, copper, vanadium, nickel, titanium etc) become brittle when exposed to hydrogen. It's of crucial concern in metal manufacturing where the embrittlement can cause severe cracking in castings and in welding processes etc.
During hydrogen embrittlement, hydrogen is introduced to the surface of a metal and individual hydrogen atoms diffuse through the metal structure. Because the solubility of hydrogen increases at higher temperatures, raising the temperature can increase the diffusion of hydrogen. When assisted by a concentration gradient where there is significantly more hydrogen outside the metal than inside, hydrogen diffusion can occur even at lower temperatures."
Source : Wikipedia
It was first described in 1875, and since then at least 8 (competing) theories have been proposed to explain the chemical / physical mechanism. None of the theories is currently considered to be a definitive explanation.
See: Wikipedia