Susceptible alloys, especially steels, react with hydrogen sulfide (H2S), forming metal sulfides (MeS) and atomic hydrogen (H•) as corrosion byproducts.
Since sulfur is a hydrogen recombination poison, the amount of atomic hydrogen which recombines to form H2 on the surface is greatly reduced, thereby increasing the amount of diffusion of atomic hydrogen into the metal matrix.
This aspect is what makes wet H2S environments so severe.
[1] Since SSC is a form of hydrogen embrittlement, it is most susceptibile to cracking at or slightly below ambient temperature.
Sulfide stress cracking has special importance in the gas and oil industry, as the materials being processed there (natural gas and crude oil) often contain considerable amounts of hydrogen sulfide.