Exposure to hydrogen causes various metals, most importantly steel, to become brittle and crack. This corrosion process, called hydrogen embrittlement, is not completely understood. Detection of hydrogen embrittlement in welds and fabricated parts is more difficult than detection of oxidation (rust).
The basic corrosion mechanism begins with hydrogen molecules diffusing through the metal. In steel (an alloy of iron and carbon), hydrogen will diffuse along grain boundaries and combine with the carbon to form tiny pockets of methane. This methane does not readily diffuse out of the metal, and collects in small voids on the grain boundaries at terrifically high pressure. These pockets then initiate cracks in the bulk material.
High-strength steels, aluminum and titanium alloys are most susceptible.
If the metal hasn't already started to crack, the condition can be reversed by removing the ambient hydrogen source to cause the hydrogen within the metal to diffuse out and restore ductility to the metal.