Hydrogen atoms are minutely small and able to diffuse (tunnel) through iron, however a proportion remain trapped, causing reduced ductility in high strength steel by restricting internal ‘slippage’ along crystal planes. Low strength steels may blister due to the pressure of gas molecules, which are orders of magnitude larger than individual atoms. Austenitic stainless steels are immune to embrittlement because their FCC structure has 4 more slip planes than BCC iron.
Hydrogen may be present in steel from manufacture, or it may enter due to corrosion or from process gas. Sour (H2S) corrosion is more virulent because the sulphur inhibits the formation gas molecules, making available substantially more atomic hydrogen. Molecules dissociate to atomic hydrogen above 205°C resulting in hydrogen ‘charging’, which doesn’t reduce ductility whilst hot but can when cold or if welding is undertaken. Above ~240°C hydrogen combines with the carbon in steel, forming methane, surface fissuring and decarburisation (HTHA). Hydrogen can be driven out of steel by heating to above ~300°C, but HTHA is irreversible.
The mysteries of hydrogen damage remain because, really, we don’t know the first thing about how atoms work, hence we ended up believing quantum entanglement is actually a thing!