High temperature hydrogen attack (HTHA), also called hot hydrogen attack or methane reaction, is a problem which concerns steels operating at elevated temperatures (typically above 400 °C (752 °F)) in hydrogen-rich atmospheres, such as refineries, petrochemical and other chemical facilities and, possibly, high pressure steam boilers.
[1] If a steel is exposed to very hot hydrogen, the high temperature enables the hydrogen molecules to dissociate and to diffuse into the alloy as individual diffusible atoms.
[4] Surface oxide layers are ineffective as a protection as they are immediately reduced by the hydrogen, forming water vapour.
Later-stage damage in the steel component can be seen using ultrasonic examination, which detects the large defects created by methane pressure.
[5][4] These large defects in a stressed component are usually the cause of failure in service: which is usually catastrophic as hot flammable hydrogen gas escapes rapidly.