High Temperature Hydrogen Attack
High Temperature Hydrogen Attack (HTHA) is a degradation mechanism that occurs in steel when hydrogen atoms diffuse into the material at elevated temperatures. Under these conditions, hydrogen can combine with carbon in the steel to form methane gas. Since methane cannot diffuse back out of the material as readily as atomic hydrogen, it accumulates in internal voids and grain boundaries, creating pressure that eventually causes micro-cracking and internal fissuring. This process weakens the structural integrity of the steel without necessarily showing surface-level corrosion or obvious damage.
Conditions and Susceptibility
HTHA typically becomes a concern in equipment operating above 200°C in hydrogen-containing environments, though risk increases significantly at higher temperatures. Steels with higher carbon content and coarser grain structures are generally more susceptible to the attack. The mechanism is particularly relevant in petrochemical refining, hydrogenation processes, and high-pressure hydrogen systems where both elevated temperatures and hydrogen atmospheres are present together.
Detection and Mitigation
Internal cracking caused by HTHA can be difficult to detect by visual inspection alone, making non-destructive testing methods such as ultrasonic examination important for condition monitoring. Mitigation strategies include material selection—using lower-carbon steels or alloys with improved hydrogen resistance—controlling operating temperatures, and in some cases applying surface treatments. Regular inspection intervals are typically established for equipment in known HTHA risk environments to identify damage before critical failure occurs.