High temperature hydrogen attack is a form of degradation caused by hydrogen reacting with carbon from pearlite to form methane in a high temperature environment.  The methane forms tiny pockets at internal surfaces like grain boundaries and voids.  This methane does not diffuse out of the metal, and collects in the voids at high pressure and can initiate cracks in the steel. This process is known as hydrogen attack and often leads to decarburization of the steel and loss of strength.  High-strength low-alloy steels are particularly susceptible to this mechanism, which leads to embrittlement of the bulk parent metal (typical carbon-1/2 Mo steels).  This embrittlement can be severe, resulting in the material having very little lower shelf fracture toughness and high ductile to brittle transition temperatures.  The damage can also occur relatively localized at the heat affected zone (HAZ) regions. The embrittlement in the material can result in a catastrophic brittle fracture of the asset.

The API 579-1/ASME FFS-1 fitness-for-service standard currently does not address HTHA.  In the absence of a recognized standard assessment methodology, experience is required to manage assets where this degradation mechanism occurs.  Quest Reliability has advanced NDT capabilities for detecting and characterizing HTHA. We also have substantial experience advising clients on suitable means for addressing HTHA when it occurs.

For further information about HTHA, please contact us.