High temperature hydrogen attack (HTHA) is the focus of the November Asset Intelligence Report by Inspectioneering.com. The article serves as an informative primer to provide an understanding of HTHA. This primer contains the following sections:
- Susceptible Areas
- Nelson Curve
- Effective Inspection Techniques
- Codes, Standards, and Best Practices
- Further Reading
HTHA is a time dependent intergranular damage mechanism that can occur in process equipment that is exposed to hydrogen at elevated temperatures (at least 400ºF or 204ºC), when hydrogen disassociates into nascent (atomic) hydrogen, which is then driven into the steel by the temperature and pressure of the environment. The atomic hydrogen reacts with unstable carbides in steel to form methane gas, resulting in the formation of gas pockets at grain boundaries or inclusions that lead to material degradation. HTHA is a time-temperature-pressure function, meaning that the longer a piece of equipment is exposed to temperatures and/or pressures above its resistance limit, the more damage accumulates in the steel. The damage that results from HTHA ranges from superficial surface decarburization of the inside diameter (ID) surface to severe material embrittlement, loss of tensile strength and toughness, and cracking.
As with other Asset Intelligence Reports, this document is not intended to serve as a comprehensive guide, but rather an introductory primer on HTHA that can be referenced by you and your co-workers at your convenience.