High-Temperature Hydrogen Attack (HTHA)
Definition - What does High-Temperature Hydrogen Attack (HTHA) mean?
A high-temperature hydrogen attack (HTHA) is the loss of ductility and strength of steel caused by high temperature. This reaction results in hydrogen absorption by the carbides existing in the steel, leading to internal fissures and decarburization.
In this form of attack, atomic hydrogen forms a reaction with unstable carbides, producing methane gas that is responsible for gas pockets and further degradation.
HTHA takes place in low alloy metals like carbon when exposed to hydrogen at extremely high temperatures and high pressure for long periods of time.
Nelson curves serve as a tool to assist in choosing the proper steel grade as well as its safe hydrogen partial pressure and safe operating limits.
High-temperature hydrogen attack is also known as decarburization or hot hydrogen attack.
Corrosionpedia explains High-Temperature Hydrogen Attack (HTHA)
In a high-temperature hydrogen attack, atomic hydrogen disseminates into the steel. These atoms are created through molecular hydrogen dissociation and corrosion. Atomic hydrogen forms a reaction with metal carbides or dissolved carbon to build methane at:
- Crystal imperfections
- Grain boundaries
- Other types of flaws
Due to the buildup of pressure caused by methane, intergranular cracks, blisters and fissures are formed and this usually extends throughout the steel surface. Furthermore, the process of decarburization results in carbon loss within the steel and thus, results in decreased ductility and tensile strength. Amazingly, carburization which is the reverse process, can also take place in hydrocarbon-hydrogen mixtures. This usually occurs in fuel-refining industries.
HTHA typically attacks steels with low alloys and the most common is carbon steel that is operating above high temperature limits. Also, areas that are close to the exterior nozzle of catalytic machinery or interior nozzle of an exchanger as well as welds are the main concerns for HTHA.
High-temperature hydrogen attack, fissuring and decarburization can normally be prevented via:
- Making use of high alloy steel or materials
- Avoiding use of high carbon steel
- Abiding by the limits on safe operations as specified in the Nelson curves
- Utilizing 30°C as a margin when making use of Nelson curves