HTHA (High Temperature Hydrogen Attack)

Definition: HTHA (High Temperature Hydrogen Attack) is a hydrogen-induced material degradation that occurs at elevated temperatures and hydrogen partial pressures. Atomic hydrogen reacts with carbides in the steel to form methane, leading to decarburisation and the formation of internal microcracks. The damage is usually not directly visible from the outside.

Practical relevance: Low-alloy steels in refineries, ammonia or hydrocracking plants are particularly affected. The assessment is based on operating parameters (temperature, H₂ partial pressure) in accordance with API 941 (Nelson curves). Metallographic examinations reveal decarburisation zones, intergranular cracking and methane bubbles. Undetected HTHA can lead to brittle component failure.

Decision-making perspectives:

• Technical decision-makers: Material selection, inspection intervals and FFS assessments for hydrogen-carrying plants.
• Purchasing/project management: Specification of suitable Cr-Mo steels or higher-alloyed materials.
• Science: Analysis of hydrogen diffusion, carbide stability and microstructural degradation.
• Insurance/law: Verification of standard-compliant material selection and inspection strategy.

Typical testing or verification methods: Metallography (replica technique), ultrasonic testing (UT), hardness testing, fracture mechanics assessment.

FAQ:

• How is HTHA detected?
• By metallographic microstructure examination, supplemented by ultrasonic testing and assessment in accordance with API 941.