High Temperature Hydrogen Attack
High Temperature Hydrogen Attack (HTHA) is a degradation mechanism affecting steel and other ferrous alloys exposed to hydrogen-rich environments at elevated temperatures, typically above 200°C. The process occurs when hydrogen atoms diffuse into the steel matrix and react with carbon present in the material to form methane gas. Unlike atomic hydrogen, which can diffuse back out of steel, methane molecules are too large to escape through the material’s lattice structure and become trapped internally.
Mechanism and Effects
The accumulation of pressurized methane gas creates internal bubbles and voids within the steel. These gas pockets weaken the material’s structural integrity and act as stress concentration points, initiating micro-cracks that propagate under load or thermal cycling. Over time, these cracks coalesce, leading to embrittlement and potential catastrophic failure. The extent of damage depends on factors including temperature, hydrogen partial pressure, material composition, and exposure duration.
Industrial Significance
HTHA is a critical concern in refineries, petrochemical plants, and other facilities where steel equipment operates in hydrogen-rich conditions, particularly in hydrocracking and desulfurization units. Materials selection is the primary prevention strategy, with low-carbon steels and alloys containing molybdenum or chromium showing greater resistance to HTHA. Regular inspection and monitoring of susceptible components are essential for maintaining equipment reliability and safety.