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 formation of methane creates high-pressure bubbles at grain boundaries and other microstructural defects. As these bubbles grow and coalesce, they generate significant internal stress that exceeds the material’s strength, leading to the formation of microcracks. These cracks propagate through the metal, resulting in a loss of ductility and tensile strength. Over time, this internal damage can lead to catastrophic structural failure, even if the external appearance of the component remains unchanged.
Prevention and Mitigation
Preventing HTHA primarily involves selecting materials with appropriate resistance to hydrogen attack, such as low-carbon steels or alloys containing stabilizing carbide-forming elements like chromium, molybdenum, and vanadium. These elements form stable carbides that do not react with hydrogen, thereby preserving the structural integrity of the material. Additionally, controlling the operating temperature and hydrogen partial pressure within safe limits, as defined by standards such as Nelson curves, is critical for ensuring the longevity of equipment in petrochemical and refining industries.