Forms of Hydrogen Attack

High-temperature hydrogen attack (HTHA) is a form of degradation caused by hydrogen reacting with carbon to form methane in a high-temperature environment. 

The methane forms and stays in grain boundaries and voids however, it does not diffuse out of the metal. Once accumulated in the grains and voids, it expands and forms blisters, weakens the metal strength, and initiates cracks in the steel. 

High-strength low-aUoy steels are particularly susceptible to this mechanism, which leads to embrittlement of the bulk parent metal (typical C-0.5 Mo steels). The embrittlement in the material can result in a catastrophic brittle fracture of the asset. 

The overall effect of hydrogen attack is the partial depletion of carbon in pearUte (decarbuiiza-tion) and the formation of fissures in the metal. Hydrogen attack is accompanied by loss of tensile strength and ductility. Consequently, unexpected failure of equipment without prior warning signs is the primary cause for concern. 

Hydrogen attack can take several forms within the metal structure, depending on the severity of the attack, stress, and the presence of inclusions in the steel. The following discussion will illustrate these. General surface attack occurs when equipment that is not under stress is exposed to hydrogen at elevated temperatures and pressures. As a rule, decarburization is not uniform across the surface or through the thickness instead, it takes place at various locations within the structure. 

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