High Temperature Hydrogen Attack

High-temperature hydrogen attack (HTHA) results from the action of hydrogen at elevated temperature and pressure, causing internal decarburization of carbon and low-alloy steels. It is typically observed in hydroprocessing units (hydrotreaters/hydrocrackers) and hydrogen-producing units, posing a serious risk to equipment integrity. Traditionally, the propensity for HTHA is determined using empirically developed curves (commonly known as Nelson curves), which define the boundaries for HTHA under various hydrogen partial pressures and temperatures. The forthcoming chapter will delve into the primary characteristics of HTHA, highlight essential parameters, and provide a tool for estimating susceptibility to HTHA.

General Information

At high temperatures and elevated partial pressures, hydrogen can attack carbon and low-alloy steels by reacting with carbon and/or carbides to form methane. These reactions may occur either on the metal surface or within the metal lattice, leading to decarburization (surface reactions) and the formation of microcracks or blisters (internal reactions). Internal defects like cracks, blisters, or voids can significantly weaken the steel’s mechanical properties, potentially resulting in catastrophic failure. 1 ,2

Mechanism

The HTHA process is driven by reactions between hydrogen and carbon (predominantly in the form of cementite, Fe₃C) present in carbon steel and low alloy steels, which can be simplistically described by Reaction 1:

\(\ce{Fe3C + 2H2 -> 3Fe + CH4} \quad \text{(Reaction 1)}\)

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References

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