The Difference Between Hydrogen Attack (HTHA) and Hydrogen Embrittlement (HE): A Comprehensive Analysis and Preventive Measures

The Difference Between Hydrogen Attack (HTHA) and Hydrogen Embrittlement (HE): A Comprehensive Analysis and Preventive Measures

In the field of industrial production, hydrogen damage to metal materials has always been a key factor affecting the safe operation of equipment. Among them, Hydrogen Attack (HTHA) and Hydrogen Embrittlement (HE), as two typical hydrogen damage phenomena, are often confused due to their association with hydrogen, but they are essentially significantly different. An in-depth understanding of their differences is of great significance for the material selection, maintenance, and safety assurance of industrial equipment.

What are Hydrogen Attack (HTHA) and Hydrogen Embrittlement (HE)?

Although both Hydrogen Attack (High Temperature Hydrogen Attack, HTHA for short) and Hydrogen Embrittlement (Hydrogen Embrittlement, HE for short) are related to hydrogen-induced damage to materials, their occurrence conditions, mechanisms, and impacts are quite different.

Hydrogen attack mainly occurs in high-temperature and high-pressure hydrogen environments and is an irreversible chemical damage process. Hydrogen embrittlement, on the other hand, can occur over a wide temperature range. It is a physical embrittlement phenomenon caused by the synergistic effect of hydrogen atoms and stress, and material properties can be restored through appropriate treatment in the early stage.

Key Differences Between Hydrogen Attack and Hydrogen Embrittlement

Occurrence Conditions

Hydrogen attack has strict requirements on ambient temperature and pressure. It usually occurs in high-temperature (≥220℃) and high-pressure hydrogen environments, where conditions promote chemical reactions between hydrogen and components such as carbides in metals. Hydrogen embrittlement has relatively loose occurrence conditions and can occur at room temperature or medium temperature. Its core is that hydrogen atoms can penetrate into the metal interior and need to act synergistically with the stress borne by the material to cause embrittlement. For example, hydrogen embrittlement may pose safety hazards in some hydrogen transmission pipelines operating at room temperature.

Differences in Damage Mechanisms

The damage mechanisms of the two are fundamentally different. The mechanism of hydrogen attack is that hydrogen reacts chemically with carbides (such as Fe₃C) in metals to generate methane (CH₄) gas. These methane gases continuously accumulate at metal grain boundaries, gradually forming local high pressure. When the pressure reaches a certain level, it will cause cracks and bulges at grain boundaries, seriously damaging the structural integrity of the metal. Hydrogen embrittlement is the accumulation of hydrogen atoms at defects such as grain boundaries and dislocations in the metal. These accumulated hydrogen atoms will interfere with dislocation movement or form high-pressure hydrogen micro-regions, thereby reducing the internal bonding force of the material, decreasing its toughness, and increasing its brittleness.

Reversibility Comparison

In terms of reversibility, hydrogen attack is an irreversible damage process. Once decarburization occurs in the metal and grain boundaries are damaged, the mechanical properties and structural stability of the material cannot be restored. Even if the hydrogen environment is removed, the damage already caused cannot be changed. Hydrogen embrittlement, however, is partially reversible. For mild hydrogen embrittlement, the performance of the material can be restored to a certain extent by dehydrogenation treatment (such as appropriate heat treatment) to remove hydrogen atoms inside the metal. But if the hydrogen embrittlement is severe and has caused obvious cracks and other damages to the material, it will become irreversible.

Common Application Scenarios

Hydrogen attack is common in high-temperature hydrogen-exposed equipment such as petroleum hydrogenation units and synthetic ammonia equipment. These equipment operate in high-temperature and high-pressure hydrogen environments for a long time, and metal materials are prone to meet the conditions for hydrogen attack. Hydrogen embrittlement has a wider range of application scenarios, mostly occurring in hydrogen transmission pipelines, various hydrogen-exposed components, and the use of high-strength steel in hydrogen-containing environments. For example, pressure vessels made of high-strength steel may face the risk of hydrogen embrittlement if they are in a hydrogen-containing environment.

Material Selection and Preventive Measures

Prevention of Hydrogen Attack

For hydrogen attack, it is necessary to select hydrogen attack-resistant alloys containing elements such as Cr and Mo. For example, 15CrMo steel is one of the commonly used hydrogen attack-resistant materials. At the same time, the Nelson curve is usually used to evaluate the applicability of materials in specific high-temperature and high-pressure hydrogen environments.

Prevention of Hydrogen Embrittlement

For the prevention and control of hydrogen embrittlement, low-strength steel or alloy steel containing Ni and Mo is preferred in material selection. The use of high-strength martensitic steel should be avoided because high-strength martensitic steel is more sensitive to hydrogen embrittlement and prone to hydrogen embrittlement failure.

Summary

In summary, although both hydrogen attack and hydrogen embrittlement are hydrogen damage phenomena, they have obvious differences in occurrence conditions, mechanisms, reversibility, application scenarios, and material selection. In industrial production, it is necessary to accurately judge the possible type of hydrogen damage according to specific operating conditions and take targeted prevention and control measures to ensure the safe and stable operation of equipment.

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