Difference between hydrogen embrittlement (HTHA) and hydrogen cracking (HE)

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Differences between hydrogen embrittlement (HTHA) and hydrogen cracking (HE): Hydrogen embrittlement HTHA and hydrogen cracking HE are two distinct types of hydrogen-induced damage; they differ significantly in terms of occurrence conditions, mechanisms, and reversibility. I. Occurrence conditions: Hydrogen attack: It usually occurs in high-temperature (≥220°C) and high-pressure hydrogen environments, where hydrogen reacts chemically with components such as carbides in the metal. Hydrogen embrittlement: It can occur at room temperature or moderate temperatures, as hydrogen atoms penetrate into the metal and, in combination with stress, cause the material to become brittle. II. Mechanism: Hydrogen attack: Hydrogen reacts with carbides in the metal (such as Fe₃C) to produce methane (CH₄). Methane accumulates at the grain boundaries, creating localized high pressures that lead to grain boundary cracks and bulging. Hydrogen embrittlement: Hydrogen atoms accumulate at defects such as grain boundaries and dislocations, interfering with the movement of dislocations or forming high-pressure hydrogen gas, thereby reducing the strength of the material. III. Reversibility of hydrogen embrittlement: It is irreversible; once decarburization and grain boundary damage occur, the material properties cannot be restored. Hydrogen embrittlement: Partially reversible; the material’s properties can be restored through dehydrogenation treatments such as heat treatment, but it can be irreversible in severe cases. IV. Typical Application Scenarios: Hydrogen embrittlement: Commonly occurs in high-temperature hydrogen-containing equipment such as petroleum hydrogenation and ammonia synthesis. Hydrogen embrittlement: It occurs frequently in hydrogen transport pipelines, components exposed to hydrogen, or when high-strength steel is used in hydrogen-containing environments. V. Material Selection – Hydrogen corrosion: It is necessary to use hydrogen-resistant alloys containing elements such as Cr and Mo (such as 15CrMo steel), and the suitability of the material is assessed using Nelson curves. Hydrogen embrittlement: Prefer low-strength steels or alloy steels containing Ni and Mo, and avoid using high-strength martensitic steels. VI. Summary: Hydrogen embrittlement is an irreversible damage caused by chemical reactions under high temperature and pressure, while hydrogen cracking is a reversible embrittlement resulting from the combined effect of hydrogen and stress at normal temperatures. Although both are related to hydrogen, their mechanisms and prevention strategies differ significantly; therefore, targeted measures must be chosen based on the specific operating conditions.