What are the similarities and differences between high-temperature hydrogen corrosion and hydrogen embrittlement corrosion?

Hydrogen corrosion refers to the phenomenon where, when steel is in prolonged contact with high-temperature, high-pressure hydrogen gas, hydrogen atoms or molecules react chemically with carbides (cementite) to form methane (Fe3C2 + H2 → 3Fe + CH4). When such a chemical reaction occurs on the surface of the steel, it is called surface decarburization; when it occurs inside the steel, it is called internal decarburization. Internal decarburization and external decarburization are collectively referred to as hydrogen corrosion. Regarding internal decarburization in steel, the methane gas generated cannot diffuse out of the steel; instead, it accumulates between the grains, creating localized high pressures that lead to stress concentration. This in turn causes microcracks or bulging in the steel, resulting in a decrease in its strength and toughness, and even making the steel more brittle. Hydrogen embrittlement is a type of embrittlement that is reversible, whereas hydrogen corrosion results in permanent embrittlement that is irreversible.

In high-temperature hydrogen-containing equipment and devices in contact with aqueous hydrogen sulfide solutions, there are processes involving the addition of hydrogen or its release, which can cause hydrogen-induced damage to such equipment. The types of hydrogen-induced damage include hydrogen blistering: hydrogen atoms penetrate into the steel, and where they encounter cracks, inclusions, and voids within the steel, they gather together to form hydrogen molecules; this leads to an increase in volume and pressure, resulting in blistering of the steel. Hydrogen bubbling can be prevented by using clean steel free from inclusions or layers. Hydrogen embrittlement is a phenomenon in which hydrogen itself causes steel to become brittle. When hydrogen atoms penetrate into steel, they reduce the bonding strength between the steel grains, resulting in a decrease in the elongation and reduction of area of the steel, or leading to delayed failure. If hydrogen is released from the steel, its mechanical properties can still be restored. Hydrogen embrittlement is temporary, and it can be eliminated by heating the steel. Surface decarburization occurs when steel comes into contact with high-temperature hydrogen. Surface decarburization does not cause cracks; its effect is a slight decrease in strength and hardness, along with an increase in elongation. Hydrogen corrosion (internal decarburization): Hydrogen penetrates the steel under high temperature and pressure, and unstable carbides form methane. Methane in steel does not easily escape, causing cracks and bubbling in the steel, as well as a significant reduction in its strength and toughness. Its corrosion reaction is irreversible, resulting in permanent embrittlement.

The original poster has covered a lot of points; let me summarize them: the common feature of high-temperature hydrogen corrosion and hydrogen embrittlement corrosion is that they both occur when the hydrogen partial pressure reaches a certain level； The differences are: (1) The mechanisms of occurrence are different: high-temperature hydrogen corrosion is mainly chemical corrosion, while hydrogen embrittlement corrosion is physical corrosion ； ⑵The conditions under which it occurs are different: high-temperature hydrogen corrosion mainly takes place at high temperatures with a certain hydrogen partial pressure, whereas hydrogen embrittlement can occur at room temperature; of course, certain conditions are also required, namely a specific hydrogen partial pressure and tensile stress ； ⑶The origins of the damage are different: high-temperature hydrogen corrosion is mainly caused by the formation of methane bubbles inside, while hydrogen embrittlement corrosion begins with internal cracking.