Key considerations for selecting high-pressure hydrogenation control valves

Hydrogen possesses various properties such as reducibility, explosiveness, and flammability. At high temperatures, hydrogen molecules break down into atomic form, allowing them to easily penetrate into steel and diffuse within it. Under these conditions, hydrogen diffuses very rapidly in steel; it can react with the carbon in the steel, causing surface decarburization and internal decarburization as well as microcracks. The effects of hydrogen on metal materials (1) Hydrogen blistering Definition: Hydrogen atoms diffuse into the interior of the metal (mostly through the wall), where they combine to form hydrogen molecules that then escape. If hydrogen atoms diffuse into the pores in steel and combine there to form hydrogen molecules, and since these hydrogen molecules cannot diffuse, they accumulate and create high internal pressures, leading to bulging or even cracking of the steel surface—a phenomenon known as hydrogen embrittlement. Low-strength steels, especially those containing a large amount of non-metallic inclusions, are most prone to hydrogen blistering. Corrosion environments that cause hydrogen bulging: The medium usually contains toxins such as hydrogen sulfide, arsenic compounds, cyanides, or phosphorus ions, which prevent the hydrogen evolution reaction. 　 Preventive measures: Eliminate toxic substances in the working environment ； If it cannot be eliminated, choose a calm steel with fewer voids ； Austenitic stainless steel with low hydrogen permeability can also be used ； Or use nickel lining, plastic protective layer, fiberglass lining, etc ； Sometimes a corrosion inhibitor is added. (2) Hydrogen embrittlement Definition: Hydrogen embrittlement occurs when hydrogen dissolved in steel aggregates into hydrogen molecules, resulting in stress concentration that exceeds the steel’s strength limit; this causes significant deformation of the metal lattice and the formation of tiny cracks within the steel, also known as white spots. Hydrogen embrittlement can only be prevented, not cured. Once hydrogen embrittlement occurs, it cannot be eliminated. Measures to avoid and eliminate it: 1. Use materials that are not sensitive to hydrogen embrittlement, such as alloy steels containing Ni and Mo. 2. Reduce the amount of hydrogen absorbed in the metal. If pickling is used, a corrosion inhibitor needs to be added to the pickling solution ； During degreasing, chemical degreasing, cleaning agents, or solvents are used ； During electroplating, the hydrogen absorption amount is low in both alkaline plating baths and plating baths with high current. 3. Use plating coatings with low hydrogen diffusivity and low hydrogen solubility. It is generally believed that when electroplating Zn, Cd, Sn, and Pb, hydrogen that penetrates into the steel tends to remain there, whereas metal platings such as Cu, Mo, Al, Ag, Au, and W have low hydrogen diffusivity and low hydrogen solubility, resulting in less hydrogen penetration. 4. Pre-plating stress relief and post-plating dehydrogenation to eliminate the risk of hydrogen embrittlement, etc. (3) Hydrogen attack: Definition: In high-temperature and high-pressure environments, hydrogen enters the metal and reacts chemically with a certain component or element, resulting in damage to the metal; this is known as hydrogen attack. Preventive measures: Use hydrogen-resistant steel. Options include 16MnR (HIC), 15CrMoR (equivalent to 1Cr-0.5Mo), 14Cr1MoR (equivalent to 1.25Cr-0.5Mo), 2Cr-0.5Mo, 2.25Cr-1Mo, 2.25Cr-1Mo-0.25V, 3Cr-1Mo-0.25V, etc. Cr and Mo in hydrogen-resistant steel can form stable carbides, thereby reducing the chances of hydrogen combining with carbon and preventing the formation of methane gas. In theory, hydrogen corrosion can be divided into the three types mentioned above, but in practice, all three types of corrosion occur almost simultaneously. Therefore, for equipment operating in hydrogen-corrosion environments (hydrogen-rich environments), material selection is generally carried out based on the Nelson curve, and this matter requires great attention. Common anti-hydrogen materials include austenitic stainless steels, precipitation-hardened austenitic alloys, low-alloy steels, aluminum alloys, and copper alloys. According to the provisions in the **Hydrogen Design Code GB50177-2005 regarding valves to be used in hydrogen pipelines: ball valves and globe valves are preferred. :Time: Design pressure (MPa). For materials <0.1, the valve body is made of ductile iron or cast steel; the sealing surface is made of alloy steel or the same material as the valve body. For values between 0.1 and 2.5, the valve stem is made of carbon steel, the valve body is made of cast steel, and the sealing surface is made of alloy steel or the same material as the valve body. For values >2.5, the valve body, valve stem, and sealing surface are all made of stainless steel. Under high temperature and pressure conditions, low-carbon stainless steels such as austenitic stainless steel, as well as low-alloy steels (Mo steel, Cr-Mo steel), are most commonly used for pipelines and valves. In addition, from economic and stress-related perspectives, low-carbon stainless steel is too expensive and prone to stress cracking; therefore, low-alloy steels are preferred for use (such as in refineries, petrochemical plants, ammonia synthesis plants, etc.).

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