Hydrogen embrittlement and hydrogen corrosion in chemical production

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Hydrogen embrittlement is a type of hydrogen corrosion. Hydrogen corrosion is divided into three types: hydrogen blistering, hydrogen embrittlement, and hydrogen erosion. Definition of hydrogen blistering: Hydrogen atoms diffuse into the metal (mostly through the wall), combine on the other side to form hydrogen molecules, and 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. These media prevent the hydrogen evolution reaction. Preventive measures: Eliminate toxic agents ； If it cannot be eliminated, use a calm steel with fewer voids, or an austenitic stainless steel with low hydrogen permeability. Or use nickel lining, rubber-lined lining, plastic protective layer, fiberglass lining, etc ； Sometimes a corrosion inhibitor is added. Definition of hydrogen embrittlement: In high-strength steel, the metal lattice is highly deformed; when hydrogen atoms enter the metal, they increase the lattice strain, thereby reducing toughness and ductility and causing embrittlement. This phenomenon is known as hydrogen embrittlement. Hydrogen embrittlement is unrelated to voids in steel, so relying solely on the use of killed steel is ineffective. Preventive measures: Use materials that are not sensitive to hydrogen embrittlement, such as alloy steels containing Ni and Mo. During the manufacturing process, try to avoid or minimize the generation of hydrogen. Definition of hydrogen embrittlement: Under high temperature and pressure conditions, hydrogen enters the metal and reacts chemically with a certain component or element, resulting in the degradation of the metal; this phenomenon is known as hydrogen embrittlement. At temperatures above 200°C, hydrogen enters low-strength steel and reacts with carbides to produce methane gas. This gas occupies a large volume, causing small cracks and voids within the metal, which in turn makes the steel brittle and prone to breaking under even slight deformation. This kind of rupture occurs without any warning and is extremely dangerous. 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 is divided into three types, but in practice, all three types of corrosion occur almost simultaneously. Therefore, for equipment operating in hydrogen-corrosion environments (hydrogen-exposed environments), material selection is generally carried out based on the Nelson curve, and this matter requires close attention. For reference: http://www.hudong.com/wiki/%E6%B0%A2%E8%85%90%E8%9A%80 http://www.39hg.com/Article/ChmSb/200709/800.html http://bbs.hcbbs.com/viewthread.php?tid=147561 This post was last edited by zhangyong6404 on 2009-3-8 21:37 ]

When hydrogen enters metal materials, it causes significant changes in their mechanical properties; both strength and ductility decrease. The fracture surface of the specimens is no longer cup-cone shaped, but instead exhibits brittle fracture. It must be noted that when steel is exposed to high-temperature and high-pressure hydrogen, the changes in its mechanical properties are caused by two factors: one is the brittleness induced by atomic hydrogen dissolved in the steel’s lattice during subsequent slow deformation (such as in tensile tests); the structure of the steel remains unchanged. If the dissolved hydrogen is removed by standing the steel aside or subjecting it to vacuum heating before deformation, this tendency toward brittleness disappears. This reversible form of brittleness is commonly known as hydrogen embrittlement. Another mechanism is the chemical change in the steel structure caused by high-temperature and high-pressure hydrogen; dehydrogenation cannot restore the properties, and this irreversible brittleness is known as hydrogen embrittlement.

Excerpt from my previous reading report: The damage caused by hydrogen to materials can be classified into two categories based on the temperature at which it occurs: ‘low temperature’ and ‘high temperature’. ※ Damage caused by low-temperature hydrogen: ◎ Hydrogen blistering: This usually occurs in acidic environments at lower temperatures, as a result of the penetration and diffusion of hydrogen atoms into the metal. As hydrogen molecules accumulate over time, pressure builds up, which can lead to localized deformation and damage near the metal surface. ◎ Hydrogen embrittlement: It occurs at lower temperatures, resulting from the penetration of hydrogen atoms into the metal; these atoms then interact complexly with the tips of existing fine cracks that are expanding, leading to a reduction in the material’s ductility and tensile strength. This situation occurs particularly easily when welding high-strength steels, and special care must be taken to avoid it. ※ Damage caused by high-temperature hydrogen: ◎ Decarburization: Typically under high temperature and pressure, decarburization occurs as a result of the reaction between humid hydrogen and the carbon in the material, thereby reducing the material’s tensile strength and hardness. Generally, the most effective preventive method is to use Nelson Diagrams to determine the temperature and pressure ranges suitable for various materials. ◎ Hydrogen attack: The focus of this discussion, detailed as follows. Mechanism of damage caused by high-temperature hydrogen attack: High-temperature hydrogen attack is a type of damage that occurs in an environment of high temperature and high pressure hydrogen. It results from the combination of carbon atoms in the workpiece with hydrogen atoms in the environment, due to the effects of high temperature and high pressure. When the temperature is relatively high and the pressure is low, surface decarburization is the dominant phenomenon ； When the pressure is relatively high and the temperature is relatively low, the main outcome is the formation of methane within the subsurface (Fe3C + 4H = 3Fe + CH4). These phenomena lead to a decrease in the tensile strength and ductility of the material, but they do not necessarily cause changes in the size and thickness of the workpiece; as a result, it is difficult to detect such changes from the outside. In most high-temperature hydrogen attacks, time is not the decisive factor; the main factors remain high temperature and high pressure. Among the methods used to prevent attacks by high-temperature hydrogen, electroplating or surface treatment via welding is ineffective, mainly because hydrogen atoms can still penetrate the protective layer and corrode the base material. An increase in carbon content further reduces resistance to high-temperature hydrogen attacks. Therefore, an effective preventive measure is to use alloy steels containing elements such as Cr, Mo, W, V, Ti, and Nb; these elements help increase the stability of carbon and reduce the extent of damage caused by high-temperature hydrogen. Additionally, since high stresses at the surface or at grain boundaries accelerate the diffusion of hydrogen atoms, reducing the residual stresses in the workpiece surface or in the heat-affected zone of welding can also help. Yet, the most effective method remains the use of appropriate alloy materials.