Hydrogen corrosion

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Hydrogen corrosion: In the ammonia synthesis industry, petroleum hydrocracking, and other chemical processes, mixed gases in which hydrogen constitutes a large proportion of the reaction medium are commonly encountered. Moreover, these chemical reactions take place under high temperature and pressure conditions; for example, the pressure in ammonia synthesis is usually 31.4 MPa, with temperatures ranging from 470 to 500°C. At lower temperatures and pressures (temperature ≤ 200°C, pressure ≤ 4.9 MPa), hydrogen does not have a significant corrosive effect on ordinary carbon steel and low-alloy steel. However, under high temperature and pressure, they are subject to corrosion, which results in a significant decrease in the mechanical strength and ductility of the material, or even its damage. This phenomenon is often referred to as \"red corrosion\" or \"hydrogen embrittlement\". The hydrogen corrosion process of iron-carbon alloys under high temperature and pressure can be divided into a hydrogen embrittlement stage and a hydrogen erosion stage. The first stage is the hydrogen embrittlement stage. At this stage, hydrogen is adsorbed by the steel when in direct contact with it, and diffuses into the steel in an atomic state, dissolving in the ferrite to form a solid solution. However, at this stage, the hydrogen dissolved in the steel does not undergo any chemical reaction with the steel, nor does it alter its microstructure; no cracks can be observed under a microscope, and there are no significant changes in the steel’s strength limit or yield limit. However, it reduces the plasticity of steel and significantly decreases its impact toughness value. This brittleness of steel is directly proportional to the amount of hydrogen dissolved in it. As long as the material is subjected to dehydrogenation treatment when it is in the hydrogen embrittlement stage, its properties can be restored to their original state. The second stage is the hydrogen erosion stage. At this point, the hydrogen dissolved in the steel reacts chemically with the cementite present in the steel to produce methane gas, thereby altering the structure of the steel. The chemical reaction equation is: Fe3C + 2H2 → 3Fe + CH4. This chemical reaction often occurs at the grain boundaries, and the methane gas produced tends to accumulate in the existing micro-pores at those boundaries, creating localized high pressures that lead to stress concentration. This, in turn, causes the grain boundaries to widen, resulting in larger cracks ； Or it may accumulate at defects such as inclusions on the surface of the steel, forming bubbles that reduce the mechanical properties of the steel. On the other hand, since the volume of cementite decreases as it is reduced to ferrite, corresponding structural stresses are generated within the steel due to these volume changes. These stresses, combined with the aforementioned internal stresses, contribute to crack propagation. The propagation of cracks, in turn, provides more favorable conditions for the diffusion and reaction of hydrogen and carbon. If this process continues repeatedly, it eventually leads to complete decarburization of the steel, the formation of a network of cracks within it, a severe decline in its mechanical properties, and even destruction. Since high pressure facilitates the dissolution of hydrogen in steel, while high temperature increases the diffusion rate of hydrogen within the steel’s structure as well as the rate of decarburization, hydrogen corrosion in iron-carbon alloys intensifies as pressure and temperature rise. Typically, hydrogen corrosion in iron-carbon alloys occurs at a certain starting temperature and pressure, which serves as an indicator of the steel’s resistance to hydrogen corrosion.