Regarding the issue of hydrogen embrittlement

Regarding hydrogen embrittlement: 1. Once hydrogen embrittlement occurs, it cannot be eliminated. 　　Hydrogen embrittlement occurs when hydrogen dissolved in steel combines to form hydrogen molecules, resulting in stress concentration that exceeds the steel’s strength limit and leads to the formation of tiny cracks within the steel, also known as white spots. Hydrogen embrittlement can only be prevented, not cured. 　　2 Internal hydrogen embrittlement: Trace amounts of hydrogen (on the order of 10–6) that enter the steel during its smelting process and during the manufacturing and assembly of parts (such as electroplating and welding) can cause the material to become brittle or even crack under the effect of residual or applied stresses. The properties of the steel can be restored through dehydrogenation treatment (for example, by heating to above 200°C for several hours to reduce internal hydrogen) before cracking occurs. Therefore, internal hydrogen embrittlement is reversible. 　　3. Situations where heat treatment is not applicable Heat treatment involves heating the workpiece to a certain temperature, holding it at that temperature for a period of time, and then cooling it slowly, so that hydrogen gradually loses its solubility and precipitates out. However, heating will damage the coating; therefore, heat treatment is not suitable for electroplated workpieces. 　　4. How to prevent and control it. 　　First, try to shorten the pickling time as much as possible ； Secondly, a corrosion inhibitor is added to reduce hydrogen production. 　　Hydrogen embrittlement (or hydrogen damage) of pressure vessels refers to the erosion of their vessel walls by hydrogen, which leads to a reduction in the material’s plasticity and strength, and consequently to cracking or delayed brittle failure. The damage caused by high-temperature and high-pressure hydrogen to steel is mainly due to hydrogen penetrating into the metal in atomic form and then recombining into molecules inside the metal, generating high pressure; in severe cases, this can lead to bulging or wrinkling of the surface ； Hydrogen combines with carbon in steel, causing decarburization of the steel, or reducing the sulfides and oxides present in it. The hydrogen that causes hydrogen embrittlement failure in pressure vessels can be present originally in the equipment; for example, moisture present during steelmaking and welding processes is reduced to produce hydrogen at high temperatures, which then dissolves in the liquid metal. Or, during electroplating or pickling, the hydrogen atoms adsorbed on the steel surface become supersaturated, allowing hydrogen to penetrate into the steel ； It can also be absorbed into the medium after use; for example, in petroleum and chemical containers, there are many mediums that contain hydrogen or impurities such as hydrogen sulfide. The characteristics of hydrogen embrittlement in steel are mainly manifested in the microstructure. On its corroded surface, decarburized ferrite of steel can often be seen, and the hydrogen embrittlement layer features corrosion cracks that extend along the grain boundaries. In containers with particularly severe corrosion, bulges caused by hydrogen embrittlement can be observed macroscopically. Whether a container containing hydrogen (or hydrogen sulfide) in a medium will suffer from hydrogen embrittlement depends mainly on the operating temperature, the partial pressure of hydrogen, the exposure time, and the chemical composition of the steel. The higher the temperature and hydrogen partial pressure, the deeper the hydrogen embrittlement layer in carbon steel, and the shorter the time before hydrogen embrittlement failure occurs; among these factors, temperature is particularly important. The higher the carbon content in steel, the greater its tendency to hydrogen embrittlement under the same temperature and pressure conditions. Adding elements such as chromium, titanium, and vanadium to steel can prevent the occurrence of hydrogen embrittlement. 　　Workpieces affected by hydrogen embrittlement can have this issue eliminated through dehydrogenation treatments such as heating; heating in a vacuum, low-hydrogen atmosphere, or inert atmosphere can also prevent hydrogen embrittlement. For example, in the dehydrogenation of electroplated parts, heating at a temperature of 200–240 degrees for 2–4 hours can remove the vast majority of hydrogen. 　　Hydrogen does not cause significant corrosion of steel at normal temperature and pressure, but when the temperature exceeds 300°C and the pressure is above 30 MPa, a corrosion defect known as hydrogen embrittlement occurs, especially under high-temperature conditions. Such as the desulfurization tower, shift tower, and ammonia synthesis tower in the ammonia synthesis production process ； Some hydrogenation reaction units in the refining process ； Methanol synthesis towers in the petrochemical production process, etc. 　　II: Hydrogen embrittlement – Hydrogen in steel can make the material’s mechanical properties more brittle; this phenomenon is known as hydrogen embrittlement. It mainly occurs in carbon steel and low-alloy steel.

[Generally, the outer cylinder is made of low-alloy high-strength steel, while all internal components are made of nickel-chromium stainless steel; this is not necessary unless required by the manufacturer.]

Hydrogen embrittlement is mainly caused by too rapid heating and cooling during the production process; in addition, hydrogen present in the process gas causes stress-induced deformation of the tower pipes.

In the initial stage of the airtightness test, low-pressure steam is used in the waste heat boiler to heat the synthesis tower to around 100 degrees Celsius. The technician said that this step is taken to prevent hydrogen embrittlement, and it is carried out correctly.

I still don’t understand why raising the temperature of the synthesis tower to 100 degrees Celsius can prevent hydrogen embrittlement.

This post was last edited by davidlzht on 2010-4-24 at 15:09. 6# mawell: Could some expert please explain why, during the warming-up process after a major overhaul, the temperature of the tower wall rises to 100 degrees as the system pressure increases?