What Is Hydrogen Embrittlement?

Hydrogen embrittlement is hydrogen dissolved in steel, which aggregates into hydrogen molecules, causing stress concentration, exceeding the strength limit of steel, and forming small cracks inside the steel, also known as white spots. Hydrogen embrittlement is only preventable and cannot be cured. Once hydrogen embrittlement occurs, it cannot be eliminated. During the smelting process of materials and the manufacturing and assembly processes of parts (such as electroplating, welding), the trace hydrogen (in the order of negative 6 to the order of 10) entering the steel interior causes the material to become brittle or even under the influence of internal residual or external stress. Cracking. In the case of no cracking, the performance of the steel can be restored by dehydrogenation treatment (for example, heating to 200 ° C for several hours can reduce the internal hydrogen). Therefore internal hydrogen embrittlement is reversible.

Hydrogen embrittlement

The mechanism of hydrogen embrittlement is still controversial in academic circles, but most scholars believe that the following effects are

First, try to shorten the pickling time as much as possible; second, add a corrosion inhibitor to reduce the amount of hydrogen produced.

Hydrogen embrittlement (or hydrogen damage) of a pressure vessel means that its wall is eroded by hydrogen, which causes the plasticity and strength of the material to decrease, and as a result, cracking or delayed brittle failure. The high-temperature and high-pressure hydrogen damage to steel is mainly due to the infiltration of hydrogen into the metal in the atomic state, and the recombination of molecules inside the metal, which generates a high pressure, which can cause the surface to bulge or wrinkle in severe cases; Combined to decarburize steel or reduce sulfides and oxides in steel. Hydrogen that causes hydrogen embrittlement in the pressure vessel can be originally present in the equipment. For example, moisture during steelmaking and welding is reduced at high temperatures to generate hydrogen, which is dissolved in liquid metals. Or when the equipment is electroplated or pickled, the hydrogen atoms adsorbed on the steel surface are supersaturated, so that the hydrogen penetrates into the steel; it can also be absorbed into the medium after use, for example, in petroleum and chemical containers, there are many mediums. Hydrogen or impurities containing hydrogen sulfide. The characteristic of hydrogen embrittlement in steel is mainly manifested in the microstructure. The decarburized ferrite of steel is often seen on its corrosion surface, and the hydrogen embrittlement layer has corrosion cracks extending along the grain boundaries. Particularly corroded containers, the bulging caused by hydrogen embrittlement can be found on a macro scale. Whether hydrogen embrittlement occurs in a container containing hydrogen (or hydrogen sulfide) in the medium depends mainly on the operating temperature, the partial pressure of hydrogen, the action time, and the chemical composition of the steel. The higher the temperature and the higher the hydrogen partial pressure, the deeper the hydrogen embrittlement layer of the carbon steel is, and the shorter the time for hydrogen embrittlement cracking to occur, among which the temperature is an important factor. The higher the carbon content of steel, the more severe the tendency for hydrogen embrittlement under the same temperature and pressure conditions. Steel is added with chromium, titanium, vanadium and other elements to prevent the generation of hydrogen embrittlement. ^[1]

Hydrogen embrittlement can be eliminated by hydrogen removal treatment (such as heating). Vacuum, low hydrogen atmosphere or inert atmosphere heating can avoid hydrogen embrittlement. For example, the dehydrogenation of electroplated parts is at a temperature of 200 to 240 degrees, and most of the hydrogen can be removed by heating for 2 to 4 hours.

Hydrogen does not cause significant corrosion of steel at normal temperature and pressure, but when the temperature exceeds 300 ° C and the pressure is higher than 30MPa, the corrosion defect of hydrogen embrittlement will occur, especially under high temperature conditions. Such as desulfurization tower, conversion tower, ammonia synthesis tower in the production of synthetic ammonia; some hydrogenation reaction devices in the refining process; methanol synthesis tower in the petrochemical production process.

Two: hydrogen embrittlement-the hydrogen in the steel will embrittle the mechanical properties of the material, this phenomenon is called hydrogen embrittlement. It mainly occurs in carbon steel and low alloy steel.

(1) To reduce the amount of hydrogen permeation in the metal, the pickling of high-strength / high-hardness steel fasteners must be minimized, because pickling can exacerbate hydrogen embrittlement. When removing rust and scale, use sand blasting as much as possible. If fasteners with Rockwell hardness equal to or greater than HRC 32 are pickled, you must ensure that the parts are immersed in acid during the pickling process. No longer than 10 minutes. The concentration of the acid solution should be reduced as much as possible, and the parts should be immersed in acid for less than 10 minutes. When removing oil, use chemical degreasing methods such as detergent or solvent degreasing to reduce the amount of hydrogen penetration. If electricity is used, Chemical degreasing, cathode first, anode first, high-strength parts are not allowed to use the cathode to degrease the oil; during the heat treatment, strictly control the dripping amount of methanol and propane; during electroplating, alkaline plating solution or high current efficiency plating solution infiltration The amount of hydrogen is small.

(2) Using a coating with low hydrogen diffusivity and low hydrogen solubility It is generally believed that when plating Cr, Zn, Cd, Ni, Sn, Pb, hydrogen that penetrates into steel parts is easy to remain, while Cu, Mo, Al , Ag, Au, W and other metal coatings have low hydrogen diffusion and low hydrogen solubility, less hydrogen permeation. In the case of meeting the technical requirements of the product, coatings that do not cause hydrogen permeation, such as mechanical zinc plating or chromium-free zinc-aluminum coating, can be used without hydrogen embrittlement, high corrosion resistance, good adhesion, and Environmental protection.

(3) Remove stress before plating and remove hydrogen after plating to eliminate hidden dangers of hydrogen embrittlement. If the internal residual stress of the part is large after quenching, welding, and other processes, tempering should be performed before plating. Tempering to relieve stress can actually reduce the internal stress of the part. The number of traps, thereby mitigating the hidden dangers of hydrogen embrittlement.

Control the thickness of the plating layer. As the plating layer covers the surface of the fastener, the plating layer acts as a hydrogen diffusion barrier to a certain extent, which will hinder the diffusion of hydrogen to the outside of the fastener. When the thickness of the coating exceeds 2.5 m, it is very difficult for hydrogen to diffuse out of the fastener. Therefore, the coating thickness of fasteners with a hardness of less than 32HRC can be required to be 12m; for high-strength bolts with a hardness of 32HRC, the coating thickness should be controlled at 8mmax. This requires that in product design, the risk of hydrogen embrittlement of high-strength bolts must be taken into account, and the type and thickness of the coating should be selected reasonably.