Causes and solutions of metal hydrogen embrittlement

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I. Hydrogen embrittlement 1. Phenomenon of hydrogen embrittlement Hydrogen embrittlement typically manifests as delayed fracture under stress. There were instances of galvanized parts such as car springs, washers, screws, and leaf springs that fractured within a few hours after assembly, with a fracture rate of 40% to 50%. Cadmium-plated components of a certain specialty product experienced batch failures due to cracking during use; a national-level research effort was launched to develop strict dehydrogenation processes. Furthermore, some cases of hydrogen embrittlement do not manifest as delayed fracture; for example, electroplating hangers (steel wires, copper wires) suffer from severe hydrogen absorption due to repeated electroplating and acid washing processes, and often experience brittle fracture as soon as they are bent during use ； The mandrel used for shot gun precision forging broke after being chromium-plated multiple times and falling to the ground ； Some quenched parts (with high internal stress) develop cracks during pickling. These parts are severely hydrogen-impregnated; cracks occur without any external stress, and it is no longer possible to restore their original toughness through dehydrogenation. 2. Hydrogen embrittlement mechanism: The occurrence of delayed fracture is due to the diffusion and accumulation of hydrogen within the part toward areas of stress concentration, where there are numerous metal defects (such as atomic lattice dislocations and vacancies). Hydrogen diffuses into these defects, where hydrogen atoms combine to form hydrogen molecules, generating tremendous pressure. This pressure, together with the residual stress within the material and any external stresses acting on it, constitutes a resultant force. When this resultant force exceeds the material’s yield strength, fracture occurs. Since hydrogen embrittlement is related to the diffusion of hydrogen atoms, and diffusion takes time, the rate of diffusion depends on the concentration gradient, temperature, and type of material. Therefore, hydrogen embrittlement usually manifests as delayed fracture. The hydrogen atom has the smallest atomic radius, allowing it to diffuse easily in metals such as steel and copper; however, hydrogen diffusion is more difficult in cadmium, tin, zinc, and their alloys. The cadmium plating layer is the most difficult to diffuse through. The hydrogen generated during cadmium plating remains initially within the plating layer and on the surface of the metal beneath it, making it very difficult for it to diffuse outward; degassing is particularly challenging. After some time, hydrogen diffuses into the interior of the metal; in particular, hydrogen that has entered the defects within the metal finds it difficult to diffuse out. At room temperature, the diffusion rate of hydrogen is quite slow, so immediate heating is required for dehydrogenation. An increase in temperature raises the solubility of hydrogen in steel; however, excessively high temperatures can reduce the material’s hardness. Therefore, when selecting temperatures for stress relief prior to plating and hydrogen removal after plating, it is essential to ensure that these temperatures do not diminish the material’s hardness, remain below the brittle tempering temperatures for certain steels, and do not compromise the properties of the plating itself. II. Measures to avoid and eliminate it: 1. Reduce the amount of hydrogen absorbed in metals. When removing rust and scale, sandblasting should be used as much as possible; if acid washing is employed, corrosion inhibitors such as rosin should be added to the acid wash solution ； During oil removal, chemical methods, cleaning agents, or solvents are used, resulting in less hydrogen absorption; if electrochemical oil removal is employed, cathodic treatment is carried out first followed by anodic treatment ； During electroplating, alkaline plating baths or plating baths with high current efficiency result in less hydrogen penetration. 2. Use plating coatings with low hydrogen diffusivity and low hydrogen solubility. It is generally believed that when electroplating Cr, Zn, Cd, Ni, Sn, Pb, hydrogen that penetrates into the steel tends to remain there, whereas metal platings such as Cu, Mo, Al, Ag, Au, W have low hydrogen diffusivity and low hydrogen solubility, resulting in less hydrogen penetration. Where the product’s technical requirements are met, coatings that do not cause hydrogen infiltration can be used; for example, Dacromet coating can replace galvanizing. It prevents hydrogen embrittlement, increases corrosion resistance by 7 to 10 times, has good adhesion, and a film thickness of 6 to 8 um – which is equivalent to a relatively thin layer of galvanizing – without affecting assembly. 3. Pre-plating stress relief and post-plating dehydrogenation are necessary to eliminate the risk of hydrogen embrittlement. If residual stresses are high inside the parts after processes such as quenching and welding, tempering should be carried out before plating to reduce the risk of severe hydrogen absorption. In principle, parts that absorb a large amount of hydrogen during the electroplating process should be dehydrogenated as soon as possible, because the hydrogen in the coating and that in the surface base metal diffuse into the steel matrix, with their quantity increasing over time. The new draft international standard specifies that “dehydrogenation should be carried out within 1 hour after plating, but no later than 3 hours”. There are also corresponding domestic standards that specify the dehydrogenation treatments before and after electro-galvanizing. The dehydrogenation treatment after electroplating commonly employs heating in an oven; the typical baking temperature ranges from 150 to 300°C, with a holding time of 2 to 24 hours. The specific treatment temperature and time should be determined based on the size and strength of the parts, the properties of the coating, and the duration of the electroplating process. Dehydrogenation is often carried out in an oven. The dehydrogenation temperature for galvanized parts is 110–220°C, and the level of temperature control should be determined based on the base material. For elastic materials, thin-walled parts with a thickness of less than 0.5 mm, and steel parts requiring high mechanical strength, dehydrogenation treatment must be carried out after galvanizing. To prevent \"cadmium embrittlement,\" the dehydrogenation temperature for cadmium-plated parts must not be too high; it is usually between 180 and 200°C. III. Issues to Consider The greater the strength of a material, the greater its susceptibility to hydrogen embrittlement; this is a fundamental concept that surface treatment technicians must keep in mind when developing electroplating process specifications. International standards require that steel with a tensile strength of σb > 105 kg/mm2 undergo corresponding pre-plating stress relief and post-plating dehydrogenation treatments. The French aerospace industry requires corresponding dehydrogenation treatment for steel components with a yield strength σｓ > 90 kg/mm2. Since there is a good correlation between the strength and hardness of steel, it is more intuitive and convenient to use material hardness to determine its susceptibility to hydrogen embrittlement than to use strength. Because both a complete product diagram and the machining process should specify the steel hardness. In electroplating, we find that steel begins to face the risk of hydrogen embrittlement fracture at a hardness of around HRC38. For parts with a hardness higher than HRC43, dehydrogenation treatment should be considered after plating. When the hardness is around HRC60, dehydrogenation treatment must be carried out immediately after surface treatment; otherwise, the steel part will crack within a few hours. In addition to the hardness of the steel, the following factors should also be taken into consideration: ① Safety factor for component use: For components where safety is of high importance, dehydrogenation processes should be intensified ； ②Geometric shape of the parts: Parts with notches that can easily lead to stress concentration, as well as those with small R-values, should be reinforced to reduce degassing ； ③Cross-sectional area of the parts: Fine spring wires and thin leaf springs are highly susceptible to hydrogen saturation; therefore, dehydrogenation measures should be strengthened ； ④Degree of hydrogen absorption in parts: For parts that generate a large amount of hydrogen during surface treatment and require a long treatment time, hydrogen removal should be intensified ； ⑤Types of coatings: For example, a cadmium coating can significantly hinder the outward diffusion of hydrogen; therefore, enhanced dehydrogenation measures are necessary ； ⑥Stress conditions during part use: Dehydrogenation should be intensified when the part is subjected to high tensile stress; hydrogen embrittlement does not occur when only compressive stress is applied ； ⑦Surface finish of parts: For parts that have high residual stresses from processes such as cold bending, drawing, cold rolling, quenching, and welding, it is necessary to carry out dehydrogenation after plating, as well as stress relief before plating ； ⑧History of the parts: Special attention should be paid to parts that have experienced hydrogen embrittlement during previous production, and relevant records must be kept. Hydrogen embrittlement is primarily caused by the \"hydrogenation\" of metals during the plating process. The substandard products you are using are not the result of an issue with the plating process itself; after all, plating (with the exception of vacuum plating) does cause metal hydrogenation. However, many metal surface treatment manufacturers have omitted the final step in this process – a step that is particularly fatal for elastic components – namely the \"dehydrogenation\" process. Under normal circumstances, metal parts that require high strength must undergo dehydrogenation before being delivered to customers. But in order to save production costs, and since customers may not be aware of this or may never have requested or inspected it, skipping this step can result in cost savings of 5–15%. Therefore, you notice that parts such as bolts and spring washers become “brittle” after being electroplated. Generally speaking, for metal parts that require certain strength levels, the dehydrogenation treatment involves maintaining the part at a temperature of 120 degrees to 220 degrees for 1 to 2 hours (after electroplating); the specific conditions must be determined according to the requirements of the particular part.