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Metal hydrogen embrittlement Reasons and methods for removing hydrogen embrittlement

Metal hydrogen embrittlement Reasons and methods for removing hydrogen embrittlement

In any electroplating solution, due to the dissociation of water molecules, there is always a certain amount of hydrogen ions present to varying degrees. Therefore, during the electroplating process, metal precipitation at the cathode (main reaction) is accompanied by the precipitation of hydrogen gas (side reaction). The influence of hydrogen evolution is multifaceted, and hydrogen embrittlement being the most significant. Hydrogen embrittlement is one of the most serious quality hazards in surface treatment, and parts with severe hydrogen evolution may break during use, causing serious accidents. Surface treatment technicians must master techniques to avoid and eliminate hydrogen embrittlement, in order to minimize the impact of hydrogen embrittlement.

1. Hydrogen embrittlement

A. Hydrogen embrittlement phenomenon

Hydrogen embrittlement typically manifests as delayed fracture under stress. Galvanized parts such as automotive springs, washers, screws, and leaf springs have experienced fractures within hours of assembly, with a fracture rate of 40% to 50%. A certain special product cadmium plated parts have experienced batch cracking and fracture during use, and a nationwide research and development was organized to formulate strict dehydrogenation processes. In addition, there are some hydrogen embrittlement phenomena that do not manifest as delayed fracture. For example, electroplating hangers (steel wires, copper wires) often suffer from severe hydrogen permeation due to multiple electroplating and acid pickling processes, resulting in brittle fracture upon bending during use; The core rod used for precision forging of hunting rifles, after multiple chrome plating, fell to the ground and broke; Some quenched parts (with high internal stress) may crack during acid washing. These parts have severe hydrogen permeation and can crack without external stress, making it impossible to restore their original toughness by removing hydrogen.

B. Hydrogen embrittlement mechanism

The occurrence of delayed fracture phenomenon is due to the diffusion and accumulation of hydrogen inside the part towards the stress concentration area, where there are many metal defects (such as atomic lattice displacement, holes, etc.). Hydrogen diffuses to these defects, turning hydrogen atoms into hydrogen molecules and generating enormous pressure. This pressure, along with residual stresses inside the material and external stresses acting on the material, forms a resultant force. When this resultant force exceeds the yield strength of the material, it can lead to fracture. Since hydrogen embrittlement is related to the diffusion of hydrogen atoms, diffusion requires time, and the diffusion rate is related to concentration gradient, temperature, and material type. Therefore, hydrogen embrittlement usually manifests as delayed fracture.

Hydrogen atoms have the smallest atomic radius and are easily diffused in metals such as steel and copper, while hydrogen diffusion is more difficult in cadmium, tin, zinc, and their alloys. The cadmium plating layer is the most difficult to diffuse, and the hydrogen generated during cadmium plating initially stays in the plating layer and the metal surface below the plating layer, making it difficult to diffuse outward, making hydrogen removal particularly challenging. After a period of time, hydrogen diffuses into the interior of the metal, especially hydrogen that enters the defects inside the metal, making it difficult to diffuse out. The diffusion rate of hydrogen at room temperature is quite slow, so it needs to be heated immediately to remove hydrogen. The increase in temperature increases the solubility of hydrogen in steel, and excessively high temperatures can reduce the hardness of the material. Therefore, the selection of pre plating stress and post plating hydrogen removal temperatures must consider not reducing the hardness of the material, and must not be at the brittle tempering temperature of certain steels, without damaging the performance of the coating itself.

2. Measures to avoid and eliminate

A. Reduce the amount of hydrogen permeation in metals

When removing rust and oxide skin, sand blowing should be used as much as possible. If acid washing is used, corrosion inhibitors such as Rhodamine should be added to the acid washing solution; When removing oil, chemical oil removal, cleaning agents or solvents are used to remove oil, with less hydrogen permeation. If electrochemical oil removal is used, the cathode should be removed first and then the anode; During electroplating, alkaline plating solutions or high current efficiency plating solutions have less hydrogen permeation.

B. Adopting a coating with low hydrogen diffusion and low hydrogen solubility

It is generally believed that hydrogen infiltrated into steel during electroplating of Cr, Zn, Cd, Ni, Sn, Pb is prone to residue, while metal coatings such as Cu, Mo, Al, Ag, Au, W have low hydrogen diffusion and solubility, resulting in less hydrogen permeation. Under the condition of meeting the technical requirements of the product, coatings that do not cause hydrogen permeation can be used, such as Dacromet coating layer, which can replace galvanizing, without hydrogen embrittlement, corrosion resistance increased by 7-10 times, good adhesion, film thickness of 6-8um, equivalent to a thinner galvanized layer, without affecting assembly.

C. Pre plating stress and post plating hydrogen removal to eliminate hydrogen embrittlement hazards

If there is significant residual stress inside the parts after quenching, welding, and other processes, tempering treatment should be carried out before plating to reduce the risk of serious hydrogen permeation.

In principle, parts with high hydrogen permeation during the electroplating process should be dehydrogenated as soon as possible, because the hydrogen in the coating and the hydrogen in the surface substrate metal diffuse into the steel substrate, and their quantities increase with time. The new international standard draft stipulates that "it is best to perform dehydrogenation treatment within 1 hour after plating, but not later than 3 hours". There are also corresponding standards in China that specify the dehydrogenation treatment before and after electroplating. The dehydrogenation treatment process after electroplating widely adopts heating baking, with a commonly used baking temperature of 150-300 ℃ and insulation for 2-24 hours. The specific processing temperature and time should be determined based on the size, strength, coating properties, and plating time of the parts. Dehydration treatment is often carried out in an oven. The dehydrogenation treatment temperature for galvanized parts is 110-220 ℃, and the temperature control should be determined according to the substrate material. For elastic materials, thin-walled parts below 0.5mm, and steel parts with high mechanical strength requirements, dehydrogenation treatment must be carried out after galvanizing. In order to prevent cadmium embrittlement, the dehydrogenation temperature of cadmium plated parts should not be too high, usually 180-200 ℃.

3. Issues to be noted

The higher the strength of the material, the greater its sensitivity to hydrogen embrittlement, which is a basic concept that surface treatment technicians must clarify when preparing electroplating process specifications. International standards require steel with a tensile strength of σ b>105kg/mm2 to undergo corresponding pre plating stress and post plating dehydrogenation treatment. The French aviation industry requires corresponding dehydrogenation treatment for steel parts with a yield strength of σ s>90kg/mm2.

Due to the good correlation between steel strength and hardness, using material hardness to determine hydrogen embrittlement sensitivity is more intuitive and convenient than using strength. Because a complete product drawing and machining process should indicate the hardness of the steel. In electroplating, we found that the hardness of steel begins to show the risk of hydrogen embrittlement fracture when it is around HRC38. For parts 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 steel, the following points should also be considered comprehensively:

       ①    Safety factor for the use of parts: For parts with high safety importance, hydrogen removal should be strengthened;

② Geometric shape of parts: Parts with notches that are prone to stress concentration, such as small R, should be strengthened for dehydrogenation;

③ The cross-sectional area of the parts: Small spring steel wires and thinner leaf springs are easily saturated with hydrogen, and hydrogen removal should be strengthened;

④ The degree of hydrogen permeation in parts: For parts that generate more hydrogen during surface treatment and have a longer processing time, hydrogen removal should be strengthened;

⑤ Coating type: If cadmium plating is applied, it will seriously block the outward diffusion of hydrogen, so it is necessary to strengthen hydrogen removal;

⑥ The force properties during the use of parts: When the part is subjected to high tensile stress, hydrogen removal should be strengthened, and hydrogen embrittlement will not occur only under compressive stress;

⑦ Surface processing status of parts: For parts with high internal residual stress such as cold bending, stretching, cold bending, quenching, welding, etc., not only should hydrogen removal be strengthened after plating, but also stress removal should be carried out before plating;

⑧ Historical situation of parts: Special attention should be paid to parts that have experienced hydrogen embrittlement in past production, and relevant records should be kept.

Remove hydrogen embrittlement

The main reason is the phenomenon of metal "hydrogenation" caused by the electroplating process, and the unqualified products you use are not caused by the electroplating process itself, because electroplating (except vacuum plating) originally causes metal hydrogenation, but currently many metal surface treatment companies have removed the last process (especially fatal for elastic components): the "dehydrogenation treatment" process, which means that under normal circumstances, metal parts with strength requirements need to be dehydrogenated before being handed over to users. However, in order to save production costs, if users do not understand or have never requested or accepted it, omitting this process can save 5-15% of costs. So you feel that the bolts, spring washers, and other parts after electroplating become brittle.

Generally speaking, the dehydrogenation treatment requirement for metal parts with strength requirements is to maintain a high temperature of 120 ° C to 220 ° C for 1-2 hours (after electroplating), and the specific situation needs to be controlled according to the requirements of the parts.