In any electroplating solution, due to the dissociation of water molecules, there is always a certain amount of hydrogen ions present. Therefore, during the electroplating process, while the metal is precipitated at the cathode (the main reaction), it is accompanied by the hydrogen gas evolution (side reaction). The impact of hydrogen evolution is manifold, the most important of which is hydrogen embrittlement. Hydrogen embrittlement is one of the most serious quality hazards in surface treatment. Parts with severe hydrogen evolution may break during use, causing serious accidents. Surface treatment technicians must master the technology of avoiding and eliminating hydrogen embrittlement, so as to minimize the impact of hydrogen embrittlement.
1. Hydrogen embrittlement
1 Hydrogen embrittlement
Hydrogen embrittlement usually manifests itself as a delayed fracture phenomenon under stress. There have been galvanized parts such as automobile springs, washers, screws, and leaf springs, which broke within a few hours after assembly, and the fracture ratio reached 40% to 50%. A batch of cracks and fractures occurred in the cadmium-plated parts of a special product during use, and a national research was organized to formulate a strict dehydrogenation process. In addition, some hydrogen embrittlement does not manifest as a delayed fracture phenomenon. For example, the electroplating rack (steel wire, copper wire) has serious hydrogen permeation due to repeated electroplating and pickling deplating, and it often occurs when it is folded during use. The phenomenon of brittle fracture; the mandrel used for precision forging of shotguns, after repeated chrome plating, fell to the ground and broke; some quenched parts (with large internal stress) cracked during pickling. These parts have serious hydrogen penetration, and cracks will occur without external stress, and the original toughness can no longer be restored by dehydrogenation.
2 Mechanism of hydrogen embrittlement
The phenomenon of delayed fracture is due to the diffusion and accumulation of hydrogen inside the part to the stress concentration part, and there are many metal defects (atomic lattice dislocation, holes, etc.) in the stress concentration part. Hydrogen diffuses to these defects, and hydrogen atoms become hydrogen molecules, generating huge pressure. This pressure, the residual stress inside the material and the external stress on the material form a resultant force. When the resultant force exceeds the yield strength of the material, it will cause Fracture happens. Since hydrogen embrittlement is related to the diffusion of hydrogen atoms, the diffusion takes time, and the diffusion speed is related to the concentration gradient, temperature and material type. Therefore, hydrogen embrittlement usually manifests as delayed fracture.
Hydrogen atoms have the smallest atomic radius and are easy to diffuse in metals such as steel and copper, but it is difficult to diffuse hydrogen in cadmium, tin, zinc and their alloys. The cadmium plating layer is the most difficult to diffuse. The hydrogen generated during cadmium plating initially stays in the plating layer and the metal surface under the plating layer, and it is difficult to diffuse outwards. It is particularly difficult to remove hydrogen. After a period of time, hydrogen diffuses into the metal, especially the hydrogen that enters the defects inside the metal, it is difficult to diffuse out. The diffusion rate of hydrogen is quite slow at normal temperature, so immediate heating is required to dehydrogenate. An increase in temperature will increase the solubility of hydrogen in steel. Excessively high temperature will reduce the hardness of the material. Therefore, the temperature selection for stress relief before plating and hydrogen removal after plating must be considered so as not to reduce the hardness of the material. The brittle tempering temperature does not destroy the performance of the coating itself.
2. Measures to avoid and eliminate
1. Reduce the amount of hydrogen permeation in the metal
When removing rust and scale, try to use sand blowing to remove rust. If pickling is used, add rhodin and other corrosion inhibitors to the pickling solution; when removing oil, use chemical degreasing, cleaning agent or solvent to remove oil. , the amount of hydrogen permeation is less, if electrochemical degreasing is used, the cathode is first and then the anode; during electroplating, the amount of hydrogen permeation is less in the alkaline bath or the bath with high current efficiency.
2. Plated coating with low hydrogen diffusivity and low hydrogen solubility
It is generally believed that when electroplating Cr, Zn, Cd, Ni, Sn, Pb, the hydrogen permeated into the steel is easy to remain, while Cu, Mo, Al, Ag, Au, W and other metal coatings have low hydrogen diffusivity and low hydrogen Solubility, less hydrogen permeation. In the case of meeting the technical requirements of the product, a coating that will not cause hydrogen permeation can be used. For example, Dacromet coating can replace galvanized coating, which will not cause hydrogen embrittlement, improve corrosion resistance by 7 to 10 times, and improve adhesion. Well, the film thickness is 6-8um, which is equivalent to a thinner galvanized layer and does not affect assembly.
3. Stress relief before plating and hydrogen removal after plating to eliminate the hidden danger of hydrogen embrittlement
If the internal residual stress of the part is relatively large after quenching, welding and other processes, it should be tempered before plating to reduce the hidden danger of serious hydrogen penetration.
In principle, the parts with more 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 matrix metal are diffusing into the steel matrix, and the amount increases with time. The new draft international standard stipulates that "it is best to carry out dehydrogenation treatment within 1h after plating, but not later than 3h". There are also corresponding domestic standards, which stipulate the dehydrogenation treatment before and after electro-galvanizing. Heating and baking is widely used in the dehydrogenation treatment process after electroplating. The commonly used baking temperature is 150-300 ° C, and the heat preservation is 2-24 hours. The specific treatment temperature and time should be determined according to the size, strength, properties of the coating and the length of the plating time. Dehydrogenation is often carried out in an oven. The dehydrogenation treatment temperature of galvanized parts is 110-220 ° C, and the temperature control level should be determined according to the base 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 brittleness", the dehydrogenation treatment temperature of cadmium-plated parts should not be too high, usually 180-200 °C.
3. Problems that should be paid attention to
The greater the strength of the material, the greater its susceptibility to hydrogen embrittlement. This is a basic concept that surface treatment technicians must clarify when compiling electroplating process specifications. International standards require that steels with tensile strength σb>105kg/mm2 should be subjected to corresponding stress relief before plating and dehydrogenation treatment after plating. The French aviation industry requires corresponding dehydrogenation treatment for steel parts with yield strength σs>90kg/mm2.
Since there is a good correspondence between steel strength and hardness, it is more intuitive and convenient to judge the hydrogen embrittlement susceptibility of materials by material hardness than by strength. Because a complete product drawing and machining process should indicate the steel hardness. In electroplating, we found that when the hardness of steel is around HRC38, there is a danger of hydrogen embrittlement fracture. For parts higher than HRC43, dehydrogenation treatment should be considered after plating. When the hardness is about HRC60, dehydrogenation treatment must be carried out immediately after surface treatment, otherwise the steel part will crack within a few hours.
In addition to steel hardness, the following points should also be considered comprehensively:
①Safety factor of parts: for parts with high safety importance, dehydrogenation should be strengthened; ②geometric shape of parts: parts with gaps that are prone to stress concentration, small R, etc. should be dehydrogenated; ③cross-sectional area of parts: Small spring steel wires and thin leaf springs are easily saturated with hydrogen, so dehydrogenation should be strengthened; ④ Hydrogen permeation degree of parts: For parts that generate a lot of hydrogen during surface treatment and have a long treatment time, dehydrogenation should be strengthened; ⑤ Coating type : For example, the cadmium plating layer will seriously block the outward diffusion of hydrogen, so it is necessary to strengthen the dehydrogenation; ⑥The mechanical properties of the parts in use: when the parts are subjected to high tensile stress, dehydrogenation should be strengthened, and hydrogen will not be produced when the part is only subjected to compressive stress Brittle; ⑦ surface processing state of parts: for parts with large internal residual stress such as cold bending, stretching, cold bending, quenching, welding, etc., not only dehydrogenation should be strengthened after plating, but also stress should be removed before plating; ⑧ parts Historical situation: Special attention should be paid to parts that have experienced hydrogen embrittlement in past production, and relevant records should be made.
The main reason is the metal "hydrogenation" phenomenon caused by the electroplating process, and the unqualified product you use is not a problem with the electroplating process itself, because electroplating (except vacuum plating) will originally cause metal hydrogenation, but there are currently many Metal surface treatment manufacturers have removed the last process (especially fatal for elastic components): that is the "dehydrogenation treatment" process, which means that under normal circumstances, metal parts with strength requirements need to be dehydrogenated before delivery. For the user, but in order to save production costs, and the user does not understand or has never requested or accepted the situation, omitting this process can save 5-15% of the cost. So you feel that the electroplated bolts, spring washers and other parts are "brittle" after electroplating.
Generally speaking: the dehydrogenation treatment requirements for metal parts with strength requirements are: 120-220 degrees high temperature for 1-2 hours (after electroplating), the specific situation needs to be controlled according to the parts requirements