Hydrogen embrittlement

1. Hydrogen embrittlement phenomenon

Hydrogen embrittlement usually manifests as delayed fracture under stress. There have been cases where galvanized parts such as automobile springs, washers, screws, and leaf springs broke one after another within a few hours after assembly, with a fracture rate of 40% to 50%. Cadmium-plated parts of a certain special product have experienced batch cracking and fracture during use. A national research project was organized to develop a strict dehydrogenation process. In addition, some hydrogen embrittlement does not manifest as delayed fracture. For example, electroplating hangers (steel wire, copper wire) have severe hydrogen permeation due to multiple electroplating and pickling and deplating. They often break brittlely when bent during use; the core rod used for precision forging of hunting rifles falls to the ground and breaks after multiple chrome plating; some quenched parts (with large internal stress) crack during pickling. These parts have severe hydrogen permeation and crack without external stress, and dehydrogenation can no longer be used to restore the original toughness.

2. Hydrogen embrittlement mechanism

The delayed fracture phenomenon occurs because hydrogen inside the parts diffuses and gathers to the stress concentration site, and there are many metal defects in the stress concentration site (atomic lattice dislocation, holes, etc.). When hydrogen diffuses to these defects, hydrogen atoms become hydrogen molecules, generating huge pressure. This pressure, together with the residual stress inside the material and the external stress on the material, forms a combined force. When this combined force exceeds the yield strength of the material, it will cause fracture. Since hydrogen embrittlement is related to the diffusion of hydrogen atoms, diffusion takes time, and the diffusion rate 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, while hydrogen is difficult to diffuse 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. It is difficult to diffuse outward, and dehydrogenation is particularly difficult. After a period of time, hydrogen diffuses into the metal, especially hydrogen that enters the defects inside the metal, and it is difficult to diffuse out. The diffusion rate of hydrogen at room temperature is quite slow, so it is necessary to heat and remove hydrogen immediately. As the temperature rises, the solubility of hydrogen in steel increases. Too high a temperature will reduce the hardness of the material. Therefore, the temperature selection for stress relief before plating and dehydrogenation after plating must be considered not to reduce the hardness of the material, and must not be at the brittle tempering temperature of certain steels, and not to damage the performance of the coating itself.

Measures to avoid and eliminate

1. Reduce the amount of hydrogen permeation in the metal

When removing rust and scale, try to use sandblasting to remove rust. If pickling is used, corrosion inhibitors such as rhodium need to be added to the pickling solution; when degreasing, use chemical degreasing, cleaning agent or solvent degreasing, the amount of hydrogen permeation is less. If electrochemical degreasing is used, first the cathode and then the anode; when electroplating, alkaline plating solution or high current efficiency plating solution has less hydrogen permeation.

2. Use coatings with low hydrogen diffusivity and low hydrogen solubility
It is generally believed that when electroplating Cr, Zn, Cd, Ni, Sn, and Pb, hydrogen that penetrates into steel parts is easy to remain, while metal coatings such as Cu, Mo, Al, Ag, Au, and W have low hydrogen diffusivity and low hydrogen solubility, and less hydrogen permeation. Under the condition of meeting the technical requirements of the product, a coating that will not cause hydrogen permeation can be used, such as Dacromet coating, which can replace galvanizing, will not cause hydrogen embrittlement, and has a corrosion resistance of 7 to 10 times higher, good adhesion, and a film thickness of 6 to 8um, which is equivalent to a thinner galvanized layer and does not affect assembly.

3. Stress relief before plating and dehydrogenation after plating to eliminate the hidden danger of hydrogen embrittlement
 

If the internal residual stress of the parts is large after quenching, welding and other processes, tempering treatment should be carried out before plating to reduce the hidden danger of serious hydrogen permeation.

In principle, parts with more hydrogen permeation during electroplating should be dehydrogenated as soon as possible, because the hydrogen in the coating and the hydrogen in the surface base metal diffuses into the steel matrix, and its amount increases with time. The new draft international standard stipulates that "it is best to carry out dehydrogenation treatment within 1 hour after plating, but no later than 3 hours". There are also corresponding standards in China, which stipulate the dehydrogenation treatment before and after electrogalvanizing. The dehydrogenation treatment process after electroplating widely adopts heating and baking, and the commonly used baking temperature is 150-300℃, and the heat preservation is 2-24h. The specific treatment temperature and time should be determined according to the size, strength, coating properties and electroplating time of the parts. Dehydrogenation treatment is often carried out in an oven. The dehydrogenation treatment temperature of galvanized parts is 110-220℃, and the temperature control level should be based on 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 embrittlement", the dehydrogenation treatment temperature of cadmium-plated parts cannot be too high, usually 180-200℃.

Issues that should be paid attention to
The greater the strength of the material, the greater its hydrogen embrittlement sensitivity. This is a basic concept that surface treatment technicians must clarify when compiling electroplating process specifications. International standards require that steel with tensile strength σb>105kg/mm2 should be subjected to corresponding stress relief before plating and dehydrogenation 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 sensitivity of materials by material hardness than by strength. Because a complete product drawing and machining process should mark the hardness of steel. In electroplating, we found that the hardness of steel begins to show the danger 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 parts will crack within a few hours.

In addition to the hardness of steel, the following points should also be considered comprehensively:
① The safety factor of parts: parts with high safety importance should be dehydrogenated; 

② The geometric shape of parts: parts with notches that are prone to stress concentration, small R, etc. should be dehydrogenated; 

③ The cross-sectional area of ​​parts: small spring wires and thin leaf springs are very easy to be saturated with hydrogen, so dehydrogenation should be strengthened; 

④ The degree of hydrogen permeation of parts: parts that produce a lot of hydrogen and take a long time to treat during surface treatment should be dehydrogenated; 

⑤ Type of coating: For example, cadmium coating will seriously block the diffusion of hydrogen outward, so dehydrogenation should be strengthened; 

⑥ The stress nature of parts in use: when parts are subjected to high tensile stress, dehydrogenation should be strengthened, and hydrogen embrittlement will not occur when only compressive stress is subjected;

⑦ The surface processing state of parts: for parts with large internal residual stress such as cold bending, stretching, cold rolling bending, quenching, welding, etc., not only should dehydrogenation be strengthened after plating, but also stress relief should be performed before plating; 

⑧ The historical situation of parts: special attention should be paid to parts that have experienced hydrogen embrittlement in past production, and relevant records should be made.

Dehydrogenation embrittlement
The main reason is the "hydrogenation" of the metal caused by the electroplating process. The unqualified products you use are not caused by the electroplating process itself, because electroplating (except vacuum plating) will cause metal hydrogenation. However, many metal surface treatment companies have removed the last process (especially fatal to elastic components): that is, the "dehydrogenation treatment" process. In other words, under normal circumstances, metal parts with strength requirements need to be dehydrogenated before they can be delivered 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 the cost. So you feel that bolts, spring washers and other parts after electroplating have become "brittle" after electroplating.

Generally speaking: the dehydrogenation treatment requirements for metal parts with strength requirements are: 120 degrees ~ 220 degrees high temperature for 1~2 hours (after electroplating), and the specific situation needs to be controlled according to the requirements of the parts.