In any electroplating solution, there is always a certain amount of hydrogen ions due to the dissociation of water molecules.
Therefore, during the plating process, metal is precipitated from the cathode (main reaction) and hydrogen is precipitated (side reaction).
The influence of hydrogen evolution is multifaceted, and the most important one is hydrogen embrittlement.
Hydrogen embrittlement is one of the most serious quality hazards in surface treatment.
Parts with serious hydrogen evolution may break during use, causing serious accidents.
Surface treatment technicians must master the technology of avoiding and eliminating hydrogen embrittlement to minimize the impact of hydrogen embrittlement.
1. Hydrogen embrittlement
(1) Hydrogen embrittlement
Hydrogen embrittlement usually appears as delayed fracture under stress.
Galvanized parts such as automobile springs, washers, screws and leaf springs have been broken within several hours after assembly, with a fracture rate of 40% ~ 50%.
In the process of using cadmium plated parts of a special product, a batch of cracks and fractures occurred.
A national breakthrough was organized and a strict dehydrogenation process was formulated.
In addition, some hydrogen embrittlement does not appear as delayed fracture.
For example, the plating hanger (steel wire and copper wire) is subject to serious hydrogen permeation due to repeated electroplating and pickling and deplating, and brittle fracture often occurs after one fold in use;
The core rod used for precision forging of hunting gun is broken after being plated with chromium for many times;
Some quenched parts (with large internal stress) will crack during pickling.
The hydrogen permeation of these parts is serious, and cracks are generated without external stress.
It is no longer possible to restore the original toughness by removing hydrogen.
(2) Hydrogen embrittlement mechanism
The occurrence of delayed fracture phenomenon is due to the diffusion and accumulation of hydrogen in the part to the stress concentration part, and there are many metal defects (atomic lattice dislocation, holes, etc.) in the stress concentration part.
When hydrogen diffuses to these defects, hydrogen atoms become hydrogen molecules, generating huge pressure.
This pressure forms a resultant force with the residual stress inside the material and the external stress on the material.
When the resultant force exceeds the yield strength of the material, fracture will occur.
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 appears as delayed fracture.
Hydrogen atoms have the smallest atomic radius and are easy to diffuse in metals such as steel and copper, while 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 layer under the plating layer, which is difficult to diffuse outward, and hydrogen removal is particularly difficult.
After a period of time, hydrogen diffuses into the metal, especially the hydrogen that enters the defect inside the metal, and it is difficult to diffuse out.
The diffusion rate of hydrogen is quite slow at normal temperature, so it needs to be heated immediately to remove hydrogen.
As the temperature rises, the solubility of hydrogen in steel will be increased.
Too high a temperature will reduce the hardness of the material.
Therefore, the selection of the temperature for stress removal before plating and hydrogen removal after plating must be considered not to reduce the hardness of the material, not to be at the brittle tempering temperature of some steels, and not to damage the performance of the coating itself.
2. Measures to avoid and eliminate
(1) Reduce the amount of hydrogen permeation in metal
During rust removal and scale removal, sand blowing shall be used as much as possible.
If acid washing is used, corrosion inhibitor such as rutin shall be added to the acid washing solution;
During oil removal, chemical oil removal, cleaning agent or solvent oil removal shall be adopted with less hydrogen permeation.
If electrochemical oil removal is adopted, the cathode shall be followed by the anode;
In electroplating, the amount of hydrogen permeation in an alkaline plating solution or a plating solution with high current efficiency is small.
(2) Plating coating with low hydrogen diffusivity and low hydrogen solubility shall be adopted
It is generally believed that when electroplating Cr, Zn, Cd, Ni, Sn and Pb, hydrogen infiltrated 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 hydrogen permeation is less.
Under the condition that the requirements of product technical conditions are met, the coating that will not cause hydrogen permeation can be used.
For example, Dacromet coating can replace zinc plating, and will not cause hydrogen embrittlement.
The corrosion resistance is improved by 7-10 times, and the adhesion is good.
The film thickness is 6-8um, which is equivalent to a thin zinc coating and will not affect the assembly.
(3) Remove stress before plating and remove hydrogen after plating to eliminate potential hydrogen embrittlement
If the internal residual stress of parts is large after quenching, welding and other processes, tempering treatment shall be carried out before plating to reduce the potential of serious hydrogen permeation.
In principle, parts with more hydrogen permeation in the plating process should be dehydrogenated as soon as possible, because the hydrogen in the plating layer and the hydrogen in the surface base metal are diffusing into the steel matrix, and the amount will increase with the extension of time.
The new international standard draft stipulates that “it is better to conduct dehydrogenation treatment within 1h after plating, but not later than 3h”.
There are also corresponding standards in China, which stipulate the dehydrogenation treatment before and after galvanizing.
The post plating dehydrogenation process is widely used by heating and baking.
The commonly used baking temperature is 150-300 ℃, and the insulation is 2-24h.
The specific treatment temperature and time shall be determined according to the size, strength, coating property and plating time of the parts.
Dehydrogenation is usually carried out in an oven.
The dehydrogenation temperature of galvanized parts is 110 ~ 220 ℃, and the temperature control shall be determined according to the base material.
For elastic materials, thin-walled parts less than 0.5mm and steel parts with high mechanical strength requirements, hydrogen removal 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. Problems needing attention
The greater the strength of the material, the greater the hydrogen embrittlement sensitivity. This is a basic concept that surface treatment technicians must make clear when preparing electroplating process specifications.
The international standard requires that the steel with tensile strength σb > 105kg / mm2 shall be subject to corresponding pre plating stress removal and post plating hydrogen removal treatment.
The French aviation industry requires corresponding dehydrogenation treatment for steel parts with yield strength σs > 90kg / mm2.
Because there is a good correspondence between the strength and hardness of steel, it is more intuitive and convenient to judge the hydrogen embrittlement sensitivity of materials by the hardness of materials than by the strength.
Because a complete product drawing and machining process should be marked with steel hardness.
In electroplating, we found that when the hardness of steel was about HRC38, it began to show the danger of hydrogen embrittlement fracture.
For parts higher than HRC43, dehydrogenation shall be considered after plating.
When the hardness is about HRC60, the dehydrogenation treatment must be carried out immediately after the surface treatment, otherwise the steel parts will crack within a few hours.
In addition to the hardness of steel, the following points shall also be considered comprehensively:
① Safety factor of parts: for parts with great safety importance, hydrogen removal shall be strengthened;
② Geometry of parts: parts with notches that are prone to stress concentration, small R, etc. shall be strengthened for hydrogen removal;
③ Cross sectional area of parts: small spring steel wire and thin leaf spring are easily saturated with hydrogen, so hydrogen removal shall be strengthened;
④ Hydrogen permeation degree of parts: for parts with more hydrogen produced during surface treatment and long treatment time, hydrogen removal shall be strengthened;
⑤ Plating type: for example, cadmium plating layer will seriously block hydrogen diffusion, so hydrogen removal shall be strengthened;
⑥ Stress property of parts in use: when the parts are subjected to high tensile stress, hydrogen removal shall be strengthened, and hydrogen embrittlement will not occur when the parts are subjected to compressive stress;
⑦ Surface processing status of parts: for parts with large internal residual stress such as cold bending, stretching, cold bending, quenching and welding, the hydrogen removal shall be strengthened after plating and the stress shall be removed before plating;
⑧ History of parts: special attention shall be paid to parts with hydrogen embrittlement in the past production and relevant records shall be made.
Hydrogen embrittlement removal
The main reason is the metal “hydrogenation” phenomenon caused by the electroplating process, and the unqualified products you use are not caused by the electroplating process itself, because electroplating (except vacuum plating) will cause metal hydrogenation, but at present, many metal surface treaters have removed the last process (especially for elastic elements): that is the “dehydrogenation” process, that is, under normal conditions, Metal parts with strength requirements can only be delivered to the user after dehydrogenation.
However, in order to save production costs, if the user does not understand or has never requested or accepted, omitting this process can save 5~15% of the cost.
Therefore, you can feel that the bolts, spring pads and other parts after plating are “brittle” after plating.
Generally speaking, the dehydrogenation treatment requirements for metal parts with strength requirements are: 120 ~ 220 ℃ high temperature for 1 ~ 2 hours (after electroplating), and the specific conditions need to be controlled according to the requirements of the parts.