1. Preface
Electroplating is a process of obtaining a relatively thin layer of other metals and alloys on the surface of some metals according to the principle of electrolysis.
It is a method of attaching a metal film to the surface of metal and other material parts by electrolysis.
As a traditional surface modification technology, chromium plating plays an important role in electroplating industry.
The chromium plating layer can effectively protect the surface of the pin shaft and avoid excessive wear and corrosion of the pin shaft of the chain during actual use.
This is because the chromium itself has excellent wear resistance and corrosion resistance and strong passivation ability.
Therefore, the chromium plating process is widely used in the surface strengthening of the pin shaft of the chain.
2. Processing process of chrome plated pin shaft
Compared with common heat-treated pins, chrome plating and centerless grinding after chrome plating are added to the processing flow of chrome plated pins.
Among them, attention should be paid to the influence of pin material and pin surface quality on the coating quality, and hydrogen elimination treatment should be carried out in time after plating.
2.1 Effect of base metal on chromium plating layer
The good combination of chromium plating layer and pin shaft is closely related to the chemical properties of pin shaft.
Some metals have passivation property, and a dense oxide film is easily formed on their surface.
If it is not activated, it is difficult to obtain a solid chromium plating layer on the surface of the pin shaft.
In addition, for some pins treated by nitriding, since a layer of white compound is formed on the surface, the adhesion of chromium atoms on the surface is significantly reduced, and activation treatment is also required during chromium plating.
2.2 Influence of surface quality of pin shaft on chromium plating layer
First, the surface of the pin shaft shall be free of pores and cracks.
This is because when there are cracks and pores on the surface of the pin shaft, when the pin shaft is plated, the electrolyte will penetrate into the gap of the pin shaft.
After a period of time, the infiltrated electrolyte will interact with the pin shaft to generate hydrogen.
If the pressure of the latter is greater than the binding force of the coating and the pin shaft, the coating will bubble and pinhole, as shown in Fig. 1.
a) Blistering of coating
b) Plated needle eye
Fig. 1 chromium plating defects (200 ×）
Secondly, the surface roughness of the pin shaft has a great influence on the surface quality of the chromium plating layer, so the pin shaft is often polished or polished.
Surface roughness refers to the micro spacing and the unevenness of wave crest and valley on the surface of parts, which is greatly affected by the processing method.
However, when determining the surface roughness of the pin shaft, the pin shaft designer often neglects the influence of the surface roughness of the pin shaft on the plating effect.
Under the same chromium plating thickness requirement, the larger the surface roughness value of the pin shaft before plating, the coarser the surface of the coating after plating, the higher the porosity, the incomplete the coating, the poor adhesion between the coating and the surface of the pin shaft, the easier the corrosion, and the impact on the service life.
When the roughness value of the pin shaft surface is too large during electroplating, the actual current density of the rough surface is smaller than the apparent current density, which will cause the potential of the rough part to not reach the metal precipitation potential, and then there will be no plating at the position.
With the decrease of the surface roughness of the pin shaft, the surface hardness of the pin shaft increases.
This is because the smaller the surface roughness of the pin shaft, the denser the plating and the higher the hardness.
In addition, the cracks of different degrees around the hardness indentation can also indicate the compactness of the coating, thus reflecting that the main factor affecting the hardness of the coating is the electrolytic product hydrogen.
The overpotential phenomenon of hydrogen is different between smooth surface and rough surface, and the overpotential on rough surface is small.
Therefore, the hydrogen on the rough pin surface is easier to complete the precipitation action, and the metal ions therein are also less likely to be electrodeposited.
In order to ensure the service life and quality of the chrome plated pin shaft, the surface roughness of the pin shaft before plating must be controlled within a reasonable range.
However, properly increasing the surface roughness value of the pin shaft can also improve the bonding force between the plating layer and the pin shaft.
Therefore, the surface roughness of the pin shaft is generally required to be less than 0.6μm.
2.3. Hydrogen elimination treatment
Since pickling and activation treatment must be carried out in the electroplating process, hydrogen evolution and hydrogen permeation are inevitable in the whole electroplating process, and the hydrogen can permeate into the coating and pin metal.
Hydrogen evolution will not only reduce the performance of the coating, produce pinholes, pockmarks, bubbles and other defects, but also reduce the toughness of the base pin shaft, which may lead to the fracture of the chain parts in the connection state far less than its normal failure stress after assembly.
Therefore, the chrome plated pin shaft must be subjected to hydrogen elimination treatment to reduce the internal stress.
When the chrome plated pin shaft is put into storage, a fast and effective inspection method is required to conduct hydrogen embrittlement inspection on the product to reduce the quality risk and quality loss.
Through comparison test, it is suggested to adopt bubble method.
The bubble method is simple in operation, short in time and low in cost.
Although the test results do not form a unified technical requirement, it can be used as a reference to reduce the risk of hydrogen embrittlement.
It is suggested that this method should be continuously supplemented and perfected to form the company’s internal control standards and even industry specifications.
Bubble method refers to heating the cleaned and dried pin shaft under test in paraffin (or silicone oil or vaseline at a temperature of about 180 ° C) for 3-5min, observing the generation and escape of bubbles on the surface of the pin shaft, and judging the hydrogen content of the pin shaft under test. See Table 1.
Table 1 generation and escape of air bubbles on the surface of pin shaft and their determination
| Generation and escape of air bubbles on the surface of pin shaft | Determine |
| There are dense and continuous bubbles escaping from the surface of the tested pin shaft and the duration is long (there are still bubbles escaping after heating for 3min). | If the hydrogen content of the tested pin shaft is high, the batch pin shaft is judged to be suspicious or unqualified. |
| After 3min, no bubble is seen on the surface of the pin shaft. | If the measured pin shaft does not contain hydrogen or the hydrogen content is extremely low, the batch pin shaft is judged to be acceptable. |
| Put the tested pin shaft into the solution heated to the specified temperature and keep it for 3min. There are a few bubbles, but no bubbles escape after 3min. | Pin shaft judgment acceptable. |
| If the pin shaft under test is not cleaned, individual bubbles will form on the pin shaft under test at the initial stage of heating, or bubbles that adhere to the surface of the pin shaft under test for a long time without rising. | Invalid bubble, the pin shaft is determined to be acceptable. |
When observing and judging the generation and escape of bubbles on the surface of the pin shaft with the bubble method, the following matters shall be noted.
1) The tested pin shaft shall be cleaned, washed and dried with alcohol and other cleaning agents to confirm that the surface of the pin shaft is free of pollutants and dry.
2) When the light transmittance of the solution drops to less than 75% of the original solution due to the impurities brought into the solution due to the long service time of the solution or the pin shaft is not cleaned, the observation will be affected.
Therefore, the solution needs to be replaced regularly.
3) This method can only judge whether the pin shaft contains hydrogen, but it cannot judge the hydrogen content per unit volume.
The phenomenon of a large number of bubbles during the test does not mean that 100% of the hydrogen embrittlement fracture will occur, but it can be clear that the risk of hydrogen embrittlement fracture in this state is greatly increased.
Pin shafts judged as suspicious or unqualified shall be treated with caution.
3. Detection of chrome plated pin shaft
3.1. Thickness of chromium plating layer
Coating thickness is an important performance index of pin shaft, which often affects the corrosion resistance and wear resistance of pin shaft.
At present, there are many methods for detecting the thickness of the coating.
Due to the high accuracy of the metallographic method, according to the requirements of GB / T 6462-2005 microscopic method for measuring the thickness of metal and oxide coatings, the coating thickness of various types of chrome plated pins was tested, and the uniformity of the coating thickness was found to be unsatisfactory (see Fig. 2).
Fig. 2 shows the plating thickness of a certain type of chain pin shaft, and the difference between the thickest and thinnest parts is 0.06mm.
It can be seen that the uniformity of the plating thickness seriously affects the roundness of the pin shaft.
Zhan Ruiqiu et al. showed that the surface internal stress of the chromium plating layer was tensile stress.
With the increase of the thickness of the chromium plating layer, the surface internal stress of the chromium plating layer increased first, then decreased, and then increased and then decreased.
With the increase of the thickness of the chromium plating layer, the hardness of the chromium plating layer increases slightly at first, then rapidly and then slowly.
a) The thickest part of chromium plating layer
b) The thinnest part of chromium plating layer
Fig. 2 plating thickness at different positions of chrome plated pin shaft (200 ×）
3.2 Hardness of chromium plating layer
When measuring the hardness of chromium plating layer, a micro Vickers hardness tester is often used. According to the plating thickness, a small pressing load of 5-200g (0.049-1.96N) can be selected to make the indentation depth reach 1 / 10-1 / 7 of the plating thickness, so as to ensure the accuracy of the plating hardness measurement.
When the thickness of chromium plating layer is more than 100μm, rockwell hardness tester can also be used for testing.
Different hardness tester shall be used for hardness test according to part size, base material, coating thickness, indentation diameter and load size.
Therefore, the value of hardness value of chromium plating layer varies greatly.
Table 2 shows the hardness test of chromium plating layer on different types of chromium plated pin shafts with a load of 200g (1.96N) and a Japan Sanfeng HM-200 micro Vickers hardness tester.
It can be seen from table 2 that the maximum hardness difference of chrome plated pins of the same model is 113.7 HV.
Table 2 plating hardness of chrome plated pin shaft (HV)
| Model | Detection value | Average value | Difference | ||||
| A | 922.1 | 882.2 | 933.9 | 871.4 | 871.3 | 896.18 | 62.6 |
| B | 882.2 | 939.9 | 887.9 | 871.3 | 887.8 | 893.82 | 68.6 |
| C | 882.3 | 876.7 | 876.7 | 887.8 | 850.2 | 874.74 | 37.6 |
| D | 845.0 | 810.0 | 781.8 | 781.8 | 772.7 | 798.26 | 72.3 |
| E | 904.7 | 882.2 | 819.8 | 791.0 | 834.8 | 846.50 | 113.7 |
At present, it is known that the main reason for the high hardness of chromium plating layer is that a large amount of hydrogen is adsorbed on its surface.
Therefore, the pin shaft must be heat treated to remove hydrogen within 4h after chrome plating.
The practical demonstration shows that the total amount of hydrogen elimination is the most when the temperature is kept at 200 ℃ for a proper time.
Although the hardness is slightly reduced, the porosity and crack network of the coating will be increased, but it has little impact on the use of the chrome plated pin shaft.
3.3 Binding force of chromium plating layer
The bonding force of chromium plating layer is an important mechanical property of the coating.
If the bonding force is low, it will be difficult to achieve the purpose of electroplating even if the other properties of chromium plating layer are good.
In GB / T 5270-2008 review of test methods for adhesion strength of Electrodeposited and chemically deposited metal coatings on metal substrates, the test methods for adhesion strength of electrodeposited and chemically deposited metal coatings on metal substrates are specified, but most of them are qualitative measurements.
At present, the commonly used detection methods include steel needle scratch method, bending method, impact method and grinding wheel method.
Due to the high hardness of chromium plating layer, the grinding wheel method is often used in daily detection.
Grind the chrome plated pin shaft on the grinding wheel until the pin shaft is exposed to the base.
If the chrome plated layer is not peeled, it is qualitatively proved that the bonding force of the chrome plated layer is good.
Zhang Chunhua et al. showed that the surface roughness of the substrate has a great influence on the adhesion of the coating.
With the decrease of the surface roughness of the substrate, the bonding strength of the chromium plating layer is improved.
4. Conclusion
Due to the particularity of the chrome plating process, it is easy to cause the failure of the chrome plating pin shaft.
Therefore, it is necessary to strictly control the relevant processes and strengthen the detection of the chrome plating pin shaft to ensure that the chrome plating layer has good corrosion resistance, wear resistance and other properties, and thus reduce the occurrence of chain quality problems.
