NdFeB surface treatment: electrophoresis

Electrophoretic coating is one of the anti-corrosion surface treatment technologies widely used in sintered NdFeB permanent magnets and bonded NdFeB permanent magnets.

What is electrophoresis?

The electrophoretic coating is to immerse the parts in a water-soluble electrophoresis bath, insert the anode and cathode electrodes into the bath at the same time, and apply a direct current between the two electrodes to produce an electrochemical reaction, so that the water-soluble paint is evenly deposited on the parts, forming An anti-corrosion coating composed of resin particles, or a high-molecular polymer anti-corrosion layer.
Electrophoretic coating not only has good bonding force with porous magnet surface, but also has corrosion resistance such as salt spray, acid and alkali resistance. Compared with spray coating, it has the following advantages:

  • 1. High coating utilization rate. During the spraying process, a large amount of spray paint is scattered, and about half of the paint is lost. However, the use of ultrafiltration technology in electrophoretic coating can make the paint utilization rate reach 90%-95%.
  • 2. Electrophoretic paint is water-soluble paint, no organic solvent is needed, energy and raw materials are saved, and environmental pollution is reduced.
  • 3. High production efficiency. After pretreatment, the bonded magnet parts can be electrophoresed directly without drying, shortening the coating time.
  • 4. Good coating effect for complex parts. Electrophoretic coating relies on the distribution of the electric field in the electrophoresis bath for electrodeposition. Since the deposited coating film is insulating, a uniform coating can be formed on all parts of the part.
  • 5. The coating quality is good, the corrosion resistance is strong, and the bonding force is good.

Electrophoretic coating also has disadvantages:

  • 1. The one-time investment cost is high, the maintenance and management of the electrophoresis tank is complicated, and various process parameters need to be carefully controlled.
  • 2. Electrophoretic coating cannot cover up the defects of the magnet surface, such as cavities, scratches, dents, etc., so the surface conditions of the product are relatively high.
  • 3. Hooking and unhooking require more manual labor.

20210104073047 74401 - NdFeB surface treatment: electrophoresis

Classification of electrophoretic coating

Put the electrophoretic paint in the electrophoresis tank, including polymer resin, organic solvent, color suspended particles and water, etc., and stir them to form a uniform solution.
Electrophoretic coating has two types: anode coating and cathode coating. Anodic electrophoresis uses a sintered NdFeB magnet as the anode. In this way, the base metal and surface treatment film (such as phosphating film) are precipitated and dissolved at the same time during the electrophoresis process, which reduces the corrosion resistance of the electrophoretic deposited film, so in most cases Cathodic electrophoresis is used, that is, sintered NdFeB as the cathode, and cationic electrophoretic paint is used. This paint is a water-soluble coating formed by neutralizing synthetic resins containing amino groups with organic acids.
The cathodic electrophoresis layer uses organic acid as a neutralizer, which has low toxicity. The mechanical strength of the wet paint film itself and the bonding force with the workpiece are higher than that of the anode electrophoretic coating. However, the cost of cathodic electrophoretic coating is high, and the electrophoresis bath is acidic, which has a corrosive effect on equipment. The cathodic electrophoresis tank should be made of corrosion-resistant materials, such as stainless steel or acid-resistant plastics.

Process flow of sintered NdFeB electrophoretic coating

The process flow of electrophoretic coating is mainly divided into three parts: surface pretreatment, electrophoresis and drying and curing. The specific process flow is: roughdegreasingwashingsealing treatmentwashingphosphatingwashingelectrophoresiswashingdrying and curing.
Smooth and clean surface is the basis of high-quality electrophoretic coating, and the purpose of surface treatment is to create good surface conditions. The electrophoretic coating process is a complex chemical change process. As the coating is continuously deposited on the surface of the magnet, the composition of the electrophoresis tank is constantly changing. The solid components, the PH value of the electrophoresis tank, temperature, electrophoresis voltage, electrode spacing, etc.

Comparison of bonding strength between various coatings and magnets

  • Painting: 13.18/Mpa
  • Electroplating NiCuNiZn: 10.73/Mpa
  • Electroplating NiCuNi: 6.83/Mpa
  • Zn plating: 12.98/Mpa
  • Electrophoresis epoxy (black) coating: 28.32/Mpa
  • Phosphating surface treatment (phosphating film): 27.43/Mpa

Improvement of NdFeB Electroplating Technology

The current NdFeB electroplating production technology basically meets the needs of the market and meets the requirements of most products. However, some inherent disadvantages of the process restrict the stability of the magnet surface quality and increase the production cost, so Need to be further improved and improved.

Galvanized

To solve or improve the problem of poor corrosion resistance of trivalent chromium passivation film:

  • 1) Looking for a pure zinc electroplating process with low impurity content, and plating directly on the neodymium iron boron substrate;
  • 2) If the pure zinc electroplating layer process cannot be directly plated on the neodymium iron boron substrate, the method of potassium chloride zinc plating or dipping zinc base and then pure zinc layer can be used. After adopting this idea to conduct preliminary production experiments, the effect is more consistent with expectations.

In addition, the following ways can be used to improve the corrosion resistance of the NdFeB zinc coating.

  • 1) Using alkaline zinc plating. Alkaline zinc coating has high density, which is of greater significance when the NdFeB zinc coating depends on low porosity (rather than anode protection) to ensure its corrosion resistance, and the alkaline zinc coating contains organic impurities. Less attachment is beneficial to the formation of trivalent chromium passivation film.
  • However, the alkaline zinc plating process cannot be directly plated on the neodymium iron boron substrate. It needs to be dipped in zinc or potassium chloride for pre-plating.
  • 2) Adopt zinc-nickel alloy coating. The anodic protection of the zinc-nickel alloy coating on the NdFeB substrate is not obvious, but it has excellent corrosion resistance (up to about 10 times higher than that of the pure zinc layer), and can be used as a high-end coating of NdFeB zinc plating.

Current problems:

  • a. The acid zinc-nickel alloy coating contains high nickel content and good corrosion resistance, but it is difficult to barrel plating;
  • b. Alkaline zinc-nickel alloys are suitable for barrel plating, but the plating layer has low nickel content and poor corrosion resistance, and it needs to be pre-plated with zinc dip or potassium chloride.

Nickel plating

The pre-plated nickel adopts the Watt-type semi-bright nickel plating process. The main purpose is to ensure a stable bonding force between the plating layer and the substrate, and a good deep plating ability to lay a solid foundation for subsequent copper plating.
However, neodymium iron boron is susceptible to corrosion in the semi-bright nickel plating bath of simple salt bath type and pollutes the plating bath. In addition to taking measures (from the bath, drum and operation) to minimize the corrosion of the magnet, it should also be dissolved to the plating bath. The iron ions in the liquid take measures:

  • 1) Treat the plating solution regularly, but it will increase the cost of processing the solution, cause some solution loss, and affect the continuous production;
  • 2) The impurity masking agent is used to form a chelate with the iron ions in the plating solution, so that iron and nickel are co-deposited, turning harm into profit, which can prolong the solution treatment cycle, maintain stable production quality, and reduce non-productive cost loss.

The thicker bottom nickel layer increases the thickness of the total nickel layer and increases the magnetic shielding effect on the magnet. Thinning the thickness of the bottom nickel layer can reduce its impact, but increases the risk of the substrate being corroded by the subsequent copper plating solution.

To this end, the following measures can be taken:

  • 1) Try to use the pre-nickel plating process with good dispersion performance, and the thickness of the lower part of the coating layer will increase accordingly, and the average thickness of the bottom nickel layer can be reduced;
  • 2) Try to choose a copper plating process that is less corrosive to the magnet, and the requirement for the bottom plating layer is reduced, and the thickness of the bottom nickel layer can be reduced. The thicker surface layer of bright nickel is the main factor that affects the magnetic properties of the magnet, but it cannot be reduced by simply thinning the surface nickel layer, because too thin surface nickel will reduce the corrosion resistance of the coating, and the gain is not worth the loss.

A better solution is to use non-magnetic coating as much as possible:

  • 1) First, keep the copper plating layer δ at 7-9μm;
  • 2) Secondly, use non-magnetic high-phosphorus chemical nickel instead of bright nickel plating, and the δ is 8-10μm. In this way, it can not only improve the corrosion resistance of the coating, but also reduce the influence on the magnetic properties of the magnet.

Copper plating

At present, a key problem of NdFeB copper plating is that the copper plating solution corrodes the magnet seriously, affecting the bonding force of the plating layer, and the corrosion products will contaminate the copper plating solution. Therefore, the quality of the bottom nickel layer (thickness, covering capacity and no leakage plating) Etc.) Higher requirements.
In view of the particularity of NdFeB, finding a more stable complexing agent and developing a new non-cyanide alkaline copper plating process is the key to solving or improving the problem.
The new cyanide-free alkaline copper process uses a complexing agent with strong complexing ability with Cu2+, does not produce a substitution reaction with the neodymium iron boron matrix, can accept a bottom nickel layer with a δ of 1 to 2 μm, and always maintain a good bonding force.
Even the impulse current method can be used to directly plate copper on the surface of the neodymium iron boron magnet, and the coating adhesion is good.
At the same time, the new non-cyanide alkaline copper plating process also has the following advantages.

  • 1) The ρ(Cu2+) in the plating solution is 7-8g/L, the Baume degree of the solution is about 22, and the viscosity is low, so the carry-out loss is small, the solution stability is good, the process cleaning is simple, and it is suitable for continuous production use in automated production lines.
  • 2) There is no need to add brightener, the coating is naturally leveled and emits light, the brightness is high enough, and the production control is easier.
  • 3) The solution is easy to control, no complicated complexing agent analysis is required, and the test is convenient; the complexing agent is stable and does not hydrolyze, and there is no interference of hydrolysate.
  • 4) The deep plating ability and the even plating ability are excellent, the copper plating layer has high purity, low stress, and the overall effect is better than cyanide copper plating.

After more than a year of production practice, the new cyanide-free alkaline copper plating process is a very suitable copper plating process used in the production of NdFeB surface treatment.

Electroless nickel plating

At present, the high-phosphorus alloy technology used in the electroless nickel plating of NdFeB parts mostly borrows the conventional technology of other industries, which has defects such as high operating temperature, easy metalization of equipment plastic parts, and poor solution stability.
In view of the particularity of NdFeB, the development of an electroless nickel plating process with low temperature, high stability and suitable for barrel plating is the direction of improvement of NdFeB electroless nickel plating.
A more feasible complete set of NdFeB electroless nickel plating process, the main process is as follows.
Pre-nickel plating 2μm→copper plating 4μm→electroless nickel plating 1μm→electroless nickel-copper-phosphorus alloy plating 10-15μm.
This process can obtain at least the NdFeB electroless nickel plating layer that meets the requirements of the 96h neutral salt spray test:

  • 1) The coating is not magnetic and has little effect on the magnetic properties of the magnet;
  • 2) The solution operating temperature is low (65~72℃), and the service life is long;
  • 3) No roller and fluoroplastic heater are metalized, suitable for continuous production;
  • 4) The user prepares nickel supplement concentrate by himself, and the production cost is low;
  • 5) The plating layer is lead and cadmium-free, which fully meets the requirements of the RoHS directive. The use of this set of technological solutions is an effective way to solve the problem of NdFeB electroless nickel plating.

Production equipment and methods

There are many problems with the automatic line used in the production of NdFeB barrel plating. Now only the following improvement measures are proposed for the cross-contamination of various plating solutions.
1) Realize sufficient water washing.
Changing the drum during manual line changeover plating can completely solve the cleaning problem, and the automatic line does not change the drum halfway, it is easy to cross contamination.
Therefore, the automatic line production should realize sufficient water washing, and adopt two-stage recovery and four-stage countercurrent rinsing every time the plating species is changed. Each stage should be cleaned more than three times during the washing process, and the drum rotates more than three times in the washing tank each time.
The cleaning water needs to be precisely filtered, and the pure water has a better effect. The nickel recovery tank adds a nickel ion recovery device to reduce the rate of nickel ion rise.
2) Choose a process that is more suitable for automatic lines to solve the problem of cross-contamination of nickel and copper solutions.
For example, pre-nickel plating uses a low-concentration process to reduce viscosity and reduce the amount of carry out, which is conducive to the cleaning effect of the washing process; copper plating adopts a new non-cyanide alkaline copper plating process, which has low copper ion concentration, low solution viscosity, and less carry out. Favorable for washing.
3) Each plating type adds on-line continuous electrolysis purification link to pass the inevitable cross-contamination in production through the purification device to realize self-maintenance of the solution and maintain the quality of the solution.
For example, next to the copper plating and nickel plating tanks, a circulating electrolytic purification tank is added, 24 hours of continuous uninterrupted low current electrolytic purification solution.
NdFeB semi-automatic barrel plating production line is a method suitable for barrel plating production of NdFeB parts with special properties.
Each kind of neodymium iron boron plating is separately formed into wire (such as galvanized wire, bottom nickel-plated wire, copper-plated wire, bright nickel-plated wire, etc.), the drum plating still uses the original barrel plating machine form, and the corresponding auxiliary tank is added, and the drum is transported Use semi-automatic driving.
This form is between the manual line and the automatic line, taking into account the advantages of both, strong flexibility, no cross-contamination problems, at the same time the labor production efficiency is improved, the labor intensity of the workers is reduced, and high quality barrel plating of NdFeB plating parts can be achieved. The dual purpose of high productivity.
At present, many domestic enterprises are using this method for production.

Source: China Permanent Magnet Manufacturer – www.ymagnet.com

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