Deep analysis of NdFeB electroplating technology

Table of Contents

• Difficulties in NdFeB electroplating

• Production status of NdFeB electroplating technology

• Pretreatment

• Galvanization

• Nickeling

• Coppering

Since the mid-1980s, rare earth NdFeB (Nd2Fe14B) permanent magnet materials have been widely used in various industries due to their unique characteristics such as high magnetic energy product, high coercivity, and high remanence. NdFeB material is a fine alloy body mainly composed of iron Fe (about 65%), boron B (about 1%), and rare earth metal Re (containing neodymium Nd, praseodymium Pr, dysprosium Dy, terbium Tb, etc., with a total content of about 33%). There is a potential difference between the neodymium rich and boron rich phases in the material and the main phase of the magnet alloy, which can easily form a primary battery and cause electrochemical corrosion on the material surface. In addition, NdFeB magnets are produced using powder metallurgy technology, which makes the actual density of the material unable to reach the theoretical density. There are small pores and voids inside, which are easily affected by oxidation in the atmosphere and damage the alloy components. These corrosion and component damage can lead to the attenuation and loss of magnetic properties, thereby affecting the performance and lifespan of the entire machine. Therefore, strict anti-corrosion treatment must be carried out before use. At present, NdFeB anti-corrosion treatment usually adopts methods such as electroplating, chemical plating, chemical conversion film, electrophoresis, spraying, etc. Among them, electroplating is widely used as a mature metal surface treatment method.

Difficulties in NdFeB electroplating

NdFeB magnets are usually small parts with a mass between 0.25g and 20g. Therefore, in the electroplating production process, roll plating is usually used as the main method, supplemented by hanging plating. However, compared to ordinary steel parts, rolling plating of NdFeB parts is more difficult.

This is because neodymium is very active chemically, and NdFeB parts even produce hydrogen gas and corrode when in contact with water. Therefore, the following points need to be noted:

1. Do not use too strong acids or alkalis for pre plating treatment, otherwise it may cause corrosion of the material substrate. In addition, due to the strong reaction between chlorine and neodymium in hydrochloric acid, it is necessary to avoid using hydrochloric acid.

2. Before pre plating or direct plating, a simple salt plating solution (such as Watt nickel, potassium chloride zinc plating) should be selected. This plating solution is prone to oxidation of parts, thereby affecting the adhesion between the coating and the substrate, and the parts are prone to corrosion, while also polluting the plating solution.

3. Difficulty in selecting large-sized drums can affect production capacity, and the mixing cycle can also be affected, leading to severe oxidation of parts.

Production status of NdFeB electroplating technology

At present, the electroplating production of NdFeB magnets mainly adopts three main processes or their combinations: zinc plating, "nickel+copper+nickel" plating, and "nickel+copper+chemical nickel" plating. Other processes, such as gold plating, silver plating, tin plating, and black nickel plating, typically require additional electroplating treatment on top of the three basic processes mentioned above.

Pretreatment

Due to the unique characteristics of NdFeB materials (such as strong chemical activity and porous surface), pre-treatment has always been one of the bottlenecks that NdFeB electroplating technology is difficult to overcome. However, after years of effort, this problem has been largely resolved.

1. Polishing treatment: also known as chamfering treatment, can make the surface of the part flat and smooth, while also reducing the micro surface area, which is conducive to rapid, uniform, and continuous deposition of the coating. The commonly used chamfering equipment includes horizontal planetary rolling machine and vibration polishing machine, which adopt planetary motion and vibration principles respectively, and can achieve the purpose of polishing without damaging the quality of the parts. Horizontal planetary polishing machines are usually used for the polishing treatment of small-sized NdFeB parts, while vibration polishing machines are usually used for the polishing treatment of large-sized NdFeB parts.

2. Degreasing, pickling, and activation: The acidity and alkalinity of the degreasing and pickling fluids should not be too strong to avoid corrosion of the parts. Additionally, some substances that can complexe with neodymium need to be added to the treatment solution to prevent neodymium oxidation. During the pickling and activation process, it is necessary to avoid the use of hydrochloric acid.

3. Ultrasonic treatment: The cavitation effect of ultrasound can completely remove oil stains, acids and alkalis in NdFeB materials. Therefore, a small or large amount of ultrasonic treatment technology is often used before NdFeB plating. In practical operation, a small amount of parts are usually placed in a plastic mesh, manually shaken and cleaned, and then loaded into a drum for electroplating after completing steps such as ultrasonic degreasing, acid washing, and water washing. Although this operation method requires high labor intensity for workers, it can ensure that the parts are thoroughly cleaned and the treatment effect is excellent.

Galvanization

Due to the extremely negative potential of neodymium in NdFeB materials, zinc plating cannot provide strong anodic protection to the substrate as it does for ordinary steel parts. This means that the corrosion resistance of NdFeB zinc plating heavily depends on the density of the coating. Although the density of alkaline zinc plating layer is high, alkaline zinc plating solution is not suitable for direct application on porous NdFeB surface due to its low current efficiency. At present, the commonly used technology is the potassium chloride zinc plating process, but this process often encounters problems such as insufficient bonding force, corrosion of parts, and contamination of the plating solution. In order to solve these problems, various measures have been taken, such as using plating solutions with high current density limits, using small-sized slender rollers, charging into grooves, high current impact, and uninterrupted operation between processes, to quickly coat the surface of parts.

After 2007, the EU RoHS directive required the elimination of the traditional heavily polluting hexavalent chromium passivation process and the adoption of a new type of lightly polluting trivalent chromium passivation process. With the development of commercial solutions for trivalent chromium passivation, a trivalent chromium passivation film system mainly consisting of blue white and color has been formed. However, after using the "potassium chloride zinc plating+trivalent chromium passivation" process, the corrosion resistance of the trivalent chromium passivation film on NdFeB zinc plating decreased significantly. This is because the hexavalent chromium passivation film is thick and has self-healing ability, while the trivalent chromium passivation film is thin and has a clear reaction to impurities in the coating. It is necessary to generate a continuous covering film layer on the surface of pure zinc. However, the presence of a large amount of organic impurities in the potassium chloride zinc coating is not conducive to the formation of a qualified trivalent chromium passivation film. Therefore, the decrease in corrosion resistance of the trivalent chromium passivation film cannot be avoided.

Nickeling

The current process of NdFeB nickel plating is generally completed using "nickel+copper+nickel" (i.e. "pre nickel plating+intermediate copper+surface bright nickel"). In this combination system, the function of pre plating nickel is to provide a positive potential and dense structure of the bottom coating, ensuring the normal plating of copper and preventing corrosion of the substrate. However, the current pre copper plating process is difficult to apply to the bottom plating of NdFeB matrix, as it belongs to the type of complex plating solution with low current efficiency and cannot obtain continuous and qualified copper coatings on the porous NdFeB matrix. In contrast, pre nickel plating mostly uses Watt nickel plating process and moderate use of semi bright nickel additives. The purpose of using additives is not to pursue brightness, but to use a high current density, which is conducive to the rapid deposition of the coating. Watt nickel is a simple type of salt plating solution, so it can be directly plated on a NdFeB substrate. Similar to NdFeB zinc plating, for NdFeB pre nickel plating, multiple requirements (such as plating solution, drum, and operation) are similar to NdFeB zinc plating.

The surface layer of bright nickel mostly adopts the standard bright nickel plating process. Now, the bright nickel process is mature enough and there is no need to elaborate further. Very few manufacturers use the sulfamate nickel plating process.

Generally speaking, the average thickness of NdFeB pre plated nickel layer is required to be no less than 4-5 microns to ensure complete coverage of the low area coating of the part and prevent corrosion of subsequent copper plating solution. The thickness of the surface nickel layer is 8-10 microns to ensure the corrosion resistance of the coating. In this way, the total nickel layer thickness reaches 12-15 microns. Nickel is a ferromagnetic metal, and its coating not only does not produce magnetic output, but also shields the magnetic output of NdFeB magnets. The thicker the coating, the greater the shielding effect. Through the current NdFeB "nickel+copper+nickel" coating combination system, the magnetic properties of small-sized magnets below 0.5g can be reduced by 10-15%. How to reduce the usage of nickel plating without affecting subsequent copper plating and coating corrosion prevention is a major challenge faced by NdFeB nickel plating.

Coppering

For NdFeB copper plating, it refers to increasing the total thickness of the entire coating through the intermediate layer between pre plating nickel and surface nickel. The advantages of doing this are: firstly, copper is a non conductive magnetic metal compared to nickel, which has a smaller impact on the magnetic shielding of the magnet. Replacing some nickel with copper can reduce the magnetic energy loss caused by the magnetic shielding of the nickel layer; Secondly, compared to nickel, copper has a lower porosity, which can improve the corrosion resistance of the coating; Thirdly, the cost of NdFeB copper plating will also decrease; Finally, in small products with a large area to volume ratio, especially in ultra small size products, the nickel layer has a greater impact on the magnetic properties of the magnet, so the significance of reducing the thickness of the nickel layer is more important.

However, using the NdFeB "nickel+copper+nickel" combination process for copper plating poses an unstable problem. Although the widely used cyanide copper plating process is stable, has strong pollution resistance, good deep plating ability, and its coating is uniform, soft, and low stress performance is relatively balanced and stable, cyanide is a highly toxic substance, and the country has strict management and usage restrictions on it, so only a few manufacturers use this process. On the other hand, the acid copper process has extremely high requirements for pre coating, and a slight lack of control may lead to corrosion of the NdFeB matrix. Moreover, the adhesion of the acid copper plating semi bright nickel as a base is poor. Currently, NdFeB generally uses semi bright nickel as a base. Therefore, in the current NdFeB electroplating "nickel+copper+nickel" system, this process is not recommended for roller copper plating (hanging copper plating will be discussed separately).

At present, the commonly used processes for NdFeB copper plating include pyrophosphate copper plating and citrate copper plating. Among them, over 85% of NdFeB copper plating adopts the pyrophosphate copper plating process, followed by the citrate copper plating process developed in recent years. Through years of production practice, it can be found that under precise control, these two processes can basically meet the requirements of NdFeB copper plating. However, there are still unsatisfactory aspects to these two processes.

Pyrophosphate copper plating has the following problems: firstly, its solution concentration is high, the copper ion content is above 18g/l, the solution Baume degree is around 35, and the solution viscosity is high, which leads to a large amount of carryover during the production process and complex process cleaning. It is suitable for manual production and not easy to use in automatic production. Secondly, its solution parameters change rapidly, and the range of parameters required by the process is narrow, making it difficult to accurately control. In addition, pyrophosphate analysis is difficult and has low accuracy, often making it difficult to accurately determine whether the ratio between pyrophosphate and copper ions is within the normal range. In addition, its application range is narrow, there are few manufacturers of brighteners, and the technology of brighteners is not mature, making it much more difficult to control the addition of brighteners than nickel plating. In short, the pyrophosphate copper plating process is often difficult to achieve advanced control. It always relies on experience to adjust and recover the coating quality problems, and it is difficult to timely add and adjust the solution parameters through analysis to restore the stability of the solution. Therefore, it is easy to cause frequent production quality accidents.

NdFeB citrate copper plating is a new process developed and applied after 2003, which is rarely used in other industries and is almost a specialized copper plating process for the magnet industry. This process also faces difficulties in analysis and lack of control measures, but the main issue is the problem of bacterial growth in the solution. Improper treatment can cause local blurring of the surface brightness of the workpiece, thereby affecting product quality.

The common problem with the above two processes is that pyrophosphate and citrate are weak complexing agents of copper ions, so both processes have high requirements for the quality of the bottom nickel layer (thickness, coverage, leakage, etc.), otherwise it will cause copper displacement and affect the adhesion of the coating, and pollute the copper plating solution. At present, it has been found in production that the stability of the NdFeB barrel plating coke copper solution is relatively poor (steel barrel plating coke copper), indicating that the NdFeB matrix is gradually corroded by the coke copper solution, and the products contaminate the solution. Generally speaking, NdFeB copper plating is produced using the above two processes, both of which require an average thickness of the bottom nickel layer not less than 4-5 μm to ensure complete coverage of the bottom coating and avoid the occurrence of copper displacement. However, excessive thickness of the bottom nickel layer increases the total thickness of the nickel layer, especially for small-sized products, which has a significant magnetic shielding effect, leading to a significant decrease in the magnetic properties of the magnet.