With the rapid development of rare earth NdFeB magnets, in order to solve the problem of NdFeB magnets that are easy to corrode, a variety of magnet coating methods have been produced, such as nickel, chromium, gold, black zinc, epoxy resin, and so on. From the point of view of corrosion prevention, chromium is difficult to react with other substances, and corrosion prevention is very good, but the cost of chromium is very high, so the application is not much. Because of controllable cost, mature technology, and good anti-corrosion effect, nickel + copper + nickel coating has become a widely used NdFeB magnet coating treatment.
Image Source: Stanfordmagnets
Characteristics of NdFeB Magnet Parts
| Usually, NdFeB magnet parts are small, the mass is generally 0.25g~20g.
| The chemical activity of neodymium elements is extremely strong.
| The surface of the NdFeB magnet is loose and porous, rough and uneven.
| NdFeB material is brittle and the surface is easy to be damaged.
According to the characteristics of NdFeB magnet parts, nickel+copper+nickel coating adopts electroplating to attach nickel and copper to the surface of NdFeB magnets. Electroplating is mainly by roll plating and hang plating. There are a series of requirements and specifications before and during plating.
Pre-plating Treatment of NdFeB Magnet
Chamfering | This process can make the surface of the parts flat, and smooth, which is conducive to rapid, uniform, and continuous deposition of the plating layer. The equipment mostly adopts horizontal rolling chamfering machines and vibration chamfering machines, which are collectively known as the chamfering machines, which adopt the principles of rolling and vibration, respectively, to achieve the purpose of smooth edges and corners without damaging the parts. The rolling chamfering machine is mostly used for small-size NdFeB parts processing, and the vibration chamfering machine is mostly used for large-size NdFeB parts chamfering processing.
Degreasing, pickling, activation | Degreasing solution and pickling solution should not be too strong in acidity and alkalinity, so as to avoid corrosion of the parts, and also, substances with chelating properties for neodymium should be added to the liquid to prevent the oxidation of neodymium. For pickling solution and activation solution, do not use hydrochloric acid.
Ultrasonic treatment | Ultrasonic cavitation can completely remove the oil, acid, and alkali in the microporous NdFeB. Before plating NdFeB magnets, ultrasonic treatment can be used moderately according to the situation. After completing the steps of ultrasonic degreasing, acid washing, and water washing, then loaded into the drum plating. Sometimes, there will be manual shaking cleaning. Although the manual labor intensity is slightly greater, the parts are cleaned thoroughly, and the coating effect is good
Image Source: Anchor-fasteners
Due to the current copper plating process being of the complex solution type, the solution has a low current efficiency. It is unable to achieve a continuous and qualified copper plating layer on the porous NdFeB surface, making it unsuitable for use as an underlayer for NdFeB coatings. The purpose of the nickel plating on the surface is to provide a positively charged and structurally dense underlayer for subsequent copper plating. This ensures the normal electroplating of copper and prevents corrosion of the underlayer by the copper plating solution. The focus is primarily on the adhesion and deep plating capability between the coating and the surface.
The method of underlayer nickel-plating is the Watt nickel-plating process. The appropriate amount of semi-bright nickel additives is added, the purpose of using additives is not for the brightness, but for the formation of a large current density, which is conducive to the rapid deposition of the coating. Watt nickel-plating also belongs to the type of simple salt plating solution, so the operation process is more or less the same as NdFeB zinc plating.
Middle Layer---Copper Plating
NdFeB copper plating refers to the process of intermediate copper plating between the base layer of nickel plating and surface nickel. At present, NdFeB copper plating mainly adopts the pyrophosphate copper plating process. The copper plating has some advantages as follows:
Reduce the loss of magnetic performance | As copper is a diamagnetic material, compared with nickel, the magnetic shielding effect on the magnet is smaller. Therefore, by replacing a portion of the nickel with copper, the loss of magnetic properties of the magnet can be reduced.
Improved corrosion resistance of plating | Copper has a lower porosity than nickel, so copper plating can improve the corrosion resistance of the plating and make it more durable.
Reduces plating costs | Using the copper plating process reduces the thickness of the nickel layer on the face, thus reducing the cost of the plating.
For small sizes with a high area-to-volume ratio, the thickness of the nickel layer has a greater influence on the magnetic properties of the magnet: in this case, the copper plating process can reduce the magnetic performance degradation of the magnet by increasing the thickness of the copper layer.
Image Source: Pubs.rsc & Amazon
Top Layer---Nickel Plating
The top layer of bright nickel is mostly plated using the standard bright nickel plating process, which is now sufficiently mature. Very few manufacturers use the sulfamate nickel plating process.
Usually, the average thickness of the NdFeB nickel underlayer is required to be not less than 4～5µm to ensure the complete coverage of the parts and prevent the corrosion of the subsequent copper plating solution. The thickness of the nickel top layer is 8～10µm to ensure the corrosion resistance of the plating layer. In this way, the total thickness of the nickel layer reaches 12～15µm. nickel belongs to ferromagnetic metal, which shields the magnetic performance of the NdFeB magnet. The thicker the coating, the greater the shielding effect. With the current NdFeB nickel+copper+nickel plating method, for the small size magnets below 0.5g, the magnetic performance can be attenuated by 10~15%.
So, how to reduce the use of nickel and reduce the shielding effect of the plating on the magnet, but not affect the subsequent copper plating and corrosion, is the development direction of NdFeB magnet nickel + copper + nickel coating.