Surface coating treatment of NdFeB
Neodymium iron boron magnet, simply speaking, is a kind of magnet. Different from the magnets we usually see, it is called “magnet king” because of its excellent magnetic properties. Nd-Fe-B contains a lot of rare earth elements nd, Fe and B, which are hard and brittle. Because the surface is easy to be oxidized and corroded, NdFeB must be coated. The following are the common surface treatment methods of NdFeB in the magnetic material industry: electroplating (electroless plating), conversion coating, coating and vapor phase coating.
Electroplating (electroless plating)
Nickel plating and zinc plating are the main methods of NdFeB electroplating.
Nickel plating can be divided into single nickel, double nickel, nickel copper nickel, etc. according to the coating structure, and dark nickel, semi bright nickel and bright nickel according to the appearance color. Generally, the nickel plating layer is nickel copper, and the bright nickel and semi bright nickel are the most. The advantage of nickel plating is that the coating has better wear resistance and environmental resistance. The worse is that the nickel coating is ferromagnetic material, which has a certain impact on the magnetic properties, especially for thin and small products. The improvement method is to use part of the copper coating instead. The ferromagnetism of electroless nickel and electroplated nickel phosphorus alloy is weak, but it is troublesome to deplate, so they are not used as mainstream coatings.
Electroless nickel is more uniform than electroplated nickel. However, it can not be directly plated on NdFeB substrate at present. Moreover, electroless nickel uses a lot of complexing agent and reducing agent, and has short bath life, low driving force and high energy consumption, so it can not be used as mainstream coating.
After nickel plating, according to the requirements of the coating, the products can also be plated with gold, silver, tin, substituting chromium, imitating gold, black nickel, etc.
Generally, NdFeB zinc plating adopts chloride zinc plating, and zinc plating is the largest one in terms of domestic production, especially for magnetic steel of electric bicycle. The advantages of the coating are cheap, convenient and the zinc layer is not ferromagnetic. The disadvantage is that the wear resistance and weather resistance are not as good as nickel plating. After galvanizing, the coating should be passivated. Because of the elimination of hexavalent chromium passivation, the common passivation methods are trivalent chromium blue white passivation and trivalent chromium color passivation. Of course, zinc passivation can be as rich as hardware; however, the current mainstream is trivalent chromium blue white passivation.
Zinc nickel alloy has been used in electroplating, but it can not be used as a kind of conventional plating of NdFeB.
NdFeB strong magnet coating generally includes nickel, copper, chromium, gold, black zinc, blue white zinc, epoxy resin, etc. Different electroplating, the color of the magnet surface will vary, and the storage time will be long and short.
The effects of Ni, Zn, epoxy and Parylene-C coatings on magnetic properties of NdFeB magnets in three solutions were studied. We found that in acid, alkali and salt environment, the protective effect of polymer coating on magnet was the best, epoxy resin was relatively poor, Ni coating was the second, and Zn coating was relatively poor.
- Zinc: the surface looks silver white, can be used for 12-48 hours salt fog, and can be used for bonding with some glue (such as abglue), and can be stored for 2-5 years if electroplating is good.
- Nickel: it looks like stainless steel color, it is difficult to be oxidized on the surface in the air, and it has good appearance and good gloss. The plating can pass 12-72 hours salt spray test. The disadvantage is that it can not be used for bonding with some glue, which will make the coating fall off and accelerate oxidation. Now, the market is mostly nickel copper nickel plating, do 120 – 200 hours salt fog.
- Copper: it is mostly found in hardware industry, and few people use NdFeB magnetic material industry, and its appearance is yellow.
- Chromium: chromium plating industry is also relatively rare, its electroplating cost is very high, general companies can not accept. But it has a strong ability of releasing and corrosion, and it is difficult to react with other substances. It is mainly used in places with strong pH.
- Black zinc: according to the requirements of customers, the surface of NdFeB magnetic material can be treated as black. In the electroplating process, it is mainly based on galvanizing and chemical treatment and a layer of black protective film. This film can also be used to protect the product, increase the salt spray time and prolong the oxidation time. But its surface is easy to be scratched and its protection function is lost. There are few people who are using it now, and most of them are replaced by resin.
- Gold: some of the little stalls we saw on the street have yellow gold hand decorations mostly gilded or copper. Gilding makes the appearance of products look like gold is very beautiful, and it is generally used in the hand decoration industry. Our products can also be gilded and used for magnetic hand decoration.
- Epoxy resin: it is to add a layer of resin paint on the outside after nickel plating. The industry has only been in the market for nearly two years. It has a rapid development speed. There are many people using various electroplating products. The biggest advantage is that you can achieve the color you want.
When choosing which material to use, the appropriate coating should be selected according to different use environment. The coating of the unused manufacturer is different, the main reason is the chamfering and the length of plating time. In the absence of understanding the magnet plating, we should judge the quality of the coating, in fact, there is a simplest and fast way to see the coating, the brighter the coating means the better the quality.
Magnet surface treatment table:
Heat and humidity resistance
Salt fog resistance
Corrosion resistance evaluation
Blue white zinc
Blue and white color
Nickel copper nickel
Nickel + gold
The conversion film of NdFeB mainly includes phosphating and passivation film. As the conversion film belongs to workshop protection film, many products will have flash rust when it is used for a long time.
The coating referred to here refers to the electrodeposition, spraying, brushing and dip coating process, excluding the vapor phase coating. At present, the most common coatings are epoxy coatings, followed by everlube coatings, Teflon coatings and chromium free Dacromet coatings.
Due to the different types of epoxy coatings, salt spray resistance test, hardness, color and so on are different. Everlube coating is generally golden yellow with metallic luster. It is an improved solid lubricating film produced in the United States. The comprehensive performance of the coating is excellent and has been commercialized. Teflon coating has also been commercialized. Advanced chromium free Dacromet is also a promising alternative to electroplating.
Vapor phase coating
Vapor coatings include physical vapor deposition (PVD), which can be divided into evaporation, sputtering and ion plating to form al, Zn and Cr coatings; chemical vapor deposition (CVD) can form nitride and carbide coatings of Ti and Cr; chemical vapor deposition (CVD) can form polymer coatings such as Perrin.
Precautions for magnet use
When selecting Nd-Fe-B magnets, customers should choose their own magnet brand according to its working temperature. If the working temperature is higher than the bearing temperature of the magnet brand, the magnet will demagnetize slowly until it is demagnetized. Please make sure to confirm the working temperature and suitable brand before ordering.
If the magnet is in sealed state, the magnet with non electroplating or white zinc plating on the surface can be selected to save cost. If the magnet is exposed to humid air, especially in salt spray state, it is recommended to consider the magnet with nickel plating on the surface. Magnets should be stored in a dry and constant temperature environment. Plastic, wood chips, hard paper, foam and so on should be separated and wrapped. Keep away from iron and easily magnetized objects, such as monitor, bank card, computer, TV, mobile phone, etc.
3. Handling and stacking
Nd-Fe-B strong magnetic material is hard, brittle and strong in magnetic field. It should be handled with care (especially large size and thin sheet). When the strong magnet itself or it is attracted or separated from other ironware, it should be operated carefully without impact. Otherwise, it is easy to cause collision and damage the magnet or pinch fingers. Because NdFeB is a multi metal alloy material with density between 7.45, it is heavy and brittle. Therefore, special care should be taken when handling to avoid magnetic fragmentation. Storage should also keep the environment dry, not directly stacked on the ground, it is best to pad cardboard or placed on the shelf.
4. Instructions for access
When the magnets cannot be separated from each other, it is recommended to push horizontally and stagger, and do not pry hard. Fix the whole magnet with the left hand, use the index finger and thumb with the right hand, move out the magnet horizontally. After moving out, do not throw the magnet back into the magnet pile immediately, please isolate the magnet immediately. Handle it with care. If you are far away from iron objects or magnetic objects (about 16 cm) or use paper to wrap the removed paper, it will be safer. If you need to put the magnet back into the whole magnet, please put it back slowly and move it in close from the edge. Be sure to hold the magnet in your hand until it has moved in.
Magnet ordering notes:
- 1. Magnet performance.
- 2. Dimension and tolerance.
- 3. Do you want to magnetize? If you want to magnetize, what is the way, axial direction? Radial?
- 4. Maximum temperature of working environment of magnet.
- 5. Order quantity.
- 6. Surface treatment. Zinc plating, nickel plating?
- 7. If you have any other special requirements, please let us know.
- 8. It is better to provide drawings.
Source: China Permanent Magnet Manufacturer www.rizinia.com