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Surface treatment and coating of sintered NdFeB

The sintered neodymium iron boron permanent magnet material is produced by powder metallurgy technology. It is a kind of powder material with very strong chemical activity. There are tiny pores and cavities inside, which are easily corroded and oxidized in the air. The experimental results show that a 1cm³ sintered NdFeB permanent magnet will be completely oxidized and corroded if placed in the air at 150°C for 51 days. After the magnetic material is corroded or damaged by the components, the magnetic properties will be attenuated or even lost over time, which will affect the performance and life of the whole machine. Therefore, strict anti-corrosion treatment must be carried out before use.

The quality of NdFeB electroplating is closely related to its pre-treatment effect. The pre-treatment generally includes abrasive grinding and chamfering-soaking chemical degreasing-pickling oxide film-weak acid activation, etc., interspersed with ultrasonic cleaning during the process, and after the above treatment, The neodymium iron boron magnet is exposed to the basic surface that is clean and suitable for electroplating, and then it can be electroplated. If one part of the pre-treatment process is not cleanly handled, it will bring latent defects to the final electroplated product, causing problems such as blistering and peeling of the electroplated layer.

Compared with ordinary steel parts, the pre-treatment of NdFeB products is more difficult. The reason is that the surface is rough, loose and porous, and the dirt is difficult to completely remove, which adversely affects the bonding force of the NdFeB coating and the substrate. At present, the pretreatment of NdFeB plating generally adopts multi-channel ultrasonic cleaning. The cavitation effect of ultrasonic can completely remove the oil, acid and alkali in the micropores of the NdFeB; in addition, ultrasonic cleaning is also beneficial to remove the acid in the NdFeB. The boron ash generated on the surface during washing further eliminates the hidden dangers of binding force.

NdFeB strong magnetic electroplating can use different electroplating processes according to the different use environment of the product, and the surface coating is also different, such as zinc plating, nickel plating, copper plating, tin plating, precious metal plating, epoxy resin, etc., generally zinc plating , Nickel plating + copper + nickel, nickel plating + copper + electroless nickel plating three processes are the mainstream.
Only zinc and nickel are suitable for direct plating on the surface of neodymium iron boron magnets, so multi-layer electroplating technology is generally implemented after nickel plating. At present, the technical difficulty of direct copper plating of NdFeB has been broken. Direct copper plating and then nickel plating is a development trend. Such a coating design is more conducive to the thermal demagnetization index of NdFeB components to meet customer needs.

The most commonly used coatings of neodymium iron boron strong magnets are zinc plating and nickel plating. They have obvious differences in appearance, corrosion resistance, service life, and price:

Polishing difference: Nickel plating is superior to zinc plating on polishing, and the appearance is brighter. Those with high product appearance requirements generally choose nickel plating, while some magnets are not exposed, and those with relatively low product appearance requirements generally choose zinc plating.

Difference in corrosion resistance: Zinc is an active metal and can react with acid, so it has poor corrosion resistance; after nickel-plated surface treatment, its corrosion resistance is higher.

Difference in service life: Due to the different corrosion resistance, the service life of zinc plating is lower than that of nickel plating. The main manifestation is that the surface coating is easy to fall off after a long time of use, causing the magnet to oxidize, thereby affecting the magnetic performance.

Hardness difference: Nickel plating is higher than zinc plating, which can greatly avoid corners and chipping of the powerful neodymium iron boron magnets caused by collisions during use.

Price difference: Zinc plating is very advantageous in terms of price. The prices of NdFeB coatings are arranged from low to high: zinc plating, nickel plating, and epoxy resin.

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