Surface treatment and coating of sintered NdFeB

I believe that these problems have been buried in the hearts of many friends for a long time, and today yueci is here to tell you about the coating problems of 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 performance will be attenuated or even lost over time, which will affect the performance and life of the whole machine, so strict anti-corrosion treatment must be carried out before use.
Currently, NdFeB anti-corrosion treatment generally adopts electroplating, electroless plating, electrophoretic coating, phosphating treatment and other methods, and electroplating is widely used as a mature metal surface treatment method. The NdFeB electroplating process includes two important links: pretreatment and electroplating.

Pretreatment

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., during which ultrasonic cleaning is interspersed. After the above treatment, The neodymium iron boron magnet is exposed to a clean basic surface suitable for electroplating, and then it can be electroplated. If a link in 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, because 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 NdFeB micropores. In addition, ultrasonic cleaning is also beneficial to remove the acid and alkali in the NdFeB. The boron ash generated on the surface during washing further eliminates hidden dangers of binding force.

Plating

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 + chemical 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 multilayer 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.
Compared with traditional magnetic materials such as samarium cobalt, sintered NdFeB permanent magnetic materials have poor corrosion resistance. In most applications, sintered NdFeB requires surface anti-corrosion treatment. Electroplating is the most common surface protection measure. The characteristics of different types of plating and the description of their use environment are shown in the table below.1.
Table.1 The characteristics of different types of coatings and their use environment description

Plating The characteristics of the coating and its use environment description
Nickel plating Nickel is a magnetically conductive material, and the coating has a magnetic shielding effect, which has a slightly greater impact on sheet products. The coating has strong resistance to damp heat and high pressure accelerated aging test. It is suitable for customers who are exposed to the atmospheric environment that may produce condensation and have high requirements for long-term stability of appearance and internal performance.
Zinc plating Blue and white passivation Zinc is a non-magnetic material. The coating has good salt spray resistance, and the product is prone to produce powder on the surface after long-term use, which has certain limitations for applications with high requirements for surface particles. Used in environments that may cause slight corrosion, the coating has only limited corrosion resistance against short-term pollution and discoloration.
Color passivation Compared with the blue and white passivated zinc coating, the corrosion resistance is significantly improved. It is suitable for more severe atmosphere environment, such as organic corrosive atmosphere environment.
Nickel + copper + nickel plating Compared with a single nickel layer, it has better corrosion resistance, but the process is relatively complicated; because the current cost of Cu is lower, the total cost is not high under the same condition of the total plating thickness, so single-layer electroplating nickel has been less used.
Nickel + tin plating layer Good appearance and solderability. It is used in situations where electrical contact is required and the surface is weldable.
Nickel + silver plating Good appearance and solderability, low contact resistance, poor surface resistance to discoloration. It is used in situations where electrical contact is required and the surface is weldable.
Nickel + gold plating Good decoration, the surface is not easy to change color, the contact resistance is small, and the cost is high. It is used in occasions where it is in contact with electricity, and the surface is required to be weldable, and the appearance and decoration requirements are high.

Before electroplating, the sintered neodymium iron boron permanent magnet material must undergo surface cleaning processes such as chamfering, degreasing, pickling, and ultrasonic cleaning, so that subsequent electroplating can obtain a coating with good bonding, low porosity, and long-lasting corrosion resistance.
When choosing an electroplating process, it is necessary to consider the production practicability of the process and the protection of the coating, as well as the environmental impact and damage caused by production emissions. The electroplating process is generally divided into the following four steps:

Chamfer

Small parts can be chamfered with a centrifugal finishing machine, and larger parts can be chamfered with a spiral vibration or centrifugal vortex finishing machine after grinding the edges with a grinder. Chamfering should be carried out in a neutral or slightly alkaline medium, and never use acidic or corrosive media for chamfering.

Degreasing

It should be carried out in neutral and weakly alkaline degreasing agent.

Pickling

Use dilute nitric acid solution or weak organic acid for short-term pickling, but should not pickling in strong halogen acid, and avoid pickling in high concentration strong acid.

Ultrasonic cleaning

Ultrasonic cleaning should be carried out after each treatment process to ensure the cleaning effect and prevent contamination of the next process.

Low-corrosive plating

Low-corrosive plating solution should be used for electroplating, and it is best to use multiple layers of composite plating to meet actual requirements. It can be post-treated if necessary to further improve the corrosion resistance.

The corrosion resistance of different coatings has certain differences, as follows:

Coating category Coating code Time to start corrosion (h)
Neutral salt spray test Damp heat test 85, 85%RH High pressure accelerated aging test 120, 100%RH2atm
Nickel plating (barrel plating) Ni 48 168 48
Nickel plating layer (rack plating) Ni 16 168 48
Nickel copper nickel plating layer (barrel plating) NiCuNi 48 168 48
Nickel copper nickel plating layer (rack plating) NiCuNi 16 168 48
Zinc plating Blue and white zinc Zn.L 24
Color zinc Zn.C 48
Nickel + tin plating layer NiSn 72 168 96
Nickel + silver plating NiAg 72 168 96
Nickel + gold plating NiAu 72 168 96
Nickel copper nickel + tin plating layer NiCuNiSn 72 168 96
Nickel + chemical nickel plating (rack plating) Ni+AP.Ni 24 168 48
Nickel + chemical nickel plating (barrel plating) Ni+AP.Ni 72 168 48
Physical vapor deposition aluminum plating PVD.Al 24 168 24
  • Note 1: The damp heat test (7.3.2) and high-pressure accelerated aging test (7.3.4) are generally not done for zinc coatings;
  • Note 2: The indicators of neutral salt spray test, damp heat test, high-pressure accelerated aging test, etc. refer to the results of coating the wafers and square magnets of general specifications (such as Φ10×3, 10×10×4mm, etc.) . When there is a change in the specifications of the magnet (such as the unit weight is greater than 20g or the unit weight is less than 2g or other shapes such as tile-shaped or special-shaped magnets, etc.), the test indicators will be adjusted accordingly.

The most commonly used coatings of NdFeB 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 in 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 properties.
  • 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 from low to high are: zinc plating, nickel plating, and epoxy resin.

20210103232458 62580 - Surface treatment and coating of sintered NdFeB
When using neodymium iron boron strong magnets, it is necessary to consider which coating to choose according to the use temperature, environmental impact, corrosion resistance characteristics, product appearance, coating adhesion, adhesive effect and other factors.
Source: China Permanent Magnet Manufacturer – www.ymagnet.com

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