How to choose the coating of NdFeB magnet

• Why does sintered NdFeB need coating?
• How to choose coating materials? Zinc plating, nickel plating, or other materials?
• What are the differences between different coatings?

Today, this article will address issues related to coatings for NdFeB magnetic steel.
NdFeB magnetic steel is produced using powder metallurgy technology and is a material with very strong chemical activity, containing tiny pores and voids internally, which are easily corroded and oxidized in air. Corrosion or component degradation of the material will cause a decline or even loss of magnetic properties over time, thus affecting the performance and lifespan of the entire device, hence strict anti-corrosion treatment is necessary before use.

Currently, anti-corrosion treatment for NdFeB commonly employs methods such as electroplating, chemical plating, electrophoresis, and phosphating, among which electroplating, as a mature metal surface treatment method, is the most widely used.

Close Relationship Between Electroplating Quality and Pretreatment

The electroplating process for NdFeB includes pretreatment and electroplating. The quality of electroplating for NdFeB is closely related to its pretreatment. This pretreatment process generally includes abrasive grinding and chamfering, chemical degreasing immersion, acid pickling oxide film, weak acid activation, etc., interspersed with ultrasonic cleaning. Through the above treatments, the NdFeB magnet exposes a clean surface suitable for electroplating, and then electroplating treatment can be carried out. If any step of the pretreatment process is not clean, it may bring latent defects to the final electroplated product, resulting in problems such as blistering and peeling of the electroplated layer.

Compared with ordinary steel parts, the pretreatment of NdFeB products is more difficult because their rough and porous surfaces are prone to “hiding dirt.” Therefore, these “dirt” must be thoroughly removed, otherwise it will adversely affect the adhesion between the NdFeB coating and the substrate. Currently, the pretreatment of NdFeB commonly adopts multi-stage ultrasonic cleaning. The cavitation effect of ultrasonic waves can thoroughly remove oil, acid, and alkali substances inside the micropores of NdFeB, and it is also beneficial for removing boron ash generated on the surface during acid pickling, further eliminating adhesion hazards.

Types and Characteristics of Coatings for NdFeB Magnets

Different electroplating processes are adopted for NdFeB according to different product usage environments, resulting in different surface coatings, such as zinc plating, nickel plating, copper plating, tin plating, precious metal plating, etc. The mainstream processes generally include zinc plating, nickel plating+copper+nickel, and nickel plating+copper+chemical nickel plating. Only zinc and nickel are suitable for direct plating on the surface of neodymium iron boron, so multi-layer electroplating technology is generally implemented after nickel plating. The technological barrier of directly plating copper on NdFeB has now been overcome, and the trend is to directly plate copper and then nickel, which is more conducive to meeting the thermal demagnetization index requirements of NdFeB components as needed by customers.

Different coatings have certain differences in corrosion resistance:

Zinc Coating VS. Nickel Coating

The most commonly used coatings for NdFeB strong magnets are zinc plating and nickel plating, which have significant differences in appearance, corrosion resistance, service life, and price:

• Polishing: Nickel plating is superior to zinc plating in terms of polishing and has a brighter appearance. Products with high appearance requirements generally choose nickel plating, while those with relatively low appearance requirements generally choose zinc plating.
• Corrosion resistance: Zinc is an active metal that reacts with acid, so its corrosion resistance is poor; after treatment, nickel plating has higher corrosion resistance.
• Service life: Due to different corrosion resistance, the service life of zinc plating is lower than that of nickel plating, mainly manifested in the easy peeling of the surface coating after prolonged use, leading to oxidation of the magnet and affecting magnetic properties.
• Hardness: Nickel plating has higher hardness than zinc plating, which can greatly reduce damage to NdFeB magnets caused by collisions during use.
• Price: In terms of price, zinc plating has a significant advantage, with zinc plating, nickel plating, and epoxy resin listed in ascending order of price.

When choosing NdFeB strong magnets, factors such as operating temperature, environmental impact, corrosion resistance, product appearance, coating adhesion, and adhesive effects should be comprehensively considered to determine the most suitable coating.