Sintered Nd-Fe-B magnets are relatively inferior on corrosion resistance comparing with the traditional magnets such as Sm-Co, Alnico and ferrite. For general applications, sintered Nd-Fe-B magnets may need to be coated or surface treated depending on their intended uses. Coating treatment for the sintered Nd-Fe-B magnets not only improves their corrosion resistance, but also appearance, wear resistance as well as appropriate for applications in clean room conditions.
There are various coatings suitable for the sintered Nd-Fe-B magnets, ranging from plating coating layers such as Ni, Zn, Au to organic electro-deposition (epoxy for example), physical vacuum deposition (PVD) of Al, galvanic tin, chemical vapor deposition (CVD), or any combinations of these.
Besides of coating protection, the sintered Nd-Fe-B magnets can also be surface treated by chromatation, phosphatization, and other chemical passivation treatments to improve their corrosion resistance.
Not all types of coating or passivation treatment will be suitable for every magnet, and the final choice will depend on their applications and environment. For example, Ni coating has good corrosion resistance for a variety of atmospheres, but it shields part of the magnetic flux because nickel is a ferromagnetic material. Furthermore, the affinity of Ni coating glued to other metals is inferior to that of Zn coating. Comparing with Ni coating, Zn coating has advantages of good gluing ability, less magnetic flux shielding so that the magnet has good homogenous apparent flux features from piece to piece. But the corrosion resistance of Zn coating is inferior to Ni coating when the magnet is used in stronger corrosive environment. If the magnet is intended to be used in high humid and strong corrosive surroundings, it is suggested that the magnet be coated with multiple layers such as Ni/Cu/Ni, Ni/epoxy, Zn/epoxy et al. If the magnet is intended to be used in dry atmosphere and weak corrosive environment, Zn coating is recommended.
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