NdFeB magnets are produced using powder metallurgy technology. They are a very chemically active powder material with tiny pores and cavities inside, making them easily corroded and oxidized in the air. After the material is corroded or the components are damaged, the magnetic properties will be attenuated or even lost over time, thus affecting the performance and life of the entire machine. Therefore, strict anti-corrosion treatment must be carried out before use.
At present, the anti-corrosion treatment of NdFeB generally adopts electroplating, chemical plating, electrophoresis, phosphating, and other methods. Among them, electroplating, as a mature metal surface treatment method, is the most widely used.
The electroplating quality of NdFeB magnets is closely related to pre-treatment
The NdFeB electroplating process includes pre-treatment and electroplating. The quality of NdFeB electroplating is closely related to its pre-treatment. This pre-treatment process generally includes abrasive grinding and chamfering-immersion chemical degreasing-pickling oxide film-weak acid activation and other processes, during which ultrasonic cleaning is interspersed. After the above-mentioned treatment, the NdFeB magnet exposes a clean basic surface suitable for electroplating, and then it can be electroplated. If any part of the pre-treatment process is not handled cleanly, 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-plating treatment of NdFeB products is more difficult. The reason is that its rough, loose, and porous surface easily "harbors dirt". If this "dirt" is not completely removed, it will cause damage to the NdFeB products. The bonding force between the NdFeB coating and the substrate has adverse effects. At present, multi-channel ultrasonic cleaning is generally used for pre-treatment of NdFeB plating. The cavitation effect of ultrasonic waves completely removes oil, acid alkali, and other substances in the micropores of NdFeB. In addition, ultrasonic cleaning also helps to remove the acid in NdFeB. The boron dust produced on the surface during washing further eliminates the hidden danger of bonding.
Coating types and characteristics of NdFeB magnets
NdFeB electroplating adopts different electroplating processes according to the different use environments of the products, and the surface plating is also different, such as zinc plating, nickel plating, copper plating, tin plating, precious metal plating, etc. Generally, zinc plating, nickel plating + copper plating + nickel, nickel plating + copper + electroless nickel plating are the three processes that are mainstream. Only zinc and nickel are suitable for direct plating on the surface of NdFeB magnets, so multi-layer plating technology is generally implemented after nickel plating. Nowadays, the technical difficulty of direct copper plating of NdFeB has been broken through. Direct copper plating and then nickel plating is the development trend. This kind of plating design is more conducive to achieving the thermal demagnetization index of NdFeB components to meet customer needs.
Characteristics of different coatings and usage environments
| Coating category | Features and usage environment |
| 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 humid heat and high-pressure accelerated aging tests. It is suitable for use by customers who have higher requirements for the long-term stability of appearance and internal performance when exposed to atmospheric environments that may produce condensation. |
| Blue and white zinc | Zinc is a non-magnetic material, and the coating has good salt spray resistance. The surface of the product is prone to produce powder after long-term use. The requirements for surface particles are high, and the application has certain limitations. It is suitable for use in environments where slight corrosion may occur. The coating only has limited anti-corrosion capabilities against short-term pollution discoloration. |
| Color zinc | Compared with blue and white zinc, the anti-corrosion ability is significantly improved, and it is suitable for harsher atmospheres, such as organic corrosive atmospheres. |
| Nickel + copper + nickel plating layer | Compared with a single nickel layer, it has better corrosion resistance, but the process is relatively complicated. |
| Nickel+tin plating layer | It has a good appearance and weldability and is suitable for occasions where electrical contact is required and the surface is required to be weldable. |
| Nickel plating + silver plating | The appearance and weldability are good, the contact resistance is small, and the surface discoloration resistance is poor. Suitable for occasions with electrical contact. The surface requirements are weldable. |
| Nickel+gold plating | It has good decorative performance, the surface is not easy to change color, the contact resistance is small, and the cost is high. It is suitable for the following occasions Electrical contact is required, the surface is required to be weldable, and a decorative appearance is required. |
There are certain differences in the corrosion resistance between different coatings, as follows:
| Coating category | Coating code | Typical thickness μm |
Neutral salt spray test |
Damp heat test h |
High pressure accelerated aging experiment h (unsaturated mode) |
| Nickel plating (barrel plating) | Ni | 5-20 | 48 | 168 | 48 |
| Nickel electroplating (rack plating) | Ni | 5-20 | 16 | 168 | 48 |
| Nickel copper nickel electroplating (barrel plating) | NiCuNi | 5-20 | 48 | 168 | 48 |
| Nickel copper nickel electroplating (rack plating) | NiCuNi | 5-20 | 16 | 168 | 48 |
| Blue and white zinc | Zn.L | 4-15 | 24 | - | - |
| Color zinc | Zn.C | 4-15 | 48 | - | - |
| Nickel+tin plating layer | NiSn | 5-20 | 72 | 168 | 96 |
| Nickel + silver plating | NiAg | 5-20 | 72 | 168 | 96 |
| Nickel + gold plating | NiAu | 5-20 | 72 | 168 | 96 |
| Nickel copper nickel + tin plating layer | NiCuNiSn | 5-20 | 72 | 168 | 96 |
| Nickel + electroless nickel plating (rack plating) | Ni+AP.Ni | 3-20 | 24 | 168 | 48 |
| Nickel + electroless nickel plating (barrel plating) | Ni+AP.Ni | 3-20 | 72 | 168 | 48 |
| Physical vapor deposition aluminum plating | PAD.AI | 2-15 | 24 | 168 | 24 |
Galvanized vs. nickel-plated
The most commonly used coatings for NdFeB powerful magnets are zinc plating and nickel plating. They have obvious differences in appearance, corrosion resistance, service life, price, etc.
The difference in polishability: Nickel plating is superior to zinc plating in terms of polishing, and the appearance is brighter. Those with high requirements on product appearance generally choose nickel plating, while some magnets are not exposed, and those with relatively low requirements on product appearance are generally galvanized.
Difference in corrosion resistance: Zinc is an active metal and can react with acid, so its corrosion resistance is poor; after nickel plating surface treatment, its corrosion resistance is higher.
Difference in service life: Due to different corrosion resistance, the service life of zinc plating is lower than that of nickel plating. The main reason is that the surface coating easily falls off after a long time of use, causing the magnet to oxidize, and thereby affecting the magnetic performance.
Difference in hardness: Nickel plating is higher than zinc plating. During use, it can greatly avoid collisions and other situations, which may cause the NdFeB powerful magnets to fall off, break, etc.
Price difference: Galvanizing is extremely advantageous in this regard. The prices from low to high are galvanizing, nickel plating, epoxy resin, etc.
When choosing NdFeB magnets, you need to consider which coating to use based on factors such as operating temperature, environmental impact, corrosion resistance, product appearance, coating bonding strength, and adhesive effect.