Sintered NdFeB permanent magnets are produced by powder metallurgy and are highly chemically active, with internal micro-pores and voids. They are prone to corrosion and oxidation in air. Experimental results show that a 1 cm³ sintered NdFeB magnet can be completely oxidized after 51 days at 150°C in air. Once corroded or structurally degraded, magnetic performance will deteriorate or even be lost over time, affecting the performance and service life of the final product. Therefore, strict anti-corrosion treatment is essential before use.
Currently, common corrosion protection methods for NdFeB include electroplating, electroless plating, electrophoretic coating, and phosphating. Among these, electroplating is widely used due to its maturity as a metal surface treatment process. The electroplating process consists of two key stages: pre-treatment and plating.
The quality of electroplating is highly dependent on pre-treatment. Typical steps include abrasive grinding and chamfering, chemical degreasing, acid pickling to remove oxide layers, and mild acid activation, often combined with ultrasonic cleaning. These processes ensure a clean surface suitable for plating. Any insufficient cleaning may lead to defects such as blistering or peeling.
Compared to conventional steel parts, pre-treatment of NdFeB is more challenging due to its rough, porous structure, which traps contaminants and reduces coating adhesion. Multi-stage ultrasonic cleaning is commonly used, as cavitation effectively removes oil, acid, and other residues from micro-pores. It also helps eliminate boron-rich residues formed during pickling, improving coating adhesion.
Different electroplating processes are selected based on the application environment, resulting in various coating systems such as zinc, nickel, copper, tin, precious metals, and epoxy. The most common processes are zinc plating, Ni–Cu–Ni, and Ni–Cu–electroless Ni.
Only zinc and nickel can be directly plated onto NdFeB surfaces, so multilayer coatings are typically applied after an initial nickel layer. Advances have enabled direct copper plating on NdFeB, followed by nickel plating. This Cu + Ni structure is an emerging trend, as it better supports thermal demagnetization performance to meet application requirements.
| Characteristics and Application of Different Coatings for NdFeB Magnets | |||
| Types | Characteristics and Application | ||
| Nickel (Ni) Coating: | Magnetic; may cause magnetic shielding effects. Moderate resistance to humidity, heat, and pressure aging. Suitable for applications requiring stable appearance and performance over time. | ||
| Zinc (Zn) Coating: | Zn. L | Non-magnetic; good heat resistance but prone to white rust over time, affecting appearance. Suitable for mildly corrosive environments with basic protection needs. | |
| Zn. C | Improved corrosion resistance compared to blue/white Zn; suitable for more demanding atmospheric environments (e.g., organic corrosive conditions). | ||
| Ni–Cu–Ni Coating: | Better corrosion resistance than single-layer Ni; widely used, though the process is more complex. | ||
| Ni–Sn Coating: | Good appearance and solderability; suitable for applications requiring electrical contact and soldering. | ||
| Ni–Ag Coating: | Good appearance and solderability, low contact resistance, but relatively poor tarnish resistance. Suitable for electrical contact applications. | ||
| Ni–Au Coating: | Excellent decorative finish, stable surface, low contact resistance, but higher cost. Suitable for high-end applications requiring both electrical contact and premium appearance. | ||
| Corrosion Resistance of Common Coatings | |||||
| Coating Type | Thickness (μm) |
Salt Spray (hrs) |
Humidity Test (hrs) |
PCT / Pressure Test (hrs) |
|
| Ni (Barrel Plating) | 5-20 | 48 | 168 | 48 | |
| Ni (Rack Plating) | 5-20 | 16 | 168 | 48 | |
| NiCuNi (Barrel Plating) | 5-20 | 48 | 168 | 48 | |
| NiCuNi (Rack Plating) | 5-20 | 16 | 168 | 48 | |
| Zn | Zn. L | 4-15 | 24 | / | / |
| Zn. C | 4-15 | 48 | / | / | |
| NiSn | 5-20 | 72 | 168 | 96 | |
| NiAg | 5-20 | 72 | 168 | 96 | |
| NiAu | 5-20 | 72 | 168 | 96 | |
| NiCuNiSn | 5-20 | 72 | 168 | 96 | |
| Ni+AP. Ni (Rack Plating) | 3-20 | 24 | 168 | 48 | |
| Ni+AP. Ni (Barrel Plating) | 3-20 | 72 | 168 | 48 | |
| PVD. AI | 2-15 | 24 | 168 | 24 | |
The most commonly used coatings for NdFeB magnets are zinc (Zn) and nickel (Ni). They differ significantly in appearance, corrosion resistance, service life, hardness, and cost:
• Appearance:
Nickel plating provides better polishability and a brighter finish. It is preferred for applications with high aesthetic requirements. Zinc plating is typically used where appearance is less critical or the magnet is not visible.
• Corrosion Resistance:
Zinc is a reactive metal and more prone to corrosion, especially in acidic environments. Nickel plating offers higher corrosion resistance.
• Service Life:
Due to its lower corrosion resistance, zinc-plated magnets generally have a shorter lifespan. Over time, the coating may degrade and peel off, leading to oxidation and reduced magnetic performance. Nickel-plated magnets have a longer service life.
• Hardness:
Nickel plating has higher hardness than zinc, helping to reduce chipping and cracking caused by impact during use.
• Cost:
Zinc plating is more cost-effective. In general, coating costs increase in the order: Zn < Ni < Epoxy.
When selecting coatings for NdFeB magnets, factors such as operating temperature, environmental conditions, corrosion resistance, appearance requirements, coating adhesion, and bonding performance should be comprehensively considered.
