Why Does a Small Magnet Need a "Gold Coating"?
In air at a high temperature of 150°C, an unprotected NdFeB magnet can be completely oxidized and corroded in just 51 days, ultimately losing its magical magnetic force.
As the "King of Magnets" in modern industry, NdFeB magnets are widely used in new energy vehicles, wind power generation, consumer electronics, and other fields due to their excellent magnetic properties. However, this powerful magnet has a fatal weakness: it is highly susceptible to corrosion and oxidation.
Without surface treatment, NdFeB magnets rapidly oxidize in air, leading to the decay or even complete loss of magnetic properties, ultimately affecting the performance and lifespan of the entire machine.
01 Why Is Surface Treatment Necessary?
NdFeB magnets are produced using powder metallurgy processes, making them a highly chemically reactive powder material with internal micro-pores and voids. This porous structure causes the magnet to act like a miniature sponge, easily absorbing moisture and pollutants from the air.
Experimental results show that a 1cm³ sintered NdFeB permanent magnet placed in air at 150°C for 51 days will be completely oxidized and corroded. Even at room temperature, unprotected NdFeB magnets gradually oxidize, leading to a decline in magnetic properties.
When magnetic materials are corroded or their composition is damaged, it will eventually cause the decay or even complete loss of magnetic properties, thereby affecting the performance and lifespan of the entire machine. Therefore, surface treatment is not just a matter of aesthetics but a key technology to ensure the long-term reliability of magnets.
02 Preparations for Surface Treatment
The quality of NdFeB electroplating is closely related to the effectiveness of its pre-treatment. Pre-treatment is the most critical and most prone to issues in the entire surface treatment process.
Pre-treatment generally includes processes such as abrasive grinding and deburring, chemical degreasing by immersion, acid washing to remove oxide films, and weak acid activation, interspersed with ultrasonic cleaning. The purpose of these processes is to expose a clean basic surface of the NdFeB magnet suitable for electroplating.
Compared to ordinary steel parts, the pre-treatment for NdFeB products is more difficult due to their rough and porous surface, which makes it hard to completely remove dirt. These "contaminants" can adversely affect the bonding force between the NdFeB coating and the substrate.
Currently, NdFeB pre-treatment generally involves multiple stages of ultrasonic cleaning. The cavitation effect of ultrasound thoroughly removes oil stains, acids, alkalis, and other substances from the micropores of NdFeB. This method also effectively removes boron ash generated on the surface of NdFeB during acid washing.
03 Diversified Surface Treatment Technologies
There are various methods for anti-corrosion treatment of NdFeB, commonly including electroplating, electroless plating, electrophoretic coating, phosphating treatment, etc. Each method has its unique advantages and applicable scenarios.
Passivation Treatment
Passivation involves forming a protective film on the surface of Nd magnets through chemical methods to achieve anti-corrosion purposes. The passivation process includes: degreasing → water rinsing → ultrasonic water rinsing → acid washing → water rinsing → ultrasonic water rinsing → pure water rinsing → passivation treatment → pure water rinsing → dehydration → drying.
Traditional passivation treatments mostly use chromic acid and chromates as treating agents, known as chromate passivation. The chromate conversion film formed on the metal surface after treatment provides good anti-corrosion protection for the base metal.
Phosphating Treatment
Phosphating treatment involves generating an insoluble phosphate protective film on the metal surface through a chemical reaction. This method has relatively low cost and simple operation, but its anti-corrosion performance is poorer compared to electroplating.
An improved method involves passivation treatment after phosphating, where the phosphated product is immersed in a mixed solution of molten stearic acid derivatives and epoxy resin. The protective film obtained by this method has strong adhesion, a uniform surface, and significantly improved corrosion resistance.
Electroplating Treatment
As a mature metal surface treatment method, electroplating is widely used. NdFeB electroplating can adopt different electroplating processes depending on the product's usage environment.
Surface coatings also vary, such as zinc plating, nickel plating, copper plating, tin plating, precious metal plating, epoxy resin, etc. The three mainstream processes are generally zinc plating, nickel + copper + nickel plating, and nickel + copper + electroless nickel plating.
Only zinc and nickel are suitable for direct plating on the surface of NdFeB magnets, so multilayer electroplating technology is generally implemented after nickel plating. The technical challenge of direct copper plating on NdFeB has now been broken through, and direct copper plating followed by nickel plating is a development trend.
04 Performance Comparison of Different Coatings
The most commonly used coatings for powerful NdFeB magnets are zinc plating and nickel plating. They have obvious differences in appearance, corrosion resistance, service life, price, etc.
Characteristics of Zinc Plating
Zinc plating is the most cost-effective option. Its main advantage is low cost, making it suitable for applications where appearance is not a high priority.
However, zinc is an active metal that can react with acids, so its corrosion resistance is relatively poor. Over time, the surface coating is prone to falling off, causing oxidation of the magnet and thereby affecting its magnetic properties.
Characteristics of Nickel Plating
Nickel plating is superior to zinc plating in terms of polishing and has a brighter appearance. Those requiring high product appearance usually choose nickel plating.
After nickel plating surface treatment, its corrosion resistance is higher. Due to the difference in corrosion resistance, the service life of nickel plating is longer than that of zinc plating. Nickel plating has higher hardness than zinc plating, which can largely avoid chipping, cracking, and other phenomena in powerful NdFeB magnets caused by impact during use.
05 How to Choose the Right Coating?
When selecting powerful NdFeB magnets, it is necessary to comprehensively consider factors such as operating temperature, environmental impact, corrosion resistance requirements, product appearance, coating adhesion, adhesive effect, etc., to decide which coating to use.
For applications with high appearance requirements, such as consumer electronics products, nickel plating is usually chosen because it has a brighter appearance and better corrosion resistance.
For applications where the magnet is not exposed and product appearance requirements are relatively low, zinc plating can be considered to reduce costs.
In high-temperature, high-humidity, or corrosive environments, it is necessary to choose coatings with better corrosion resistance, such as multilayer electroplating (nickel + copper + nickel).
06 Development Trends of Surface Treatment Technology
NdFeB surface treatment technology is constantly developing and innovating. In recent years, the requirements for the corrosion resistance of NdFeB conversion films have become increasingly higher, making it difficult to meet demands relying solely on passivation technology.
A commonly used process is composite conversion film technology, which involves phosphating first followed by passivation. By filling the pores of the phosphating film, the corrosion resistance of the composite conversion film is effectively improved.
Direct copper plating followed by nickel plating is a development trend. Such a coating design is more conducive to achieving the thermal demagnetization indicators of NdFeB components.
Researchers are also developing new environmentally friendly treatment technologies to reduce environmental impact. When selecting an electroplating process, not only the protective nature of the process and production practicality should be considered, but also the impact and damage degree of electroplating emissions on the environment.
Now, the surface treatment technology for NdFeB magnets can already enable coatings to withstand 500-1000 hours of salt spray testing, significantly extending the service life of the magnets.
Surface treatment technology is still continuously improving. Direct copper plating followed by nickel plating is a development trend, as such a coating design is more beneficial for achieving the thermal demagnetization indicators of NdFeB components.
In the future, with increasing environmental protection requirements, new green surface treatment technologies will become a research and development focus, allowing us to better protect our planet while enjoying the conveniences of technology.
