At the beginning of last century, people mainly used carbon steel, tungsten steel, chromium steel and cobalt steel as permanent magnetic materials. At the end of the 1930s, the successful development of aluminum-nickel-cobalt (AlNiCo) permanent magnet materials enabled the large-scale application of permanent magnetic materials. In the 1950s, the occurrence of barium ferrite not only reduced the cost of permanent magnet materials, but also broadened the application of permanent magnetic materials to high-frequency fields. Until the 1960s, the emergence of rare earth cobalt permanent magnets opened up a new era for the application of permanent magnetic materials: In 1967, Schneider and others from the University of Dayton in the United States used powder bonding method to successfully make samarium-cobalt ( SmCo) permanent magnet material marks the arrival of the rare earth permanent magnet era.
To date, rare-earth permanent magnets have undergone the first generation of samarium cobalt (SmCo) and the second generation of precipitation-hardened samarium cobalt (Sm2Co17) and have evolved into third-generation neodymium-iron-boron (Nd-Fe-B) permanent magnet materials.
At present, NdFeB permanent magnet materials occupy a leading position in the market for new energy automotive drive motors, wind power generators, servo motors, voice coil motors, nuclear magnetic resonance, and other high-end applications.
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According to data from 2014, the market size of NdFeB products reached 3.28 billion yuan, accounting for 98.8% of the total market share of rare earth magnetic materials, and the remaining 1.2% was occupied by SmCo permanent magnets and iron-nitrogen permanent magnet materials. In the rare earth neodymium-iron-boron products, the market size of sintered neodymium-iron-boron products reached 2.86 billion yuan, accounting for 86.1% of the total market for rare earth permanent magnet materials, and bonded neodymium iron boron reached 420 million yuan, accounting for 12.7 percent of the total market. %.
NdFeB Permanent Magnets Challenges
Since niobium is one of the most chemically active elements, its standard potential E0 is approximately between -2.2 and -2.5 V. Under normal conditions, the NdFeB permanent magnet material has a slower corrosion reaction, but in a warm and humid environment, NdFeB permanent magnet materials are prone to corrosion in an electrochemical environment or under a long-time high temperature environment. The following figure shows a simple model diagram of NdFeB permanent magnet material corrosion.
(Corrosion resistance model of permanent magnet material)
Corrosion has prevented the further widespread application of NdFeB permanent magnet materials, and has become a common problem in the industry. Therefore, it is of great significance to the research on anti-corrosion of NdFeB permanent magnet materials.
Neodymium iron boron permanent magnetic materials current anti-corrosion method
At present, there are two types of anti-corrosion methods for NdFeB permanent magnet materials: The first type is to change the corrosion resistance of NdFeB permanent magnet materials, such as: using effective process measures to increase the density of permanent magnetic materials; Means to optimize the microstructure of permanent magnetic materials; alloying method is used to improve the anti-corrosion performance of permanent magnetic materials by adding other trace elements (such as yttrium Dy elements);
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The second category is the protective coating method, which includes metal coatings and organic coatings. The metal coatings are generally nickel (Ni), zinc (Zn), and aluminum (Al) by electroplating, electroless plating or physical vapor deposition. Nickel-iron (Ni-Fe), nickel-copper (Ni-Cu), nickel-copper-nickel (Ni-Cu-Ni) and other metals or alloys and compounds coated on the surface of permanent magnetic materials; organic coatings are generally used In the more severe corrosive environment, the permanent magnetic material or the sound insulation when the surface of the permanent magnetic material is required to be electrically insulated in certain use environments is generally used for self-deposition, dip coating, spray coating, etc. The polymer coating is applied to the surface of the permanent magnetic material.
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Both protective coatings have their own advantages and disadvantages. Metal coatings (such as electroplating) have a long history of mature technology, high hardness, high temperature resistance, good oil resistance, uniform film thickness, and beautiful appearance, but it is difficult to avoid water, acid, and When the alkali and plating solution penetrate into the permanent magnetic material and when the permanent magnetic material is used as a cathode, hydrogen is precipitated, resulting in hydrogen embrittlement, whitening, bubbling, etc., of the permanent magnetic material, which affects the corrosion resistance, and the insulation is poor;
At present, the organic coatings on the market (such as electrophoresis, ordinary spraying) and permanent magnet materials have relatively good adhesion, corrosion resistance and insulation are also higher than the plating, but the high temperature resistance and oil resistance are poor, and the hardness is low. Therefore, the wear resistance and scratch resistance of NdFeB permanent magnet materials are not satisfactory. In order to achieve the desired corrosion resistance, it is necessary to use a thicker dry film thickness than plating (electroplating is generally 5-7 microns, and the electrophoresis needs to reach 30-50 microns);
Based on the above circumstances, in the harsh environment of offshore turbines and other corrosion resistance requirements (standard salt spray test requires more than 1,000 hours), many permanent magnet manufacturers have to use nickel-copper + epoxy composite coating; but the use of composite Coatings are not only costly, but also complicated processes lead to quality instability.
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Source: Popular Website