Nd-fe-b permanent magnetic material is produced by powder metallurgy process. It is a kind of active powder material with chemical properties. There are tiny pores inside. After the material is corroded or the component destroys, the day long will cause the magnetic energy attenuation even to lose, thus affects the whole machine service performance and the life, therefore must carry on the strict anti-corrosion treatment before the use.
The surface protection treatment of NdFeb can be divided into two categories: dry and wet
Wet method is the surface protection treatment of magnets in pure water, inorganic solution or organic solution, such as electroplating, electroless plating, electrophoresis, spraying and dipping. The dry method is a surface protection treatment that the magnet does not contact the solution and is performed by physical or chemical processes, such as physical vapor deposition (PVD) and chemical vapor deposition (CVD). Physical vapor deposition includes vacuum evaporation, vacuum sputtering and ion deposition (IVD). Different coating methods have their own advantages and need to be selected according to different application environment requirements.
Wet surface treatment of NdFeb
The main methods include phosphating, electroplating, electroless plating, electrophoresis, spraying and dipping, which can be used alone or in combination.
1. The phosphating
Phosphating is a process in which a magnet is put into a solution containing phosphate, which reacts with the surface of the magnet to form a phosphating film. Phosphating film is generally gray, difficult to water, so it can improve the water absorption and corrosion resistance of the magnet, can be used alone to deal with the short-term corrosion resistance requirements or use environment requirements are not high occasions.
2. The plating
The electroplating process is to connect the magnet to the cathode of the DC power supply and immerse the magnet in the solution containing the cations of the coating material. Under the action of electric field, the cations migrate to the cathode and turn into metal atoms to crystallize on the surface of the magnet.
Sintered NdFeb are commonly used in zinc and nickel plating, and nickel copper nickel plating.
Zinc is more stable in dry air, in moist air or water containing oxygen will form zinc carbonate film, can delay the corrosion rate of zinc, but in acid and alkali salt solution, oceanic atmosphere, high temperature and high humidity air corrosion resistance is poor, passivation treatment can significantly improve the corrosion resistance of zinc coating. Nickel is easy to form an extremely thin passivation film with oxygen in the air, and has good corrosion resistance to atmosphere, alkali and some acids at room temperature. Therefore, nickel plating has become the most common electroplating method for sintered NdFeb. However, nickel is an excellent soft magnetic material, which can form a shield against the magnetic matrix, especially when the magnet is small or very thin. The erosion risk of single layer nickel is relatively high. Multi-layer or composite plating can solve this problem well. Copper chemical properties, easy to rust, so generally not used alone, but as a bottom coating or intermediate layer to improve the adhesion of the base and surface coating.
3. The chemical plating
Electroless plating, like electroplating, is also through REDOX reaction. Metal ions in the plating solution are reduced to atoms and attached to the surface of the magnet. The difference is that there is no current to attract ions and enhance atomic adhesion. No power supply is the biggest advantage of electroless plating, it can form uniform thickness coating on the surface of complex magnets with high hardness, small void and high chemical stability.
4. Electrophoresis
Different from phosphating and electroplating, electrophoresis, spraying and leaching form a surface coating of organic matter. Electrophoresis is outer electric field under the action of conductive and charged colloid particles in the dispersion medium to the vision to the phenomenon of directional moving electrode, electrophoresis coating use this feature to make charged organic molecular (usually) epoxy resin coating firmly adsorbed on the surface of the magnet that electrode as the opposite sex, magnet from the electrophoresis tank and then form a dense protective film after curing.
Ndfeb dry surface treatment
In dry coating, the steps of chamfering and deoiling of magnets are similar to those of wet coating. However, it is not necessary to make the chamfering of magnets much larger in dry coating, because the film thickness of dry coating is more uniform, the film thickness is lower under the same corrosion condition, and the edge effect is relatively small.
1. Physical Vapor deposition (PVD) -- Vacuum ion evaporation plating (IVD)
Vacuum ion evaporation plating is a kind of physical vapor deposition. Vacuum evaporation refers to the evaporation of metal materials to be plated by means of resistance heating, arc heating or sputtering in a vacuum environment. The atomized metal is deposited on the surface of the magnet and crystallizes to grow into a complete metal film covering the magnet. The most mature technology is resistance evaporation aluminum plating. Vacuum ion plating accelerates the deposition rate of aluminum vapor by loading high negative bias between the evaporation source and the magnet, and further improves the adhesion of the magnet to the aluminum film.
2. Chemical Vapor Deposition (CVD)
In recent years, the application of chemical vapor deposition of Parylene polymer (Parylene) has been developed to the field of surface protection of Neodymium iron boron. The coating process is carried out in a vacuum environment. The heating and sublimation of Parylene into meteorological monomer are then deposited on the surface of the magnet at low temperature, and then the polymer is formed into solid Parylene film. Parylen offers excellent protection against moisture and corrosion due to its extremely low water vapor transmission rate.
