We all know that the sintered NdFeB permanent magnet material is produced by powder metallurgy technology. It is a kind of material with relatively active chemical properties. There are tiny pores inside, and the Nd-rich phase is easy to oxidize in the air. After the material is corroded or damaged by the components, the magnetic properties will be attenuated or even lost over time, which will affect the performance and life of the whole machine, so strict anti-corrosion treatment must be carried out before use.

Surface protection treatment can be divided into two categories: wet and dry

The wet method is the surface protection treatment of the magnet 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 performed by physical or chemical processes on the magnet without contacting the solution, such as physical vapor deposition (PVD) and chemical vapor deposition (CVD). Among them, physical vapor deposition includes vacuum evaporation plating, vacuum sputtering plating and ion plating (IVD). Different coating methods have their own strengths and need to be selected for different application environment requirements.

Wet surface treatment

The main methods include phosphating, electroplating, electroless plating, electrophoresis, spraying, and dipping. They can be used alone or in combination.

1. Phosphating

The phosphating process is a process in which the magnet is placed in a solution containing phosphate, and the solution reacts with the surface of the magnet to form a phosphating film. Phosphating film is generally gray and difficult to contain water, so it can improve the water absorption and corrosion resistance of the magnet, and can be used alone to deal with short-term corrosion resistance requirements or occasions where the use environment is not high.

2. Electroplating

The electroplating process is to connect the magnet to the cathode of the DC power supply and immerse it in a solution containing cations of the coating material. The cations will migrate to the cathode under the action of an electric field and be transformed into metal atoms to crystallize on the surface of the magnet.

Sintered NdFeB commonly used zinc plating and nickel plating, and nickel-copper-nickel composite plating.

Zinc is relatively stable in dry air, and a zinc carbonate film is formed in moist air or oxygen-containing water, which can delay the corrosion rate of zinc, but it has better corrosion resistance in acid-base salt solutions, marine atmosphere, and high-temperature and high-humidity air. Poor, passivation treatment can significantly improve the corrosion resistance of the zinc coating.

Nickel easily forms a very thin passivation film with oxygen in the air, and has good corrosion resistance to the atmosphere, alkalis 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 will form a shield to the magnetism of the substrate, especially when the magnet is small or thin. The corrosion risk of single-layer nickel is relatively high. Multi-layer plating or composite plating can solve this problem well.

Copper is chemically active and easy to rust, so it is generally not used alone, but as a bottom coating or an intermediate layer to improve the bonding force between the substrate and the surface coating.

3. Electroless plating

Electroless plating is the same as electroplating. It also undergoes oxidation-reduction reaction. The metal ions in the plating solution are reduced to atoms and adhere to the surface of the magnet. The difference is that there is no current to attract ions and enhance the adhesion of atoms. Therefore, a reducing agent is required to coexist in the plating solution. , The surface of the substrate also needs catalysis. No power supply is the biggest advantage of electroless plating. It can form a coating with a uniform thickness on the surface of a magnet with complex shapes. The coating has high hardness, small voids and high chemical stability.

4. Electrophoresis

Unlike phosphating and electroplating, the surface protective layer formed by electrophoresis, spraying and dipping is organic. Electrophoresis is a phenomenon in which the charged colloidal particles in the conductive dispersion medium move towards the opposite electrode under the action of an external electric field. Electrophoretic coating uses this feature to make the charged organic paint molecules (usually epoxy resin) firmly adsorb on the opposite electrode. On the surface of the magnet, the magnet is taken out from the electrophoresis tank and then cured to form a dense protective film.

Dry surface treatment

In dry coating, the chamfering and degreasing steps of magnets are similar to wet coating, but dry coating does not need to make the chamfer of the magnet too large, because the film thickness of dry coating is more uniform. The film thickness is low under the same corrosion conditions, and the corner effect is relatively small.

1. Physical vapor deposition (PVD)-vacuum ion evaporation (IVD)

Vacuum ion evaporation is a kind of physical vapor deposition method, developed from the vacuum evaporation process. Vacuum evaporation is to evaporate the metal material to be plated by resistance heating, arc heating or sputtering in a vacuum environment. The atomic metal is deposited on the surface of the magnet, and the crystal grows into a metal film that completely covers the magnet. The process is the most mature It is resistance heating evaporation aluminum plating. Vacuum ion plating accelerates the deposition rate of aluminum vapor by loading a 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)

The application of chemical vapor deposition of paraxylene polymer (Parylene parylene) has been developed into the field of NdFeB surface protection in recent years. The coating process is carried out in a vacuum environment. The parylene is heated and sublimated into a gas phase monomer. It is deposited on the surface of the magnet with lower temperature and polymerized into a solid parylene film. Parylen has a very low water vapor transmission rate and can provide excellent moisture and corrosion protection.