The surface protection of NdFeB permanent magnets has three aspects:
1. Improve the corrosion resistance of NdFeB and use various surface protections to resist corrosion such as humid environments, acids, alkalis, salts, and harmful gases;
2. Thoroughly clean the magnet surface, such as removing loose magnetic particles on the surface, etc., to form a smooth surface to prevent loose magnetic particles from affecting the function or damaging the magnetic system;
3. Operational protection to prevent magnetic particles from falling off during assembly or work.
NdFeB passivation is similar to phosphating. It uses chemical methods to form a protective film on the surface of the workpiece on the NdFeB surface to achieve the purpose of anti-corrosion of the workpiece. Surface chemical conversion coating technology is a common method of surface treatment. At present, NdFeB surface chemical conversion coating mainly uses phosphating technology, which is mostly ordinary zinc-based or iron-based phosphating. The phosphating film itself has poor corrosion resistance and often It does not independently undertake the anti-corrosion function but cooperates with electrophoresis, coating, etc. as an outer layer of composite anti-corrosion; at the same time, the phosphating process uses a large amount of phosphoric acid and phosphate compounds, which is prone to phosphorus pollution and causes eutrophication of water bodies and other adverse consequences. Production The process environment is not friendly. Compared with phosphating agents, passivating agents have simple ingredients and do not contain phosphoric acid and phosphate compounds, making them more environmentally friendly.
The NdFeB passivation process is generally: oil removal → water washing → ultrasonic washing → pickling → water washing → ultrasonic washing → pure water washing → pure water washing → passivation treatment → pure water washing → pure water washing → dehydration → drying.
Degreasing
Degreasing can only remove grease, dust, sweat, and attached metal shavings on the surface of NdFeB, but cannot remove rust on the magnet. The oil stain on the surface of NdFeB is brought from the processing of the material, such as the anti-rust grease used to prevent rust during storage and transportation, the cutting fluid that parts come into contact with during cutting, etc.
The oil on the surface of NdFeB must be cleaned before further surface treatment, otherwise, it will affect the quality of surface chemical conversion, electroplating, or painting. Since oil pollution comes from a wide range of sources and types, and the degree of pollution varies greatly, the problem of oil removal is very complex. In order not to cause corrosion and subsequent residues during the degreasing process, try to use degreasing agents with low free alkalinity and total alkalinity.
Pickling
The purpose of pickling is to remove the black dust and rust remaining on the surface of NdFeB. The pickling solution generally uses 2%~4% nitric acid, and the time is 0.5~2.0 minutes. High acid concentrations and long pickling times are detrimental to magnets.
passivation
The passivation treatment method is to place the magnet in a container containing passivation liquid, soak or spray it for some time, or use the magnet as an anode to energize it to achieve passivation, that is, a passivation film is formed on its surface. As an important surface anti-corrosion treatment method, passivation is widely used on many metals, especially aluminum, zinc, cadmium, tin, magnesium, and other metals and their alloy materials.
Most traditional passivation treatments use chromic acid and chromate as treatment agents, which is called chromate passivation. The chromate conversion film formed on the metal surface after treatment has a good anti-corrosion protection effect on the base metal. As a separate protective film, passivation treatment is simple, practical, and low-cost, and was widely used in the early stages of NdFeB development. However, its fatal weakness is that it contains toxic chromium Cr (VI). The harm to the human body and the environment has prompted people to actively research effective alternative technologies. In recent years, there have been many passivating agent patents in my country. There is a passivating agent for NdFeB magnets, including oxalic acid, surfactant, and complexing agent. It has simple ingredients and has the functions of degreasing, rust removal, and passivation at the same time, Without the use of phosphoric acid and phosphate compounds, it is a more convenient and environmentally friendly passivation agent for NdFeB magnets.
In recent years, the corrosion resistance requirements for NdFeB conversion coatings have become higher and higher. It is difficult for passivation technology alone to meet the requirements. The commonly used process is to use composite conversion coating technology, that is, phosphating first and then passivation, and filling it with phosphorus. The pores of the conversion film effectively improve the corrosion resistance of the composite conversion film.