NdFeB Surface Treatment – Passivation Coating
The role of NdFeB permanent magnet surface protection 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 surface of the magnet, 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 destroying the magnetic system;
3-Operation protection, prevent magnetic particles from falling off during assembly or work.
In the previous article, I have introduced in detail several common surface treatment methods of NdFeB, such as phosphating, electroplating, electrophoresis, and vapor deposition. Today we will talk about passivation.
NdFeB passivation is similar to phosphating. It forms a protective film on the surface of the workpiece through chemical methods 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, phosphating technology is mainly used for NdFeB surface chemical conversion coating treatment, mostly ordinary zinc or iron phosphating. The corrosion resistance of the phosphating film itself is poor, and often It does not independently undertake the anticorrosion function, but cooperates with electrophoresis, coating, etc. as the outer layer for composite anticorrosion; at the same time, phosphoric acid and phosphate compounds are used in large quantities in the phosphating process, which is prone to phosphorus pollution, causing adverse consequences such as eutrophication of the water body, and production The process environment is not friendly. Compared with the phosphating agent, the passivating agent has simple components, does not contain phosphoric acid and phosphate compounds, and is more environmentally friendly.
NdFeB passivation process is generally: degreasing→water washing→ultrasonic water washing→pickling→water washing→ultrasonic water washing→pure water washing→pure water washing→passivation treatment→pure water washing→pure water washing→dehydration→drying.
Degreasing can only remove grease, dust, sweat and attached metal shavings on the surface of NdFeB, but cannot remove the rust on the magnet. The oil stain on the surface of NdFeB is brought from the process of material processing, such as the antirust grease used for antirust during storage and transportation, and the cutting fluid that parts come into contact with during cutting. NdFeB Surface Treatment – Passivation Coating
The oil on the surface of NdFeB must be cleaned before further surface treatment, otherwise it will affect the quality of surface chemical transformation, electroplating or coating. Due to the wide variety of sources and types of oil pollution, and the degree of pollution varies greatly, the problem of oil removal is very complicated. In order not to cause corrosion and subsequent residues during the degreasing process, try to use a degreasing agent with low free alkalinity and total alkalinity.
The purpose of pickling is to remove the black ash and rust remaining on the surface of NdFeB. The pickling liquid generally uses 2% to 4% nitric acid, and the time is 0.5 to 2.0 min. High acid concentration and long pickling time are not good for magnets.
The passivation treatment method is to place the magnet in a container containing a passivation solution, soak or spray for a period of time, or use the magnet as an anode to energize and polarize to achieve passivation, that is, to form a passivation film on its surface. Passivation, as an important means of surface anti-corrosion treatment, is widely used in many metals, especially metals such as aluminum, zinc, cadmium, tin and magnesium and their alloy materials.
Most of the traditional passivation treatment uses chromic acid and chromate as the treatment agent, which is called chromate passivation. After treatment, the chromate conversion film formed on the metal surface has a good anti-corrosion protection effect on the base metal. . As a separate protective film, passivation treatment is simple, practical, and low in cost, and it was widely used in the early stage of NdFeB development. But its fatal weakness is that it contains toxic chromium Cr (Ⅵ), which is harmful to the human body and the environment, which has prompted people to actively carry out research on 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, surfactants and complexing agents. Without the use of phosphoric acid and phosphate compounds, it is a more convenient and environmentally friendly passivator for NdFeB magnets. NdFeB Surface Treatment – Passivation Coating
In recent years, the corrosion resistance requirements for NdFeB conversion coatings have become higher and higher, and it is difficult to meet the requirements with a single passivation technology. The commonly used process is to use composite conversion coating technology, that is, phosphating and then passivation. The pores of the chemical film can effectively improve the corrosion resistance of the composite conversion film. NdFeB Surface Treatment – Passivation Coating