Surface treatment of NdFeB: passivation
Similar to phosphating, NdFeB passivation is to form a protective film on the surface of the workpiece by chemical method to achieve the purpose of anti-corrosion. Surface chemical conversion coating technology is a common means of surface treatment. At present, phosphating technology is mainly used in the surface chemical conversion coating treatment of NdFeB, most of which are ordinary zinc or iron phosphating. The phosphating film itself has poor corrosion resistance and often does not bear the anti-corrosion function independently, but cooperates with electrophoresis and coating as the outer composite anti-corrosion. At the same time, phosphoric acid and phosphoric acid are widely used in the phosphating process Salt compounds, easy to produce phosphorus pollution, resulting in water eutrophication and other adverse consequences, the production process is not friendly. Compared with the phosphating agent, the passivating agent has simple composition, and does not contain phosphoric acid and phosphate compounds, so it is more environmentally friendly.
The process flow of NdFeB passivation is as follows: 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.
Oil removal can only remove grease, dust, sweat and metal chips on the surface of NdFeB, but not 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 in the process of storage and transportation, the cutting fluid contacted by parts during cutting, etc.
Before further surface treatment, the oil on the surface of NdFeB must be cleaned, otherwise the quality of chemical conversion, electroplating or coating will be affected. Due to the wide source, many kinds of oil pollution, the degree of pollution varies greatly, so the problem of oil removal is very complex. In order to avoid corrosion and residue during degreasing, degreaser with low free alkalinity and total alkalinity should be used as far as possible.
The purpose of pickling is to remove the residual black ash and rust on the surface of NdFeB. Pickling solution generally uses 2% ~ 4% nitric acid for 0.5 ~ 2.0 min. High acid concentration and long pickling time are unfavorable to magnets.
The passivation treatment method is to put the magnet in the container containing passivation solution, soak or spray for a period of time, or use the magnet as the anode to electrify and polarize, then passivation can be realized, that is, to form a passivation film on its surface. Passivation, as an important surface anti-corrosion treatment, is widely used in many metals, especially aluminum, zinc, cadmium, tin, magnesium and their alloys.
The traditional passivation treatment mostly uses chromic acid and chromate as the treatment agent, 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, the passivation treatment is simple, practical and low cost, which is widely used in the early development of NdFeB. But its fatal weakness is that it contains toxic chromium Cr (VI). The harm to human body and environment has prompted people to actively carry out the research of effective alternative technology. In recent years, there are many patents of passivator in China. There is a kind of NdFeB magnet passivator, including oxalic acid, surfactant and complexing agent. Its composition is simple, and it has the functions of degreasing, rust removing and passivation at the same time. There is no phosphoric acid and phosphate compound. It is a more convenient and environmental friendly NdFeB magnet passivator.
In recent years, the corrosion resistance of Nd-Fe-B conversion coating is required to be higher and higher, and the single passivation technology is difficult to meet the requirements. The common process is to use the composite conversion coating technology, that is, phosphating first and then passivation. By filling the pores of the phosphating film, the corrosion resistance of the composite conversion coating can be effectively improved.
Source: China Permanent Magnet Manufacturer www.rizinia.com