Adding plating or coating to a magnet can have a huge benefit on its longevity. Below are three frequently asked questions about plated or coated magnets.
1. Why should the magnet be electroplated or coated?
There are two main reasons for electroplating magnets:
(1) Plating magnets helps protect them from corrosion from extreme temperatures or exposure to liquids or vapors. Plating can also help maintain the desired magnetic field in less-than-ideal environments.
(2) Protective plating prevents chipping, peeling or other damage caused by magnets colliding with each other during transportation or after final use.
2.What are the most common types of plating and coating for magnets?
Given the susceptibility of the material to corrosion, protective coatings are most commonly found on NdFeB magnets. For some extreme environments, samarium cobalt can be plated. The most common ones are: nickel plating (black nickel), galvanizing (color zinc), epoxy resin, gold plating, etc.
3.What is the difference between passivated and plated/coated magnets?
Passivation is a common practice to protect magnets for a short period of time (typically 3-6 months). This process is often used to protect magnet materials from corrosion during transportation. Plating or coating is a long-term solution to protect magnets. Unlike passivation, once a magnet has been plated or coated, it generally cannot be remanufactured.
Metal Film Conversion Technology
Metal Conversion Coating is a dense compound protective film formed on the metal surface by chemical or electrochemical methods. For example, the oxidation and phosphating of steel, the oxidation of aluminum and magnesium alloys, etc., all belong to metal conversion coating technology. In the early stage of NdFeB development, hexavalent chromium passivation technology was widely used for surface treatment. This method is simple, practical, and low in cost. Chromate passivation film (belonging to chemical conversion film) has certain anti-corrosion protection for magnets. effect.
However, with the development of technology, a single passivation film can no longer meet the higher and higher anti-corrosion requirements of NdFeB. Therefore, in recent years, the treatment of NdFeB conversion film often adopts composite conversion film technology, that is, phosphating first and then passivation. By filling the pores of the phosphating film, the corrosion resistance of the composite conversion film can be effectively improved.
Organic Coating Spraying Technology
NdFeB organic coatings usually use epoxy resin, or acrylic, polyamide and other resin coatings, among which epoxy resin has the best water resistance and chemical corrosion resistance. The coating construction of organic coating usually adopts the method of spraying.
NdFeB can be directly coated on the substrate by organic coating technology, that is, substrate-coating. However, the main problem of this solution is that the coating adhesion is not good and the corrosion resistance is not high, because the NdFeB substrate cannot provide the good surface state required to obtain high-quality coatings. NdFeB phosphating-coating, NdFeB phosphating-passivation-coating and other technologies can be used. However, the coating-coating technology can provide a better surface state for the organic coating, and the corrosion resistance is also better. Studies have shown that after NdFeB electroplating, zinc-based phosphating treatment and then coating, that is, NdFeB electroplating-phosphating-coating, this composite coating scheme can obtain better paint film adhesion and corrosion resistance.
Metal Plating Technology
The metal coating is thin, generally only micron level, but it can meet the high anti-corrosion requirements of NdFeB magnets, and endows it with a variety of beautiful appearances, so it is widely used in NdFeB surface treatment. NdFeB metal coating can be obtained by electroplating, chemical plating, vacuum ion plating and other methods.
1. Electroplating As a mature and cheap metal surface treatment method, electroplating is widely used in the field of anti-corrosion of NdFeB magnets. NdFeB is mostly small parts, so NdFeB electroplating is mainly barrel plating.
2. Electroless plating ，Generally, the anti-neutral salt spray test of NdFeB nickel-copper-nickel coating combination is more than 72h. If the requirement is higher, the thickness of the coating needs to be increased. However, if the coating is too thick, the influence on the magnetic properties of the magnet will increase. The plating time is long and the production efficiency is low, so it is not a wise choice. At this time, electroless nickel plating with higher corrosion resistance is generally used to solve the problem, and the electroless nickel plating controls the phosphorus content, so that the coating can be non-magnetic, and the impact on the magnetic properties of the magnet can be minimized at this time.
3. Vacuum ion plating Vacuum ion plating belongs to physical vapor deposition technology, rather than conventional liquid phase chemical or electrochemical deposition technology. The technical process is simple, the film layer is uniform and dense, the bonding force with the substrate is good, and the corrosion resistance is good.
This method of NdFeB is mainly ion-plated aluminum. The advantage is that the coating has good adhesion, good corrosion resistance, and no residual acid and alkali on the surface of the magnet, thus reducing the chance of substrate corrosion after plating.