Neodymium magnets, or rather NdFeB, are products produced by powder metallurgy and based on neodymium, iron and boron, which makes magnets that are impressive in relation to their strong magnetic field. They are actually the strongest permanent magnets that can be produced today.
What is not as well known is that neodymium magnets are susceptible to corrosion in their raw state. When the magnets are exposed to humidity they can corrode, which can destroy the magnet if the magnets are not treated correctly from the outset. The magnets can be surface treated to prevent corrosion.
Corrosion is a very negative characteristics in relation to permanent magnets in NdFeB and SmCo alloys. A lot of research has gone into the development of suitable surface treatment processes to reduce the risk of corrosion, but recently new types of neodymium magnet material have been developed with a lower risk of corrosion.
Two types of corrosion affect NdFeB magnets. Red corrosion that arises in roughly the same way as rust appears on iron parts and white corrosion which penetrates the magnet by passing through the grain boundaries in the magnetic material. White corrosion can ultimately lead to the magnet being turned into powder and thus ceasing to be a magnet.
The basic problem arises from the fact that magnets are largely based on rare earth metals such as samarium or neodymium. These elements have a strongly negative electric potential (-2.2 to -2.5 V) with the result that if they are exposed to a higher temperature along with a high humidity, the corrosion of the magnets can drastically increase. Samarium is often alloyed with cobalt and therefore has better protection against corrosion, whereas neodymium magnets are more sensitive because the magnetic grains in the material are held together mechanically by a neodymium-rich grain boundary. This grain boundary is approximately 5% of the total volume of the magnet and can be regarded chemically as pure neodymium. A raised temperature and air humidity can rapidly lead to greatly accelerated corrosion and this in turn can result in the disintegration of the magnet where the remains become neodymium hydroxide and loose NdFeB grains.
It is easy to appreciate from this information that it is vital to choose the right base material in the magnet to avoid future corrosion problems. If the magnets are surface treated correctly and the application does not require high temperatures or a strong counter-field, you can choose a simple NdFeB material, but the requirements are often higher both in terms of the temperature, counter-field and humidity. A thorough review together with a magnet supplier is therefore very important in order to choose the right material.
As stated above there are different NdFeB alloys. The lower grades of magnet consist mainly of the basic alloy NdFeB and have a poorer temperature resistance and poorer corrosion properties. If you increase in alloy grade, this is often achieved by adding different alloying elements such as dysprosium and praseodymium. These alloying elements can be used both to increase the temperature resistance and to improve the corrosion properties. Most magnets are made in China nowadays and have designations which are roughly similar to the ones below.
|Temperature and corrosion properties
|N 35, 40… 52
|approx. 80°C and requiring surface treatment
|40, 50 M
|up to 100°C and requiring surface treatment
|up to 120°C and requiring surface treatment
|up to 150°C and requiring surface treatment
|up to 180°C and requiring surface treatment
|up to 220°C and requiring surface treatment
|up to 180°C and with better corrosion properties
|up to 200°C and with better corrosion properties
|up to 200°C and with much better corrosion properties
A manufactured NdFeB magnet is highly sensitive to humidity in its basic state. It is therefore customary to surface treat the magnet in all circumstances even if no further coating is applied. This process is called passivation and is carried out on all NdFeB magnets so that they can withstand transport from the factory to the end customer. All it takes is some sweat from your hands that is left in the magnets for them to corrode, so passivation is extremely important. It is also essential for the magnets to be treated correctly before surface treatment takes place. The manufacturer must ensure that the magnet is not exposed to humidity before surface treatment since this can start corrosion and the magnets will corrode under the surface coating.
Surface treatment of magnets not only reduces corrosion, but also brings other advantages. Small magnetic particles are always present on the surface of the magnet which can make bonding more difficult, for example. Through surface treatment these particles can easily be removed in a subsequent cleaning process. Magnetised magnets are brittle and must be handled carefully. The magnets can easily have their edges crushed or parts of the magnets can become chipped. The surface treatment also has a positive effect in this case.
Nickel coating in China usually consists of an Ni-Cu-Ni or Ni-Sn coating. The coating is applied galvanically with a thickness of between 10 and 30 µm. It produces a hard, abrasion-resistant, shiny surface, which can easily be cleaned without any problems.
A Zn coating is also applied galvanically with a thickness of approximately 6-15 µm. Zn does not provide such good corrosion protection as Ni-Cu-Ni and produces a white surface.
Epoxy coating produces a black surface 10-30 µm thick which provides extremely good corrosion protection. Epoxy also provides good abrasion resistance and good electrical insulation.
There are a number of other types of surface treatment for magnets but those described above are the most common ones. You can consult us at Compotech in relation to the surface treatment of both neodymium and samarium cobalt magnets. We will help you find the right surface treatment for the right application!