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Why Neodymium Magnets Need Coating-Magnets And Corrosion
Nov 29, 2017

Neodymium magnets, are products produced by powder metallurgy and based on neodymium, iron and boron. They are actually the strongest permanent magnets that can be produced today.


But 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. This is the reason why neodymium magnets need coating.


The reasons for corrosion in magnets are due to rare earth metals


Corrosion is a very negative characteristics in relation to permanent magnets. A lot of research has gone into the development of suitable surface treatment processes to reduce the risk of corrosion.


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.


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. A thorough review together with a magnet supplier is therefore very important in order to choose the right material.