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Spectacular magnet failure

12 November 2017 - Consumer watchdog

klik hier voor de afbeelding op ware grootteThe other day I bought a set of magnets that failed the moment I unpacked them. At 4 euro's not the end of the world but the material failure was spectacular. The button-sized pieces of metal just shattered on any impact, made worse by their large magnetic strength. So before returning them to the store, exactly what did I buy?
With some research done (the packaging just said 'super strong magnets' and nothing else) my bet is that I am dealing with neodymium magnets (NdFeB). The Wikipedia page mentions susceptibility to corrosion. To get some insight into the production process this clip was a delight. A lot can go wrong! In this clip SuperMagnetMan (his real name) visits a Chinese magnet factory. With Chinese can-do a mixture of Neodynium, iron and boron was first strip-casted at 1400°C to a flake-like material, then these flakes were disintegrated in a hydrogen decrepitation step. A jet milling step turned everything into a fine powder and (never knew that) the magnets were pressed into shape with pressures up to 23000 psi. Next followed a sintering process, a machining step , a coating step and finally magnetisation. Watch that clip. The decrepitation step (a metal hydride forms) is an interesting one by itself and can be seen in other youtube clips here and here.

If the magnets are supposed to be coated with nickel or epoxy to protect it from corrosion it is not obvious from visual inspection. Even if everything went well, the magnets can have been over-magnetized. It is almost impossible to pry them apart (they had already cluttered together in the packaging), making them near useless even if they do not shatter. This blog sticks with ferrite from now on.