Hydrogen Decrepitation is a process step used in the production of Neodymium magnets to create extremely small grains in the material.
Neodymium magnets must have very small grains averaging about 5 micrometers in order to have the best magnetic properties. But the grain sizes after casting are much larger than that, so they must be ground down to a much smaller size after casting.
So how do you do that? And how do you do it economically?
When Neodymium magnets were first discovered, there were two teams of researchers analyzing alternate paths to make the magnets by the best and most cost-competitive means. One team followed the path of conventional powder metallurgy, and the other team tried Hydrogen Decrepitation.
Both methods work –and the story has more details we won’t get into here- but after much research, Hydrogen Decrepitation became the dominant –and eventually- the only method. Economics and technical factors play a part in the selection of this technology.
Hydrogen is a very small and very reactive atom (the smallest atom- comprised of just one proton and one electron) that easily penetrates the grain boundaries of many metals. In most situations, metallurgists try to prevent hydrogen from entering the metal.
Hydrogen embrittles metals by entering the grain boundaries and creating pressure at the weakest point. This causes micro-cracks that begin to propagate through the grain structure.
Then why would you want to introduce hydrogen into a metal? Because we can use the same property to achieve our goal of reducing the grain size. If we deliberately introduce a large amount of hydrogen into the metal, it actually falls apart –decrepitates- because the hydrogen is so effective at breaking the metal down into very small pieces.
While the grains are very small from strip casting with rapid solidification, the material from strip casting comes out of the caster in flakes that must be reduced to powder in order to make magnets. These metal flakes are placed in a vessel under vacuum and a small amount of hydrogen is introduced into the vessel.
As the hydrogen collects in the grain boundaries, the metal begins to disintegrate as can be seen in the short video above.
The decrepitated material is now ready for the next step in the process, where it will be jet-milled to achieve the correct grain size and shape.