Two invisible gases decide a lot about a neodymium magnet's quality: oxygen and hydrogen. Neither can be detected by normal metal analysis, so they need their own test — and the results say a lot about how well the magnet was made.
Oxygen: too much makes magnets weak
NdFeB starts life as a very fine metal powder — particles only a few micrometres across. Fine powder has a huge surface area, and surface means oxidation. If too much oxygen gets in, the magnet sinters poorly: it comes out less dense, weaker, and with lower magnetic performance. That is why the powder is handled under inert gas, and why suppliers keep powder oxygen below about 1500 ppm. Oxygen is also a great early-warning signal: if the number creeps up across batches, something in the process is leaking.
Hydrogen: useful in production, dangerous if left behind
Hydrogen is actually used on purpose. A step called hydrogen decrepitation makes the alloy crumble into powder by absorbing hydrogen. The catch: the hydrogen must be driven back out by heating in vacuum before sintering. If too much stays behind, the magnet turns brittle — parts can crack during machining or assembly. Leftover hydrogen is a classic suspect when a batch of magnets suddenly starts breaking.
How the test works
Both gases are measured by the same type of instrument: an inert-gas fusion analyzer. A small sample is dropped into a graphite crucible and melted at extreme heat. The released gas is swept away and measured — oxygen by infrared absorption, hydrogen by thermal conductivity. The result is a simple ppm number that tells you whether the process was healthy.
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