Neodymium Iron Boron and Hydrogen do not work well together. Occasionally it may be important to use a magnet in areas where the atmosphere has been purged with Hydrogen for reasons such as purity. If a NdFeB magnet has been selected to be used in this environment, special care needs to be taken.
The Mechanism of Failure
There are two mechanisms that cause failure due to hydrogenation of the NdFeB magnet.
A Neodymium Iron Boron magnet has two phases that make the material. The Nd2Fe14B phase is the predominant phase. This is the basic molecular lattice structure of the magnet. In between this is the neodymium rich phase. The hydrogen molecule penetrates the basic molecular structure causing a “mechanical stress” inside the molecule. The molecule expands and internal cracking begins.
The neodymium rich phase at the boundary absorbs the hydrogen. When the neodymium and hydrogen combine, the resultant material is a powder rather than a solid material. This mechanism leads to a total mechanical failure and visually appears as if the magnet was simply crumbling away.
Note that at elevated temperatures, this process of disintegration is highly accelerated. At temperatures in excess of the Curie temperature, the disintegration from a solid material to powder can occur within a matter of minutes, and sometimes faster if the surface area/weight ratio is high.
Several different methods have been explored to determine the best methods of isolating the magnet from the hydrogen atmosphere. Various types of coatings and platings have been tested. To date, all systems have shown eventual failure. The failure rate may be retarded, but only temporarily. Once the hydrogenation process begins, the rate of disintegration increases exponentially. As mechanical breakdown is introduced, the actual surface area continues to increase, thereby causing a “chain-reaction” effect.
The most successful system has been hermetically sealing the magnet. In this case, a thin boundary wall is placed around the magnet. This method ends up isolating the magnet from the hydrogen atmosphere entirely. The walls can be laser welded or electron beam welded. If the magnet is being welded in the magnetized condition, it is important to use laser welding since the field emanating from the magnet will deflect the electron beam. Though costly, this is currently the only method to ensure longevity of the Neodymium Iron Boron magnet in a Hydrogen atmosphere.