Hydrogen Decrepitation of Neodymium Magnets

Introduction

The hydrogen decrepitation (HD) process is employed in the manufacturing and recycling of sintered NdFeB neodymium magnets. This process is highly effective for extracting Noedymium-Iron-Boron (NdFeB) powder from non-functional electronic and magnetic products, such as hard disk drives, headphones, loudspeakers, electric motors, phones, and tablets, as illustrated in Figure 1 and Video 1.. The HD process facilitates the recovery of NdFeB powder without the application of mechanical forces, thereby allowing for its recycling in the production of new electronic products.

On the other hand, in recent years, the utilization of sintered NdFeB magnets in hydrogen and vacuum environments has increased due to advances in magnetic technologies in different industries, such as the semiconductor industry, space industry, medical industry, etc. This implies that NdFeB magnets should be protected from hydrogen diffusion to prevent their decrepitation or their entire disintegration. In this article, we will discuss some methods to protect sintered NdFeB magnets from HD.

Hydrogen Decrepitation of NdFeB Magnets

The small atom of hydrogen (H₂) is very reactive and penetrates the grain boundaries of many metals. Generally, we try to prevent hydrogen from penetrating into the metal grain regions to avoid brittleness and brittle fractures. The mechanism of this type of failure is that hydrogen easily diffuses into the grain boundaries and creates pressure at the weakest point, which leads to microcracks that begin to propagate in the grain structure. As a result of this propagation, the specimen experiences inelastic strain, which significantly decreases the fracture strength. This property of hydrogen (H₂) forms the basis of the targeted hydrogenation process for the decomposition and decrepitation of a wide range of materials, including NdFeB magnets.

When a sintered NdFeB magnet (composed of Nd₂Fe₁₄B grain regions and Nd-rich phase) is exposed to hydrogen gas (H₂), the hydrogen decrepitation (HD) or hydrogenation process is initiated at the Nd-rich phase. During the hydrogenation process, the Nd₂Fe₁₄B grain regions and the Nd-rich phase absorb hydrogen (H₂), producing Neodymium Hydrides and causing the expansion and separation of the Nd-rich phase regions from the Nd₂Fe₁₄B grain regions by hydrogen (H₂) diffusion, producing cracks and collapsing and disintegrating the NdFeB magnet structure, see Video 2.

Figure 2 shows a NdFeB magnet structure before HD process, formed by Nd₂Fe₁₄B grain regions and an Nd-rich phase, and the new collapsed NdFeB magnet structure after the HD process, formed by separated Nd₂Fe₁₄BH_x grain regions and NdH_x phase regions. In this final step the NdFeB structure is disintegrated.

HD Protection Methods for NdFeB Magnets

For sintered and non-coated NdFeB magnets employed in vacuum conditions as well as in H₂ environments, it is recommended to protect them with appropriate metallic enclosures or cans hermetically sealed with a suitable laser-welding technique, ensuring the absence of can surface defects. This prevents hydrogen gas (H₂) from penetrating the NdFeB magnet structure and offers a robust mechanical framework. This method effectively prevents HD of the NdFeB magnets.

Numerous failures in NdFeB magnet systems with insufficient protection against HD have been identified. The entire disintegration of NdFeB magnets enclosed in hermetically sealed cans with surface defects or cracks, operating under vacuum conditions and high temperatures under H₂ environment have been observed. These magnets were subjected to strong magnetic fields, which led to significant induced eddy currents and power losses, causing elevated temperatures and considerable mechanical and thermal stress. Metallic cans with defects allow hydrogen to penetrate the structure of the NdFeB magnets, resulting in their breakdown inside the metallic cans, see Figure 3.

In numerous cases, the pulverization of NdFeB magnets leads to significant NdFeB magnet powder contamination across the vacuum system, making it extremely challenging to clean, as the magnet powder can sometimes remain magnetized.

I recommend the sue of thick enclosures made of Titanium or 316L stainless steel due to their excellent corrosion resistance and capacity to create robust seals in high-pressure vacuum settings. To minimize heat exposure during welding and prevent potential degradation or demagnetization of the NdFeB magnets, laser welding should be applied to the titanium enclosures. In addition, we recommend using an additional extra surface coating for the NdFeB magnets like doble protection against HD.

Please contact us if you need assistance with magnet assemblies under hydrogen and vacuum conditions

References

[1] Habibzadeh, Alireza and Kucuker, Mehmet Ali, and Gokelma, Mertol, "Review on the Parameters of Recycling NdFeB Magnets via a Hydrogenation Process," ACS Omega, vol. 8, no. 20, pp. 17431-17445, 2023.

[2] Habibzadeh, A., Kucuker, M.A., Çakır, Ö. et al. "Microstructural Investigation of Discarded NdFeB Magnets After Low-Temperature Hydrogenation," J. Sustain. Metall. vol. 10, pp. 1141–1155, 2024.

[3] Grau L, Fleissner P, Kobe S, and Burkhardt C., "Processability and Separability of Commercial Anti-Corrosion Coatings Produced by In Situ Hydrogen-Processing of Magnetic Scrap (HPMS) Recycling of NdFeB," Materials (Basel), 17, (11):2487, May 2024.

[4] Falang Lin, Zexin Lu, Yingjie Ding, Jianjun Jiang, Bizhang Zheng, Lijing Yang, Anli Lin, Shengzhi Dong, Guangjian Peng, Zhenlun Song, "Study on the hydrogen damage of sintered NdFeB and the fabrication of protective Al coating," Journal of Magnetism and Magnetic Materials, Vol. 626, 2025.

[5] "Hermetically Sealed Magnetic Assemblies for Vacuum and Hydrogen Applications," - Dexter Mag Techs, 2024.

[6] I. Ambriz, H. King, A. Norris, and C. Yaeger, "A comparison of welding methods and their use on magnetized assemblies," Dexter Mag Tech Article.