Effect of DyF3on the corrosion behavior of hot-pressed Nd-Fe-B permanent magnets

The grain boundary diffusion process (GBDP) with DyF3 is a promising method to obtain excellent magnetic properties in Nd–Fe–B permanent magnets. Compared to conventionally prepared (Dy,Nd)–Fe–B magnets the overall content of the precious heavy rare earth Dy is reduced. Thus, the GBDP with DyF3 is highly attractive to cut down costs and save resources. However, it is not well known how the addition of DyF3 affects the corrosion resistance of Nd–Fe–B permanent magnets. In this work, the anodic polarization behavior of hot-pressed Nd–Fe–B magnets with and without DyF3 additions was investigated in electrolytes of different pH values. The results were compared with measurements on a commercially sintered Nd–Fe–B permanent magnet. In the polarization studies the only observed significant impact of DyF3 additions is an increased passive current density in alkaline media. The pitting susceptibility in NaOH solutions containing chloride ions is not affected by DyF3. Also in contrast to expectations, first free corrosion studies indicated no significant impact of the DyF3 additions on the dissolution rate

Impact of magnetization state on the corrosion of sintered Nd–Fe–B magnets for e-motor applications

We have created an accelerated corrosion environment for sintered Nd–Fe–B magnets in e-motor applications. E-motor working conditions are complex, and standard magnet corrosion tests only cover a small subset of possible parameters (e.g., samples are usually tested in the demagnetized state). In this work magnetized and demagnetized sintered Nd–Fe–B magnets were placed in gearbox oil, and exposed to temperature cycles (θmax = 130 °C) using an autoclave. Beforehand the magnets were pre-immersed in saturated water-based salt solution to account for water and de-icing salt that might interfuse gearbox oil over time. The corrosive behavior was studied for two commercial magnet grades, “high grade” (8.9 wt% dysprosium) and “low grade” (3.1 wt% dysprosium); and monitored by weight loss, structural analysis (scanning electron microscopy and energy dispersive X-ray) as well as magnetic characterization. The magnetized samples corroded significantly faster than their demagnetized counterparts. Strong differences in the corrosion rates of the “low grade” and “high grade” material are discussed. We concluded that the magnetization state is one key parameter that needs to be considered in corrosion tests for e-motor applications