A sustainable phosphorization treatment on the high-temperature oxidation resistance of Nd-Fe-B magnetic powders

To address the operational safety hazards associated with using high-concentration inorganic phosphoric acid as a surface treatments to enhance the high-temperature oxidation resistance of Nd-Fe-B magnetic powders, this study innovatively proposes an environmentally friendly organic phosphonic acid (methylene diphosphonic acid, MDPA) as an alternative solution. By comparing the effects of inorganic phosphoric acid and MDPA surface treatments on the magnetic properties and thermal stability of magnetic powders, it was found that phosphating-treated samples exhibited slight decrease in coercivity H cj and maximum magnetic energy product (BH) max at room temperature compared to untreated samples. However, at 340 °C, the (BH)max and Hcj of untreated samples dropped sharply by 98.55 % (from 4.97 to 0.07 MGOe) and 92.72 % (from 9.62 to 0.70 kOe), respectively. In contrast, the inorganic phosphate-treated samples showed only a slight decline of 13.55 % (from 4.53 to 3.92 MGOe) and 1.22 % (from 9.39 to 9.28 kOe). The MDPA-treated samples exhibited the best thermal stability, with minimal losses in both (BH)max and Hcj, decreasing by only 6.88 % (from 4.66 to 4.34 MGOe) and 0.69 % (from 9.43 to 9.36 kOe), respectively. Microstructural analysis revealed that the dense metal phosphate conversion layer formed during phosphating was the key to maintaining high-temperature magnetic performance. Specifically, inorganic phosphoric acid treatment preferentially corroded Nd-rich intergranular phases, resulting in a higher Nd content in the conversion layer. In contrast, the iron phosphate salt formed by MDPA treatment dominates the transformation layer, effectively reduces the damage to the microstructure of the magnetic powder. Our research provides a surface treatment solution that combines safety and efficiency for the performance protection of rare-earth magnetic powders in high-temperature injection molding processes.