Study On Surface Modification Of Neodymium Iron Boron Magnets

Due to its excellent performance of magnetism and comparatively high magnetic energy, the neodymium iron boron magnets, the third generation of permanent-magnet material developed in the early 1980s, have been widely used in many areas like microwave techniques, miniature motors, disc drives, car sound equipments and nuclear magnetic resonance. In terms of the richness in mineral resources, neodymium ranks at the third place as a rare earth metal. Therefore, magnet, composed of iron and neodymium, can be regarded as the ideal material low at price but good at quality. However, as a rare earth element neodymium's standard redox potential was-2.431V and it was one of the most unstable non-radioactive elements. Polyphase powder metallurgical materials are used in sintering NdFeB permanent magnets. Lots of minute capillary bores do exist in the surface as well as inside the magnets. As a result, NdFeB performs poor corrosion resistance and has great limitation in its application.This experiment improved the corrosion resistance of neodymium iron boron magnets by means of phosphatization and passivation. The effects of solution-composing, processing con-dition on the phosphate layer and passivation layrer were also studied. The quality of phosphate layer and passivation layrer was tested by taking the experiments of dropping, porosity test, immerging experiment and weight-losing test. The best formulation of phosphatizing decided by orthogonal experiments were as following:50g/L Na2H2PO4·2H2O, 3mol/L85%H3PO4,0.02g/LNa2MoO4,0.03g/LZn(NO3)2·6H2O,0.02g/LCH3(CH2)10CH2OSO3N a. The paper compares the effects of three different passivation processes of Cr6+, Cr3+and none Cr on the performances of the layer. A conclusion that the corrosion resistance of none Cr processing was close to that of the passivation processing of Cr6+ was drawn. A deeper study of none Cr processing for passivation was done and the corresponding formulation was gained by orthogonal experiments:20g/L Na2MoO4·2H2O,3.5g/L NH2CH2CH2OH, pH=5.The shape and the distribution of the layer was observed through SEM and EDX. The results were as following:the phosphate layer and the passivation layer were symmetrical, without any obvious cracks. The composition of the layer are some elements such as Mo,P,Zn,O and so on.