Research On Bidirectional Pulse Electrodeposition Of Ni-based Alloy Coatings On NdFeB Magnet And Its Corrosion Performance

As the third generation of rare earth permanent magnet material,"Nd Fe B"is not only a key material in the"Made in China 2025"development plan,but also can realize the storage,conversion,transmission of information and energy,and has a wide range of applications.However,due to its special composition and microstructure,the corrosion resistance of magnets is poor,which greatly restricts its further development.To improve the protective performance and application stability of magnets,this paper is based on bidirectional pulse electrodeposition technology to rapidly deposit Ni-Cr and Ni-P alloy coatings on the surface of sintered Nd Fe B magnets.The effects of different pulse parameters and Cr and P containing main salt concentrations on the corrosion resistance and microstructure of the two alloy coatings were investigated.The corrosion mechanism and dynamic corrosion failure process of the optimal Ni-Cr and amorphous Ni-P allloy coatings in salt solution were revealed.The specific research conclusions are as follows:（1）The Ni-Cr alloy coating on sintered Nd Fe B surface was rapidly deposited by bidirectional pulse electrodeposition.The results show that when the p H,current density,frequency,forward duty ratio,and Cr Cl₃.6H₂O concentration were 4.5,0.2 A/cm²,800 Hz,60%,and 25 g/L,respectively,the Ni-Cr alloy coating has high surface quality,excellent corrosion resistance,and a Cr content of up to 8.05 at.%.It is found that the optimal Ni-Cr alloy coating has a significantly smaller grain size,significantly improved corrosion resistance,increased hardness,and better wear resistance compared to the Ni coating.（2）Electrochemical testing found that the Ni-Cr coating produced the best protective n-type semiconductor passivation film after potentiostatic polarization at–0.4 V for 1 hour,and its carrier concentration N_d significantly reduced.After static immersion in 3.5 wt.%Na Cl solution for 48 hours,only slight local pitting occurred,and the corrosion products were Ni（OH）₂,Cr₂O₃,and Cr O₃.The dynamic corrosion failure process showed that after 72 hours of static corrosion,the surface corrosion products of the optimal Ni-Cr coating significantly increase and there is obvious pitting corrosion inside.Spot scanning revealed that Cl element was enriched in the pores,indicating that Cl element could induce pitting corrosion.The deep corrosion mechanism is that when Cl^-adsorbs on the surface of the passivation film,ion exchange occurs with the oxide lattice and cation vacancy is generated,which makes the cation more easily migrate under the electric field and finally forms pore corrosion.At the same time,the coating also has good mechanical properties,with a microhardness of up to 601 HV_0.3 and a friction coefficient reduced to 0.42.（3）The Ni-P alloy coating on the sintered Nd Fe B surface was rapidly deposited by bidirectional pulse electrodeposition.The results show that when the p H,current density,frequency,and H₃PO₃ content were 2.75,0.1 A/cm²,700 Hz,and 40 g/L,respectively,the Ni-P alloy coating has high surface quality and excellent corrosion resistance.After amorphous transformation of the optimal crystalline Ni-P alloy coating,the coating exhibits a nanoscale and P content increases to a maximum of 16.61 wt.%,with significant improvement in corrosion resistance,wear resistance,and hydrophobicity,and a reduction in surface roughness.（4）The electrochemical test shows that the amorphous Ni-P coating has the best corrosion resistance after potentiostatic polarization at 0.1 V for 1 h.After static corrosion in 3.5 wt.%Na Cl solution for 72 h,the coating exhibited slight overall corrosion,with a smooth and flat surface and only a small amount of corrosion products.XPS indicated that the corrosion products（passivation film）were Ni O,Ni（OH）₂,and Ni₃（PO₄）₂.The passivation film is denser than the crystalline coating,and has better protective effects on magnets.The reason is that nanocrystallization promotes the nucleation process and growth rate of passivated film,promotes the rapid enrichment of passivated elements and improves the density of passivated film,and significantly improves the resistance to local corrosion.