Electrochemical corrosion kinetics of spark plasma sintered Nd–Fe–B permanent magnets in NaCl electrolyte

Abstract

Static state immersion experiments, polarisation curve and electrochemical impedance spectroscopy tests have been applied to investigate the corrosion kinetics for both spark plasma sintered (SPS) and conventional sintered Nd–Fe–B magnets in NaCl electrolyte. Effect of microstructure modification on their chemical stability of the magnets was discussed. The electrochemical reactions of both magnets are controlled by the step of active substances adsorption process at the open circuit potential and the anodic potential, which turn to diffusion process at the cathodal potential. Although both magnets are susceptible to corrosion in saline electrolytes, SPS magnets are more corrosion resistant than conventional sintered magnets due to their special microstructure that is different from those of conventional sintered magnets. In SPS magnets, the grain size of the Nd₂Fe₁₄B main phase is fine and uniform, only a few Nd rich phase form along the grain boundaries of Nd₂Fe₁₄B phase, while most of them agglomerate into triple junctions as small particles. Such microstructure effectively restrains the aggressive intergranular corrosion along Nd rich phases. As a result, the SPS magnet possesses excellent corrosion resistance in NaCl electrolyte.     

Korrosionsverhalten von Nd‐Fe‐B‐Dauermagneten

Corrosion behaviour of Nd‐Fe‐B permanent magnets The corrosion behaviour of Nd‐Fe‐B permanent magnetic alloys as well as of single phases of these alloys have been investigated in sulphuric acid at room temperature and humid air at 150 °C using mass loss and electrochemical techniques. Scanning electron microscopy and scanning probe microscopy were used to study the surface topography. The electrostatic surface potential was examined by scanning probe microscopy using tapping mode. A correlation between the electrostatic surface potential and the corrosion rate of these alloys was found. The higher the value of the electrostatic surface potential of the intergranular phases the higher is the corrosion sensitivity. The strength of the corrosion attack on the phases of sintered permanent magnetic alloys is as follows: Ferrmagnetic phase < B‐rich phase < Nd‐rich phase. The differences in the chemical composition as well as in the preparation of these magnetic alloys have distinct influence on the corrosion resistance of the magnetic Nd‐Fe‐B alloys.     

A comparative study on the corrosion behavior of NdFeB magnets in different electrolyte solutions

Sintered NdFeB magnets possess excellent magnetic properties. However, the corrosion resistance property of NdFeB is very poor due to its multiphase microstructure consisting of matrix phase Nd₂Fe₁₄B, Nd‐rich phase, and B‐rich phase. The corrosion behavior of NdFeB magnets in sodium hydroxide (NaOH), sodium chloride (NaCl), nitric acid (HNO₃), and oxalic acid (H₂C₂O₄) solutions was investigated by immersion and electrochemical tests. HNO₃is the strongest corrosive electrolyte compared with the other three solutions. The increase in HNO₃concentration can accelerate the corrosion of NdFeB magnets. NaCl belongs to medium corrosion electrolyte. A NaCl concentration of 0.5 M shows the severest corrosive feature in comparison with other concentrations of NaCl solution. NdFeB hardly suffers corrosion in NaOH and H₂C₂O₄solutions owing to the formation of passivation films on the surface of magnets. Based on the corrosion behavior of NdFeB in different electrolytes, the possible corrosion mechanisms are discussed.     

High-coercivity ultrafine-grained anisotropic Nd–Fe–B magnets processed by hot deformation and the Nd–Cu grain boundary diffusion process

The grain boundary diffusion process using an Nd₇₀Cu₃₀ eutectic alloy has been applied to hot-deformed anisotropic Nd–Fe–B magnets, resulting in a substantial enhancement of coercivity, from 1.5 T to 2.3 T, at the expense of remanence. Scanning electron microscopy showed that the areal fraction of an Nd-rich intergranular phase increased from 10% to 37%. The intergranular phase of the hot-deformed magnet initially contained ～55 at.% ferromagnetic element, while it diminished to an undetectable level after the process. Microscale eutectic solidification of Nd/NdCu as well as a fine lamellae structure of Nd₇₀(Co,Cu)₃₀/Nd were observed in the intergranular phase. Micromagnetic simulations indicated that the reduction of the magnetization in the intergranular phases leads to the enhancement of coercivity in agreement with the experimental observation.     

Magnetic properties of hard magnetic nanoparticles of Nd2Fe14B synthesized using self-assembled block copolymers

Nd₂Fe₁₄B nanoparticles are widely used in bonded magnets for actuators in many types of electric equipment, motors in mobile phones and hard disk drives. Magnetic nanoparticles are also used in magnetic storage devices, drug delivery systems, and ferrofluids. Improvement of the magnetic properties of Nd₂Fe₁₄B rare-earth magnets would allow for production of small, lightweight, high-torque motors for use in energy-saving household appliances. However, because of the chemical instability of Nd₂Fe₁₄B nanoparticles, it is challenging to synthesize them with high purity by chemical synthesis or by conventional metallurgy methods using Nd₂Fe₁₄B alloys. In addition, the high reduction potential of Nd³⁺ makes reduction difficult. In this study, block copolymers were used as templates for chemical synthesis of Nd₂Fe₁₄B nanoparticles. In the synthesis, tris(acetylacetonato) iron(III) as an iron precursor, neodymium tris(acetylacetonate) hydrate as a neodymium precursor, and 1,1-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)ferrocene as a boron precursor were introduced into a polystyrene-b-poly(2-vinylpyridine) block copolymer template. Then, the block copolymer was removed by oxidation. After reduction with CaH₂ and washing with water to remove residual Ca species, highly pure Nd₂Fe₁₄B nanoparticles were obtained. Under the optimized experimental conditions, nanoparticles with a coercivity of 3.5 kOe and a saturation magnetization of 158 emu/g were obtained. The saturation magnetization was 93.5% of the theoretical value of Nd₂Fe₁₄B (169 emu/g).     

Corrosion mechanism of Mo/Nd16Fe71B13/Mo film in a simulated marine atmosphere

This study describes the corrosion mechanism of Mo/Nd₁₆Fe₇₁B₁₃/Mo film induced by sodium chloride particles in 80% relative humidity (RH) environment. The deliquescence of sodium chloride particles on the Mo/Nd₁₆Fe₇₁B₁₃/Mo film caused the step by step attacks. Initial loosening of the Mo layer allows permeation of electrolyte into Nd–Fe–B layer, resulting in cavitations of electrolyte and subsequent film failure. The second failure step involves corrosion of Nd element in the Nd–Fe–B layer, with Fe element remaining beneath the corrosion product. Corrosion of Fe constitutes the third-step failure, forming a mixture of Nd and Fe corrosion product.     

Effect of post-sinter annealing on the coercivity and microstructure of Nd–Fe–B permanent magnets

To understand the mechanism of the increase in coercivity caused by post-sinter annealing of Nd–Fe–B-based magnets, we have investigated the microstructures of commercial sintered magnets by high-resolution scanning electron microscopy, transmission electron microscopy and atom probe tomography. Continuous thin layers of a Nd-rich amorphous phase were found along the grain boundaries in the post-sinter annealed sample, the chemical composition of which was determined to be Nd₃₀Fe₄₅Cu_24.1B_0.9. A fine Cu-enriched shell was also confirmed in the Nd-rich phase grain, suggesting the Nd₂Fe₁₄B grains are completely enveloped by the Cu- and Nd-enriched layers. Furthermore, a lamellar microstructure of the Cu-enriched phase was confirmed in some Nd-rich phase grains. The mechanism of the coercivity increase caused by post-sinter annealing is discussed based on these characterization results.     

Grain boundary and interface chemistry of an Nd-Fe-B-based sintered magnet

The compositions of grain boundaries (GBs) and other interfaces surrounding Nd₂Fe₁₄B grains in commercial Nd-Fe-B sintered magnets have been investigated by laser-assisted three-dimensional atom probe to understand the mechanism of the coercivity enhancement by post-sinter annealing. While only a slight segregation of Nd and Pr to the GBs was confirmed in the as-sintered sample, a thin Nd-rich amorphous phase layer was observed along the GBs with Cu segregation to the interfaces in the annealed sample. The segregation of Cu to NdO_x/Nd₂Fe₁₄B interfaces was also found, suggesting that the Nd₂Fe₁₄B grains are enveloped by a Cu-enriched layer after the annealing. The concentration of Fe + Co in the thin GB layer was found to be as high as 65 at.%, and a model amorphous film processed by sputtering with the same composition as the thin GB layer was found to be ferromagnetic. Ferromagnetic behavior of the thin GB layer suggested that Nd₂Fe₁₄B grains are magnetically coupled. The coercivity mechanism of the sintered magnets is discussed based on these new findings.     

Effect of Pr and Dy substitution on the impact resistance of sintered Nd–Fe–B magnets

The impact resistance of sintered Nd–Fe–B magnets with nominal compositions (Nd_1−xDy_x)₁₆Fe₇₈B₆ (x = 0, 0.05, 0.10, 0.15) and (Nd_1−yPr_y)₁₆Fe₇₈B₆ (y = 0, 0.33, 0.67, 1) has been investigated by the falling-weight impact test. The bending strength and the Vickers hardness were measured. It shows that the Dy substituted magnets have better impact resistance than the Pr substituted magnets. The impact resistance of the investigated magnets improves monotonously with increasing Dy content, and reduces with increasing Pr content, which seems to relate closely to the change of the density or of the hardness.     

Permanent magnet materials—Developments during the past 12 months

Information about real and potential permanent magnet materials continues to grow at a rapid pace, and developments in the following areas are discussed. Co- Zr, although not comparable to the best high-performance magnets, has attracted interest as a rare earth free alloy. Carbon and nitrogen are becoming more prominent as the metalloid in iron- rare earth magnets. This leads to cast magnets based on Fe-Nd-C, bonded magnets based on Fe₁₇Sm₂N_x, a renewed interest in the tetragonal phase (Fe,Ti)_12Nd, now with added nitrogen, and a nitrogen treatment for sintered Fe₁₄Nd₂B that raises the Curie temperature by more than 100 K. Alloying additions to Fe₁₄Nd₂B improve the coercivity. Finally, developments in corrosion protection and processing are highlighted. This article was presented at the ASM Symposium on Soft and Hard Magnetic Materials, which was held 23 Oct at the combined 1991 ASM Materials Week and TMS Fall Meeting in Cincinnati, Ohio.     

The influence of Co and Ga additions on the corrosion behavior of nanocrystalline NdFeB magnets

Isotropic nanocrystalline Nd₁₄Fe₈₀B₆ and Nd₁₂Dy₂Fe_73.2Co_6.6Ga_0.6B_5.6 magnets with different grain sizes in the range of 60-600 nm have been produced from melt-spun materials by hot pressing at 700 °C and subsequent annealing at 800 °C for 0.5-6 h. The microstructure has been characterized using XRD, SEM, energy dispersive X-ray analysis, and Kerr microscopy. The corrosion behavior of NdFeB magnets has been examined on 0.1 M H₂SO₄ by in situ inductively coupled plasma solution analysis, gravimetric and electrochemical techniques. The corrosion hydrogen absorption/desorption behavior has been investigated by thermal desorption analysis and hot extraction methods. Partial substitution of Fe with Co and Ga leads to an improvement in corrosion resistance and reduces the affinity and binding energy for hydrogen in these materials. Coarsening of the microstructure results in a better corrosion performance of these materials. The corrosion behavior of the magnets in relation to phase composition, phase distribution and grain size is discussed in terms of dissolution, hydrogenation and mechanical degradation.     

New permanent magnets and new processing techniques

Since the discovery of Nd₂Fe₁₄B in 1984, a number of new phases with uniaxial anisotropy like Sm(Fe,X)₁₂, Sm₅(Fe,Ti)₁₇, (Sm,Zr)Fe₃, and Sm₂Fe₁₇N₃ have been investigated. Their intrinsic magnetic properties compare favorably with Nd₂Fe₁₄B. Magnetic hardening is possible by rapid quenching or mechanical alloying, which both result in isotropic materials with coercivities of up to 51 kA/cm. Mechanical alloying is a new technique to prepare high- performance permanent magnets at low temperatures. For Nd- Fe- B, isotropic and anisotropic magnets with properties comparable to sintered or melt spun materials can be produced.     

Characterisation of oxidation products of modified Nd–Fe–B type magnets

In order to study the influence of Co addition on oxidation behaviour, magnets with the compositions Nd₁₆Fe₇₆B₈, Nd₁₆Co₃Fe_73.3B_7.7, Nd₁₆Co₅Fe_71.5B_7.5, Nd₁₆Co₇Fe_69.6B_7.4, Nd₁₆Co₁₀Fe_66.9B_7.1, Nd₁₄Dy₃Co_9.7Fe_66.4B_6.9, Nd_13.9Dy₃Co_9.7Fe_66.2B_6.9Zr_0.3 and Nd_13.9Dy₃Co_9.7Fe_66.2B_6.9V_0.3 in powder and solid pellet forms were oxidised at 400 °C for 96 h. The resultant phases were identified using X-ray diffraction (XRD) and Mössbauer spectroscopy. α-Fe₂O₃ is the main final oxidation product of the iron content of these magnets, while Fe₃O₄ is the intermediate phase of this oxidation procedure. In addition the presence of several Nd oxides, as well as Dy oxides in the compositions with Dy additions was confirmed in all samples. B seems to prefer to react with Nd with the formation of oxide phases, while a minority Fe₂B phase is also detected in the powder samples. A delay of the oxidation process is found for the case of compact pelletised samples, and this is extended for those samples containing Dy in their structure.     

Effect of Dy2O3 intergranular addition on thermal stability and corrosion resistance of Nd–Fe–B magnets

Sintered Nd–Fe–B magnets with and without Dy₂O₃ were prepared by powder blending method. The temperature-dependent magnetic properties, thermal stability, microstructure and corrosion resistance of sintered magnets were investigated. The temperature-dependent magnetic properties revealed that with intergranular addition of Dy₂O₃, the reversible temperature coefficients β and α in the range of 293–373 K were both lowered, indicating that the thermal stability was effectively improved. This was also verified by the decreased irreversible flux loss (h_irr) and the increased maximum operating temperature (MOT). Moreover, the electrochemical and accelerated corrosion results clearly evidenced that the corrosion resistance of Nd–Fe–B magnet was also modified by addition of Dy₂O₃. Furthermore, the related mechanisms on improved thermal stability and corrosion resistance were systematically discussed.     

Passivity and its breakdown of sintered NdFeB-based magnets in chloride containing solution

Passivity and its breakdown of sintered Nd_11.6Dy_3.6Fe₇₉B_5.78 and Nd_11.3Dy_2.3Fe₇₇Co₃Cu_0.1Ga_0.5B_5.8 magnets were investigated in 0.5 M NaCl solution of pH 8.4, using polarization and electrochemical noise measurements. The microstructure of the magnets was characterized using XRD, SEM and EDX techniques. Both magnets were spontaneously passive and showed metastable pitting at freely corroding conditions as well as intergranular corrosion propagation above the pitting potential. Characteristic charge (Q) and frequency (F) of the events constituting the corrosion process at freely corroding conditions were estimated from the potential and current power spectra of the noise signals and presented as useful probes for metastable pitting characterization. The time dependence of the characteristic charge was described by Q(t)=Q(0)exp(at) while that of the characteristic frequency was well described by F(t)=F(0)exp(bt), 0?t<τ and F(t)=F(τ)exp(−ct), t?τ. Crystallographic tunneling was found to be the way for both pitting and intergranular corrosion propagation. The partial replacement of Fe with minor amount of Co, Cu and Ga beneficially enhanced the passivity and overall localized corrosion resistance of the magnets. The beneficial effect of alloying addition was attributed to the reduction in the strength of galvanic coupling effect between ferromagnetic matrix and Nd-rich intergranular phases and subsequently in the lateral heterogeneity on the magnet surface.     

Corrosion resistance of Nd‐Fe‐B permanent magnetic alloys. Part 1: Role of alloying elements

The electrochemical behaviour of Nd‐Fe‐B magnetic alloys was investigated in acid and neutral solutions. The differences in the chemical composition of these materials have distinct influence on the corrosion rate and polarization behaviour. Small additions of cobalt, aluminium and gallium increase the corrosion resistance of the magnetic alloys at high cathodic potential. It was also observed that the increase of hydrogen evolution rate on the surface of the magnetic materials raises the rate of dissolution of these magnets. The electrostatic surface potential was examined by scanning probe microscopy. A relation between the electrostatic surface potential and electrochemical behaviour of these alloys was found. The high values of electrostatic surface potential of the intergranular phases reflect higher corrosion attack. Auger electron spectroscopy was used to analyse the surface layer which formed during anodic polarization of the magnet containing alloying additives. The result indicates the formation of (Nd,Fe)‐oxide with small amounts of cobalt and aluminium.     

Effects of boron concentration on the corrosion resistance of Fe-B alloys immersed in 460 °C molten zinc bath

In order to investigate the effects of boron concentration on the corrosion resistance of Fe-B alloys in molten zinc, Fe-B alloys, with the boron concentrations of 1.5 wt.%, 3.5 wt.% and 6.0 wt.% respectively, were dipped into a pure molten zinc bath at 460 °C and kept in different time intervals. The results show that, in comparison with 1Cr18Ni9Ti stainless steel, Fe-B alloy with 3.5 wt.%B exhibits excellent corrosion resistance, due to the dense continuous network or parallel Fe₂B phase which hinders the Fe/Zn interface reaction in Fe-B alloys. The energy dispersive spectrum (EDS) results indicate that the coarse and compact δ phase with the length about 40 μm generates near the matrix of Fe-B alloy and massive ζ phase occurs close to the liquid zinc. The corrosion process includes Fe/Zn reaction and the isolation and fracture of Fe₂B. The failure of boride is mainly caused by the microcrack.     

晶界添加Ho63.3Fe36.7 对NdFeB再生磁体组织与性能的影响

采用一种高效、绿色的物理方法对NdFeB废旧磁体表面进行清理并回收利用。通过晶界添加低熔点Ho_63.3Fe_36.7合金制备NdFeB再生磁体。在未添加Ho_63.3Fe_36.7的磁体中,没有足够的富Nd相隔离Nd₂Fe₁₄B相,从而导致磁体性能较差;随着Ho_63.3Fe_36.7合金的加入,晶界相变得清晰且连续。在质量分数2%Ho_63.3Fe_36.7添加量下,钕铁硼再生磁体获得最佳磁性能[(BH)_max+H_cj=1756.07]。此时矫顽力增加123 kA/m（约提高9.1%）,磁体的最大能积由290.94 kJ/m³下降到281.07 kJ/m³,而剩磁少量下降。通过对再生磁体显微组织和成分的分析可知,磁体晶界处形成了(Nd, Pr, Ho)₂Fe₁₄B壳层,这能够提高磁体的矫顽力。而X射线衍射分析表明,磁体I₍₀₀₆₎/I₍₁₀₅₎的衍射峰强度比从0.92提高到1.32。这说明磁体取向度提高,可以减弱对剩磁的影响,从而使得再生磁体在保持剩磁的同时提高磁体的矫顽力。     

Nb对非晶态Fe-Co-Nd-B合金的晶化动力学的影响

在Fe-Co-Nd-B非晶合金中添加4%的Nb(原子分数)可延迟其晶化过程, 提高晶化温度, 并使其热稳定性显著提高. Nb抑制Fe₃B晶化相的形核, 但促进Fe₂₃B₆相的形核及长大. Nb可使晶化相的平均晶粒尺寸从30-60 nm减至14-20 nm. Nb使由初始晶化温度计算的晶化激活能明显降低. Fe-Co-Nd-B合金中, α-Fe(Co), Fe₃B和Nd₂(Fe, Co)₁₄B晶化相的形核过程要难于长大过程, 而加Nb后α-Fe(Co), Fe₂₃B₆和Nd₂(Fe, Co)₁₄B晶化相的长大过程要难于形核过程, 但Nb基本未改变晶化相的形核及长大机制. 非晶合金的晶化主要是一维界面控制的形核以及形核率随时间减小的三维长大过程.     

Effect of Structure on the Magnetic Properties of Rapidly Hardened Powders of the Nd – Fe – B System

The structure and magnetic properties of rapidly hardened powders from alloys based on Nd₂Fe₁₄B compounds are studied. Centrifugally sprayed powders with a multiphase structure bearing Nd₂Fe₁₄B, Nd(OH)₃, and α-Fe phases are described. A decrease in the size of crystals of the magnetic phases (Nd₂Fe₁₄B and α-Fe) and the amount of α-Fe are shown to increase the magnetic properties of such powders. At the same time, powders obtained by melt spinning are shown to be single-phase ones with more dispersed crystals and higher magnetic properties.