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.     

Characterisation of electrochemical interactions between single phases of Nd‐Fe‐B permanent magnets

The electrode functions (anode or cathode) of coupled single phases (ferromagnetic phase: Nd₂Fe₁₄B; boron rich phase: Nd_1+εFe₄B₄; neodymium rich phase: Nd₄Fe) of Nd‐Fe‐B permanent magnets have been investigated. The electrochemical potential differences and current densities between the three phases were determined. Furthermore, the corrosion current density of the magnetic materials in 0.1 M H₂SO₄ was calculated from measurements of current density‐potential curves. The neodymium rich phase works as the anode in contact with both the ferromagnetic and the boron rich phase. The calculation of corrosion current densities for systems of three coupled phases from the current density‐potential curves of single phases measurements demonstrates very high values for the small area of the corroding neodymium rich phase at the grain boundaries. An increase of corrosion resistance of Nd‐Fe‐B‐magnets is possible only by an increase of the corrosion resistance of neodymium rich phase. The presented results do not demonstrate an important influence of the alloying elements gallium or dysprosium on the corrosion rate of the Nd‐Fe‐B magnets. Therefore, new concepts have to be developed to increase the corrosion resistance of these magnets.     

Determination of the weaker phase in the pitting corrosion of non‐standard low‐Ni high‐Mn‐N duplex stainless steels

In this paper, the use of Energy Dispersive Spectrometry (EDS) is proposed to determine the partition coefficients of the elements of a new family of duplex stainless steels that are characterized by having low contents of nickel, together with high levels of manganese and nitrogen. From the values of the partition coefficients, the chemical compositions of the constituting phases have been determined, in order subsequently to calculate the value of the Pitting Resistance Equivalent Number (PREN) of each phase. The proposition put forward in this study is that the phase having the lower PREN determines the pitting corrosion behaviour of these types of steels. Results obtained by means of optical and scanning electron microscopy have provided confirmation that the pitting corrosion behaviour of these new materials gets determined by the resistance of the weaker phase and consequently by the phase having the lower PREN value. Lastly it has been proved possible to determine the existence of an exponential relationship between the alloys pitting potential (Ep) and the weaker phase PREN; this can be utilized for the low‐nickel duplex stainless steels design in which the pitting corrosion resistance is controlled.     

Improving corrosion resistance of Al‐11mass%Si alloy through a large number of ECAP passes

Ultrafine‐grained (UFG) Al‐11mass%Si alloy, processed by multi‐pass equal‐channel angular pressing (ECAP) at 573 K, was investigated on corrosion behavior in 0.6 M NaCl solution. Potentiodynamic polarization tests and scanning electron microscopy observation showed that a large number of ECAP passes resulted in lower corrosion current density, more positive corrosion potential, and rather smooth corroded surface with shallow corrosion pits. The uniform distribution of fine secondary‐phase particles on UFG Al matrix weakened the susceptibility to pitting corrosion while inhibited general microgalvanic reactions. The present results indicate that grain refinement of aluminum matrix to the UFG state and uniform redistribution of broken particles (including eutectic silicon and secondary phases), via severe plastic deformation at elevated temperature undergoing dynamic recrystallization, can significantly improve the corrosion resistance of Al alloys, besides the known exceptional mechanical advantages. The simple and effective ECAP procedure makes UFG Al alloys more attractive for high strength structural application in corrosive environment.     

ZrV1.5Ni1.5 as electrode material in nickel-metal hydride batteries An in situ scanning tunnelling microscopy investigation

We have investigated the alloy ZrV_1.5Ni_1.5 by means of scanning electron microscopy (SEM) with electron probe X-ray microanalysis (EPMA), X-ray diffraction (XRD), in situ STM (scanning tunnelling microscopy in an electrolyte under controlled electrochemical potential) and electrochemical charge discharge measurements. By means of EPMA we found that the alloy is composed of three different crystallographic phases. The main phase (≈75 vol.%) is ZrV_0.81Ni_1.47, the second phase (≈20 vol.%) is V₉₂Ni₈ and the third (≈5 vol.%) is a ZrNi based phase. Using in situ STM we investigated the different corrosion behaviour of the phases. At a potential of −600 mV versus the Hg/HgO reference electrode we observed the corrosion of the vanadium rich phase while the other two phases passivated.     

PHASE COMPONENT AND CURIE TEMPERATURE OF Nd_4(Fe_(1-x)Cr_x)_(77.5)B_(18.5)ALLOYS

1.IntroductionThenewhard-magneticNdFeBall0yswithlowNdc0ncentration(35at.%)havebeenobtainedbycrystallization0fam0rphous.ibbo.sI1-3].Theirmagneticpr0pertiesatr0omtemperatureareasfollows:coercivityHc=16O-24OkA/m,remanenceB,=1'2-1.25T,andmagneticenergy(BH).ax=64108kJ/m'.ThevaluesofB.and(BH).axaremuchhigherthanth0se0fthewidelyusedanis0tropic(Ba,Sr)Fe12Ol9ferritesandAlNiC0alloys.Inpreyi0uspapers['-'1,wehavereportedthestructuralandmagneticpr0perties.Recentlywestudiedtheinfluenceofthesubstit…     

MICROSCOPIC CORROSION STUDIES OF DUPLEX STAINLESS STEELS

Electrochem,cal scanning tunneling microscopy and scanning electrochemical microscopy have been used for in situ monitoring of localized corrosion processes of different Duplex stainless steels (DSS) in acidic chloride solutions. The techniques allow imaging of local dissolution events with micrometer resolution, as opposed to conventional electrochemical techniques, which only give an overall view of the corrosion behavior. In addition, combined scanning Kelvin probe force microscopy and magnetic force microscopy were used for mapping the Volta potential variation over the surface of DSSs. A significant difference in Volta potential between the austenite and ferrite phases suggests galvanic interaction between the phases. A compositional gradient appears within 2 micrometers across the phase boundary, as seen with scanning Auger microscopy (SAM). In all, the studies suggest that higher alloyed DSS exhibit a more homogeneous dissolution behavior than lower alloyed DSS, due to higher and more similar corrosion resistance of the two phases, and enhanced resistance of the ferrite/austenite phase boundary regions.     

Effect of Nd content on the microstructure and coercivity of hot-deformed Nd–Fe–B permanent magnets

The microstructure of hot-deformed Nd–Fe–B permanent magnets with different Nd contents was investigated in order to correlate them with the hard magnetic properties. A thick distinct Nd-rich grain boundary (GB) layer was observed in a high Nd content sample by scanning electron microscopy and transmission electron microscopy. Three-dimensional atom probe results showed a significant increase in the Nd content in the GB as the overall Nd content in the alloy increased. We found a clear correlation between the Nd concentration in the GB layer and the coercivity. The mechanism of the coercivity increase is discussed based on the microstructure characterization and micromagnetic simulation results.     

HDDR法制备NdFeB永磁体的高矫顽力研究

采用X射线衍射分析、扫描电镜和BH测试仪分别研究HDDR法制备的NdFeB永磁体微结构、晶粒表面形貌及其磁性能。结果表明,HDDR法制备的磁粉再经1080 ℃高温真空烧结所获得的NdFeB永磁体,主要由四方相Nd2Fe14B（P42/mnm）和少量的富稀土相构成,扫描电镜相片显示主相平均晶粒尺寸约为12.3 μm;采用Horta法计算得到样品（006）晶面的极密度因子约为3.5,表明该样品具有较高的c轴取向;不同温度下退磁曲线研究表明,室温下合金有较好的磁性能：磁能积 (BH)max=264 kJ/m3,剩磁Br=1.17 T,矫顽力达到Hcj=2038 kA/m;随温度的升高,磁性能各参数都单调下降,特别是矫顽力降低最为显著,从295 K升温到448 K过程中其值下降了1496 kA/m;Hc(T)/Ms(T)与H(T)/Ms(T)(Kronmüller-Plot)关系曲线研究表明,该合金的矫顽力机制为畴成核反转机制,其中微磁参数αk和Neff分别为1.39和1.75,是决定该合金高矫顽力的关键因素。     

Effect of Nd on the corrosion behaviour of AM50 and AZ91D magnesium alloys in 3.5 wt.% NaCl solution

The corrosion performance of AM50 and AZ91D alloys containing up to 1.5 wt.% Nd was investigated by electrochemical and gravimetric measurements in 3.5 wt.% NaCl at 22 °C. The alloys were characterized by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD) and surface potential maps. In Nd-containing alloys, formation of Al₂Nd and Al–Mn–Nd intermetallic compounds reduced the volume fraction and modified the morphology of the β-Mg₁₇Al₁₂ phase. The addition of Nd improved the corrosion resistance of the alloys due to increased passivity of the surface film and suppression of micro-galvanic couples.     

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.     

SKPFM investigations of intermetallic compounds of innovative Er‐ and Zr‐containing Al–Si–Mg alloys and their influence on corrosion localization in saline solution

The paper aims at characterizing the influence of intermetallic compounds on the corrosion localization of innovative Al–Si–Mg Er‐ and Zr‐containing casting alloys. Samples of the investigated materials were studied by means of optical and scanning electron microscope micrographs, immersion tests, and scanning Kelvin probe force microscope (SKPFM) analyses in the T6 temper. Combination of immersion tests and SKPFM analyses allowed to identify those classes of intermetallic compounds promoting localization of the corrosion process. It was found that intermetallic compounds richer in Fe were the most critical for corrosion localization; furthermore, additions of Er caused a marked decrease of the potential difference of intermetallic compounds with respect to the Al matrix and a consequent less intense microgalvanic coupling, which translates into slower corrosion kinetics. Further, Zr additions slightly increased the potential difference of intermetallic compounds with the Al matrix, promoting a faster corrosion process.     

Crystallization kinetics and magnetization behavior of RE_(3.5)Fe_(66.5)Co_(10)B_(20)(RE=Pr,Nd) nanocomposite ribbons

The influence of hard magnetic phase on the crystallization kinetics and magnetization behavior in nanocomposite RE3.5Fe66.5Co10B20(RE = Pr, Nd) ribbons prepared by melt-spinning was studied. Differential scanning calorimeter(DSC) measurement of the as-cast meltspun amorphous ribbons during the crystallization process shows that precipitation energy of Pr2Fe14 B phase is higher than that for Nd2Fe14 B phase, confirmed by X-ray diffraction(XRD) patterns. It can be explained by the different radii of Pr and Nd atoms. Scanning electron microscopy(SEM)images indicate that the average grain size in Pr3.5Fe66.5Co10B20 ribbon is smaller than that in Nd3.5Fe66.5Co10B20,resulting in an enhancement of exchange coupling between hard and soft phases. It is responsible for the better hard magnetic properties in Pr3.5Fe66.5Co10B20. In addition, the process of magnetization reversal of nanocomposite RE3.5Fe66.5Co10B20(RE = Pr, Nd) ribbons was discussed in detail by the recoil loops.     

Nd-Fe-B合金的腐蚀及防蚀表面处理

研究了在环境腐蚀试验条件下Ｎｄ－Ｆｅ－Ｂ合金的腐蚀行为及腐蚀对其磁性能的影响,并提出了防止腐蚀的表面处理方法。试验结果表明,Ｎｄ－Ｆｅ－Ｂ合金有较高的腐蚀速率,腐蚀后Ｎｄ－Ｆｅ－Ｂ合金的磁性能降低。化学镀Ｎｉ－Ｐ及电泳涂装的复合表面处理方法,具有良好的防蚀效果。     

熔体温度对快淬法制备纳米晶复合Nd_(8.5)Fe_(77)Co_5Zr_3B_(6.5)合金微观结构和磁性能的影响

将Nd_(8.5)Fe_(77)Co_5Zr_3B_(6.5)(at%)合金熔化至不同温度后,以18 m/s的甩带速度快淬,对淬态条带进行了退火处理,分析了其微观结构和磁性能的变化。结果表明,熔体温度对淬态及其退火态合金的微观结构和磁性能可以产生重要影响,熔体温度为1210℃时制备的快淬条带由Nd_2Fe_(14)B相和部分非晶相组成,具有一定的硬磁性;随着快淬时熔体温度的升高,淬态条带中非晶相的质量分数逐渐增加,其磁性逐渐转变为软磁性。几种合金经退火处理后均由大量Nd_2Fe_(14)B相与少量软磁相组成,熔体温度较低的合金退火后其晶粒尺寸较小,磁性能较好。熔体温度为1210℃时制备的合金退火后磁性能最佳,内禀矫顽力Hci为559.2 kA/m,剩余磁化强度Br为0.98 T,最大磁能积(BH)_(max)为127.8 kJ/m~3。     

Preparation and characterization of TiO_2/acrylic resin composite coatings on sintered NdFeB permanent magnets by electrochemical deposition

The sintered Nd Fe B permanent magnets are widely used in various industries because of their excellent magnetic and comprehensive properties. However, they are seriously hindered to further developments and applications owing to its poor corrosion resistance. In this paper,TiO2/acrylic resin composite coatings were successfully prepared on the sintered Nd Fe B permanent magnets by electrochemical deposition for the first time, and the corrosion behaviors were investigated by using immersion testing and potentiodynamic polarization tests. It is found that the coating is a composite, being composed of polymerized acrylic resin as the matrix and nano-rutile TiO2particles(-200 nm) as the reinforcements. It is about25 lm in thickness and its surface and cross-section exhibit uniform and smooth with no pores, cracks, and other defects. Compared with those of the uncoated Nd Fe B magnets, the weight loss of the coated samples immersed in H2SO4 solution reduces by two orders of magnitude,corrosion current density decreases by four orders of magnitude, and the corrosion potential of the coated sample is shifted in the noble direction by 400 m V. The mechanisms of corrosive resistance enhancement were discussed.     

Effect of aging time and temperature on exfoliation corrosion of aluminum alloys 2024‐T3 and 7075‐T6

Two types of aluminum alloys, 2024‐T3 and 7075‐T6, have been selected in this study to investigate the effect of metallurgical aspects on exfoliation corrosion. To determine and evaluate the metallurgical effects of heat treatments on corrosion behaviour of these alloys, G34 ASTM test was selected to investigate the exfoliation corrosion behaviour. The results showed that with increasing the aging time for the aluminum alloy type 2024‐T3 the susceptibility to exfoliation corrosion increases, while for type 7075‐T6 decreased. These results refer to precipitation of the intermetallic compound phases such as CuAl₂, and MgZn₂, in 2024‐T3 and 7075‐T6 respectively. The amount of these phases increases with increasing the aging time for both alloys. The investigations showed the phases that initiate in 2024‐T3 act as anode sites while in 7075‐T6 they act as cathode sites.     

Structural and corrosion characterization of biodegradable Mg–RE (RE=Gd,Y, Nd) alloys

Binary Mg–Gd (up to 5% Gd in mass fraction), Mg–Nd (up to 9% Nd in mass fraction) and ternary Mg–Gd-Y (up to 5% Gd, 1% Y) alloys with precisely determined contents of cathodic impurities (Fe, Ni, Cu, Co) were studied. The alloys were studied in the as-cast state (cooling rate of 500 K/min) and after solution heat treatment (T4). Structures were investigated by optical and scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction and glow discharge spectrometry. Structural investigation was completed by Vickers hardness measurements. Corrosion behavior in the simulated physiological solution (9 g/L NaCl) was assessed by immersion tests and potentiodynamic measurements. It was found that the structures of the as-cast alloys were dominated by fine α-Mg dendrites and eutectic Mg–RE phases. The dendrites exhibited RE-concentration gradients which were most pronounced in the Mg–Gd alloys. For this reason, the T4 heat treatment of the Mg–Gd alloy led to the formation of a new cuboidal Mg₅Gd phase. The corrosion resistance was significantly improved by Gd. The effect of Nd was weak and the addition of Y to Mg–Gd alloys had harmful effect on the corrosion resistance. The T4 heat treatment strongly accelerated the corrosion of Mg–Gd alloys. Its effect on the corrosion of Mg–Nd alloys was not significant. The observed corrosion behavior of the alloys was discussed in relation to their structural states and contents of cathodic impurities.     

Ti对高B含量纳米复合Nd9.4Fe79.6-xTixB11永磁合金显微结构和磁性能的影响

研究了Ti添加对Nd9.4Fe79.6-xTixBu(x=0,1,2,4,6)合金显微结构和磁性能的影响.结果表明,添加Tj能抑制Nd9.4Fe79.6-xTixB11合金中NdzFe23B3和Fe3B相的形成及a-Fe相的析出和长大,促进Nd2Fe14B相的形成.当Ti添加到一定量时,Ti能以TiB2质点的形式从合金中析出,TiB2质点能够抑制晶粒的长大,改善合金的显微结构.综合性能比较佳的Nd9.4Fe75.6Ti4B11合金薄带最佳退火工艺下剩磁Br为0.87T,矫顽力Hcj达到931 kA/m,磁能积(BH)max为115.4kJ/m3.     

烧结含镝钕铁硼永磁体时效后的微观组织分析

本文采用场发射扫描电镜、能谱仪和金相显微镜等研究了时效对烧结含镝钕铁硼永磁体的相组成和显微组织的影响。实验结果表明,烧结含镝钕铁硼永磁体经一级、二级时效后,组成相有Nd2Fe14B主晶相和在晶界交隅处析出的较多富钕相及少量富硼相。时效后Nd2Fe14B主晶相的晶界较烧结态变得清晰、平直,这是其内禀矫顽力明显增高的主要原因。