grain growth effects on the corrosion behavior of nanocrystalline ndfeb magnets

Grain growth effects on the corrosion behavior ofnanocrystalline NdFeB magnets

A.A. El-Moneim a,b, A. Gebert a,*, F. Schneider a,O. Gutfleisch a, L. Schultz a

a Institute for Solid State and Materials Research Dresden, P.O. Box 270016, D-01171 Dresden, Germanyb Department of Physical Chemistry, National Research Center, Cairo, Egypt

Received 16 May 2001; accepted 20 June 2001

Abstract

Isotropic nanocrystalline Nd14Fe80B6 magnets with different grain sizes in the range of 100–

600 nm have been produced from melt-spun materials by hot pressing at 700 �C and subse-

quent annealing at 800 �C for 0.5–6 h. The microstructures have been characterized using

XRD, SEM, EDX and Kerr microscopy. The effect of grain size on the corrosion behavior of

nanocrystalline magnets has been examined in N2-purged 0.1 M H2SO4 electrolyte by in situ

inductively coupled plasma solution analysis, gravimetric and electrochemical techniques and

hot extraction [H]-analysis. The corrosion resistance increases with increasing grain size of the

hard magnetic phase. Nanocrystalline magnets showed an increase in absorbed hydrogen by

anodic polarization and abnormal dissolution by cathodic polarization. The corrosion be-

havior of the magnets in relation to their microstructure is discussed in terms of dissolution,

hydrogenation and mechanical degradation. � 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Nanocrystalline NdFeB magnet; Grain size; Corrosion; Hydrogen

1. Introduction

NdFeB-magnets find a rapidly growing variety of applications, because of theirhigh energy density (BH)max, which is sensitive to the degree of alignment of crys-tallographic c-axis of the tetragonal Nd2Fe14B phase. Such a texture can be achieved

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Corrosion Science 44 (2002) 1097–1112

*Corresponding author. Tel.: +49-351-4659-275; fax: +49-351-4659-320.

E-mail address: [email protected] (A. Gebert).

0010-938X/02/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.

PII: S0010 -938X(01 )00123-8

by the conventional powder metallurgy route including pre-alignment and sintering[1] or by hot deformation [2]. For hot deformation a fine-grained microstructure ofNdFeB alloys is required. There are different possibilities for the preparation of sub-micron grain-sized NdFeB powders such as melt-spinning [3], mechanical alloying [4]and the Hydrogenation–Disproportionation–Desorption–Recombination (HDDR)process [5,6]. Detailed studies on the influence of the microstructure, particularlygrain size on the magnetic properties and deformation behavior showed that inde-pendent on the preparation process, very small and homogeneous grain sizes arefavorable in order to obtain optimum magnetic properties and minimum deforma-tion stresses [7,8].

On the other hand, corrosion of magnets is one of the key parameters assessingthe applicability of NdFeB-type magnets. The corrosion behavior of microcrystalline(sintered) magnets has been studied extensively in different media [9,10]. Such studieshave shown that the low corrosion resistance of NdFeB magnets can be attributed tothe high content of neodymium and their complex multiphase microstructure. Themicrostructure comprises of ferromagnetic phase matrix (Nd2Fe14B) and the mostcorrosion sensitive Nd- and B-rich phases in the grain boundary region [11]. Balaet al. reported that sintered magnets are subjected at free corrosion conditions in 0.5M H2SO4 solution to initial rapid intergranular attack, which causes the grains of theferromagnetic phase to lose contact with the surface of bulk magnets [12]. Thus,structure integrity of the magnet is lost. Furthermore, the magnets exhibited anabnormal dissolution behavior at high cathodic potential [13,14].

Due to differences in the powder fabrication method, the chemical composi-tion and microstructure, and the corrosion and electrochemical behavior of sin-tered and nanocrystalline magnets may differ drastically. A few investigationshave been devoted for the understanding of the corrosion and electrochemi-cal behavior of nanocrystalline magnets. Most previous investigations haveonly addressed the evaluation of corrosion resistance in dependence on alloy-ing additions and processing routes [15,16]. Results of these investigations re-vealed that the partial replacement of iron with cobalt, aluminium and galium has apositive effect on the corrosion resistance. This improvement in corrosion resis-tance results mainly from the electrochemical stabilization of the intergranu-lar phases. It was also found that magnets made from HDDR powder exhibiteda comparable corrosion resistance in acid solution than magnets made from melt-spun and intensively milled powders [15]. It was assumed that the significance ofthis result lies in the microstructural differences (i.e. grain size, shape and distribu-tion of phases). However, no direct experimental evidence was presented to sub-stantiate this claim. Consequently, there is a necessity to optimize the microstructurenot only with respect to the magnetic properties but also taking into account thecorrosion aspects.

The present work was undertaken to understand the inherent corrosion behaviorof nanocrystalline melt-spun and hot pressed NdFeB magnets in 0.1 M H2SO4

solution under various conditions such as free corrosion conditions, and anodic andcathodic polarization. The effect of grain growth due to annealing on the micro-structure and corrosion behavior is discussed in this paper.

1098 A.A. El-Moneim et al. / Corrosion Science 44 (2002) 1097–1112

2. Experimental

2.1. Preparation of magnets and microstructural analysis

Commercial nanocrystalline melt-spun powder of the nominal compositionNd14Fe80B6 (MQP-A) was used as starting materials. The powders were hot pressedin vacuum at 700 �C, applying a pressure of 150 MPa to obtain a highly densifiedprecursor with a diameter of 8 mm and a height of 8 mm. In order to prepare NdFeBmagnets with different grain sizes, hot pressed materials were further annealed at 800�C for varying times from 0.5–6 h in evacuated silica tube. The microstructure of themagnets was investigated by X-ray diffraction using a Philips PW 1050 diffracto-meter with CoKa radiation. Further characterization regarding grain sizes, shapes andphase distribution were carried out on hot pressed and annealed samples using Kerreffect-optical microscopy and scanning electron microscopy (SEM, JEOL JSM-6400)complemented with energy dispersive X-ray (system Noran-Voyager) analysis(EDX). Prior to each experiment, NdFeB-magnets were mechanically polished up to1 lm using diamond paste. After polishing, test specimens were ultrasonicallycleaned with ethanol and dried in air.

2.2. Corrosion testing

The corrosion behavior of nanocrystalline NdFeB magnets was characterized inN2-purged 0.1 M H2SO4 solution at 25 �C, using the following methods:

Analytical measurements were carried out in 100 ml of the test solution by usingon-line inductively coupled plasma (ICP). This solution analysis technique permitsin situ measurements of the dissolution behavior of NdFeB magnet components.Gravimetric measurements were performed by weighing test specimens before andafter immersion in N2-purged 0.1 M H2SO4 using a microbalance with an accuracyof �10 lg.

2.3. Electrochemical measurements

An electrochemical cell with standard three electrode arrangements consists ofNdFeB-sample as a working electrode, a platinum net counter electrode and a cal-omel electrode (SCE) as a reference electrode. All potentials in this paper are referredto the SCE.

In all experiments, NdFeB samples were embedded in rotating disc electrode of adiameter of 12 mm. The rotation speed of the disc electrode was 720 rpm in order toavoid surface screening by corrosion products or hydrogen bubbles and maintainchemical homogeneity of the test solution.

Potentiodynamic polarization tests were carried out at two different scan rates,that is, 0.2 and 2 mV/s. Galvanostatic polarization technique was also used tocharacterize the electrochemical corrosion behavior of NdFeB magnets at variousapplied anodic and cathodic current densities.

A.A. El-Moneim et al. / Corrosion Science 44 (2002) 1097–1112 1099

The hydrogen content absorbed by the treated magnets in N2-purged 0.1 MH2SO4 solution at 25 �C was measured by hot extraction using Leco 402 analyzer.

3. Results and discussion

3.1. Microstructure characterization

A detailed analysis of the measured XRD patterns revealed that all magnets re-tained Nd2Fe14B structure and that no evidence for new phases was observed afterannealing at 800 �C.

Fig. 1(a)–(d) shows Kerr-effect micrographs of hot pressed (a) and annealed(800 �C for 0.5, 1 and 6 h) NdFeB magnets. The microstructure of hot pressedmagnet is homogeneous with extremely fine grains (Fig. 1(a)). Upon annealing, graingrowth takes place and flake boundaries act as points for coarsening process (Fig.1(b)–(d)). Thus, the microstructure becomes very inhomogeneous.

The mean average grain sizes of the hot pressed magnet before and after an-nealing at 800 �C for 0.5, 1, 2 and 6 h have been determined using scanning electronmicroscopy and were estimated to be 100, 230, 330, 460 and 600 nm, respectively [7].

Fig. 1. Kerr-effect micrographs for polished NdFeB magnets hot pressed (a) and annealed at 800 �C for

0.5 h (b), 1 h (c) and 6 h (d).


It can, therefore, be said that heat treatment sequences provide grain sizes still in thesubmicron range.

Fig. 2(a) and (b) shows SEM micrographs in the backscattered mode of polishedsample surfaces after hot pressing (a) and after annealing at 800 �C for 6 h (b), re-spectively. In both micrographs, the regions of the white contrast were identifiedby EDX analysis as Nd-rich phase and gray regions represent fine particles mainlyconsisting of the ferromagnetic phase (Nd2Fe14B). The black regions are pores,which appear during surface finishing. In Fig. 2(a), the microstructure of hot pressedprecursor shows clusters of finely dispersed and continuous Nd-rich regions with athickness of about 1 lm at the flake boundaries. Li and Graham defined the mi-crostructure of melt-spun and hot pressed NdFeB magnet in more detail before andafter die upsetting by means of TEM investigations [17,18]. For the hot pressedprecursor, it was observed that Nd2Fe14B grains with sizes in the range of 50–100 nm

Fig. 2. SEM micrographs (backscattering mode) of polished, hot pressed NdFeB magnets before (a) and

after (b) annealing at 800 �C for 6 h, respectively.


are surrounded by a thin, continuous and very uniformly distributed layer of Nd-richgrain boundary phase. The thickness of this layer amounts to 1–5 nm [17,18]. Thiscontinuous layer cannot always be identified with excessive grain growth of phasematrix or upon die upsetting.

Upon annealing, a random distribution of Nd-rich grain boundary phase isgenerally noticed in the micrometer scale due to the heterogeneity in the graingrowth of ferromagnetic phase. Their sizes appear more agglomerated in non-con-tinuous form and increase with the extension in annealing time (Fig. 2(b)).

In view of the present results, grain growth due to annealing of nanocrystallinemelt-spun and hot pressed magnets results in heterogeneity of the microstructurewith a reduction in the fractions of Nd-rich intergranular phase.

3.2. Corrosion and electrochemical characterization

3.2.1. Corrosion behavior under open circuit conditionsFig. 3 shows the corrosion rates of NdFeB magnets with average grain sizes of 100

and 600 nm as a function of immersion time in N2-purged 0.1 M H2SO4 solution at asample rotation speed of 720 rpm. The corrosion rates presented in Fig. 3 representthe sum of partial dissolution rates of magnet components instantaneously measuredduring immersion in the test solution using on-line ICP solution analysis. In general,both magnets exhibit a fast corrosion process and their corrosion rate curves aredistinguished by three stages of dissolution. A detailed analysis of ICP results duringthe dissolution regimes along with careful examination of magnets surfaces by SEM(SE-mode) and EDX (Fig. 4(a)–(c)) revealed that the initial stage (I) of about 1–3min results from a beginning of preferential attack of Nd-rich intergranular region(Fig. 4(a)). This attack is known to occur predominately by galvanic interaction with

Fig. 3. Corrosion rates measured by ICP solution analysis for NdFeB-magnets with grain sizes of about

100 and 600 nm in N2 purged H2SO4 at 25 �C and 720 rpm as a function of immersion time.


Fig. 4. SEM micrographs (SE-mode) after etching the surface of NdFeB magnets with grain size of about

600 nm in N2 purged H2SO4 at 25 �C and 720 rpm for 1 min (a), 5 min (b) and 10 min (c), respectively.


the ferromagnetic phase matrix [11]. The second stage (II) corresponding to a rapidincrease in the corrosion rate is due to the preferential dissolution of Nd-rich in-tergranular regions and the partial undermining of phase matrix particles (Fig. 4(b)).After 6–7 min, the corrosion rate attains its steady state (III), where all magnetcomponents dissolve in the same proportion as they appear in the alloy formula. Thesteady state is attributed to the undermining and falling away of ferromagnetic phaseparticles that lose their contact with magnet surface after the initial incubationperiod of intergranular corrosion ðIþ IIÞ, as shown in Fig. 4(c).

It should also be noted that the corrosion rate of the annealed NdFeB magnetwith the mean grain size of 600 nm is significantly lower than that of hot pressedsample with 100 nm grain size (Fig. 3). This indicates a beneficial effect of graingrowth on the corrosion resistance of NdFeB magnets. This fact is further confirmedby Fig. 5, which summarizes the corrosion rates estimated gravimetrically afterimmersion for 1 and 10 min in 0.1 M H2SO4 solution at 720 rpm, as a function of themean average grain size of the ferromagnetic phase. It is obvious that the corrosionrates of NdFeB magnets generally decrease with the increase in the grain size ofmatrix phase. Such a behavior of corrosion inhibition with grain growth is attributedto the observed change in the microstructure upon annealing, i.e. the heterogeneityand the reduction in the volume fraction of Nd-rich intergranular phase.

Furthermore, all nanocrystalline magnets are found to be highly reactive to theattack of hydrogen released during the acid corrosion process. Fig. 6 shows thevariation of hydrogen concentration of the magnets after immersion in 0.1 M H2SO4

for 2 and 20 min in dependence on the average of grain sizes. After 2 min, the hy-drogen concentration is very low and independent of the grain size of the ferro-magnetic phase. After 20 min, the amount of hydrogen absorbed is significantlyincreased and strongly influenced by the grain size of the ferromagnetic phase, i.e.the hydrogen concentration remarkably decreases with increasing grain size. The

Fig. 5. Change in the corrosion rates measured gravimetrically after 1 and 10 min of magnets immersion

in N2 purged H2SO4 at 25 �C and 720 rpm as a function of mean average grain size of ferromagnetic phase.


high affinity of nanocrystalline magnets with smaller grain size to absorb hydrogenis in favor for the larger accessible surface area of a fine-grained microstructure,besides the larger fractions of Nd-rich intergranular phase, which act as additionalinter-diffusion paths for hydrogen.

XRD patterns of the magnets immersed for 20 min in 0.1 M H2SO4 solutionrevealed a distinct shift of the reflections of Nd2Fe14B towards smaller diffractionangles. This indicates that the absorbed hydrogen atoms diffuse interstitially inNd2Fe14B-type phase and cause a lattice dilatation with a decrease in the interatomiccohesion. This additionally (besides the preferential dissolution of the Nd-rich phase)promotes pulverization and increases corrosion susceptibility of NdFeB magnets,particularly those with smaller grain sizes.

In summary, nanocrystalline magnets undergo fast corrosion processes in acidicsolution under open circuit conditions. The process consists of selective dissolutionof the Nd-rich intergranular phase, hydrogenation of the hard magnet phase and itssubsequent mechanical degradation, i.e. pulverization. Magnets with larger grainsizes retard the fast corrosion process due to the heterogeneity and smaller fractionsof Nd-rich intergranular phase in the microstructure, besides the hard surface hy-drogenation of more coarse-grained magnets.

3.2.2. Corrosion under polarization conditionsThe understanding of the effect of grain growth on the electrochemical behavior of

NdFeB magnets is also of fundamental importance. Fig. 7 presents potentiodynamicpolarization curves measured in N2-purged 0.1 M H2SO4 at 25 �C at a scan rate of 2mV/s for NdFeB magnets with grain sizes of about 100 and 600 nm. The polarizationcurve of pure iron is also shown for comparison. It can be seen that the polarizationcurves are well defined, having an activation controlled region of about three decadesof the measured current prior to the onset of the diffusion controlled region at more

Fig. 6. Dependence of hydrogen concentration absorbed by NdFeB magnets after immersion for 2 and

20 min in N2 purged H2SO4 at 25 �C and 720 rpm on the mean grain size of ferromagnetic phase.


than 500 mA/cm2. A distinct difference in the electrochemical behavior between theexamined magnets occurs mainly in the region near the rest potential, and becomesinsignificant at higher anodic potential. Therefore, it is interesting to follow thevariation in the electrochemical behavior of NdFeB magnets with respect to theirgrain sizes at a region near to the rest potential. Fig. 8 shows typical examples ofpotentiodynamic polarization plots measured for NdFeB magnets with various grainsizes in N2-purged 0.1 M H2SO4 using a scan rate of 0.2 mV/s, which gives the bestdefinition and maintains quasi-steady conditions. In Fig. 8, a progressive decrease in

Fig. 7. Potentiodynamic polarization curves of NdFeB-magnets with an average grain size of 100 and 600

nm in N2 purged H2SO4 at 25 �C, 720 rpm using scan rate of 2 mV/s. A potentiodynamic polarization

curve for pure Fe is shown for comparison.

Fig. 8. Potentiodynamic polarization curves of NdFeB magnets with various grain sizes in N2 purged

H2SO4 at 25 �C and 720 rpm using a potential sweep rate of 0.2 mV/s.


both anodic and cathodic activities of NdFeB-magnets occurs with the increase in thegrain size of the hard magnetic phase. The estimated corrosion current density de-creases with increasing the grain size from about 5 to 0.2 mA/cm2, indicating a sharpreduction in the corrosion rate. This observation is in full agreement with analyticaland gravimetric results and suggests that grain growth of the ferromagnetic phaseconsiderably influences the corrosion behavior of nanocrystalline NdFeB magnets.

3.2.2.1. Corrosion behavior under galvanostatic anodic polarization. In order to studythe anodic polarization behavior of NdFeB magnets with different grain sizes in moredetail, samples were polarized galvanostatically in 0.1 M H2SO4 for 10 min and masslosses at i ¼ 50–300 mA/cm2 (ffi active region) were determined by two ways: (1)weighing the working electrode before and after the test and (2) ICP solution analysis.Solution analysis after anodic polarization for 10 min revealed no selective dissolu-tion of magnet components. Therefore, the total current density of the magnet dis-solution (iD) is simply defined by using Faraday’s law [14], which states that

iD ¼ DmFX

PxZx

.tA

XMxPx ð1Þ

where Dm is the mass loss of the magnet at the open circuit potential or at a givenapplied current density; t, exposure time; A, area of electrode; F , Faraday’s constant;Px, atomic percentage of element x in the magnet formulae; Zx, number of electronstransferred per one atom x; Mx, atomic weight of element x in the magnet.

Fig. 9 presents the change in the dissolution current density determined from massloss measurements (Dm) using Eq. (1) of NdFeB magnets with various grain sizes asa function of the applied anodic current density. Dissolution currents of magnets arehigher than those calculated from Faraday’s Law (dashed curve in Fig. 9), especiallyfor magnets with smaller grain sizes. This result implies that the total dissolution rate(from mass loss) is higher than the electrochemical dissolution rate. The difference

Fig. 9. Dissolution current density (iD) of NdFeB magnets with various grain sizes in N2 purged H2SO4 at

25 �C and 720 rpm after 10 min as a function of applied anodic current density.


must be attributed to the magnet pulverization process (mechanical degradation)and the subsequent dissolution of the detached particles in the solution.

The variation of hydrogen concentration absorbed by NdFeB-magnets with anaverage grain size of 100 nm during spontaneous dissolution at the open circuitpotential or upon anodic polarization at different current densities is presented inFig. 10. It can be seen that the hydrogen concentration increases markedly with theincrease in the applied anodic current density. Since there is a direct relationshipbetween the amounts of hydrogen absorbed and the hydrogen evolution reaction(HER) rate, such behavior appears to be contrary to the very basics of electro-chemical theory. Normally, the rate of the cathodic partial reaction of the corrosionprocess, i.e. the hydrogen reduction decreases upon anodic polarization. The abovedescribed phenomenon is similar to that observed for other highly reactive metals,such as magnesium, which is known as negative difference effect (NDE) [19]. Sincethe magnet surface in acidic solution is film free and since the strength of the galvaniccoupling effect is usually reduced by anodic polarization, the NDE may be attributedto the previously justified magnet pulverization that is enhanced with anodicpolarization (Fig. 9). In other words, the easier hydrolysis of the particles detachedfrom the magnet surface in acidic solution, particularly near magnet alloy/electrolyteinterface may result in extra-hydrogen production. The more pulverization withanodic polarization the more hydrogen production and subsequently the moreabsorbed.

3.2.2.2. Corrosion behavior under galvanostatic cathodic polarization. Atomic nascenthydrogen produced during cathodic reduction of protons (Hþ) causes surface de-gradation of sintered NdFeB magnets. This is known as abnormal dissolutionprocess [13,14]. To investigate the abnormal dissolution process in the present study,nanocrystalline magnets with different grain sizes were cathodically polarized for

Fig. 10. Dependence of hydrogen concentration absorbed by NdFeB magnet with an average grain size of

100 nm on the applied anodic current density.


10 min in 0.1 M H2SO4 solution with a subsequent determination of Fe2þ ions content

in the test solution using ICP analysis. The apparent dissolution current density ofiron (iFe) was estimated using Faraday’s law as stated in Eq. (1). The obtained resultsshowed that iFe values are significantly increased with increasing cathodic polar-ization current densities, indicating abnormal dissolution behavior. In this context, ithas to be mentioned that the potential values recorded during galvanostatic cathodicpolarization are relatively more noble than the standard redox potentials of thesystems Nd=Nd5þ (E0 ¼ �2:65 (SCE)) and B=B3þ (E0 ¼ �1:11 (SCE)) and morenegative than that of the Fe=Fe2þ system (E0 ¼ �0:681 V (SCE)), indicating that thecalculated iFe values are mainly the results of non-current dissolution.

On the other hand, it is known that the rate of hydrogen permeation into thecrystal lattice of a metal is usually proportional to the square root of the hydrogenreduction current (i0:5H ) [20]. If the abnormal dissolution is mainly the result of surfacehydrogenation, it should hold that

iFe ¼ ki0:5H ð2Þ

where k is the permeation constant. The dependence of iFe versus i0:5H for NdFeBmagnets with various grain sizes is presented in Fig. 11. For all samples a linearrelationship can be derived, confirming the relation given in Eq. (2). The slope (k) ofthese curves clearly decreases with the increase in the mean average grain size of theferromagnetic phase, suggesting a more inhibited hydrogen permeation process withthe increase in mean grain size of hard magnetic phase.

SEM investigations also justify the abnormal dissolution behavior of nanocrys-talline NdFeB magnets. Fig. 12 (a)–(d) show micrographs of the surface of a magnetwith grain size of about 100 nm after immersion under open circuit conditions and

Fig. 11. Abnormal dissolution current of iron as a function of square root of applied cathodic current

density.


cathodic polarization in 0.1 M H2SO4 for 10 min at �0.1, �0.3 and �0.5 A/cm2,respectively. It is obvious that the mode of the corrosion attack after cathodicpolarization is different from that observed after current-less immersion shown inFig. 12(a). It is assumed that, the absorbed hydrogen is accumulated at the Nd-richintergranular regions and that molecular hydrogen of very high pressure is formed.This leads to blistering (Fig. 12(b)) and, when a certain critical stress level is attainedlocally due to the pressure generated in these blisters, micro and macrocracks appearas shown in Fig. 12(c). Eventually, magnets surface layers are undercut and undergofurther dissolution inside the bulk solution (Fig. 12(d)). Thus, a new surface of themagnet is exposed to the solution and the process restarts even more accelerated.

Fig. 12. SEM micrographs for NdFeB magnet with an average grain size of 100 nm after open circuit

immersion (a) and galvanostatic cathodic polarization at �0.1 A cm�2 (b), �0.3 A cm�2 (c) and �0.8A cm�2 (d) for 10 min in N2 purged H2SO4 at 25 �C, 720 rpm, respectively.


4. Conclusions

The corrosion behavior of nanocrystalline NdFeB magnets with various grainsizes prepared from melt-spun hot pressed and annealed Nd14Fe80B6 powders, in 0.1M H2SO4 under free corrosion conditions and anodic and cathodic polarizations hasbeen studied. It was found that the corrosion behavior of nanocrystalline NdFeBmagnets depends on their microstructure, particularly on the grain size of the fer-romagnetic phase. Based on the results presented in this study, the following con-clusions can be drawn:

1. The corrosion resistance of the magnets increases with increasing mean averagegrain size of the matrix phase. This improvement in the corrosion resistance is at-tributed to smaller fractions and more heterogeneous distribution of the Nd-richintergranular phase, besides less hydrogenation of the magnets surface.

2. Two interesting phenomena are detected for NdFeB magnets upon anodic polar-ization: (I) the total dissolution rate estimated from mass loss is higher than theelectrochemical dissolution rate, (II) unusual increase in the amount of hydrogenabsorbed with anodic polarization (NDE effect).

3. The magnets show abnormal dissolution behavior with cathodic polarization, in-dicating that applying any kind of cathodic protection can lead to catastrophiccorrosion of nanocrystalline NdFeB magnets.

Acknowledgements

The authors wish to express their thanks to the International Office at the FZJ€uulich for financial support. Also the authors are very grateful for A. Kirchner, A.G€uuth, A. Plotinkov and C. Vogt for their experimental assistance and V. Pancha-nathan for supplying melt-spun powders.

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grain growth effects on the corrosion behavior of nanocrystalline ndfeb magnets

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