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Corrosion behavior of NdFeB sintered magnets in HNO3–HF acid mixture solution
Materials Chemistry and Physics xxx (2015) 1e7
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Corrosion behavior of NdFeB sintered magnets in HNO3eHF acid mixture solution Junjie Ni a, b, *, Xin Cui b, Xiangfa Liu a, Hui Cui c, ZeChun Xue c, Zhengfeng Jia b, Changzheng Wang b, Jie Ma b, Shuwen Gong c a b c
School of Material Science and Engineering, Shandong University, Jinan 250061, China School of Material Science and Engineering, Institute of Non-ferrous Metal, Liaocheng University, Liaocheng 252059, China School of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng 252059, China
h i g h l i g h t s HF affected electrochemical corrosion behavior of NdFeB in HNO3eHF solution. HF effects on corrosion performance depended on its content. HF effects related to the formation of NdF3 corrosion products on magnet surface.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 September 2014 Received in revised form 26 May 2015 Accepted 8 June 2015 Available online xxx
Corrosion behavior of NdFeB magnet in HNO3eHF solutions was investigated using open circuit potential and polarization measurements and impedance technique. Results showed that the additions of HF increased open circuit potential, corrosion potential and film/inductive/charge transfer resistance, but decreased corrosion current density of magnet. The HF effects on the corrosion performance were closely related to the formation of NdF3 corrosion products on magnet surface, in turn which depended on its content. Also the corrosion mechanism of magnet was discussed on the basis of structural observation, composition and EIS analyses. © 2015 Published by Elsevier B.V.
Keywords: Alloys Electrochemical techniques Corrosion Microstructure
1. Introduction NdFeB sintered magnets have excellent permanent magnetic performance and high property/price ratio for a number of commercial applications such as industrial motor, automobile, dental appliances and so on [1e3]. However a critical limitation for these service applications is the susceptibility of NdFeB alloys to corrosion and hence there continues to be much research to improve their anticorrosion properties [4e9]. Up to now, the methods to increase the corrosion resistance basically include the alloying additions of Al, Ti, Cu, Cr, etc. to the NdFeB magnets [10e13] and the application of protective coating [14e16]. Of two approaches the former one has been found to improve the corrosion resistance of
* Corresponding author. School of Material Science and Engineering, Shandong University, Jinan 250061, China. E-mail address: [email protected] (J. Ni).
magnets only in a certain degree and possibly to deteriorate the magnetic properties. Fortunately, these drawbacks do not occur in the second method. That facts have spurred ones to apply protective coatings, like electroplating Ni, Al, Co [17e20] and electroless NieP plating [21], in the NdFeB industry production, although this method unavoidably increases the fabrication cost. Before plating, acid pickling process is necessarily performed to remove rust, grease and oxide films on the magnet surface, which is a precondition for the formation of protective coating having strong adhesion force with NdFeB material. Therefore, the acid picking is an important magnet surface pretreatment. During acid picking the sever grain boundaries (GBs) corrosion happens on the NdFeB sintered magnets, but it is detrimental to the adhesion force of plating coats. The GBs corrosion derives from the multiphase structure in NdFeB sintered magnets and much lower corrosion potential for the Nd-rich phase than the Nd2Fe14B matrix phase [22,23]. It thus is essential to inhibit the Nd-rich GBs
http://dx.doi.org/10.1016/j.matchemphys.2015.06.023 0254-0584/© 2015 Published by Elsevier B.V.
Please cite this article in press as: J. Ni, et al., Corrosion behavior of NdFeB sintered magnets in HNO3eHF acid mixture solution, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.023
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J. Ni et al. / Materials Chemistry and Physics xxx (2015) 1e7
corrosion, finally obtaining a uniformly corroded magnet surface. In other words, the corrosion of the Nd2Fe14B matrix and Nd-rich GBs phases must be synchronous. Such requirements may be satisfied by the additions of hydrofluoric acid to a commonly used nitric acid solution, partly because the matrix phase has high Fe content and the dissolution of Fe containing phase is very sensitive to HF [24]. Moreover, the formed fluoride neodymium and iron complexes may be a corrosion inhibitor of Nd-rich GBs phase in acid pickling. So the HNO3eHF mixture solution should be a promising candidate for the NdFeB acid pickling, in which the HF content is a key factor. Nevertheless, little is known about the relation of corrosion feature for NdFeB magnets with the HF content in the HNO3eHF mixed acid solutions. Considering the aforementioned aspects, the present study aims at determining the effects of HF content on the corrosion behavior of NdFeB magnets. Also in this work the corrosion mechanism of magnet in the HF-free and -containing acid solutions was discussed on the basis of electrochemical measurements, structural observation and composition analysis.
3. Results and discussion
2. Experimental
3.2. Open circuit potential
Alloys with nominal composition of Nd32.5FebalAl0.3Cu0.1B1.0(wt.%) were prepared by strip melting Nd, Fe, Al, Cu, FeeB in a vacuum atmosphere. The obtained strips were pulverized and further milled into ~5 mm powder using hydrogen decrepitation and jet milling, respectively. These powders were pressed in a magnetic field of 1.8 T under a pressure of 8.0 MPa. Afterward, the green compacts were sintered at 1070 C for 3 h in vacuum, cooled by Ar quenching. The as-sintered magnets were annealed at 900 C for 3 h, first postsintering annealing, and then annealed again at 520 C for 2 h, second postsintering annealing. In the two-step annealing treatments, the cooling rate was 90 C/min. These annealed magnets were mechanically cut into cubic specimens that were embedded in epoxy resin with one side exposed 10 10 mm2 as working surface. Prior to each experiment, this working surface was ground with a series of SiC papers up to 1200 grit, then polished with 1.0 mm diamond suspensions. Specimens were immersed first in acetone, then in deionized water and ultrasonicated for 10 min to remove any attached polishing residue and finally dried in cool air for use. In tests, 0.7%HNO3-x%HF (x ¼ 0, 0.224, 0.672, 1.344, 2.016) mixture acid solutions were prepared using distilled water and their pH values were 2.96, 3.68, 3.78, 3.76, 3.72, respectively which were measured by an acidometer. Before electrochemical measurements, these samples were immersed in 250 ml test solutions about 30 min to keep the open circuit potential (OCP) stable. Electrochemical measurements were conducted at 25 ± 0.2 C using a CHI660D electrochemical workstation and performed by a three electrode system consisting of NdFeB working electrode, Pt counter electrode and saturated calomel reference electrode (SCE). The glass shell of SCE was covered with plastic film to avoid its corrosion by HF acid. Polarization curves were recorded at a scan rate of 2.0 mV/s. Electrochemical impedance spectroscopy (EIS) measurements were conducted at the OCP with amplitude of 10 mV in the frequency range from 10 kHz to 10 mHz. Spectra analyses were performed using ZView software. All electrochemical tests were performed three times to ensure the reproducibility of the results. Microstructure of the samples was examined by scanning electron microscopy (SEM) equipped with energy dispersive spectrometer (EDS). Constituent analyses of corrosion products were investigated by X-ray diffractometry (XRD) using CuKa radiation. And the concentration of metal elements dissolved in the corrosion solution was determined using an OPTIME 2000DV-type inductively coupled plasma (ICP) atomic emission spectrometry.
Fig. 2 shows the evolution of the open circuit potential (OCP) for NdFeB magnet in HF-free and -containing solutions. Each measurement started immediately after the immersion of magnet in the test solutions and reflected its corrosion behavior. In the HFcontaining solutions the OCP smoothly moved towards more positive values and stabilized to the steady state corrosion potential (Ef) after immersion about 1500, 1200, 900, 450 s in the corrosion solutions with 0.224, 0.672, 1.344, 2.016% HF, respectively. In the HFfree solution, during the first about 70 s of immersion the OCP shifted towards more negative values then changed positively to achieve the Ef, about 1500 s after immersion. It was notable that the Ef, at which the magnet surface was corroding, reveals the dynamic balance between the advance of corrosion and the formation of the corrosion product films. And their values are shown in Fig. 3, from which it can be seen that in the HF-containing solutions the Ef moved positive direction with the increase in HF content and had more positive values than it in HF-free solution. Moreover, in the later solution the development of corrosion potential had some fluctuation at the whole immersion time, which may be due to the obvious morphologh changes of NdFeB surface. Results indicate that the additions of HF inhibited the dissolution of Nd-rich GBs phase of NdFeB magnet in nitric acid solution. Aiming to address it, the element concentration of Nd and Fe that dissolved in the test solutions during the OCP measurements of NdFeB magnet were measured using ICP technique and its results are shown in Fig. 4. This figure demonstrates the changes of Nd/Fe element
3.1. Microstructure of the prepared magnet Fig. 1 presents the SEM back-scattered micrograph of the prepared NdFeB sample. The image showed typical microstructure features, i.e. the gray and bright contrasts corresponded to Nd2Fe14B matrix phase and Nd-rich phase, respectively [25,26]. The Nd-rich phase existed in the grain boundary regions of magnet. The GBs and matrix phases compromised galvanic cell. In the multiphase structure the Nd-rich GBs phase exhibited more negative potential than the Nd2Fe14B phase [22] and acted as anode against the later during the electrochemical corrosion. Generally, the aforementioned microstructure features leaded to the serious corrosion of Nd-rich phase in HNO3 solution. However such phenomena would be obviously weakened by the additions of HF to a nitric acidic electrolyte, as described in the following sections.
Fig. 1. SEM back-scattered images of the prepared magnets.
Please cite this article in press as: J. Ni, et al., Corrosion behavior of NdFeB sintered magnets in HNO3eHF acid mixture solution, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.023
J. Ni et al. / Materials Chemistry and Physics xxx (2015) 1e7
Fig. 2. Evolution of the open circuit potential for the NdFeB magnet as a function of HF content.
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further additions of HF, reaching up to 1.385 mg/l at HF ¼ 2.016%. Such facts showed the preferential dissolution of Nd-rich GBs phase in HF-free solution than in HF-containing solutions. That is to say, HF exhibited an inhibition effect on the Nd-rich phase corrosion. Hence the ICP data well agreed with the above OCP results. Furthermore, the increase in Nd element concentration with the enhanced HF content mainly arises from the dissolved Nd2Fe14B matrix phase, as indicated by the rapid increase in Fe element concentration with larger HF content (Fig. 4b). Above positive shift of potential with the increase of the immersion time can be attributed to the adhesion of corrosion products on the Nd-Fe-B magnet. The adhesive corrosion products contained the neodymium fluorides (NdF3), as proved by the XRD results in Fig. 5. This figure described the XRD patterns of corrosion products on the magnet surface after immersion in the solution with 2.016% HF. Generally, the adhesive corrosion products on the alloy surface can effectively seal the active anodic sites, i.e. Nd-rich GBs regions against further reaction and accordingly shift the OCP towards positive value. Also that facts, coupled with the increased amount of NdF3 by adding more HF, account for the positive shift of Ef with the enhancement of HF content. However, in the case of HFfree solution the magnet surface was hard to be effectively inhibited by the corrosion products without NdF3. Thus in HF-free solution the Nd element concentration has much higher values and the OCP is more negative compared to the case of HF-containing solutions.
3.3. Potentiodyamic polarization curves and corrosion morphology
Fig. 3. Dependence of steady corrosion potential (Ef) after 30 min immersion on the HF-free and -containing solutions.
concentration as a function of HF content. Observations of Fig. 4a allow to see that as 0.224% HF was added to HNO3 solution, the Nd element concentration sharply decreased from 2.308 mg/l for the HF-free solution to 0.332 mg/l and it sequently increased with
Fig. 4. Concentration of Nd element (a) and Fe element (b) in the test solutions after the free corrosion of magnet for 30 min.
Alternatively, the electrochemical corrosion of NdFeB magnet was assessed by the potentiodynamic polarization curves. It was after 30 min immersion in the HF-free and HF-containing HNO3 solutions that the OCP of magnet were stable, then the measurements were carried out. The obtained polarization curves are shown in Fig. 6a. From these curves the corrosion potential (Ecorr) and corrosion current density (icorr) of magnet can be determined by the Tafel extrapolation method [27] and their results are illustrated in Fig. 6b. Evidently, the polarization curves had a similar shape and the beginning of anode side of polarization curves was dominated with active reaction for the NdFeB magnet in all the test solutions. It indicates that the additions of HF hardly changed the corrosion mechanism of the NdFeB magnet in the test solutions. But HF had remarkable effects on the polarization behavior of magnet.
Fig. 5. XRD patterns of corrosion products for the magnet in 2.016% HF containing solution after 48 h immersion.
Please cite this article in press as: J. Ni, et al., Corrosion behavior of NdFeB sintered magnets in HNO3eHF acid mixture solution, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.023
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Fig. 6. Potentiodynamic polarization curves of the magnet in the test solutions (a), and changes of corrosion potential and corrosion current density as a function of HF content (b).
In the HF-containing solutions the magnet exhibited more noble Ecorr, which increased from 0.453 V for HF ¼ 0.224% to 0.352 V for HF ¼ 2.016%, and had much lower icorr compared to it in the HFfree solution. Among the electrochemical parameters, Ecorr and Ef had a similar changing tendency, i.e. their values increased with HF content. Moreover, the Ecorr values were somewhat different from the Ef values (Fig. 3). It is probably due to partial removal of the adhesive corrosion products from the magnet surface as polarization scanning was started at more negative potential relative Ef. icorr decreased with the additions of HF and reached the minimum of 0.209 mA/cm2 at HF ¼ 2.016%, where its value had been decreased by one order of magnitude in comparison with that in HF-free solution. At potentials a little more positive than Ecorr, the presence of HF in the test solutions moved the anodic curves towards smaller current density. Greater content of HF decreased the anodic active dissolution before HF 1.344%, but it accelerated the cathodic process of magnet. Especially when the HF content was 2.016%, the cathodic curve was much upper than that in the HF-free solution, suggesting the promotion of cathodic reaction by the additions of hydrofluoric acid. In summary, the additions of HF indeed affected the cathodic and anodic process in the corrosion of NdFeB magnet during the polarization scan and the related effects mainly depended on its content. For the polarization corrosion behavior of NdFeB magnet in an acidic solution, the cathodic process was dominated by hydrogen evolution (2H þ þ 2e/H2 ), whose reaction rate was generally enhanced with the increase in Hþ concentration. The Hþ ions arose
from the ionization of HNO3/HF and its concentration was related to the HF content. In the mixed acid solutions, the HF acid ionized only in a part degree because it is a weak acid, leading to increase the Hþ concentration. Also, the Hþ ions were likely to combine with HF molecular, thus decreasing the Hþ concentration. The two factors affected the Hþ concentration and the pH values of the test solutions. And the possible reason for higher pH values (3.68 pH 3.78) of these HNO3eHF solutions compared with the HF-free solution is that the later factor overshadowed the effect of partial ionization of HF on the Hþ concentration. However after 30 min of immersion in the mixed acid solutions, the Hþ concentration might be enhanced with the combination of F and Nd3þ/ Fe3þ, because the decrease in the amount of F promoted the ionization of HF acid. Such phenomena might be omitted by small additions of HF acid, accounting for the similar cathodic corrosion density at the scan range of 0.85 ~ 0.6 V for the magnet in the 0.224 and 0.672% HF containing solutions as it in HF-free solution. As higher amounts of HF was added, before polarization curves measurement the concentration of Hþ ions was pronouncedly enhanced in the test solutions. Hence for the magnet in the 1.344 and 2.016% HF containing solutions, especially in the later solution, it possessed higher cathodic corrosion density than the one in the other case. On the other hand, during the anodic process the Me-containing phases (Me ¼ Nd, Fe) were dissolved in the test solutions, forming Me3þ (Me/Me3þ þ 3e). At the same time, the Me3þ ions reacted with F ions in HF-containing solutions and formed MeF3 in addition to the soluble MeNO3. Among these corrosion products, the insoluble NdF3 can be adhered to the magnet surface, as proved by the EDX mapping of magnet surface after anodic corrosion in the solution added with 2.016% HF (Fig. 7). From the Fig. 7 ones can find the uniform distribution of F and Nd elements on the magnet surface. But the Fe element has never been detected by the EDX surface scanning, indicating that the FeF3 corrosion products were dissolved in the aqueous test solutions. Moreover, the amount of MeF3 increased with the HF content and their formation might be influenced by corrosion potential, which needed further research. As described above, the EDS mapping results are good accordance with the obtained XRD data in the Fig. 5. These results showed that the NdF3 corrosion products could be formed on the magnet surface not only in free corrosion but also in polarization corrosion process. And the inhibition effects of NdF3 on the Nd-rich GBs corrosion was responsible for the more positive Ecorr, lower icorr and anodic corrosion density of the magnet in HF-containing solutions compared with the case of HF-free solution. Considering the similar analysis, it is easy to understand the above effects of HF content on the anodic corrosion behavior of magnet, since higher HF content makes the NdF3 adhesive corrosion products become denser. Fig. 8aec presents the SEM images of the NdFeB magnet after potentiodynamic polarization scan in the solutions without and with 0.224, 1.344% HF, respectively. For the magnet in HF-free solution, its Nd-rich phase was dissolved and sequently exposed to the test electrolytes, ultimately leading to some desquamation of Nd2Fe14B matrix phase grains from the sample surface. In the case of the 0.224% HF containing solution, however, almost no Nd2Fe14B matrix phase was desquamated, besides the Nd-rich phase corrosion at local regions. Difference in the corrosion morphology is related to the formation of NdF3 corrosion products on the magnet surface in HF-containing solutions. As the HF content was up to 1.344%, the shallow attack of the whole magnet surface occurred. It partly resulted from stronger dissolution of Nd2Fe14B matrix phase grains by greater amounts of HF. Moreover, denser NdF3 film benefited to inhibit the Nd-rich GBs corrosion, which was also responsible for the more uniform NdFeB surface corrosion in the case of 1.344% HF than other cases.
Please cite this article in press as: J. Ni, et al., Corrosion behavior of NdFeB sintered magnets in HNO3eHF acid mixture solution, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.023
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Fig. 8. SEM micrographs for the NdFeB magnet after anodic polarization in HF-free (a), 0.224% HF-containing (b), and 1.344% HF-containing solutions (c), respectively.
Fig. 7. SEM micrograph for the NdFeB magnet after anodic polarization in 2.016% HFcontaining solution (a), EDX mapping of F element (b) and Nd element (c).
3.4. Electrochemical impedance spectroscopy Electrochemical impedance spectroscopy (EIS) provides more information than other electrochemical techniques about the electrochemical kinetic behavior and is a powerful tool in investigating corrosion processes that occurred at the magnet surface [28,29]. Thus EIS at OCP were tested and the obtained Nyquist/Bode data are shown in Figs. 9 and 10, respectively. Some of features were common to all the Nyquist plots, in which there were two capacitive loops from high to medium frequencies and one inductance loop at low frequencies. In these plots, the capacitive loops were typically related to reaction in corrosion product films and reaction in electric double layer between electrode and solution interface, respectively, one for high frequencies and the other for
intermediate frequencies. The high frequencies loop reflected the charge transfer processes, and the intermediate frequencies loop was characterized by mass transfer through the corrosion product layer. The presence of inductive loop revealed the occurrence of the processes of adsorption and desorption for reaction intermediate in electrochemical corrosion. It thus can be said that a series of complicated corrosion processes happened to the surface of NdFeB magnet in the test solutions. Also a clear difference of the Nyquist curves was observed, in that the magnet that was immersed in HFcontaining solutions displayed larger loop radius than in HF-free solution. The radius of the captive loops increased with the enhanced HF content, meaning that the NdF3 corrosion products on the magnet surface became more compact. This benefited the resistance of Nd-rich GBs against the acidic solutions, since the corrosion resistance of any materials was determined by the combined diameters of both capacitive loops [30]. Furthermore, the impedance spectra on the Bode format (Fig. 10a) showed capacitive
Please cite this article in press as: J. Ni, et al., Corrosion behavior of NdFeB sintered magnets in HNO3eHF acid mixture solution, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.023
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Fig. 9. Nyquist plots of the magnet in HF-free and HF-containing solutions.
formation of more NdF3 products on the magnet surface. Meanwhile, ones observed that no matter HF was added or not, all the curves possessed similar shapes, which indicate the similar mechanisms of reaction between the solution and NdFeB alloys interface in the test solutions. Aiming to evaluate the EIS parameters and accordingly to characterize the corrosion process, an appropriate model of the impedance was developed to fit the test data and its equivalent electrical circuit is shown in Fig. 11a. Fig. 11b and c respectively provide the measured and simulated data of Nyquist/Bode plots for the magnet in the 2.016% HF containing solution. Clearly, one can observe a good agreement between the measured data and the simulated ones. This indicates that the proposed circuit Rs(CPEfRf) (CPEdl[Rt(LRL)]) can be used to discuss the corrosion mechanisms of NdFeB magnet in the test solutions. Here, Rs referred to the solution resistance between the working electrode and the reference electrode. CPEf was constant phase element that is mainly relevant to the characteristics of corrosion product films. Rf denoted the resistance of corrosion product films. Such an electric element, along with CPEf, corresponded to the first high frequency captive
Fig. 10. Bode plots of the magnet in HF-free and HF-containing solutions.
contribution at high/medium frequencies and inductive contribution at low frequencies. The phase angle (Fig. 10b) as a sensitive parameter indicating the presence of additional time constants in the impedance spectra, exhibited the phase lags at the high/medium frequency range. Generally, the phase lags got higher and broader with the increase in the HF content, reflecting the
Fig. 11. Equivalent circuits of impedance plots in the Fig. 9 and 10 (a), comparison between the measured and simulated data for (b) Nyquist and (c) Bode plots of the magnet in 2.016% HF-containing solution.
Please cite this article in press as: J. Ni, et al., Corrosion behavior of NdFeB sintered magnets in HNO3eHF acid mixture solution, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.023
J. Ni et al. / Materials Chemistry and Physics xxx (2015) 1e7 Table 1 Rf, Rt and RL of the proposed equivalent circuit models. HF content (%)
Rf (U$cm2)
Rt (U$cm2)
RL (U$cm2)
0 0.224 0.672 1.344 2.016
3.27 16.38 25.78 32.91 58.64
1.45 5.36 9.48 20.04 25.67
2.59 7.95 11.38 30.62 51.94
7
adhesive corrosion products on magnet surface. Such products can increase the charge transfer resistance across the magneteelectrolyte interface and reduce the formation rate of active reaction channels in Nd-rich phase regions. As a result, the Nd-rich GBs dissolution was effectively inhibited and the icorr values were decrease by one order of magnitude as high HF content was added to the nitric acid solution. Acknowledgments
loop in the Fig. 9. Rt and CPEdl described the second captive loop, respectively representing the charge transfer resistance meaning the resistance of electron transfer in the electric double layer and constant phase element associated with the magnet surface reactivity, roughness, porosity and current/potential distributions associated with the electrode geometry. RL was the inductive resistance and L was the pseudo-inductance. Among the electric elements, the Rs value is too small comparing to Rf/Rt/RL and can be neglected. Rf, Rt and RL are key parameters and their fitted values are listed in Table 1. Obviously, all the values of Rf and Rt was less than 60 U cm2. That fact, coupled with the occurrence of inductance arc, revealed that there were many active reaction channels in the anodic areas (Nd-rich phase) on the magnet surface. Moreover, in HF-containing solutions the magnet possessed much higher Rf, Rt and RL compared with it in HF-free solution. And their values increased with the HF content, which were related to the distribution of adhesive corrosion products and the amount of active reaction channels in the GBs regions of magnet. These factors in turn depended on the different amount of the Hþ and F ions in the test solutions. In HF-containing solutions, the adhesive NdF3 products can spread on the whole magnet surface, which increased the resistance of charge transfer across the interface of electrolyte and NdFeB substrate. Also the corrosion products increased the formation resistance of anodic active reaction channels in the GBs regions of magnet. In HF-free solution, however, only minor amounts of Me(OH)ads intermediate substances were absorbed on the NdFeB magnet surface. As a result, in the HF-containing solutions the magnet had higher Rf, Rt and RL than it in HF-free solution. Moreover, more NdF3 adhesive corrosion products increased the Rf and Rt values with enhancing the HF content. It is responsible for the decreasing tendency of icorr, due to the inversely proportional relations between electric resistance and corrosion current density. 4. Conclusions Good agreements were observed between the results obtained from open circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy measurements. Additions of HF increased the open circuit potential, the corrosion potential and the film/inductive/charge transfer resistance of magnet in nitric acid solution, but pronouncedly decreased the corrosion current density. The corresponding effect mechanism of HF on the corrosion performance was related to the formation of NdF3
This work was supported by the Nature Science Foundation of Shandong Province of China (Grant no. ZR2013EMM010), Project funded by China Postdoctoral Science Foundation (Grant no. 2014M551899), Scientific and Technological Projects of Shandong Province of China (Grant no. 2014GGX102016). References [1] [2] [3] [4] [5] [6] [7]
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Please cite this article in press as: J. Ni, et al., Corrosion behavior of NdFeB sintered magnets in HNO3eHF acid mixture solution, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.023
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Corrosion behavior of NdFeB sintered magnets in HNO3eHF acid mixture solution Junjie Ni a, b, *, Xin Cui b, Xiangfa Liu a, Hui Cui c, ZeChun Xue c, Zhengfeng Jia b, Changzheng Wang b, Jie Ma b, Shuwen Gong c a b c
School of Material Science and Engineering, Shandong University, Jinan 250061, China School of Material Science and Engineering, Institute of Non-ferrous Metal, Liaocheng University, Liaocheng 252059, China School of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng 252059, China
h i g h l i g h t s HF affected electrochemical corrosion behavior of NdFeB in HNO3eHF solution. HF effects on corrosion performance depended on its content. HF effects related to the formation of NdF3 corrosion products on magnet surface.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 September 2014 Received in revised form 26 May 2015 Accepted 8 June 2015 Available online xxx
Corrosion behavior of NdFeB magnet in HNO3eHF solutions was investigated using open circuit potential and polarization measurements and impedance technique. Results showed that the additions of HF increased open circuit potential, corrosion potential and film/inductive/charge transfer resistance, but decreased corrosion current density of magnet. The HF effects on the corrosion performance were closely related to the formation of NdF3 corrosion products on magnet surface, in turn which depended on its content. Also the corrosion mechanism of magnet was discussed on the basis of structural observation, composition and EIS analyses. © 2015 Published by Elsevier B.V.
Keywords: Alloys Electrochemical techniques Corrosion Microstructure
1. Introduction NdFeB sintered magnets have excellent permanent magnetic performance and high property/price ratio for a number of commercial applications such as industrial motor, automobile, dental appliances and so on [1e3]. However a critical limitation for these service applications is the susceptibility of NdFeB alloys to corrosion and hence there continues to be much research to improve their anticorrosion properties [4e9]. Up to now, the methods to increase the corrosion resistance basically include the alloying additions of Al, Ti, Cu, Cr, etc. to the NdFeB magnets [10e13] and the application of protective coating [14e16]. Of two approaches the former one has been found to improve the corrosion resistance of
* Corresponding author. School of Material Science and Engineering, Shandong University, Jinan 250061, China. E-mail address: [email protected] (J. Ni).
magnets only in a certain degree and possibly to deteriorate the magnetic properties. Fortunately, these drawbacks do not occur in the second method. That facts have spurred ones to apply protective coatings, like electroplating Ni, Al, Co [17e20] and electroless NieP plating [21], in the NdFeB industry production, although this method unavoidably increases the fabrication cost. Before plating, acid pickling process is necessarily performed to remove rust, grease and oxide films on the magnet surface, which is a precondition for the formation of protective coating having strong adhesion force with NdFeB material. Therefore, the acid picking is an important magnet surface pretreatment. During acid picking the sever grain boundaries (GBs) corrosion happens on the NdFeB sintered magnets, but it is detrimental to the adhesion force of plating coats. The GBs corrosion derives from the multiphase structure in NdFeB sintered magnets and much lower corrosion potential for the Nd-rich phase than the Nd2Fe14B matrix phase [22,23]. It thus is essential to inhibit the Nd-rich GBs
http://dx.doi.org/10.1016/j.matchemphys.2015.06.023 0254-0584/© 2015 Published by Elsevier B.V.
Please cite this article in press as: J. Ni, et al., Corrosion behavior of NdFeB sintered magnets in HNO3eHF acid mixture solution, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.023
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corrosion, finally obtaining a uniformly corroded magnet surface. In other words, the corrosion of the Nd2Fe14B matrix and Nd-rich GBs phases must be synchronous. Such requirements may be satisfied by the additions of hydrofluoric acid to a commonly used nitric acid solution, partly because the matrix phase has high Fe content and the dissolution of Fe containing phase is very sensitive to HF [24]. Moreover, the formed fluoride neodymium and iron complexes may be a corrosion inhibitor of Nd-rich GBs phase in acid pickling. So the HNO3eHF mixture solution should be a promising candidate for the NdFeB acid pickling, in which the HF content is a key factor. Nevertheless, little is known about the relation of corrosion feature for NdFeB magnets with the HF content in the HNO3eHF mixed acid solutions. Considering the aforementioned aspects, the present study aims at determining the effects of HF content on the corrosion behavior of NdFeB magnets. Also in this work the corrosion mechanism of magnet in the HF-free and -containing acid solutions was discussed on the basis of electrochemical measurements, structural observation and composition analysis.
3. Results and discussion
2. Experimental
3.2. Open circuit potential
Alloys with nominal composition of Nd32.5FebalAl0.3Cu0.1B1.0(wt.%) were prepared by strip melting Nd, Fe, Al, Cu, FeeB in a vacuum atmosphere. The obtained strips were pulverized and further milled into ~5 mm powder using hydrogen decrepitation and jet milling, respectively. These powders were pressed in a magnetic field of 1.8 T under a pressure of 8.0 MPa. Afterward, the green compacts were sintered at 1070 C for 3 h in vacuum, cooled by Ar quenching. The as-sintered magnets were annealed at 900 C for 3 h, first postsintering annealing, and then annealed again at 520 C for 2 h, second postsintering annealing. In the two-step annealing treatments, the cooling rate was 90 C/min. These annealed magnets were mechanically cut into cubic specimens that were embedded in epoxy resin with one side exposed 10 10 mm2 as working surface. Prior to each experiment, this working surface was ground with a series of SiC papers up to 1200 grit, then polished with 1.0 mm diamond suspensions. Specimens were immersed first in acetone, then in deionized water and ultrasonicated for 10 min to remove any attached polishing residue and finally dried in cool air for use. In tests, 0.7%HNO3-x%HF (x ¼ 0, 0.224, 0.672, 1.344, 2.016) mixture acid solutions were prepared using distilled water and their pH values were 2.96, 3.68, 3.78, 3.76, 3.72, respectively which were measured by an acidometer. Before electrochemical measurements, these samples were immersed in 250 ml test solutions about 30 min to keep the open circuit potential (OCP) stable. Electrochemical measurements were conducted at 25 ± 0.2 C using a CHI660D electrochemical workstation and performed by a three electrode system consisting of NdFeB working electrode, Pt counter electrode and saturated calomel reference electrode (SCE). The glass shell of SCE was covered with plastic film to avoid its corrosion by HF acid. Polarization curves were recorded at a scan rate of 2.0 mV/s. Electrochemical impedance spectroscopy (EIS) measurements were conducted at the OCP with amplitude of 10 mV in the frequency range from 10 kHz to 10 mHz. Spectra analyses were performed using ZView software. All electrochemical tests were performed three times to ensure the reproducibility of the results. Microstructure of the samples was examined by scanning electron microscopy (SEM) equipped with energy dispersive spectrometer (EDS). Constituent analyses of corrosion products were investigated by X-ray diffractometry (XRD) using CuKa radiation. And the concentration of metal elements dissolved in the corrosion solution was determined using an OPTIME 2000DV-type inductively coupled plasma (ICP) atomic emission spectrometry.
Fig. 2 shows the evolution of the open circuit potential (OCP) for NdFeB magnet in HF-free and -containing solutions. Each measurement started immediately after the immersion of magnet in the test solutions and reflected its corrosion behavior. In the HFcontaining solutions the OCP smoothly moved towards more positive values and stabilized to the steady state corrosion potential (Ef) after immersion about 1500, 1200, 900, 450 s in the corrosion solutions with 0.224, 0.672, 1.344, 2.016% HF, respectively. In the HFfree solution, during the first about 70 s of immersion the OCP shifted towards more negative values then changed positively to achieve the Ef, about 1500 s after immersion. It was notable that the Ef, at which the magnet surface was corroding, reveals the dynamic balance between the advance of corrosion and the formation of the corrosion product films. And their values are shown in Fig. 3, from which it can be seen that in the HF-containing solutions the Ef moved positive direction with the increase in HF content and had more positive values than it in HF-free solution. Moreover, in the later solution the development of corrosion potential had some fluctuation at the whole immersion time, which may be due to the obvious morphologh changes of NdFeB surface. Results indicate that the additions of HF inhibited the dissolution of Nd-rich GBs phase of NdFeB magnet in nitric acid solution. Aiming to address it, the element concentration of Nd and Fe that dissolved in the test solutions during the OCP measurements of NdFeB magnet were measured using ICP technique and its results are shown in Fig. 4. This figure demonstrates the changes of Nd/Fe element
3.1. Microstructure of the prepared magnet Fig. 1 presents the SEM back-scattered micrograph of the prepared NdFeB sample. The image showed typical microstructure features, i.e. the gray and bright contrasts corresponded to Nd2Fe14B matrix phase and Nd-rich phase, respectively [25,26]. The Nd-rich phase existed in the grain boundary regions of magnet. The GBs and matrix phases compromised galvanic cell. In the multiphase structure the Nd-rich GBs phase exhibited more negative potential than the Nd2Fe14B phase [22] and acted as anode against the later during the electrochemical corrosion. Generally, the aforementioned microstructure features leaded to the serious corrosion of Nd-rich phase in HNO3 solution. However such phenomena would be obviously weakened by the additions of HF to a nitric acidic electrolyte, as described in the following sections.
Fig. 1. SEM back-scattered images of the prepared magnets.
Please cite this article in press as: J. Ni, et al., Corrosion behavior of NdFeB sintered magnets in HNO3eHF acid mixture solution, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.023
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Fig. 2. Evolution of the open circuit potential for the NdFeB magnet as a function of HF content.
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further additions of HF, reaching up to 1.385 mg/l at HF ¼ 2.016%. Such facts showed the preferential dissolution of Nd-rich GBs phase in HF-free solution than in HF-containing solutions. That is to say, HF exhibited an inhibition effect on the Nd-rich phase corrosion. Hence the ICP data well agreed with the above OCP results. Furthermore, the increase in Nd element concentration with the enhanced HF content mainly arises from the dissolved Nd2Fe14B matrix phase, as indicated by the rapid increase in Fe element concentration with larger HF content (Fig. 4b). Above positive shift of potential with the increase of the immersion time can be attributed to the adhesion of corrosion products on the Nd-Fe-B magnet. The adhesive corrosion products contained the neodymium fluorides (NdF3), as proved by the XRD results in Fig. 5. This figure described the XRD patterns of corrosion products on the magnet surface after immersion in the solution with 2.016% HF. Generally, the adhesive corrosion products on the alloy surface can effectively seal the active anodic sites, i.e. Nd-rich GBs regions against further reaction and accordingly shift the OCP towards positive value. Also that facts, coupled with the increased amount of NdF3 by adding more HF, account for the positive shift of Ef with the enhancement of HF content. However, in the case of HFfree solution the magnet surface was hard to be effectively inhibited by the corrosion products without NdF3. Thus in HF-free solution the Nd element concentration has much higher values and the OCP is more negative compared to the case of HF-containing solutions.
3.3. Potentiodyamic polarization curves and corrosion morphology
Fig. 3. Dependence of steady corrosion potential (Ef) after 30 min immersion on the HF-free and -containing solutions.
concentration as a function of HF content. Observations of Fig. 4a allow to see that as 0.224% HF was added to HNO3 solution, the Nd element concentration sharply decreased from 2.308 mg/l for the HF-free solution to 0.332 mg/l and it sequently increased with
Fig. 4. Concentration of Nd element (a) and Fe element (b) in the test solutions after the free corrosion of magnet for 30 min.
Alternatively, the electrochemical corrosion of NdFeB magnet was assessed by the potentiodynamic polarization curves. It was after 30 min immersion in the HF-free and HF-containing HNO3 solutions that the OCP of magnet were stable, then the measurements were carried out. The obtained polarization curves are shown in Fig. 6a. From these curves the corrosion potential (Ecorr) and corrosion current density (icorr) of magnet can be determined by the Tafel extrapolation method [27] and their results are illustrated in Fig. 6b. Evidently, the polarization curves had a similar shape and the beginning of anode side of polarization curves was dominated with active reaction for the NdFeB magnet in all the test solutions. It indicates that the additions of HF hardly changed the corrosion mechanism of the NdFeB magnet in the test solutions. But HF had remarkable effects on the polarization behavior of magnet.
Fig. 5. XRD patterns of corrosion products for the magnet in 2.016% HF containing solution after 48 h immersion.
Please cite this article in press as: J. Ni, et al., Corrosion behavior of NdFeB sintered magnets in HNO3eHF acid mixture solution, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.023
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Fig. 6. Potentiodynamic polarization curves of the magnet in the test solutions (a), and changes of corrosion potential and corrosion current density as a function of HF content (b).
In the HF-containing solutions the magnet exhibited more noble Ecorr, which increased from 0.453 V for HF ¼ 0.224% to 0.352 V for HF ¼ 2.016%, and had much lower icorr compared to it in the HFfree solution. Among the electrochemical parameters, Ecorr and Ef had a similar changing tendency, i.e. their values increased with HF content. Moreover, the Ecorr values were somewhat different from the Ef values (Fig. 3). It is probably due to partial removal of the adhesive corrosion products from the magnet surface as polarization scanning was started at more negative potential relative Ef. icorr decreased with the additions of HF and reached the minimum of 0.209 mA/cm2 at HF ¼ 2.016%, where its value had been decreased by one order of magnitude in comparison with that in HF-free solution. At potentials a little more positive than Ecorr, the presence of HF in the test solutions moved the anodic curves towards smaller current density. Greater content of HF decreased the anodic active dissolution before HF 1.344%, but it accelerated the cathodic process of magnet. Especially when the HF content was 2.016%, the cathodic curve was much upper than that in the HF-free solution, suggesting the promotion of cathodic reaction by the additions of hydrofluoric acid. In summary, the additions of HF indeed affected the cathodic and anodic process in the corrosion of NdFeB magnet during the polarization scan and the related effects mainly depended on its content. For the polarization corrosion behavior of NdFeB magnet in an acidic solution, the cathodic process was dominated by hydrogen evolution (2H þ þ 2e/H2 ), whose reaction rate was generally enhanced with the increase in Hþ concentration. The Hþ ions arose
from the ionization of HNO3/HF and its concentration was related to the HF content. In the mixed acid solutions, the HF acid ionized only in a part degree because it is a weak acid, leading to increase the Hþ concentration. Also, the Hþ ions were likely to combine with HF molecular, thus decreasing the Hþ concentration. The two factors affected the Hþ concentration and the pH values of the test solutions. And the possible reason for higher pH values (3.68 pH 3.78) of these HNO3eHF solutions compared with the HF-free solution is that the later factor overshadowed the effect of partial ionization of HF on the Hþ concentration. However after 30 min of immersion in the mixed acid solutions, the Hþ concentration might be enhanced with the combination of F and Nd3þ/ Fe3þ, because the decrease in the amount of F promoted the ionization of HF acid. Such phenomena might be omitted by small additions of HF acid, accounting for the similar cathodic corrosion density at the scan range of 0.85 ~ 0.6 V for the magnet in the 0.224 and 0.672% HF containing solutions as it in HF-free solution. As higher amounts of HF was added, before polarization curves measurement the concentration of Hþ ions was pronouncedly enhanced in the test solutions. Hence for the magnet in the 1.344 and 2.016% HF containing solutions, especially in the later solution, it possessed higher cathodic corrosion density than the one in the other case. On the other hand, during the anodic process the Me-containing phases (Me ¼ Nd, Fe) were dissolved in the test solutions, forming Me3þ (Me/Me3þ þ 3e). At the same time, the Me3þ ions reacted with F ions in HF-containing solutions and formed MeF3 in addition to the soluble MeNO3. Among these corrosion products, the insoluble NdF3 can be adhered to the magnet surface, as proved by the EDX mapping of magnet surface after anodic corrosion in the solution added with 2.016% HF (Fig. 7). From the Fig. 7 ones can find the uniform distribution of F and Nd elements on the magnet surface. But the Fe element has never been detected by the EDX surface scanning, indicating that the FeF3 corrosion products were dissolved in the aqueous test solutions. Moreover, the amount of MeF3 increased with the HF content and their formation might be influenced by corrosion potential, which needed further research. As described above, the EDS mapping results are good accordance with the obtained XRD data in the Fig. 5. These results showed that the NdF3 corrosion products could be formed on the magnet surface not only in free corrosion but also in polarization corrosion process. And the inhibition effects of NdF3 on the Nd-rich GBs corrosion was responsible for the more positive Ecorr, lower icorr and anodic corrosion density of the magnet in HF-containing solutions compared with the case of HF-free solution. Considering the similar analysis, it is easy to understand the above effects of HF content on the anodic corrosion behavior of magnet, since higher HF content makes the NdF3 adhesive corrosion products become denser. Fig. 8aec presents the SEM images of the NdFeB magnet after potentiodynamic polarization scan in the solutions without and with 0.224, 1.344% HF, respectively. For the magnet in HF-free solution, its Nd-rich phase was dissolved and sequently exposed to the test electrolytes, ultimately leading to some desquamation of Nd2Fe14B matrix phase grains from the sample surface. In the case of the 0.224% HF containing solution, however, almost no Nd2Fe14B matrix phase was desquamated, besides the Nd-rich phase corrosion at local regions. Difference in the corrosion morphology is related to the formation of NdF3 corrosion products on the magnet surface in HF-containing solutions. As the HF content was up to 1.344%, the shallow attack of the whole magnet surface occurred. It partly resulted from stronger dissolution of Nd2Fe14B matrix phase grains by greater amounts of HF. Moreover, denser NdF3 film benefited to inhibit the Nd-rich GBs corrosion, which was also responsible for the more uniform NdFeB surface corrosion in the case of 1.344% HF than other cases.
Please cite this article in press as: J. Ni, et al., Corrosion behavior of NdFeB sintered magnets in HNO3eHF acid mixture solution, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.023
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Fig. 8. SEM micrographs for the NdFeB magnet after anodic polarization in HF-free (a), 0.224% HF-containing (b), and 1.344% HF-containing solutions (c), respectively.
Fig. 7. SEM micrograph for the NdFeB magnet after anodic polarization in 2.016% HFcontaining solution (a), EDX mapping of F element (b) and Nd element (c).
3.4. Electrochemical impedance spectroscopy Electrochemical impedance spectroscopy (EIS) provides more information than other electrochemical techniques about the electrochemical kinetic behavior and is a powerful tool in investigating corrosion processes that occurred at the magnet surface [28,29]. Thus EIS at OCP were tested and the obtained Nyquist/Bode data are shown in Figs. 9 and 10, respectively. Some of features were common to all the Nyquist plots, in which there were two capacitive loops from high to medium frequencies and one inductance loop at low frequencies. In these plots, the capacitive loops were typically related to reaction in corrosion product films and reaction in electric double layer between electrode and solution interface, respectively, one for high frequencies and the other for
intermediate frequencies. The high frequencies loop reflected the charge transfer processes, and the intermediate frequencies loop was characterized by mass transfer through the corrosion product layer. The presence of inductive loop revealed the occurrence of the processes of adsorption and desorption for reaction intermediate in electrochemical corrosion. It thus can be said that a series of complicated corrosion processes happened to the surface of NdFeB magnet in the test solutions. Also a clear difference of the Nyquist curves was observed, in that the magnet that was immersed in HFcontaining solutions displayed larger loop radius than in HF-free solution. The radius of the captive loops increased with the enhanced HF content, meaning that the NdF3 corrosion products on the magnet surface became more compact. This benefited the resistance of Nd-rich GBs against the acidic solutions, since the corrosion resistance of any materials was determined by the combined diameters of both capacitive loops [30]. Furthermore, the impedance spectra on the Bode format (Fig. 10a) showed capacitive
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Fig. 9. Nyquist plots of the magnet in HF-free and HF-containing solutions.
formation of more NdF3 products on the magnet surface. Meanwhile, ones observed that no matter HF was added or not, all the curves possessed similar shapes, which indicate the similar mechanisms of reaction between the solution and NdFeB alloys interface in the test solutions. Aiming to evaluate the EIS parameters and accordingly to characterize the corrosion process, an appropriate model of the impedance was developed to fit the test data and its equivalent electrical circuit is shown in Fig. 11a. Fig. 11b and c respectively provide the measured and simulated data of Nyquist/Bode plots for the magnet in the 2.016% HF containing solution. Clearly, one can observe a good agreement between the measured data and the simulated ones. This indicates that the proposed circuit Rs(CPEfRf) (CPEdl[Rt(LRL)]) can be used to discuss the corrosion mechanisms of NdFeB magnet in the test solutions. Here, Rs referred to the solution resistance between the working electrode and the reference electrode. CPEf was constant phase element that is mainly relevant to the characteristics of corrosion product films. Rf denoted the resistance of corrosion product films. Such an electric element, along with CPEf, corresponded to the first high frequency captive
Fig. 10. Bode plots of the magnet in HF-free and HF-containing solutions.
contribution at high/medium frequencies and inductive contribution at low frequencies. The phase angle (Fig. 10b) as a sensitive parameter indicating the presence of additional time constants in the impedance spectra, exhibited the phase lags at the high/medium frequency range. Generally, the phase lags got higher and broader with the increase in the HF content, reflecting the
Fig. 11. Equivalent circuits of impedance plots in the Fig. 9 and 10 (a), comparison between the measured and simulated data for (b) Nyquist and (c) Bode plots of the magnet in 2.016% HF-containing solution.
Please cite this article in press as: J. Ni, et al., Corrosion behavior of NdFeB sintered magnets in HNO3eHF acid mixture solution, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.023
J. Ni et al. / Materials Chemistry and Physics xxx (2015) 1e7 Table 1 Rf, Rt and RL of the proposed equivalent circuit models. HF content (%)
Rf (U$cm2)
Rt (U$cm2)
RL (U$cm2)
0 0.224 0.672 1.344 2.016
3.27 16.38 25.78 32.91 58.64
1.45 5.36 9.48 20.04 25.67
2.59 7.95 11.38 30.62 51.94
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adhesive corrosion products on magnet surface. Such products can increase the charge transfer resistance across the magneteelectrolyte interface and reduce the formation rate of active reaction channels in Nd-rich phase regions. As a result, the Nd-rich GBs dissolution was effectively inhibited and the icorr values were decrease by one order of magnitude as high HF content was added to the nitric acid solution. Acknowledgments
loop in the Fig. 9. Rt and CPEdl described the second captive loop, respectively representing the charge transfer resistance meaning the resistance of electron transfer in the electric double layer and constant phase element associated with the magnet surface reactivity, roughness, porosity and current/potential distributions associated with the electrode geometry. RL was the inductive resistance and L was the pseudo-inductance. Among the electric elements, the Rs value is too small comparing to Rf/Rt/RL and can be neglected. Rf, Rt and RL are key parameters and their fitted values are listed in Table 1. Obviously, all the values of Rf and Rt was less than 60 U cm2. That fact, coupled with the occurrence of inductance arc, revealed that there were many active reaction channels in the anodic areas (Nd-rich phase) on the magnet surface. Moreover, in HF-containing solutions the magnet possessed much higher Rf, Rt and RL compared with it in HF-free solution. And their values increased with the HF content, which were related to the distribution of adhesive corrosion products and the amount of active reaction channels in the GBs regions of magnet. These factors in turn depended on the different amount of the Hþ and F ions in the test solutions. In HF-containing solutions, the adhesive NdF3 products can spread on the whole magnet surface, which increased the resistance of charge transfer across the interface of electrolyte and NdFeB substrate. Also the corrosion products increased the formation resistance of anodic active reaction channels in the GBs regions of magnet. In HF-free solution, however, only minor amounts of Me(OH)ads intermediate substances were absorbed on the NdFeB magnet surface. As a result, in the HF-containing solutions the magnet had higher Rf, Rt and RL than it in HF-free solution. Moreover, more NdF3 adhesive corrosion products increased the Rf and Rt values with enhancing the HF content. It is responsible for the decreasing tendency of icorr, due to the inversely proportional relations between electric resistance and corrosion current density. 4. Conclusions Good agreements were observed between the results obtained from open circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy measurements. Additions of HF increased the open circuit potential, the corrosion potential and the film/inductive/charge transfer resistance of magnet in nitric acid solution, but pronouncedly decreased the corrosion current density. The corresponding effect mechanism of HF on the corrosion performance was related to the formation of NdF3
This work was supported by the Nature Science Foundation of Shandong Province of China (Grant no. ZR2013EMM010), Project funded by China Postdoctoral Science Foundation (Grant no. 2014M551899), Scientific and Technological Projects of Shandong Province of China (Grant no. 2014GGX102016). References [1] [2] [3] [4] [5] [6] [7]
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Please cite this article in press as: J. Ni, et al., Corrosion behavior of NdFeB sintered magnets in HNO3eHF acid mixture solution, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.023