- Email: [email protected]
Influence of the electroplating pretreatment on corrosion mechanism of NdFeB magnets
JOURNAL OF RARE EARTHS, Vol. 34, No. 2, Feb. 2016, P. 152
Influence of the electroplating pretreatment on corrosion mechanism of NdFeB magnets ZHOU Qiaoying (周巧英)1, LI Gang (李 刚)1, LIU Zhuang (刘 壮)1, GUO Shuai (郭 帅)1, YAN Aru (闫阿儒)1,*, LEE Don (李 东)1, LI Jianzhong (李建忠)2 (1. Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China; 2. Ningbo Permanent Magnetics Co., Ltd., Ningbo 315032, China) Received 14 July 2015; revised 3 November 2015
Abstract: The electroplating pretreatment of NdFeB magnets was simulated in this study, and the effect of the pickling process on element, matrix morphology and texture was discussed. The results showed that in the ordinary circumstance the acid solution concentration should be below 4 mol/L and the pickling time was within 120 s. However the concentration was lower than 1 mol/L and the time was about 30 s in the ultrasonic environment. The precoating technology sequentially was studied. The results revealed that there were two main reactions of the corrosion and displacement in the precoating process, and these two reactions were more obvious in the grain boundary. The electrochemical analysis illustrated that the corrosion and displacement reactions were affected obviously by the current density and the precoating solution temperature, usually the current density was controlled about 0.3 A/dm2, and the temperature was about 318 K in the precoating process. Keywords: NdFeB magnet; pickling; precoating; corrosion; pretreatment; rare earths
NdFeB magnets have attracted increasing attention in recent years due to their excellent magnetic properties. However, their poor corrosion resistance in climatic and corrosive environments hinders their further applications[1–6]. Theoretically NdFeB magnets are composed of three phases: the ferromagnetic matrix phase (Nd2Fe14B), which represents about 87% of the magnet volume and the most corrosion sensitive Nd-rich phase and the B-rich intergranular phase[7,8]. This means that NdFeB magnets are prone to galvanic corrosion, in particular, intergranular corrosion attack. Many investigations have been employed to improve the corrosion resistance of NdFeB magnets, such as the alloy additions and surface coatings. Compared with adding trace elements to improve the corrosion resistance of NdFeB itself[9–11], the surface coatings technology is still the most effective measures[12–16]. However, the electroplating pretreatment is very crucial in the protection process of NdFeB magnets. On the one hand, oils and harmful gases will be adsorbed in the pore on the magnet surface during the machining process, which result in poor adhesion between NdFeB magnets with the coatings. On the other hand, not only the magnetic original size will be seriously changed but the pore and selective corrosion will occur along the grain boundary if the substrate is excessively corrosed during the pretreatment
process, which will reduce the service life of NdFeB magnets. Therefore, taking reasonable pretreatment process to clean and activate the magnet surface is very necessary. In this paper, the electroplating pretreatment conditions of NdFeB magnets were simulated and the purpose was to obtain the more suitable electroplating parameter of NdFeB permanent magnets.
1 Experimental In this study the purity of 99.9% Nd, Fe, B were chosen as raw materials. NdFeB magnets were prepared by conventional powder metallurgy technique including cold isostatic pressing and 1050–1100 ºC sintering using jet milled alloy powder. The magnets used in further measurement were L10 mm×W10 mm×H2 mm, and were removed oil in 2 mol/L NaOH solution. Pickling process was conducted in the PVC cell which was divided into ten equal spaces with PP board, the samples were fixed between the adjacent plate. The pickling device is shown in Fig. 1((a) Front view; (b) Planform). Pickling solution is prepared with deionized water and analytical nitric acid, and the concentration was 0.5, 1, 2, 4, 5, 6, 8 and 10 mol/L respectively. During the pickling process the temperature inside the cell was about 293 K.
Foundation item: Project supported by Zhejiang Provincial Natural Science Foundation for Youth (LQ15E010004), Ningbo Natural Science Foundation (2014A610163), Project of Ningbo Innovative Research Team (2012B81001), Ningbo International Cooperation Project (2015D10019) * Corresponding author: YAN Aru (E-mail: [email protected]; Tel.: +86-574-87911129) DOI: 10.1016/S1002-0721(16)60008-X
ZHOU Qiaoying et al., Influence of the electroplating pretreatment on corrosion mechanism of NdFeB magnets
153
Fig. 1 Pickling device (a) Front view; (b) Planform
Precoating process was conducted in a conventional three-electrode device at about 293 K. A saturated calomel electrode Hg/Hg2SO4/SO42– (SCE) and a platinum foil (the purity of 99.99%, the specification is L30 mm×W30 mm×H2 mm) served as reference and counter electrode, respectively. NdFeB magnets were as the precoating cathode. The precoating solution was composited of NiSO4 280 g/L, NiCl2 50 g/L, H3BO3 50 g/L, SB-1H 0.8 mol/L, D-2 1.2 mol/L, the solution temperature was about 323 K, pH was 4.3 and the precoating time was from 10 to 30 min. The concentration of each element in the solution was analyzed with inductively coupled plasma-atomic emission spectrometry (ICP-MS) (type Optima2100). The surface hardness was characterized using a micro hardness tester (MHT) (type HV-1000), the morphology of magnets was observed using a field emission scanning electron microscope (SEM), the composition of the precoated was analyzed with an Oxford energy detective spectrometer (EDS) (type S-4800). The electrochemical behavior (electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization) during the precoating process was studied by electrochemical workstation (ECW) (type M273A).
2 Results and discussion 2.1 Pickling technology 2.1.1 Corrosion mechanism Two kinds of pickling process were performed respectively, one was the ordinary pickling (no ultrasonic) and the other was putting the pickling PVC cell into the ultrasonic generator. Fig. 2 shows the relations among Nd, Fe, B element concentration (C) in the solution, the pickling solution concentration (c) and the pickling time (t). It can be seen that (Fig. 2(a)) Nd, Fe, B element concentration is respectively less than 5 µg/mL after picking for 120 s as c in range of 0.5–4 mol/L. That is to say, under these conditions the magnet corrosion is slow. With c increasing from 4 mol/L to 10 mol/L the elements concentration in the solution enhanced significantly. Comparing the content of each element, the relations of C-c is CNd>CB≈CFe as 6 mol/L≥c≥0.5 mol/L and that CNd>CB> CFe as 10 mol/L>c≥8 mol/L after pickling 300 s. This suggests that Nd-rich phase and B-rich intergranular phase are preferential corrosion in these concentration
Fig. 2 Relation of element concentration, pickling solution concentration and pickling time (a) Ordinary pickling (no ultrasonic); (b) Ultrasonic environment
ranges. In addition when c=10 mol/L the elements concentration linearly increase, and the relations is CFe>CB>CNd. In other words, under this concentration the matrix phase Nd2Fe14B begins to corrode. According to Fig. 2(b) Nd, Fe, B element concentration is higher than 10 µg/mL after picking for 30 s when c is in range of 0.5–1 mol/L. Moreover, Nd, Fe, B element concentration approximates to linearly increase after pickling for 120 s as c≥2 mol/L, in particular, comparing with the ordinary pickling CNd, CFe increase by more than 10 times respectively. These results reveal that NdFeB main phases and the grain boundary are more vulnerable in the ultrasonic environment. In order to further understand the corrosion mechanism of different pickling systems, the differential transformation is conducted between element concentration (C) dC and the pickling time (t) ( v = ). As seen from Fig. 3 dt dC of each element shows a linear increase the value dt with increasing of C, especially as C≥4% mol/L in the ordinary pickling process (Fig. 3(a)). In other words, under this conditions the magnet corrosion quickens. This result can also be confirmed by Fig. 2(a). Therefore, we dC to indirectly estimate the can use the formula v = dt
154
JOURNAL OF RARE EARTHS, Vol. 34, No. 2, Feb. 2016
Fig. 3 Relation of corrosion rates and concentration of the pickling solution (a) Ordinary pickling (no ultrasonic); (b) Ultrasonic environment
Fig. 4 Relation of corrosion depth, concentration of the pickling solution and pickling time (a) Ordinary pickling (no ultrasonic); (b) Ultrasonic environment
magnet corrosion rate. Under the effect of ultrasound the magnet corrosion is relatively quick as C≤6 mol/L, dC level off when the solution conhowever the value dt centration further increase (Fig. 3(b)). This phenomenon is more typical for B element. ( cNd + cFe + cB ) × Vpicking According to formula d = , ρ magnet × S magnet
the grain boundary phase. Therefore, all these changes will lead to the magnet surface loose. NdFeB magnets are pickling for 120 s in 4 mol/L HNO3 solution, then the hardness of the magnet surface is tested before and after pickling (Table 1). The result shows that under the same pressure the average hardness decreases by 10.5% to 18% after pickling. To sum up, due to the selective corrosion the surface of NdFeB magnets becomes loose and the hardness decreases in the pickling process, but that will be favorable for the electroplating process as well as the adhesive
we estimate the corrosion depth of NdFeB magnets, the relation curves of d-c, t are shown in Fig. 4. It can be seen that the corrosion depth linearly increases with the pickling time, in addition the slope of the curves obviously increases with the solution concentration (Fig. 4(a)). It indicates that the corrosion gradually spreads to the magnet inside. Comparing with the ultrasonic environment the corrosion depth of the magnets rapidly increases as c≥4 mol/L (Fig. 4(b)). 2.1.2 Morphology and hardness To intuitively understand the initial changes during the pickling process, the profile morphology of NdFeB magnets is observed (Fig. 5). It shows that the surface of the magnets has changed obviously after pickling for 120 s in 4 mol/L solution. The pitting corrosion is serious in the grain boundary and the average corrosion depth is about 100 µm. In some areas the main phase grains fully highlight, and this mainly attributes to the destruction of
Table 1 Hardness values of NdFeB magnets surface before and after pickling for 120 s in 4 mol/L solution Sample No
Hardness (HV) Before pickling
After pickling
Average value Relative α (HV)
value δ/
αbefore αafter
%
1
665.2 636.0 672.5 570.8 595.6 564.5 657.9 577.0
12.3
2
622.6 599.8 685.4 544.1 586.6 577.4 635.9 569.4
10.5
3
699.1 602.8 663.1 552.2 533.3 555.6 655.0 547.0
16.5
4
631.3 664.2 687.8 594.7 545.2 588.0 661.1 576.0
12.9
5
675.4 588.2 693.1 576.6 601.0 549.2 652.2 575.6
11.7
6
623.1 667.7 651.1 515.1 567.8 542.1 647.3 541.7
16.3
7
629.0 596.4 658.2 521.2 541.3 574.9 627.9 545.8
13.1
8
651.0 688.2 644.1 571.1 602.2 596.6 661.1 590.0
10.8
9
632.8 671.9 636.5 535.5 521.1 535.5 647.1 530.7
18.0
10
644.0 628.5 611.6 584.6 574.5 523.8 628.0 561.0
10.7
ZHOU Qiaoying et al., Influence of the electroplating pretreatment on corrosion mechanism of NdFeB magnets
155
Fig. 5 Morphology of NdFeB magnets before (a), after (b) and cross-section (c) pickling for 120 s in 4 mol/L solution
strength of the coating and the substrate[17]. 2.2 Precoating technology
2.2.1 Precoating mechanism The precoating treatment needs to activate the magnet surface before the formal electroplating[18–20]. The surface morphology of NdFeB magnets is shown in Fig. 6 after precoating for 30 min. It can be seen that the corrosion happens firstly in the grain boundary and then diffuses along the main phase grain. In this process the corrosion depth also increases (Fig. 6(a), (b)). As seen from the higher magnification images (Fig. 6(c)) the grain boundary has been severe corroded. It reveals that the precoating process can also cause the magnets to become loose. Comparing Nd and Fe element concentration in these two processes the corrosive degree of NdFeB magnets is relatively weak in the precoating process. However, the precoating corrosion also cannot be ignored because the duration of precoating is generally longer. The EDS results are shown in Table 2, in which the magnets are immersed in the plating solution for 30 min. It can be seen that the contents of Nd, Fe and Ni are different, and Nd, Fe content is obviously lower, but Ni content is higher in the grain boundary (point a). This illustrates that the grain boundary corrosion preferentially happens, meanwhile the substitution reactions occur in the precoating process and cause Ni to adhere to the substrate. Such results reveal that the replacement reactions are easier to happen in the severe corrosive areas on the magnet surface. 2.2.2 Electrochemical mechanism Relations between the corrosion potential of NdFeB
substrates and the precoating time (V-t) are shown in Fig. 7. It can be seen that the corrosion potential is about 0.6–0.65 V when the current density is in the range of 0.1–0.3 A/dm2. Moreover, the time of reaching the stable potential reduces from 710 to 260 s, in these current scopes the corrosion mechanism of NdFeB magnets are similar. When the current density is 0.5 A/dm2 the corrosion potential becomes very unstable. It suggests that the complex transformation process happens in the precoating process, so the precoating time is difficult to control. Electrochemical impedance spectroscopy is a kind of practical experimental method to study the materials corrosion mechanism[21–22]. Fig. 8 shows the impedance spectra of NdFeB magnets which are tested at different temperatures of the precoating solution. It can be seen that the radiuses of the capacitive reactance increase with the reduction of the solution temperature. It illustrates that the corrosion resistance increases and the magnet corrosion changes slowly in this process. At low frequencies the imaginary part of the impedance spectrum turns negative, and it reveals that little corrosion occurs on the magnet surface[23,24]. This further confirms that the effect of the precoating process on NdFeB magnets is a complex process. Table 2 EDS results of NdFeB magnets after precoating for 30 min Content/wt.%
Fe
Nd
O
Ni
a b
14.97
4.86
4.03
70.39
5.74
39.33
18.99
7.91
29.32
3.36
Fig. 6 Morphology of NdFeB magnets after precoating for 30 min
C
156
JOURNAL OF RARE EARTHS, Vol. 34, No. 2, Feb. 2016
current density was controlled within 0.3 A/dm2, and the solution temperature was about of 318 K during the precoating process.
References:
Fig. 7 Relation between the corrosion potential (under different current densities) and the precoating time of NdFeB substrate (V-t curves)
Fig. 8 Electrochemical impedance spectra at different precoating solution temperatures
3 Conclusions The pickling technology before electroplating of NdFeB magnets could be optimized by selecting the reasonable pickling environment, acid solution concentration and pickling time. Normally, in the ordinary circumstances the acid solution concentration should be below 4 mol/L and the pickling time was within 120 s. However, in the ultrasonic environment the acid solution concentration should be less than 1 mol/L, the time was about 30 s. Although NdFeB magnets would be corroded and the surface became loose in the pickling and precoating process, but that would be favorable for the coating growth. There were two main reactions of the corrosion and displacement in the precoating process, which happened more easily in the grain boundary. These two reactions were affected significantly by the current density and the precoating solution temperature. The precoating time would be prolonged, which might result in severe corrosion of the magnets under the lower current density or solution temperature. On the contrary, the magnetic corrosion would become acute and the precoating time was difficult to control if the higher current density or solution temperature was applied. Usually the
[1] El-Azizm A M, Kirchner A, Gutfleisch O, Gebert A, Schultz L. Investigations of the corrosion behavior of nanocrystalline Nd-Fe-B hot pressed magnets. J. Alloys Compd., 2000, 311: 299. [2] Jacobson J, Kim A. Oxidation behavior of Nd-Fe-B magnets. J. Appl. Phys., 1987, 61: 376. [3] Minowa T, Yoshikawa M, Honshima M. Improvement of the corrosion resistance on Nd2Fe14B magnet with nickel plating. IEEE. Trans. Magn., 1985, 3: 776. [4] Yamamoto H. Metallographic study on Nd-Fe-Co-B sintered magnets. IEEE. Trans. Magn., 1987, 5: 2100. [5] El-Moneim A A. Passivity and its breakdown of sintered NdFeB-based magnets in chloride containing solution. Corros. Sci., 2004, 46: 2517. [6] Li Y F, Zhu M G, Li A H, Feng H B, Huang S L, Li W, Du A, Qi Y. Relationship between controllable preparation and microstructure of NdFeB sintered magnets. J. Rare Earths, 2014, 32: 628. [7] Huang Y L, Liu Z W, Zhong X C, Yu H Y, Gao X X, Zhu J, Zeng D C. Diffusion of Nd-rich phase in the spark plasma sintered and hot deformed nanocrystalline NdFeB magnets. J. Appl. Phys., 2012, 111: 033913. [8] El-Moneim A A, Gebert A. Electrochemical characterization of galvanically coupled single phases and nanocrystalline NdFeB-based magnets in NaCl solutions, J. Appl. Electrochem., 2003, 33: 795. [9] Yu L Q, Wen Y H, Yan M. Effects of Dy and Nb on the magnetic properties and corrosion resistance of sintered NdFeB. J. Magn. Magn. Mater., 2004, 283: 353. [10] Bala H, Pawlowska G, Szymura S, Sergeev V V, Rabinovich Y M. Corrosion characteristics of Nd-Fe-B sintered magnets containing various alloying elements. J. Magn. Magn. Mater., 1990, 87: 1255. [11] Sunada S, Majima K, Akasofu Y, Kaneko Y. Corrosion assessment of Nd-Fe-B alloy with Co addition through impedance measurements, J. Alloys Compd., 2006, 412: 1373. [12] Yan M, Ying H G, Ma T Y. Preparation of coatings with high adhesion strength and high corrosion resistance on sintered Nd-Fe-B magnets through electroless plating. Mater. Chem. Phys., 2009, 113: 764. [13] Gurappa I. Corrosion characteristics of permanent magnets in acidic environments. J. Alloys Compd., 2003, 360: 236. [14] He W T, Zhu L Q, Chen H N, Nan H Y, Li W P, Liu H C, Wang Y. Electrophoretic deposition of grapheme oxide as a corrosion in hibitor for sintered NdFeB. J. Appl. Surf. Sci., 2013, 279: 416. [15] Chen J, Xu B J, Ling G P. Amorphous Al-Mn coating on magnet: electrodeposition from AlCl3-EMIC-MnCl2 ionic liquid and its corrosion behavior. Mater. Chem. Phys., 2012, 134: 1067. [16] Dong X L, Wang D R, Zeng Y Q. Effect of mechanical attrition on microstructure and properties of electro-deposition
ZHOU Qiaoying et al., Influence of the electroplating pretreatment on corrosion mechanism of NdFeB magnets coatings on NdFeB. J. Rare Earths, 2014, 32: 867. [17] Steyaert S, Le-Breton L M. Microstrcuture and corrosion resistance of Nd-Fe-B magnets containing additives. J. Phys. D: Appl. Phys., 1998, 31: 1534. [18] Xu J L, Zhong Z C, Huang Z X, Luo J M. Corrosion resistance of the titania particles enhanced acrylic resin composite coatings on sintered NdFeB permanent magnets. J. Alloys Compd., 2013, 570: 28. [19] Ma C B, Cao F H, Zhang Z. Electrodeposition of amorphous Ni-P coatings onto Nd-Fe-B permanent magnet substrates. J. Appl. Surf. Sci., 2006, 253: 2251. [20] Schultz L, El-Aziz A M, Barkleit G, Mummert K. Corrosion behaviour of Nd-Fe-B permanent magnetic alloys. Mater. Sci. Eng., 1999, 267: 307. [21] Tamborim S M, Takeuchi D S, Azambuja A M, Costa
157
Saliba-Silva I. Corrosion protection of NdFeB magnets by phosphating with tungstate incorporation. Surf. Coat. Technol., 2006, 200: 6826. [22] Song L X, Zhang Z, Zhang J Q, Cao C N. Electroplating mechanism of nanostructured black Ni films. Acta Metall. Sin., 2011, 47: 123. [23] Ni J J, Cui X, Liu X F, Cui H, Xue Z C, Jia Z F, Wang C Z, Ma J, Gong S W. Corrosion behavior of NdFeB sintered magnets in HNO3-HF acid mixture solution. Mater. Chem. Phys., 2015, 162: 523. [24] Zccchi F, Grassi V, Rignani A, Monticelli C, Trabanelli G. Electrochemical behaviour of a magnesium alloy containing rare earth elements. J. Appl. Electrochem., 2006, 36: 201.
Influence of the electroplating pretreatment on corrosion mechanism of NdFeB magnets ZHOU Qiaoying (周巧英)1, LI Gang (李 刚)1, LIU Zhuang (刘 壮)1, GUO Shuai (郭 帅)1, YAN Aru (闫阿儒)1,*, LEE Don (李 东)1, LI Jianzhong (李建忠)2 (1. Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China; 2. Ningbo Permanent Magnetics Co., Ltd., Ningbo 315032, China) Received 14 July 2015; revised 3 November 2015
Abstract: The electroplating pretreatment of NdFeB magnets was simulated in this study, and the effect of the pickling process on element, matrix morphology and texture was discussed. The results showed that in the ordinary circumstance the acid solution concentration should be below 4 mol/L and the pickling time was within 120 s. However the concentration was lower than 1 mol/L and the time was about 30 s in the ultrasonic environment. The precoating technology sequentially was studied. The results revealed that there were two main reactions of the corrosion and displacement in the precoating process, and these two reactions were more obvious in the grain boundary. The electrochemical analysis illustrated that the corrosion and displacement reactions were affected obviously by the current density and the precoating solution temperature, usually the current density was controlled about 0.3 A/dm2, and the temperature was about 318 K in the precoating process. Keywords: NdFeB magnet; pickling; precoating; corrosion; pretreatment; rare earths
NdFeB magnets have attracted increasing attention in recent years due to their excellent magnetic properties. However, their poor corrosion resistance in climatic and corrosive environments hinders their further applications[1–6]. Theoretically NdFeB magnets are composed of three phases: the ferromagnetic matrix phase (Nd2Fe14B), which represents about 87% of the magnet volume and the most corrosion sensitive Nd-rich phase and the B-rich intergranular phase[7,8]. This means that NdFeB magnets are prone to galvanic corrosion, in particular, intergranular corrosion attack. Many investigations have been employed to improve the corrosion resistance of NdFeB magnets, such as the alloy additions and surface coatings. Compared with adding trace elements to improve the corrosion resistance of NdFeB itself[9–11], the surface coatings technology is still the most effective measures[12–16]. However, the electroplating pretreatment is very crucial in the protection process of NdFeB magnets. On the one hand, oils and harmful gases will be adsorbed in the pore on the magnet surface during the machining process, which result in poor adhesion between NdFeB magnets with the coatings. On the other hand, not only the magnetic original size will be seriously changed but the pore and selective corrosion will occur along the grain boundary if the substrate is excessively corrosed during the pretreatment
process, which will reduce the service life of NdFeB magnets. Therefore, taking reasonable pretreatment process to clean and activate the magnet surface is very necessary. In this paper, the electroplating pretreatment conditions of NdFeB magnets were simulated and the purpose was to obtain the more suitable electroplating parameter of NdFeB permanent magnets.
1 Experimental In this study the purity of 99.9% Nd, Fe, B were chosen as raw materials. NdFeB magnets were prepared by conventional powder metallurgy technique including cold isostatic pressing and 1050–1100 ºC sintering using jet milled alloy powder. The magnets used in further measurement were L10 mm×W10 mm×H2 mm, and were removed oil in 2 mol/L NaOH solution. Pickling process was conducted in the PVC cell which was divided into ten equal spaces with PP board, the samples were fixed between the adjacent plate. The pickling device is shown in Fig. 1((a) Front view; (b) Planform). Pickling solution is prepared with deionized water and analytical nitric acid, and the concentration was 0.5, 1, 2, 4, 5, 6, 8 and 10 mol/L respectively. During the pickling process the temperature inside the cell was about 293 K.
Foundation item: Project supported by Zhejiang Provincial Natural Science Foundation for Youth (LQ15E010004), Ningbo Natural Science Foundation (2014A610163), Project of Ningbo Innovative Research Team (2012B81001), Ningbo International Cooperation Project (2015D10019) * Corresponding author: YAN Aru (E-mail: [email protected]; Tel.: +86-574-87911129) DOI: 10.1016/S1002-0721(16)60008-X
ZHOU Qiaoying et al., Influence of the electroplating pretreatment on corrosion mechanism of NdFeB magnets
153
Fig. 1 Pickling device (a) Front view; (b) Planform
Precoating process was conducted in a conventional three-electrode device at about 293 K. A saturated calomel electrode Hg/Hg2SO4/SO42– (SCE) and a platinum foil (the purity of 99.99%, the specification is L30 mm×W30 mm×H2 mm) served as reference and counter electrode, respectively. NdFeB magnets were as the precoating cathode. The precoating solution was composited of NiSO4 280 g/L, NiCl2 50 g/L, H3BO3 50 g/L, SB-1H 0.8 mol/L, D-2 1.2 mol/L, the solution temperature was about 323 K, pH was 4.3 and the precoating time was from 10 to 30 min. The concentration of each element in the solution was analyzed with inductively coupled plasma-atomic emission spectrometry (ICP-MS) (type Optima2100). The surface hardness was characterized using a micro hardness tester (MHT) (type HV-1000), the morphology of magnets was observed using a field emission scanning electron microscope (SEM), the composition of the precoated was analyzed with an Oxford energy detective spectrometer (EDS) (type S-4800). The electrochemical behavior (electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization) during the precoating process was studied by electrochemical workstation (ECW) (type M273A).
2 Results and discussion 2.1 Pickling technology 2.1.1 Corrosion mechanism Two kinds of pickling process were performed respectively, one was the ordinary pickling (no ultrasonic) and the other was putting the pickling PVC cell into the ultrasonic generator. Fig. 2 shows the relations among Nd, Fe, B element concentration (C) in the solution, the pickling solution concentration (c) and the pickling time (t). It can be seen that (Fig. 2(a)) Nd, Fe, B element concentration is respectively less than 5 µg/mL after picking for 120 s as c in range of 0.5–4 mol/L. That is to say, under these conditions the magnet corrosion is slow. With c increasing from 4 mol/L to 10 mol/L the elements concentration in the solution enhanced significantly. Comparing the content of each element, the relations of C-c is CNd>CB≈CFe as 6 mol/L≥c≥0.5 mol/L and that CNd>CB> CFe as 10 mol/L>c≥8 mol/L after pickling 300 s. This suggests that Nd-rich phase and B-rich intergranular phase are preferential corrosion in these concentration
Fig. 2 Relation of element concentration, pickling solution concentration and pickling time (a) Ordinary pickling (no ultrasonic); (b) Ultrasonic environment
ranges. In addition when c=10 mol/L the elements concentration linearly increase, and the relations is CFe>CB>CNd. In other words, under this concentration the matrix phase Nd2Fe14B begins to corrode. According to Fig. 2(b) Nd, Fe, B element concentration is higher than 10 µg/mL after picking for 30 s when c is in range of 0.5–1 mol/L. Moreover, Nd, Fe, B element concentration approximates to linearly increase after pickling for 120 s as c≥2 mol/L, in particular, comparing with the ordinary pickling CNd, CFe increase by more than 10 times respectively. These results reveal that NdFeB main phases and the grain boundary are more vulnerable in the ultrasonic environment. In order to further understand the corrosion mechanism of different pickling systems, the differential transformation is conducted between element concentration (C) dC and the pickling time (t) ( v = ). As seen from Fig. 3 dt dC of each element shows a linear increase the value dt with increasing of C, especially as C≥4% mol/L in the ordinary pickling process (Fig. 3(a)). In other words, under this conditions the magnet corrosion quickens. This result can also be confirmed by Fig. 2(a). Therefore, we dC to indirectly estimate the can use the formula v = dt
154
JOURNAL OF RARE EARTHS, Vol. 34, No. 2, Feb. 2016
Fig. 3 Relation of corrosion rates and concentration of the pickling solution (a) Ordinary pickling (no ultrasonic); (b) Ultrasonic environment
Fig. 4 Relation of corrosion depth, concentration of the pickling solution and pickling time (a) Ordinary pickling (no ultrasonic); (b) Ultrasonic environment
magnet corrosion rate. Under the effect of ultrasound the magnet corrosion is relatively quick as C≤6 mol/L, dC level off when the solution conhowever the value dt centration further increase (Fig. 3(b)). This phenomenon is more typical for B element. ( cNd + cFe + cB ) × Vpicking According to formula d = , ρ magnet × S magnet
the grain boundary phase. Therefore, all these changes will lead to the magnet surface loose. NdFeB magnets are pickling for 120 s in 4 mol/L HNO3 solution, then the hardness of the magnet surface is tested before and after pickling (Table 1). The result shows that under the same pressure the average hardness decreases by 10.5% to 18% after pickling. To sum up, due to the selective corrosion the surface of NdFeB magnets becomes loose and the hardness decreases in the pickling process, but that will be favorable for the electroplating process as well as the adhesive
we estimate the corrosion depth of NdFeB magnets, the relation curves of d-c, t are shown in Fig. 4. It can be seen that the corrosion depth linearly increases with the pickling time, in addition the slope of the curves obviously increases with the solution concentration (Fig. 4(a)). It indicates that the corrosion gradually spreads to the magnet inside. Comparing with the ultrasonic environment the corrosion depth of the magnets rapidly increases as c≥4 mol/L (Fig. 4(b)). 2.1.2 Morphology and hardness To intuitively understand the initial changes during the pickling process, the profile morphology of NdFeB magnets is observed (Fig. 5). It shows that the surface of the magnets has changed obviously after pickling for 120 s in 4 mol/L solution. The pitting corrosion is serious in the grain boundary and the average corrosion depth is about 100 µm. In some areas the main phase grains fully highlight, and this mainly attributes to the destruction of
Table 1 Hardness values of NdFeB magnets surface before and after pickling for 120 s in 4 mol/L solution Sample No
Hardness (HV) Before pickling
After pickling
Average value Relative α (HV)
value δ/
αbefore αafter
%
1
665.2 636.0 672.5 570.8 595.6 564.5 657.9 577.0
12.3
2
622.6 599.8 685.4 544.1 586.6 577.4 635.9 569.4
10.5
3
699.1 602.8 663.1 552.2 533.3 555.6 655.0 547.0
16.5
4
631.3 664.2 687.8 594.7 545.2 588.0 661.1 576.0
12.9
5
675.4 588.2 693.1 576.6 601.0 549.2 652.2 575.6
11.7
6
623.1 667.7 651.1 515.1 567.8 542.1 647.3 541.7
16.3
7
629.0 596.4 658.2 521.2 541.3 574.9 627.9 545.8
13.1
8
651.0 688.2 644.1 571.1 602.2 596.6 661.1 590.0
10.8
9
632.8 671.9 636.5 535.5 521.1 535.5 647.1 530.7
18.0
10
644.0 628.5 611.6 584.6 574.5 523.8 628.0 561.0
10.7
ZHOU Qiaoying et al., Influence of the electroplating pretreatment on corrosion mechanism of NdFeB magnets
155
Fig. 5 Morphology of NdFeB magnets before (a), after (b) and cross-section (c) pickling for 120 s in 4 mol/L solution
strength of the coating and the substrate[17]. 2.2 Precoating technology
2.2.1 Precoating mechanism The precoating treatment needs to activate the magnet surface before the formal electroplating[18–20]. The surface morphology of NdFeB magnets is shown in Fig. 6 after precoating for 30 min. It can be seen that the corrosion happens firstly in the grain boundary and then diffuses along the main phase grain. In this process the corrosion depth also increases (Fig. 6(a), (b)). As seen from the higher magnification images (Fig. 6(c)) the grain boundary has been severe corroded. It reveals that the precoating process can also cause the magnets to become loose. Comparing Nd and Fe element concentration in these two processes the corrosive degree of NdFeB magnets is relatively weak in the precoating process. However, the precoating corrosion also cannot be ignored because the duration of precoating is generally longer. The EDS results are shown in Table 2, in which the magnets are immersed in the plating solution for 30 min. It can be seen that the contents of Nd, Fe and Ni are different, and Nd, Fe content is obviously lower, but Ni content is higher in the grain boundary (point a). This illustrates that the grain boundary corrosion preferentially happens, meanwhile the substitution reactions occur in the precoating process and cause Ni to adhere to the substrate. Such results reveal that the replacement reactions are easier to happen in the severe corrosive areas on the magnet surface. 2.2.2 Electrochemical mechanism Relations between the corrosion potential of NdFeB
substrates and the precoating time (V-t) are shown in Fig. 7. It can be seen that the corrosion potential is about 0.6–0.65 V when the current density is in the range of 0.1–0.3 A/dm2. Moreover, the time of reaching the stable potential reduces from 710 to 260 s, in these current scopes the corrosion mechanism of NdFeB magnets are similar. When the current density is 0.5 A/dm2 the corrosion potential becomes very unstable. It suggests that the complex transformation process happens in the precoating process, so the precoating time is difficult to control. Electrochemical impedance spectroscopy is a kind of practical experimental method to study the materials corrosion mechanism[21–22]. Fig. 8 shows the impedance spectra of NdFeB magnets which are tested at different temperatures of the precoating solution. It can be seen that the radiuses of the capacitive reactance increase with the reduction of the solution temperature. It illustrates that the corrosion resistance increases and the magnet corrosion changes slowly in this process. At low frequencies the imaginary part of the impedance spectrum turns negative, and it reveals that little corrosion occurs on the magnet surface[23,24]. This further confirms that the effect of the precoating process on NdFeB magnets is a complex process. Table 2 EDS results of NdFeB magnets after precoating for 30 min Content/wt.%
Fe
Nd
O
Ni
a b
14.97
4.86
4.03
70.39
5.74
39.33
18.99
7.91
29.32
3.36
Fig. 6 Morphology of NdFeB magnets after precoating for 30 min
C
156
JOURNAL OF RARE EARTHS, Vol. 34, No. 2, Feb. 2016
current density was controlled within 0.3 A/dm2, and the solution temperature was about of 318 K during the precoating process.
References:
Fig. 7 Relation between the corrosion potential (under different current densities) and the precoating time of NdFeB substrate (V-t curves)
Fig. 8 Electrochemical impedance spectra at different precoating solution temperatures
3 Conclusions The pickling technology before electroplating of NdFeB magnets could be optimized by selecting the reasonable pickling environment, acid solution concentration and pickling time. Normally, in the ordinary circumstances the acid solution concentration should be below 4 mol/L and the pickling time was within 120 s. However, in the ultrasonic environment the acid solution concentration should be less than 1 mol/L, the time was about 30 s. Although NdFeB magnets would be corroded and the surface became loose in the pickling and precoating process, but that would be favorable for the coating growth. There were two main reactions of the corrosion and displacement in the precoating process, which happened more easily in the grain boundary. These two reactions were affected significantly by the current density and the precoating solution temperature. The precoating time would be prolonged, which might result in severe corrosion of the magnets under the lower current density or solution temperature. On the contrary, the magnetic corrosion would become acute and the precoating time was difficult to control if the higher current density or solution temperature was applied. Usually the
[1] El-Azizm A M, Kirchner A, Gutfleisch O, Gebert A, Schultz L. Investigations of the corrosion behavior of nanocrystalline Nd-Fe-B hot pressed magnets. J. Alloys Compd., 2000, 311: 299. [2] Jacobson J, Kim A. Oxidation behavior of Nd-Fe-B magnets. J. Appl. Phys., 1987, 61: 376. [3] Minowa T, Yoshikawa M, Honshima M. Improvement of the corrosion resistance on Nd2Fe14B magnet with nickel plating. IEEE. Trans. Magn., 1985, 3: 776. [4] Yamamoto H. Metallographic study on Nd-Fe-Co-B sintered magnets. IEEE. Trans. Magn., 1987, 5: 2100. [5] El-Moneim A A. Passivity and its breakdown of sintered NdFeB-based magnets in chloride containing solution. Corros. Sci., 2004, 46: 2517. [6] Li Y F, Zhu M G, Li A H, Feng H B, Huang S L, Li W, Du A, Qi Y. Relationship between controllable preparation and microstructure of NdFeB sintered magnets. J. Rare Earths, 2014, 32: 628. [7] Huang Y L, Liu Z W, Zhong X C, Yu H Y, Gao X X, Zhu J, Zeng D C. Diffusion of Nd-rich phase in the spark plasma sintered and hot deformed nanocrystalline NdFeB magnets. J. Appl. Phys., 2012, 111: 033913. [8] El-Moneim A A, Gebert A. Electrochemical characterization of galvanically coupled single phases and nanocrystalline NdFeB-based magnets in NaCl solutions, J. Appl. Electrochem., 2003, 33: 795. [9] Yu L Q, Wen Y H, Yan M. Effects of Dy and Nb on the magnetic properties and corrosion resistance of sintered NdFeB. J. Magn. Magn. Mater., 2004, 283: 353. [10] Bala H, Pawlowska G, Szymura S, Sergeev V V, Rabinovich Y M. Corrosion characteristics of Nd-Fe-B sintered magnets containing various alloying elements. J. Magn. Magn. Mater., 1990, 87: 1255. [11] Sunada S, Majima K, Akasofu Y, Kaneko Y. Corrosion assessment of Nd-Fe-B alloy with Co addition through impedance measurements, J. Alloys Compd., 2006, 412: 1373. [12] Yan M, Ying H G, Ma T Y. Preparation of coatings with high adhesion strength and high corrosion resistance on sintered Nd-Fe-B magnets through electroless plating. Mater. Chem. Phys., 2009, 113: 764. [13] Gurappa I. Corrosion characteristics of permanent magnets in acidic environments. J. Alloys Compd., 2003, 360: 236. [14] He W T, Zhu L Q, Chen H N, Nan H Y, Li W P, Liu H C, Wang Y. Electrophoretic deposition of grapheme oxide as a corrosion in hibitor for sintered NdFeB. J. Appl. Surf. Sci., 2013, 279: 416. [15] Chen J, Xu B J, Ling G P. Amorphous Al-Mn coating on magnet: electrodeposition from AlCl3-EMIC-MnCl2 ionic liquid and its corrosion behavior. Mater. Chem. Phys., 2012, 134: 1067. [16] Dong X L, Wang D R, Zeng Y Q. Effect of mechanical attrition on microstructure and properties of electro-deposition
ZHOU Qiaoying et al., Influence of the electroplating pretreatment on corrosion mechanism of NdFeB magnets coatings on NdFeB. J. Rare Earths, 2014, 32: 867. [17] Steyaert S, Le-Breton L M. Microstrcuture and corrosion resistance of Nd-Fe-B magnets containing additives. J. Phys. D: Appl. Phys., 1998, 31: 1534. [18] Xu J L, Zhong Z C, Huang Z X, Luo J M. Corrosion resistance of the titania particles enhanced acrylic resin composite coatings on sintered NdFeB permanent magnets. J. Alloys Compd., 2013, 570: 28. [19] Ma C B, Cao F H, Zhang Z. Electrodeposition of amorphous Ni-P coatings onto Nd-Fe-B permanent magnet substrates. J. Appl. Surf. Sci., 2006, 253: 2251. [20] Schultz L, El-Aziz A M, Barkleit G, Mummert K. Corrosion behaviour of Nd-Fe-B permanent magnetic alloys. Mater. Sci. Eng., 1999, 267: 307. [21] Tamborim S M, Takeuchi D S, Azambuja A M, Costa
157
Saliba-Silva I. Corrosion protection of NdFeB magnets by phosphating with tungstate incorporation. Surf. Coat. Technol., 2006, 200: 6826. [22] Song L X, Zhang Z, Zhang J Q, Cao C N. Electroplating mechanism of nanostructured black Ni films. Acta Metall. Sin., 2011, 47: 123. [23] Ni J J, Cui X, Liu X F, Cui H, Xue Z C, Jia Z F, Wang C Z, Ma J, Gong S W. Corrosion behavior of NdFeB sintered magnets in HNO3-HF acid mixture solution. Mater. Chem. Phys., 2015, 162: 523. [24] Zccchi F, Grassi V, Rignani A, Monticelli C, Trabanelli G. Electrochemical behaviour of a magnesium alloy containing rare earth elements. J. Appl. Electrochem., 2006, 36: 201.