1 Introduction

Sintered magnets are widely used because of the excellent magnetic properties [1, 2]. It is reported that the highest magnetic energy product of the magnets has been reached as high as 4.7 × 105 J·m−3 [3]. In order to meet different application requirements, different elements are added to the magnets. The heavy rare earth (HRE) elements, such as dysprosium (Dy) and terbium (Tb), are added to Nd2Fe14B magnets for elevating the coercivity (H cj) of and reducing the temperature coefficient of magnets as (Dy/Tb)2Fe14B has higher magnetic anisotropy field than Nd2Fe14B [4,5,6,7]. Light rare earth element Ce is abundant in nature, especially in China. Much effort has been made in substitution of Nd for Ce to avoid overuse of rare earth Nd [8,9,10].

The microstructure of sintered Nd–Fe–B magnet presents ferromagnetic matrix phase (Nd2Fe14B) which is surrounded by Nd-rich intergranular phase [11, 12], and the magnet is highly susceptible to corrosion because of the high reactivity of Nd and the galvanic coupling between Nd-rich phase and Nd2Fe14B phase [13,14,15].

An optimized heat treatment allowed the creation of an extremely uniform RE-rich film in the grain boundary interfaces (GBs) and made it possible to reach a high coercivity [16]. Moreover, the action makes the small-sized (Pr, Nd)-rich phase grains harmoniously distribute at the intergranular triple junctions (TJs) [17]. The corrosion rate of RE-rich phase at the TJs is higher than that at GBs [17, 18]. From the perspective of the distribution with Re-rich phase, the act of the annealing process improves the corrosion resistance of the magnet. But the fine grid structure with Nd2Fe14B and Nd-rich phase reduces the corrosion resistance of magnets by heat treatment [19]. So it is dubious as to how the annealed process can influence corrosion resistance.

For the dual-main-phased (NdxCey)2Fe14B magnet, the cerium element is more easily distributed at the triple-point junctions than at neodymium [20]. The RE-rich phase has a lower melting point than the Nd-rich phase, meaning that RE-rich phase has a better liquidity than Nd-rich phase. Therefore, the distribution of intergranular phase can be significantly affected by annealing process.

The corrosion of the dual-main-phased (NdxCey)2Fe14B has already been studied [21]. However, the influence of annealing process on corrosion has rarely been reported to our best knowledge. The present work studies the corrosion resistance of as-sintered magnets (SM) and as-annealed magnets (AM) of (Nd0.8Ce0.2)2Fe14B magnets referred to as Ce20. The mass loss was measured in accelerated corrosion tests. The electrochemical character was researched in 0.1 mol·L−1 HCl and 3.5 wt% NaCl.

2 Experimental

Ce20 magnets were prepared by conventional dual-main-phased magnet preparation methods. The temperature of sintered magnet was determined at 1000 °C, and the secondary annealing temperature was defined at 450 °C without first annealing treatment.

The dimensions of the magnet specimens used for accelerated corrosion tests were Φ10 mm × 10 mm. Accelerated corrosion tests were performed under conditions of 120 °C and 100% relative humidity for 12, 24, 36, 48 and 60 h, respectively. The corrosion products were removed according to the standard of GB/T24195-2009. The magnetic properties of the samples before and after corrosion were analyzed by a hysteresis loop instrument (NIM-2000, National Institute of Metrology, China). The back-scattered electron (BSE) images were captured by Phenom ProX. Content of the triple junction was estimated by the ratio of the RE-rich phase (white contrast) area (A 1) to the whole area (A 0) in the typical back-scattered images of the magnet using the Digimizer software. The potentiodynamic polarization curves were tested by electrochemical analyzer (CHI660D). The electrochemical impedance spectra (EIS) tests were performed by Gamry Co. interface 1000. The amplitude of the sinusoidal perturbation applied to the electrode was 10 mV. All electrochemical specimens were kept in 3.5 wt% NaCl solution for 0.5 h to stabilize the stationary potential of the specimens. The electrochemical measurements were taken by a conventional three-electrode electrochemical cell with a saturated calomel electrode (SCE) as the reference, a Pt foil as the auxiliary electrode and the samples Ce20 as the working electrode.

3 Results and discussion

BSE images of SM and AM samples are shown in Fig. 1. The dark regions refer to the main phases, and the light regions represent the intergranular phases. For SM samples, the content of TJs is ~9.2 vol%. But for AM samples, the content % of TJs decreases to ~5.6 vol%. Obviously, the annealing process significantly changes the distribution of RE-rich phase.

Fig. 1
figure 1

BSE images of dual-main-phased a SM and b AM (Nd0.8Ce0.2)2Fe14B

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Figure 2 shows the results of accelerated corrosion tests under conditions of 120 °C and relative humidity of 100%. The results show that the mass loss increases with increasing corrosion time. However, the mass loss of SM samples increases sharply when the corrosion time increases to 48 h. The rate of mass loss for AM samples is the highest in the first 12 h and then gradually reduces with further corrosion. The mass loss rate of SM sample has two inflection points at 12 and 48 h. Compared to that of AM samples, mass loss of SM samples is larger, which means that intergranular phases of SM samples are more likely to dissolve in the initial 12 h. The inflection point of 48 h for SM samples shows that SM samples crack more severe than AM samples. It is reported that pits start at Nd-rich phase and then propagate along GBs, and the combination of grain boundary failure, pitting and hydrogenation leads to a degradation of Nd–Fe–B magnets under conditions of 85 °C and relative humidity of 80% [22]. The differences in the degradation of the two samples ought to be associated with pitting and the propagation of pitting.

Fig. 2
figure 2

a Mass loss and b corrosion rate of SM and AM samples with time at 120 °C in 100% relative humid atmosphere

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Figure 3a demonstrates potentiodynamic polarization curves of SM and AM Ce20 in 0.1 mol·L−1 HCl aqueous solutions. The corrosion potential (E corr) and corrosion current density (i corr) are listed in Table 1 calculated by Tafel linear extrapolation method. The E corr of AM samples is more positive compared to that of SM samples. The i corr of SM samples is larger than that of AM samples. As anode, RE-rich phase loses electrons. From the E-pH diagram [23], the RE displays RE3+ when it loses electron, it can be got that the reflections Ce→Ce3++3e−, Nd→Nd3++3e− and Fe→Fe2+ + 2e−and the main phases Nd2Fe14B and Ce2Fe14B gains electrons as cathode and the reaction is H++e−→H [24]. Shedding particles can be seen clearly in the test process.

Fig. 3
figure 3

Typical potentiodynamic polarization curves of SM and AM samples in a 0.1 mol·L−1 HCl and b 3.5 wt% NaCl

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Table 1 Electrochemical parameters of SM and AM in 0.1 mol·L−1 HCl aqueous solutions and 3.5 wt% NaCl aqueous solutions
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The corrosion mechanism of Nd–Fe–B magnets in HCl solution is mainly selectively intergranular corrosion [25], and the particles fall off [26]. From BSE images, it can be seen that the TJs content of SM samples is larger than that of AM samples. And the corrosion rate at TJs is higher than that at GBs [18]. So, the corrosion current of SM samples is larger than that of AM samples.

Figure 3b shows potentiodynamic polarization curves of SM and AM samples in 3.5 wt% NaCl aqueous solutions. Table 1 shows that the free corrosion voltage (E corr) of SM and AM samples is −0.7786 and −0.7818 V, respectively, and the moving direction of corrosion potential is similar to the results in 0.1 mol·L−1 HCl. The free corrosion current (i corr) of SM and AM samples is 8.282 × 10−6 and 1.179 × 10−5 A·cm−2, respectively. The free corrosion current of SM samples is smaller than that of AM samples, which is opposite in HCl aqueous solution. The anode loses electron, and the reactions is Nd → Nd3++3e−, Ce → Ce3++3e− and Fe → Fe2++2e−. The main course of Nd2Fe14B and Ce2Fe14B as cathode is the reaction of O2 + 2H2O + 4e−→4OH− [27] in NaCl aqueous solution. The cations Nd3+, Ce3+ and Fe2+ combine with OH− to form passive film [28].

Applying a small alternating current (AC) signal and measuring the response of the electrochemical system can be used to study the corrosion of metals [29]. The EIS tests of the samples were not conducted in HCl solution, because the disturbance of open-circuit voltage is greater than 10 mV which is comparative to voltage amplitude.

Figure 4 displays the Nyquist diagrams of EIS for SM and AM samples in 3.5 wt% NaCl. The Nyquist diagrams show the imaginary impedance values (Z″) plotted against the real impedance (Z′) values obtained at each polarization frequency. The radius of SM samples is larger than that of AM samples, meaning that SM samples perform a greater transfer resistance. Inductive loop at low frequency of SM samples is observed which originated from the pitting corrosion on the working electrode as a result of the adsorptions of Cl−.

Fig. 4
figure 4

Nyquist diagrams for SM and AM samples in 3.5 wt% NaCl

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To fit the EIS results accurately, constant phase angle element (CPE, designated as Q) was used instead of the pure capacitance. The admittance of CPE (Y Q) can be expressed by the following equation:

$$Y_{Q} = Y_{0} (j\omega )^{n}$$
(1)

where j is the imaginary number, ω is the angular frequency, and Y 0 and n are defined as the admittance constant and the empirical exponent of the CPE respectively. Inductive loop is equivalent to inductance (L) and inductive reactance (R L) which represent pitting corrosion. The equivalent circuit is shown in Fig. 5. In this circuit, R s is the electrolyte resistance between the working electrode and the reference electrode, and R ct is the transfer resistance of electrons of double electrode layer. The data fitted by Gamry’s software are listed in Table 2. The value of L is relevant to the thickness of the passivation film [30]. From the fitting parameters of EIS, the electron transfer resistance (R ct) of AM samples is lower than that of SM samples in 3.5 wt% NaCl. So, the annealed process reduces the electron transfer resistance (R ct).

Fig. 5
figure 5

Equivalent circuit for EIS data of a SM and b AM samples

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Table 2 Fitting parameters of Nyquist diagrams
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The magnetic properties of the corrosion samples were analyzed by NIM-2000 hysteresis loop instrument. Figure 6 shows that the remanence of the samples increases slowly as accelerated corrosion time increases in the initial 24 h, and then, it decreases in the following corrosion process. The intrinsic coercivity has a significant decline for both samples over the initial 12 h corrosion period, then the reduction rate slows.

Fig. 6
figure 6

Magnetic parameters of SM and AM samples after corrosion

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Intergranular corrosion mainly occurs in the initial stages, and the oxides are formed by combined ionization of rare earth elements, Fe element with water and oxygen. The oxides adhere to the magnets, which increases the remanence of the samples [21]. When accelerated corrosion time reaches 48 h, the remanence of SM samples is significantly reduced, which is consistent with the mass loss. The coercivity of both samples reduces with the corrosion time, and the reducing rate becomes slower. The reduction in coercivity is caused by the corrosion of intercrystalline phase which acts as demagnetization coupling.

Besides, the corrosion morphology of samples with accelerated corrosion time of 60 h is observed after gently brushing samples surface by 2000-grit sandpaper. The black region in Fig. 7 refers to corrosion loss region of samples. The area of black region of SM samples is larger than that of AM samples, particularly the position marked by the white circle. The magnified images (Fig. 7c, d) show that corrosion depth of single corrosion area of SM samples is obviously deeper than that of AM samples. The deeper and larger corrosion areas of SM samples show more serious pitting and easier propagation of pitting.

Fig. 7
figure 7

BSE images for corrosion morphology of a, c SM and b, d AM samples with accelerated corrosion time for 60 h

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4 Conclusion

The corrosion behavior of as-sintered and as-annealed (Nd0.8Ce0.2)2Fe14B magnets was studied. The mass loss of SM is larger than that of AM after accelerated corrosion tests. The free corrosion current of SM is larger than that of AM in 0.1 mol·L−1 HCl, but is smaller than that of AM in 3.5 wt% NaCl. The reason is that the annealing process reduces electrochemical activity of RE-rich phase and the electron transfer resistance which play the key role, respectively, in different aqueous solutions.

The inductance of Nyquist diagrams for SM in 3.5 wt% NaCl in the low-frequency region indicates that the pitting occurs in SM magnets. The deeper and larger corrosion areas of SM show more serious pitting and easier propagation of pitting. So the mass loss of SM has an obvious increase when the corrosion time is up to 48 h.