Startseite Comparison of the corrosion resistances of chromium-passivated and cerium-passivated Zn/NdFeB magnets
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Comparison of the corrosion resistances of chromium-passivated and cerium-passivated Zn/NdFeB magnets

  • Pengjie Zhang , Jing Chen , Hongyi Yang , Guangqing Xu EMAIL logo , Jun Lv , Jiewu Cui , Wei Sun , Bingshan Li , Dongmei Wang und Yucheng Wu EMAIL logo
Veröffentlicht/Copyright: 9. Januar 2023
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Corrosion Reviews
Aus der Zeitschrift Corrosion Reviews Band 41 Heft 2

Abstract

Chromium-free passivation of Zn coating on NdFeB magnets becomes a research hotspot due to the serious harm of chromium ions to the human body. Chromium-based and cerium-based passivation technologies are conducted on electroplating Zn/NdFeB respectively. Morphologies, elemental compositions and phase structures of the two passivated coatings are characterized by scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy, respectively. The corrosion resistances of the two passivated specimens are compared by neutral salt spray test, accelerated aging test and electrochemical measurements. A complete and smooth passivation film can be obtained on the surface of Zn/NdFeB, filling the gaps and pores in Zn coating. Compared with un-passivated Zn/NdFeB, Zn(Ce)/NdFeB and Zn(Cr)/NdFeB possess excellent corrosion resistance. In comparison, Zn(Ce)/NdFeB possesses excellent anti-corrosion performance, increasing the red-rust appearing time from 288 to 432 h, which is still lower than that of Zn(Cr)/NdFeB (528 h). Therefore, the self-repair effect of cerium passivation technology during the corrosion process should be further studied to achieve the purpose of replacing chromium passivation technology.

Keywords: cerium passivation; corrosion resistance; trivalent chromium passivation; Zn/NdFeB magnet

1 Introduction

Sagawa (Sagawa et al. 1984) of Japan first prepared sintered NdFeB permanent magnets by powder metallurgy in 1984, creating the first third-generation permanent magnets. NdFeB is referred to as the “magnet king” of permanent magnets due to its high remanence, high-energy product and high coercivity (Chuewangkam et al. 2019; Dai et al. 2020; Hu et al. 2015; Jiang et al. 2018; Song et al. 2019), and is widely used in many fields such as new energy technology, electronics, automobiles and medical treatment (Lin et al. 2020; Shen et al. 2018; Zeng et al. 2019). However, NdFeB magnets are extremely susceptible to corrode in a variety of environments and possess the disadvantages of poor temperature and corrosion resistance (Ding et al. 2016; J. Li et al. 2020). NdFeB magnets are prepared by a sintering method, which induces the intrinsic defects of low density with a large number of pores (Dai et al. 2020; Xu et al. 2017). A multi-phase crystal structure consisting of main magnetic phase (Nd2Fe14B), Nd-rich phases (Nd4Fe) and B-rich phases (Nd1.1Fe4B4) (Wang et al. 2011) exists in NdFeB magnets, which can easily produce electrochemical corrosion due to the potential differences among the different phases (Derewnicka-Krawczyńska et al. 2018; Zhou et al. 2018). In addition, as a chemically reactive metal, Nd-rich phases in the grain boundary are prone to oxidation reactions (Ouyang et al. 2019; Yu and Chen 2006). Corrosion of the active phases in the grain boundary will cause the shielding of the main-phase grains and the efflorescence failure of the magnets. The common corrosion forms of sintered NdFeB magnets are shown in Table 1.

Table 1:

Common corrosion forms of sintered NdFeB magnets.

Corrosion forms Occurrence condition Corrosion characteristics
High temperature oxidation corrosion Under dry and high temperature conditions Both the main phase and the Nd-rich phase of the magnet are oxidized and corroded (Nababan et al. 2021).
Wet environments corrosion Under warm and humid conditions The Nd-rich phase of the NdFeB magnet is first corroded by water vapor, and then the main phase of NdFeB is corroded (Xue et al. 2014).
Hydrogen absorption corrosion In acidic or alkaline solutions The main phase of the magnet causes lattice expansion after hydrogen absorption, and the grain boundary expansion after the Nd-rich phase absorbs hydrogen produces grain boundary stress (Parmar et al. 2018).
Electrochemical corrosion Under electrochemical environment Compared with the main phase, the Nd-rich phase and B-rich phase are smaller in volume, but there is a large corrosion current passing through them (Liu et al. 2014).

The anti-corrosion performances of NdFeB magnets can be greatly improved by surface protection technologies, including electroplating, metal coatings, electrophoresis organic coatings and vapor deposition coatings, without deteriorating the magnetic performances (Cao et al. 2017; Chen et al. 2020), which are widely used in the industry. The properties of the different coatings are compared in Table 2. Specifically, metallic coatings (e.g., Zn, Al, Ni, Ni–P, Zn–Al and Ni–Cu–P) are the mostly used coatings in the rare earth permanent magnet industry (Hu et al. 2003; Lin et al. 2020; Wang et al. 2020). Electroplating Zn coating is widely used in actual production, which requires following passivation process for improving surface density and chemical stability.

Table 2:

Comparison of different surface protective coatings.

Surface protection type Service conditions Advantages Disadvantages
Metallic coating Common corrosive environments Simple process and low cost Low production efficiency and passivation treatment required (Cao et al. 2021).
Organic coating Under conditions of severe corrosion and need for insulation Excellent corrosion resistance Poor heat and humidity resistance, poor wear resistance (Yang et al. 2020)
Ceramic coating Harsh corrosive environment Excellent corrosion and tribocorrosion resistance Poor adhesion and high cost (Cao et al. 2017)
Composite coating Complex and harsh working environment Multi-layer protection Complex process (Li et al. 2009)

The chromate conversion film was first proposed by Sauer and Vogel in Germany in 1915 and was widely used for the passivation of metals in the fields of aerospace, automotive industries and electronics (Piao et al. 2019). The chromate conversion film mainly consists of insoluble trivalent chromium and soluble hexavalent chromium, which exhibits a strong binding force and good stability (Lim et al. 2020). At the same time, when the conversion film is damaged, the exposed metal matrix can react with the soluble hexavalent chromium compound, and the damaged portion can be passivated again to achieve a self-repair effect. However, the hexavalent chromium ion is a carcinogen that can cause serious harm to the human body and has been banned by the European Union (Hesamedini and Bund 2019; Huang et al. 2018; Mao et al. 2020; Nie et al. 2020). Trivalent chromium coating possesses similar structure and anticorrosion performances with low toxicity compared with hexavalent chromium coating, which has been studied as a primary alternative for hexavalent chromium passivation (Shruthi and Swain 2019; Walton et al. 2019; Xu et al. 2020). However, trivalent chromium passivation creates Cr3+ and Cr6+ ions during the production process, causing harmful pollution to the environment. Thus, the development of a chromium-free passivation method is of importance.

Rare earth passivation is a simple, chromium-free process that is considered to be safe. As a new metal surface treating technology with good environmental performance (Dastgheib et al. 2019), it is regarded as one of the most promising transformation membranes that can replace chromate (Arthanari and Shin 2018). Cen Lu (Lu et al. 2020) prepared Ce and Ce–Mo conversion coatings on aluminum alloy 6063 by immersing the alloy in alkaline conversion baths. The optimized Ce–Mo conversion coating exhibits the lowest current density of 0.24 μA/cm2, about two orders of magnitude lower than that of the bare sample. So far, this Ce passivation technology has not been used in production. The differences of the anti-corrosion performances and corrosion mechanism between cerium passivation and chromium passivation should be deeply studied, which is benefit for indicating the direction of further study and achieving replacement of chromium passivation in industry.

Herein, the surface compositions and corrosion performances of cerium passivation and trivalent chromium passivation on Zn/NdFeB magnets were deeply studied, and the mechanisms of the two passivation methods were compared.

2 Materials and methods

2.1 Specimen preparation

NdFeB specimens in the demagnetization state (N42UH, (PrNd)25.6Dy9.8FebalB1M, Anhui Earth-Panda Advance Magnetic Co., Ltd.) were prepared using a powder-sintered method by an industry manufacturer. The specimens were cut into 12 nm × 13 nm × 2 mm sections for further experiments.

Prior to the Zn electroplating process, all the specimens were dipped in 0.6 mass% HNO3 solution for 15 s and then put into a plating tank with a bath composition of 65 g/L ZnCl2, 215 g/L KCl and 37 g/L H3BO3. The electroplating process was conducted at room temperature with a current density of 1 A/dm2 for 50 min, and the obtained specimens were defined as Zn/NdFeB.

Before passivation, Zn/NdFeB specimens were acidified in 0.6 mass% HNO3 solution for 10 s followed by ultrasonication for 3 min. The acidified Zn/NdFeB specimens were immersed in a mixed solution of nitric acid and chromium nitrate (pH 2.5) at 30 °C for 30 s. Then, the specimens were rinsed in ethanol, dried in air at 40 °C, and denoted as Zn(Cr)/NdFeB.

The acidified Zn/NdFeB specimens were immersed in an aqueous solution of 5 g/L cerium nitrate and 3 mass% hydrogen peroxide, and the pH was adjusted to 3.0 with hydrochloric acid. The passivation process was conducted for 20 min at room temperature. After being rinsed with deionized water, the specimens were dried in air at 40 °C and denoted as Zn(Ce)/NdFeB.

2.2 Characterization

Phase structures and surface morphologies of the passivated specimens were characterized by X-ray diffraction (Dutch Panax D/MAX2500V, 40 kV, 40 mA) and field emission scanning electron microscopy (SU8020 500, Japan). An X-ray photoelectron spectroscope (Thermo ESCALAB250Xi, USA) was utilized to identify the surface compositions.

Electrochemical tests, including the potentiodynamic polarization curve and electrochemical impedance spectroscopy, were performed in 3.5 mass% NaCl solution (without further aeration) at room temperature to study the anti-corrosion performance of different specimens. Before the tests, all specimens were immersed in the electrolyte for 30 min to stabilize the potential. The electrochemical tests were conducted in a three-electrode system with saturated calomel as the reference electrode, Pt foil as the counter electrode and the specimen as the working electrode (size of 1 mm × 1 mm). All the specimens were tested for 3 times to make sure the stability of polarization curves and the Nyquist plots.

The anti-corrosion performances of the specimens were measured by neutral salt spray (NSS) test and accelerated aging test. The NSS test was conducted in a standard salt spray cabinet by spraying NaCl solution (50 g/dm3) at 35 °C. The specimens were observed and recorded until the appearance of red rust. The accelerated aging tests were conducted in a highly accelerated aging Tester (HAST-S, KSON) with parameters at 120 °C, 2 × 105 Pa and 100% RH. All the specimens were accurately weighed before the test (M0), and were taken out and weighed at every 48 h during the aging test (M1). The mass gain (MG) was calculated according to the following formula:

(1)MG=1000(M1−M0)/S

3 Results and discussion

3.1 Phase structure and composition

Figure 1 shows the X-ray diffraction (XRD) patterns of Zn/NdFeB, Zn(Cr)/NdFeB and Zn(Cr)/NdFeB specimens. In Zn/NdFeB, diffraction peaks at 38.47°, 44.58° and 64.87° can be ascribed to the NdFeB substrate, while the diffraction peaks of Zn (PDF card NO. 87-0713) appearing at 36.32°, 39.02° and 43.29° originate from the electroplating Zn coating on the substrate. No additional diffraction peaks can be observed in the Zn(Cr)/NdFeB and Zn(Ce)/NdFeB specimens, which may be due to the low crystallinity and low content of the passivation components on the Zn coatings.

Figure 1: XRD patterns of Zn/NdFeB, Zn(Cr)/NdFeB and Zn(Ce)/NdFeB specimens.
Figure 1:

XRD patterns of Zn/NdFeB, Zn(Cr)/NdFeB and Zn(Ce)/NdFeB specimens.

Figure 2 shows the scanning electron microscopy (SEM) morphologies and energy dispersion spectra (EDS) analysis of the Zn/NdFeB, Zn(Cr)/NdFeB and Zn(Ce)/NdFeB specimens and their corresponding cross-sectional images (inset). As seen from the morphologies and elemental composition of the Zn/NdFeB (Figure 2(i)), electroplated Zn coating is unsmooth with gaps between the dendrites and many pores in the coating, which may lead to the infiltration of the corrosion medium. Zn matrix and small amount of O can be obtained from EDS analysis, indicating the natural oxidation of Zn in air. However, this zinc oxide layer is thin and discontinuous, unable to provide the complete protection of Zn coating and NdFeB substrate.

Figure 2: SEM morphologies and EDS analysis of (i) Zn/NdFeB, (ii) Zn(Cr)/NdFeB and (iii) Zn(Ce)/NdFeB specimens (inset: corresponding cross-sectional images).
Figure 2:

SEM morphologies and EDS analysis of (i) Zn/NdFeB, (ii) Zn(Cr)/NdFeB and (iii) Zn(Ce)/NdFeB specimens (inset: corresponding cross-sectional images).

After passivation in chromate solution, surface of the Zn coating becomes smooth with a thin passivation layer on it, as shown in Figure 2(ii). The loose structure of the Zn coating is still unchanged due to the room-temperature condition of the passivation process. However, the dense passivation layer covering on the Zn coating can prevent the infiltration of corrosion media. The EDS analysis confirms the existence of Cr in the passivation layer of the Zn coating.

The surface of the Zn(Ce)/NdFeB is relatively smooth compared to that of the Zn/NdFeB and Zn(Cr)/NdFeB specimens (Figure 2(iii)), and a complete passivation film can be observed in the cross-sectional morphologies shown in the inset. EDS analysis shows the Ce content of 5.0 mass% in the passivation film. Considering the detection depth of the EDS analysis, Ce content in the passivation film should be higher than the average data.

To determine the chemical state of the passivated Zn/NdFeB specimens, X-ray photoelectron spectroscopy (XPS) analysis was conducted. The survey patterns and the high-resolution patterns of Zn 2p electrons, O 1s electrons, Cr 2p electrons and Ce 3d electrons are shown in Figure 3. All of the patterns are normalized with the binding energy and intensity of C 1s electrons. Surfaces of Zn/NdFeB is mainly composed of Zn and O, as shown in Figure 3(i). After passivation, Cr and Ce can be detected in their respective specimens. Quantitative analysis shows that the content of Cr and Ce elements in the passivation layers are 16.11 and 4.64% respectively.

Figure 3: XPS patterns of Zn/NdFeB, Zn(Cr)/NdFeB and Zn(Ce)/NdFeB specimens. (i) Survey patterns; high resolution patterns of (ii) Zn 2p, (iii) O 1s, (iv) Cr 2p and (v) Ce 3d electrons.
Figure 3:

XPS patterns of Zn/NdFeB, Zn(Cr)/NdFeB and Zn(Ce)/NdFeB specimens. (i) Survey patterns; high resolution patterns of (ii) Zn 2p, (iii) O 1s, (iv) Cr 2p and (v) Ce 3d electrons.

Figure 3(ii) shows the high-resolution patterns of Zn 2p electrons in the three specimens. There are two characteristic peaks with binding energies at 1022 and 1045 eV, corresponding to Zn 2p3/2 and Zn 2p1/3 electrons respectively. In Zn/NdFeB, two peaks with binding energies at 1022.2 and 1023.3 eV can be obtained by a Gaussian decomposition, corresponding to Zn2+ ions and Zn respectively, which can be ascribed to the partial oxidation of Zn with O2 in the air. After passivation, both the Zn 2p3/2 electrons of Zn(Cr)/NdFeB and Zn(Ce)/NdFeB show only one characteristic peak at about 1022 eV (Zn2+), indicating that the passivation layers completely cover the metallic Zn in both specimens.

Figure 3(iii) shows the high-resolution XPS spectra of O 1s electrons in the three specimens. Both Zn/NdFeB and Zn(Cr)/NdFeB showed only one characteristic peak located at 532 eV, corresponding to the oxides of Cr and Zn. As for Zn(Ce)/NdFeB, two peaks corresponding to hydroxyl and oxide respectively, can be observed at 529.5 and 531.6 eV, indicating that Ce3+ ions exist present in Zn(Ce)/NdFeB specimens as Ce(OH)3 and Ce2O3.

Figure 3(iv) shows the high-resolution spectrum of Cr 3d electrons in the Zn(Cr)/NdFeB. Peaks at 577.8 and 587.0 eV corresponded to Cr 2p3/2 and Cr 2p1/2 electrons respectively (Cho et al. 2007), which are characteristic of Cr(III) ions. Only one peak can be observed at 577.8 eV, corresponding to the Cr 2p3/2 electrons of Cr2O3, indicating that there is no Cr(VI) in the passivation layer of Zn(Cr)/NdFeB (Huang et al. 2018).

The high-resolution patterns of Ce 3d in the Zn(Ce)/NdFeB specimens are shown in Figure 3(v). Peaks at 882.7 and 898.48 eV corresponded to Ce 3d5/2, while a characteristic Ce 3d3/2 peak is located at 916.9 eV, which fit well with the reported results (C. Li et al. 2020).

3.2 Anti-corrosion performance

Figure 4 shows photographs of Zn/NdFeB (i), Zn(Cr)/NdFeB (ii) and Zn(Ce)/NdFeB (iii) specimens after NSS tests for different time. For Zn/NdFeB, the surface becomes unsmooth at 24 h of NSS test, and white spots appear on the surface at 168 h. Red rust, indicating the complete failure of protective coating, appear after 288 h. Zn(Cr)/NdFeB shows much better anti-corrosion performances without any change after 120 h of NSS test. The passivation layer begins to dissolve at 360 h and disappears completely after 480 h. Red rusts appear in this specimen after 528 h, indicating the corrosion of the NdFeB matrix. For Zn(Ce)/NdFeB, the passivation layer becomes damaged in a large area after NSS test for 120 h. Red rusts appear at 432 h, which is in less time than that of Zn(Cr)/NdFeB.

Figure 4: Photographs of (i) Zn/NdFeB, (ii) Zn(Cr)/NdFeB and (iii) Zn(Ce)/NdFeB specimens after NSS treatment for different times.
Figure 4:

Photographs of (i) Zn/NdFeB, (ii) Zn(Cr)/NdFeB and (iii) Zn(Ce)/NdFeB specimens after NSS treatment for different times.

The potentiodynamic polarization curves of Zn/NdFeB, Zn(Cr)/NdFeB and Zn(Ce)/NdFeB specimens were measured to evaluate the corrosion resistance in 3.5 wt% NaCl solution, and the results were shown in Figure 5. Current density (Jcorr), potential (Ecorr) and polarization resistance (Rcorr) of the specimens are listed in Table 3. Compared with the Zn/NdFeB, Ecorr of the Zn(Cr)/NdFeB and Zn(Ce)/NdFeB shift positively of 0.17 and 0.11 V respectively, indicating the decreased corrosion tendency of the passivation layers on both Zn(Cr)/NdFeB and Zn(Ce)/NdFeB. The shift data shows a better passivation effect of the Zn(Cr)/NdFeB than that of Zn(Ce)/NdFeB, fitting well with the results of NSS tests. Both corrosion current densities of passivated Zn/NdFeB specimens are lower than that of Zn/NdFeB, indicating the low corrosion rates of passivated Zn/NdFeB specimens. It should be noted that the Jcorr of Zn(Ce)/NdFeB is one order of magnitude lower than that of Zn(Cr)/NdFeB, indicating the higher corrosion rate of Zn(Cr)/NdFeB, which may be ascribed to the oxidation of Cr(III) ions to Cr(IV) ions in the passivation layer.

Figure 5: Potentiodynamic polarization curves of Zn/NdFeB, Zn(Cr)/NdFeB and Zn(Ce)/NdFeB.
Figure 5:

Potentiodynamic polarization curves of Zn/NdFeB, Zn(Cr)/NdFeB and Zn(Ce)/NdFeB.

Table 3:

Current densities, potential and polarization resistances of Zn/NdFeB, Zn(Cr)/NdFeB and Zn(Ce)/NdFeB.

J corr (A/cm2) E corr (V) Cathodic slope Anodic slope R corr (Ω)
Zn/NdFeB 3.82 × 10−4 −1.30 4.97 0.93 193.1
Zn(Cr)/NdFeB 1.35 × 10−4 −1.13 3.22 11.26 222.4
Zn(Ce)/NdFeB 5.42 × 10−5 −1.19 3.28 13.16 488.0

Figure 6 shows Nyquist plots of Zn/NdFeB, Zn(Cr)/NdFeB and Zn(Ce)/NdFeB and inset is the equivalent circuit diagram. The results after simulation are tabulated in Table 4, where Rs represents solution resistance. The similar data in the three tests indicate electrolyte stability during the test. Radius of the arc curve reflects the electron transfer process on the surface. A larger radius represents greater anti-corrosion performance with a lower electron transfer rate. Rct data of passivated Zn/NdFeB are significantly higher than that of Zn/NdFeB. And Zn(Ce)/NdFeB shows the highest Rct of 867.1 Ω/cm2, which is almost an order of magnitude higher than that of un-passivated Zn/NdFeB.

Figure 6: Nyquist plots of Zn/NdFeB, Zn(Cr)/NdFeB and Zn(Ce)/NdFeB.
Figure 6:

Nyquist plots of Zn/NdFeB, Zn(Cr)/NdFeB and Zn(Ce)/NdFeB.

Table 4:

Impedances of Zn/NdFeB, Zn(Cr)/NdFeB and Zn(Ce)/NdFeB specimens after equivalent circuit simulation.

R s (Ω/cm2) R ct (Ω/cm2)
Zn/NdFeB 2.74 99.25
Zn(Cr)/NdFeB 3.54 207.3
Zn(Ce)/NdFeB 2.42 867.1

Figure 7 shows the mass gain plots of Zn/NdFeB, Zn(Cr)/NdFeB and Zn(Ce)/NdFeB during the highly accelerate aging test. And the mass gain (MG) and average corrosion rate (V) of the three specimens after aging tests for 480 h are listed in Table 5.

Figure 7: Mass gain plots of Zn/NdFeB, Zn(Cr)/NdFeB and Zn(Ce)/NdFeB during the highly accelerated aging tests with parameters of 120 °C, 2 × 105 Pa and 100% relative humidity.
Figure 7:

Mass gain plots of Zn/NdFeB, Zn(Cr)/NdFeB and Zn(Ce)/NdFeB during the highly accelerated aging tests with parameters of 120 °C, 2 × 105 Pa and 100% relative humidity.

Table 5:

Mass gain results of the Zn/NdFeB, Zn(Cr)/NdFeB and Zn(Ce)/NdFeB after aging test for 480 h.

M G (mg/cm2) V (mg/(cm2·h))
Zn/NdFeB 2.39 ± 0.02 (4.97 ± 0.03) × 10–4
Zn(Cr)/NdFeB 2.00 ± 0.04 (4.14 ± 0.06) × 10–4
Zn(Ce)/NdFeB 2.12 ± 0.02 (4.41 ± 0.03) × 10–4

M G of un-passivated Zn/NdFeB is always high and has the highest average corrosion rate of 4.97 × 10–4 mg/(cm2 h), indicating the poor corrosion resistance. The passivated specimens exhibit lower MG and V data compared with that of Zn/NdFeB, i.e., 2.0 mg/cm2 and 4.14 × 10–4 mg/(cm2 h) for Zn(Cr)/NdFeB and 2.12 mg/cm2 and 4.41 × 10–4 mg/(cm2 h) for Zn(Ce)/NdFeB. The mass gain results agree well with those of the NSS test. Both passivated Zn/NdFeB specimens possess better anti-corrosion performances than that of un-passivated Zn/NdFeB, and Zn(Ce)/NdFeB showed slightly lower anti-corrosion performance than that of Zn(Cr)/NdFeB.

3.3 Corrosion processes of the passivated Zn/NdFeB specimens

Figure 8 shows the potentiodynamic polarization curves and the surface morphologies of Zn(Cr)/NdFeB after NSS test for different time. The Jcorr and Ecorr are listed in Table 6. As shown in the polarization curve of Figure 10(i), when the NSS time is 0, 48 and 120 h, the values of Jcorr and Ecorr are very close. After NSS test for 360 h, the Ecorr negatively shifts from −1.1 to −1.36 V. The Jcorr and Ecorr keep stable in the first stage of NSS test due to the passivation layer on the surface of the Zn(Cr)/NdFeB specimens. When the passivation layer is corroded, the corrosive medium enters the Zn coating and even NdFeB matrix, leading to a negative shift in the Ecorr. When further increasing the NSS time, the corrosion product is passivated again at 480 h, with positive shift of Ecorr. Figure 8(ii)–(iv) shows SEM photographs of the surface of Zn(Cr)/NdFeB at 48, 360 and 528 h of NSS test, respectively. After 48 h of NSS test, small cracks can be seen on the surface of Cr passivation layer (Figure 8(ii)). The passivation layer disappears and many pores appear on the surface of the Zn coating. When Zn coating is thoroughly corroded and the NdFeB matrix is exposed, the Zn coating completely loses its protective function, shown in Figure 8(iv).

Figure 8: (i) Potentiodynamic polarization curves and (ii)–(iv) surface morphologies of Zn(Cr)/NdFeB specimens after NSS test for different times: (ii) 48 h, (iii) 360 h and (iv) 528 h.
Figure 8:

(i) Potentiodynamic polarization curves and (ii)–(iv) surface morphologies of Zn(Cr)/NdFeB specimens after NSS test for different times: (ii) 48 h, (iii) 360 h and (iv) 528 h.

Table 6:

J corr and Ecorr of Zn(Cr)/NdFeB specimens after NSS test for different times.

J corr (A/cm2) E corr (V)
0 h 1.35 × 10−4 −1.13
48 h 3.07 × 10−5 −1.10
120 h 8.77 × 10−5 −1.10
360 h 5.94 × 10−4 −1.36
480 h 9.18 × 10−5 −1.11
528 h 6.76 × 10−4 −1.15

To investigate the changes of Zn(Cr)/NdFeB specimens during the corrosion process, XPS analysis of Zn(Cr)/NdFeB after different NSS test time was performed, as shown in Figure 9. All the spectra core normalized according to the C 1s peak at 284.75 eV. Figure 9(i) shows the survey patterns of Zn(Cr)/NdFeB after NSS test, in which no other elements can be observed in addition to Zn, O, Cr and C. As the NSS time increased, the peaks of the O 1s electrons corresponding to the oxide show no significant change (Figure 9(ii)). After NSS test for 480 h, a new peak appears with a binding energy at 529.5 eV, corresponding to O in OH− group. Figure 9(iii) shows the high-resolution patterns of Cr 2p electrons. Cr remains in the surface layer of Zn(Cr)/NdFeB until the NSS test time of 480 h. After 528 h, no Cr ions can be detected on the surface of Zn(Cr)/NdFeB, indicating that the Cr passivation layer has been completely corroded. As shown in Figure 9(iv), the signal of Fe 2p electrons appears after NSS test for 480 h in the form of a hydroxide. At this time, the NdFeB matrix had been corrupted.

Figure 9: XPS patterns of Zn(Cr)/NdFeB after NSS test for different times. (i) Survey pattern; high-resolution patterns of (ii) O 1s, (iii) Cr 2p and (iv) Fe 2p electrons.
Figure 9:

XPS patterns of Zn(Cr)/NdFeB after NSS test for different times. (i) Survey pattern; high-resolution patterns of (ii) O 1s, (iii) Cr 2p and (iv) Fe 2p electrons.

XPS analysis of the corrosion process of the Zn(Cr)/NdFeB reveals that corrosion processes of passivation layer and Zn coating occurs simultaneously, which can be confirmed that Cr element exists in the specimens after NSS test for 480 and 528 h.

Figure 10 shows potentiodynamic polarization curves and surface morphologies of Zn(Ce)/NdFeB after NSS test for different time. Jcorr and Ecorr obtained according to the results of Figure 10(i) are listed in Table 7. After 48 h, the Jcorr increase by an order of magnitude, and the Ecorr also negatively shifts from −1.19 to −1.34 V. The degradation of the electrochemical properties indicate that the passivation film of Zn(Ce)/NdFeB has been damaged, exposing the Zn coating. As time extends, the passivation layer is consumed gradually with the increase of the corrosion current. As shown in Figure 11(ii), almost no complete passivation film can be observed on the surface of Zn(Ce)/NdFeB after NSS test for 48 h. After 432 h, Zn coating is completely consumed in part area, leaving only the NdFeB matrix.

Figure 10: Potentiodynamic polarization curves (i) and surface morphologies of Zn(Ce)/NdFeB after NSS test for different times: (ii) 48 h, (iii) 264 h and (iv) 432 h.
Figure 10:

Potentiodynamic polarization curves (i) and surface morphologies of Zn(Ce)/NdFeB after NSS test for different times: (ii) 48 h, (iii) 264 h and (iv) 432 h.

Table 7:

J corr and Ecorr of Zn(Ce)/NdFeB specimens after NSS test for different times.

J corr (A/cm2) E corr (V)
0 h 5.42 × 10−5 −1.19
48 h 1.50 × 10−4 −1.34
120 h 1.48 × 10−4 −1.39
264 h 4.32 × 10−4 −1.31
384 h 4.12 × 10−5 −1.21
432 h 1.37 × 10−4 −1.14
Figure 11: XPS patterns of Zn(Ce)/NdFeB after NSS treatment for different times. (i) Survey pattern; high-resolution patterns of (ii) O 1s, (iii) Ce 3d and (iv) Fe 2p electrons.
Figure 11:

XPS patterns of Zn(Ce)/NdFeB after NSS treatment for different times. (i) Survey pattern; high-resolution patterns of (ii) O 1s, (iii) Ce 3d and (iv) Fe 2p electrons.

Figure 11 shows the XPS patterns of Zn(Ce)/NdFeB after NSS test for different time. In the survey pattern of Figure 11(i), only Ce, O, Fe, Zn and C elements can be detected. And the disappearing of Ce and appearing of Fe occurs gradually as time extends. When the NSS time is 48 h, XPS patterns of O 1s electrons still keep characteristic two peaks with binding energies of 529.5 and 531.6 eV, indicating the complete passivation layer, as shown in Figure 11(ii). At 120 h, only one peak at 531.6 eV corresponding to O in ZnO can be detected, indicating the failure of the passivation layer, which also can be confirmed by the absence of Ce element after 120 h NSS time in Figure 11(iii). After NSS time of 384 h, Fe element signals appear in XPS pattern at 711 eV, as shown in Figure 11(iv). At the same time, double peaks of O 1s electrons can be observed again in Figure 11(ii), indicating that the corrosion product of NdFeB substrate is ferric hydroxide.

From the analysis of the corrosion process, the corrosion of passivation layer containing Ce ions is prior to the corrosion of Zn coating. The passivation layer can provide protection to Zn coating in a period of time. And even after the complete consumption of passivation layer, the corrosion tendency of Zn coating is still lower than that of unpassivated Zn coating.

4 Conclusions

In this work, the corrosion resistance and corrosion process of Zn/NdFeB after chromium passivation and cerium passivation were compared. The NSS tests show that Zn(Cr)/NdFeB has better corrosion resistance than that of Zn(Ce)/NdFeB, lasting 96 h longer than the Zn(Ce)/NdFeB. After NSS test for different times, XPS and electrochemical test analysis show that the improved anti-corrosion ability of Zn(Cr)/NdFeB is caused by the synergistic effect of the chromium passivation film and the Zn coating. And the cerium passivation film on the surface of Zn(Ce)/NdFeB can provide short-term protection for the Zn coatings and prolong its corrosion resistance. In conclusion, although the chromium-free passivation can enhance the corrosion resistance of NdFeB specimens, it still has a certain gap compared with the chromium passivation.

Corresponding authors: Guangqing Xu and Yucheng Wu, School of Materials Science and Engineering, Hefei University of Technology, Hefei230009, China; and Key Laboratory of Advanced Functional Materials and Devices of Anhui Province, Anhui Provincial International S&T Cooperation Base for Advanced Energy Materials, Hefei University of Technology, Hefei230009, China, E-mail: ,

Funding source: Hefei Municipal Natural Science Foundation

Award Identifier / Grant number: 2021026

Funding source: The Key Project of BGRIMM Technology Group Co. Ltd

Award Identifier / Grant number: 20190898000002

Funding source: The Key Research and Development Project of Anhui Province

Award Identifier / Grant number: 202004a05020048, 202004a05020051

Funding source: The Fundamental Research Funds for the Central Universities

Award Identifier / Grant number: JZ2019HGBZ0142, PA2019GDPK0043, PA2020GDJQ0026

Acknowledgments

The authors thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript.

  1. Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.

  2. Research funding: This work was financially supported by the National Key R&D Program of China (2021YFB3502400), Hefei Municipal Natural Science Foundation (2021026) and the Key Research and Development Project of Anhui Province (202004a05020048, 202004a05020051).

  3. Conflicts of interest: The authors declare no conflicts of interest regarding this article.

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Received: 2022-03-06
Accepted: 2022-09-27
Published Online: 2023-01-09
Published in Print: 2023-04-25

© 2022 Walter de Gruyter GmbH, Berlin/Boston

Artikel in diesem Heft

  1. Frontmatter
  2. Reviews
  3. Molten salt corrosion of candidate materials in LiCl–KCl eutectic for pyrochemical reprocessing applications: a review
  4. Effect of surface oxides on tritium entrance and permeation in FeCrAl alloys for nuclear fuel cladding: a review
  5. Original Articles
  6. Comparison of the corrosion resistances of chromium-passivated and cerium-passivated Zn/NdFeB magnets
  7. Long-term state-driven atmospheric corrosion prediction of carbon steel in different corrosivity categories considering environmental effects
  8. Bond of corroded reinforcement in strain resilient cementitious composites
  9. Effect of environmental variables and main alloying elements on the repassivation potential of Ni–Cr–Mo–(W) alloys 59 and 686
  10. Properties of sodium molybdate-based compound corrosion inhibitor for hot-dip galvanized steel in marine environment
https://doi.org/10.1515/corrrev-2022-0014
Schlagwörter für diesen Artikel
cerium passivation; corrosion resistance; trivalent chromium passivation; Zn/NdFeB magnet

Artikel in diesem Heft

  1. Frontmatter
  2. Reviews
  3. Molten salt corrosion of candidate materials in LiCl–KCl eutectic for pyrochemical reprocessing applications: a review
  4. Effect of surface oxides on tritium entrance and permeation in FeCrAl alloys for nuclear fuel cladding: a review
  5. Original Articles
  6. Comparison of the corrosion resistances of chromium-passivated and cerium-passivated Zn/NdFeB magnets
  7. Long-term state-driven atmospheric corrosion prediction of carbon steel in different corrosivity categories considering environmental effects
  8. Bond of corroded reinforcement in strain resilient cementitious composites
  9. Effect of environmental variables and main alloying elements on the repassivation potential of Ni–Cr–Mo–(W) alloys 59 and 686
  10. Properties of sodium molybdate-based compound corrosion inhibitor for hot-dip galvanized steel in marine environment
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