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Original Article
:19;
6062025
doi:
10.25259/AJC_606_2025

Efficient protection of sintered NdFeB enabled by incorporation of polyethyleneimine-grafted graphene oxide into silane films

, , , , , , , ,
aCollege of Materials and Chemistry, China Jiliang University, No. 258, Xiyuan Street, Qiantang District, Hangzhou City, Zhejiang Province, Hangzhou, Qiantang, China
bSchool of Materials and Energy, Foshan University, College of Materials and Energy, Foundation Laboratory Building, No. 18 Jiangwan Road, Chancheng District, Foshan City, Guangdong Province, Foshan, Chancheng, China

*Corresponding author: E-mail address: jiangli@cjlu.edu.cn (L. Jiang)

Received: , Accepted: ,
© 2026 Arabian Journal of Chemistry - Published by Scientific Scholar
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This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-Share Alike 4.0 License, which allows others to remix, transform, and build upon the work non-commercially, as long as the author is credited and the new creations are licensed under the identical terms.

Abstract

Polyethyleneimine modified graphene oxide (PEI-GO) was designed as an additive and subsequently co-deposited with bis-silane on sintered neodymium-iron-boron (NdFeB) magnets to prepare hydrophobic and anti-corrosive PEI-GO/silane composite films via an electrochemically assisted sol-gel method. Structural and compositional characterizations were carried out using fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), and Raman spectroscopy. Results confirmed successful grafting of PEI onto GO. Silane coupling agents linked to the PEI-GO additives, as well as the sintered NdFeB substrate, through covalent bonds. The corrosion resistance of the fabricated composite films in a 3.5 wt.% NaCl solution was assessed through electrochemical impedance spectroscopy and potentiodynamic polarization tests. The corrosion current density of PEI-GO/silane film under optimal deposition conditions was 8.365×10-8 A·cm-2, which is two orders of magnitude smaller than that of the neat silane film. Enhanced anti-corrosion performance of PEI-GO/silane composite film can be primarily attributed to the grafted PEI, which significantly enhanced the barrier effect of GO by improving its dispersion within the sol-gel system. Meanwhile, the co-deposited PEI-GO remarkably reinforced the structural integrity and compactness of the composite film. Furthermore, the composite coating exhibits outstanding hydrophobicity, with a water contact angle reaching 148.5°, and simultaneously maintains the magnetic properties of the sintered NdFeB magnets without detectable compromise.

Keywords

Corrosion resistance
Modified graphene oxide
Silane film
Sintered neodymium-iron-boron

1. Introduction

Sintered neodymium-iron-boron (NdFeB) magnets represent a new energy material renowned for their exceptional magnetic properties, presenting extensive applications in aerospace, electroacoustics, industrial, and electronics sectors [1]. However, the polyphase structure of NdFeB magnets, characterized by significant differences in electrochemical activity between the rare-earth-rich phase and the main phase, makes them prone to selective and rapid corrosion in electrochemical environments [2].

To enhance the durability of NdFeB permanent magnets against corrosion, the process of alloying, which entails the incorporation of supplementary metallic elements like Al, Mg, Cu, and Zn into the magnet [3-5], is employed to modify their composition and microstructure, thereby improving their resistance to corrosion. This adjustment decreases the electrochemical potential gap between the primary phase (Nd2Fe14B) and mitigates interphase corrosion. However, it plays a negative role in the reactivity of the grain boundary phase, as well as the magnetic energy product and remanence characteristics of the NdFeB materials [6]. Alternatively, surface coating methods such as chemical conversion [7], electroplating [8], and physical vapor deposition [9] are utilized. These techniques generate a protective layer (organic coatings [10], metal coating [11,12], composite coating [13], etc) to safeguard the substrate from erosion, effectively preventing corrosive media such as O2, H2O, or Cl- from penetrating the protective layer into the substrate [14].

Silane coupling agents feature inorganic and organic functional groups, enabling strong bonding with non-metallic and metallic materials [11]. The molecules of silane bond to the metal surface through the creation of covalent metal-siloxane linkages (Si-O-Fe) through hydrogen bonding between the hydrolyzed Si-OH group and the surface metal hydroxyl group (Me-OH) [12]. Thus, silane coupling agents function as specialized organic compounds for surface protection. Nevertheless, the anti-corrosion and mechanical performance of single-layer silane sol-gel films applied to NdFeB magnets are often suboptimal, frequently displaying defects such as holes and cracks, and the buildup of corrosive agents at the film-metal substrate interface promotes degradation through chemical reactions. The incorporation of nanofillers in film production, like zirconia [13], montmorillonite [15], benzotriazole [16], diamines, and carbon nanotubes [17], enhances their protective performance.

Among various nanomaterials, graphene oxide (GO) has been explored for preventing metal substrates from corrosion due to its 2D sheet structure, which enhances the barrier effects of composite films [18]. However, GO has a tendency to aggregate, attributed to its extensive surface area and significant influence of van der Waals forces. An effective strategy to improve dispersibility involves functionalizing GO, leveraging its numerous functional groups, which act as reactive sites for chemical modifications such as hydroxyl, carboxyl, carbonyl, and epoxide moieties positioned on the surface of GO nanosheets [19]. Amino-silanized GO has been synthesized using the sol-gel method. The material exhibits good dispersion within the silane film. The functionalized GO coating with amino groups shows improved anti-corrosion properties compared to the unfilled silane film. In another study, the modification of GO with polydopamine (PDA) through p-p interactions was investigated, and results indicate that incorporating evenly distributed GO-PDA nanosheets significantly enhances the corrosion resistance of coatings [20].

Polyethyleneimine (PEI) consists of a high density of amine groups along its linear macromolecular chains, enabling it to be chemically bonded to GO surfaces through condensation reactions involving carboxyl and amino functional groups. In recent years, investigations into PEI-GO nanosheets have predominantly emphasized their potential as advanced nanomaterials for fabricating high-performance composites, which exhibit superior mechanical properties and gas adsorption capabilities [21]. To date, current literature reveals a significant gap in research on the application of PEI-functionalized GO for enhancing anti-corrosion performance of sol-gel protective films. Furthermore, specific mechanisms by which PEI-GO nanosheets contribute to corrosion protection within these films remain unexplored.

Inspired by previous works [22,23], GO modified with PEI as an additive was prepared, and the anti-corrosive PEI-GO/silane film based on tetraethoxysilane (TEOS)/decyltrimethoxysilane (DTMS) was developed through an electrochemically assisted sol-gel method. The hybrid film acts as a unified, dense, and durable barrier, ensuring the protection of metallic surfaces in corrosive environments. The influence of PEI-GO incorporation on the microstructure and functionality of PEI-GO/silane film was investigated, along with an exploration of the film`s protective mechanism against corrosion.

2. Materials and Methods

2.1. Materials

Tetraethylorthosilicate (≥98%, TEOS), dodecyltrimethoxysilane (≥93%, DTMS), polyethyleneimine (99%, M.W. 10000), potassium nitrate (≥99%, KNO3), and GO were sourced from Aladdin Reagent Co., Ltd. The sintered NdFeB surface (with dimensions of 35.0 × 10.0 × 2.0 mm) underwent progressive polishing using 500, 800, 1000, and 1200-grit sandpaper. Subsequently, the samples were subjected to ethanol decontamination followed by nitrogen drying.

2.2. Preparation of modified GO

GO was prepared by oxidizing graphite, adopting an adapted version of Hummer’s technique [24]. The additive designated as PEI-GO was fabricated by functionalizing GO with PEI. In detail, 100 mL of the GO suspension (1.0 mg·mL-1) was mixed with PEI solution and stirred continuously at 60.0°C for 12.0 h. Thereafter, the modified GO was subjected to centrifugation and repeatedly washed several times to eliminate any remaining PEI. Finally, the PEI-functionalized GO sheets were isolated through freeze-drying.

2.3. Preparation of PEI-GO/silane film

Figure 1 illustrates the electrochemically assisted sol-gel procedure to fabricate the PEI-GO/silane film on the NdFeB surface. Initially, a 2.0 mg·mL-1 suspension of PEI-GO was formed by dispersing 20.0 mg of PEI-GO in absolute ethanol and sonicating it for 2.0 h. A neat bis-silane (TEOS/DTMS) solution consisting of 2.5 mL of TEOS, 2.5 mL of DTMS, 75 mL of absolute ethanol, and 20 mL of 0.2 M KNO3 solution was prepared. 10 mL of the PEI-GO suspension was added dropwise, while the pH was meticulously regulated to 3.5∼ 4.0. The adjusted mixture was then sonicated for 1.0 h to ensure uniform dispersion and then continuously stirred for 36 h at 35°C. The deposition process was conducted using a CHI660e (CH Instruments), applying a standard deposition potential of -1.4 V for 300 s, unless otherwise specified. The working electrode was a sintered NdFeB material, whereas a platinum plate with an area of 20.0 mm2 was utilized as the counter electrode. A saturated calomel electrode (SCE) functioned as the reference electrode in the experimental setup. Subsequently, the treated NdFeB specimen was washed with deionized water several times, cured at 60°C for 3.0 h, and finally kept in a dry container. Neat silane and GO/silane films were also prepared for comparison.

Schematic illustration of the PEI-GO/silane film preparing process.
Figure 1.
Schematic illustration of the PEI-GO/silane film preparing process.

2.4. Characterization

Fourier transform infrared spectroscopy (FTIR, Nicolet 470) and X-ray photoelectron spectroscopy (XPS, K-Alpha) were employed to assess the chemical composition of the specimens. Raman spectroscopy was conducted with a confocal Raman spectrometer (RS, HR Evolution) utilizing a 514 nm laser. Thermogravimetric data were revealed employing a thermal gravimetric analyzer (TGA, Mettler TGA/DSC1) at a constant rate of 10°C·min-1, extending across a temperature interval from 30°C to 600°C under nitrogen atmosphere. The crystal structures of GO and PEI-GO were analyzed using an X-ray powder diffraction (XRD, D8 Advance) with Cu-Kα radiation, covering a scan range between 5° and 90° with a 5°·min-1 rate. Micro-structure at various magnifications and element composition were analyzed through scanning electron microscope (SEM, ZEISS sigma 300) coupled with energy-dispersive X-ray spectroscopy (EDS, Oxford EDS 7426 system). KLA-Tencor stylus profiler (P-6) was utilized to determine the thickness and roughness of the specimens with and without films. Water contact angle tests were performed utilizing an FCA500B device; each specimen was treated with 5.0 μL of deionized water, and measurements from three randomly locations were averaged to determine a representative value. The magnetic characteristics of both the neat substrate and NdFeB with the PEI-GO/silane film were assessed using the VSM (LakeShore7404), and the corresponding demagnetization curves were then analyzed.

2.5. Corrosion resistance measurements

Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization experiments were achieved on a CHI660e equipped with a three-electrode system in a 3.5 wt.% NaCl solution. The setup comprised a working electrode (coated and uncoated NdFeB specimens), a reference electrode (SCE), and a counter electrode (platinum plate). Before measurements, the specimens with an exposed area of 10.0 mm2 were submerged in the electrolyte to allow for stabilization. During potentiodynamic polarization experiments, a scanning rate of 20.0 mV·min-1 was applied, and which data were collected within a potential interval of Ecorr ± 0.5 V for each sample (Ecorr represents the corrosion potential at equilibrium). EIS was performed using a sinusoidal waveform with a 50 mV amplitude, spanning a frequency spectrum between 100 kHz and 10 mHz. The collected data were subsequently processed and evaluated utilizing Z-View software.

3. Results and Discussions

3.1. Characterization of modified GO

FTIR analysis was utilized to identify the chemical structure of PEI-modified GO. As illustrated in Figure 2(a), distinct absorption bands associated with GO were detected. The critical peak at 3390 cm-1 was assigned to O-H stretching vibrations. Some characteristic absorption peaks at 1720 cm-1, 1585 cm-1, and 1041 cm-1 were related to C=O, C=C, and C-O stretching vibration, respectively [25]. After the functionalization with PEI, the C=O absorption band at 1720 cm-1 vanished, and a new peak corresponding to the amide linkage N-C=O emerged at 1650 cm-1, which confirmed the successful grafting of PEI to the matrix of GO through the formation of amide bonds [26]. Additionally, the weakened C-O peak at 1041 cm-1 and the presence of a bond attributed to C-N stretching vibration at 1355 cm-1 provided evidence of the integration of PEI onto the GO surface. This integration is likely facilitated by ring-opening reactions involving the amine groups in PEI molecules and the epoxy groups on GO [27].

(a) FTIR of PEI, GO, and PEI-GO nanosheets, (b) Raman spectra of GO and PEI-GO nanosheets.
Figure 2.
(a) FTIR of PEI, GO, and PEI-GO nanosheets, (b) Raman spectra of GO and PEI-GO nanosheets.

Raman spectroscopy was utilized to differentiate the disordered and defected graphene structures, with the resulting data shown in Figure 2(b). GO displayed two discernible peaks at approximately 1348 cm-1 (D band) and 1587 cm-1 (G band), resulting from structural imperfections linked to oxygen-based functional groups and the in-plane vibrational modes of sp2-hybridized carbon atoms, respectively [28]. After the incorporation of PEI, the positions of these peaks remained virtually identical to those observed in GO. As reported, the ratio of D band to G band intensities (ID/IG) is commonly recognized as a dependable metric for evaluating the level of structural disorder in graphene materials [29]. The modified GO nanosheets showed a higher ID/IG value of 1.647, in contrast to that of 1.242 for GO, indicating successful incorporation of PEI. This phenomenon suggests that the attached PEI chains displaced certain sp2-hybridized carbon atoms, leading to the formation of new sp3-hybridized carbon structures [30].

Figure 3 illustrates the XPS spectra for GO and PEI-GO nanosheets. The observation of nitrogen element for the PEI-GO sample provided direct evidence for the successful modification of GO by PEI. In the N 1s spectrum, distinct peaks corresponding to C-N (399.8 eV), N-C=O (400.6 eV), and N-H (401.5 eV) bonds were detected [31]. The C 1s spectrum of GO revealed five peaks at 288.6 eV (O-C=O), 287.5 eV (C=O), 286.5 eV (C-O), 285.6 eV (C-O-C), and 284.7 eV (C-C) [32]. In contrast, the C 1s spectra of PEI-GO showed new features: one peak attributed to C-NHR at 286.5 eV and another assigned to N-C=O at 287.6 eV, indicating the occurrence of ring-opening reactions and the formation of amide bonds through interactions between GO and PEI [26].

XPS spectra of (a) survey scan, C1s for (b) PEI-GO and (c) GO nanosheets, (d) N 1s for PEI-GO nanosheets.
Figure 3.
XPS spectra of (a) survey scan, C1s for (b) PEI-GO and (c) GO nanosheets, (d) N 1s for PEI-GO nanosheets.

SEM was utilized to characterize the morphological features of GO nanosheets before and after modification, whereas EDS was used to assess the elemental composition and distribution of PEI-GO. Figures 4(a, b) illustrate that GO nanosheets possessed a 2D structure characterized by a smooth surface and an exceptionally large surface area. After processing with PEI, the surface of PEI-GO nanosheets became rougher, with wrinkles becoming more pronounced (Figures 4c,d). Additionally, elemental analysis in Figures 4(e-h) confirmed a uniform distribution of C, O, and N elements across the surface of PEI-GO. As seen from Figure 4(i), GO and PEI-GO dispersions of 1.0 mg·mL-1 were ultrasonically treated in the sol-gel for 0.5 h. The stable dispersions were then stored smoothly for 24 h without disturbance. Following PEI modification, the color of the GO suspension transitioned from brown to black, indicating significant precipitation for GO and minimal delamination for PEI-GO. The reduction of hydroxyl and carboxylic groups occurred when PEI was grafted onto GO nanosheets. The lipophilic nature of the amine functional groups in PEI facilitates the effective dispersion of PEI-GO in various organic solvent systems [33]. From these observations, it is evident that PEI was effectively attached to the GO surface, thereby enhancing its compatibility and dispersibility after modification.

(a, b) SEM images of GO nanosheets and (c, d) PEI-GO nanosheets; (e-h) element mappings of the corresponding C, O, and N of PEI-GO; (i) comparison between the dispersion behavior of GO and PEI-GO nanosheets.
Figure 4.
(a, b) SEM images of GO nanosheets and (c, d) PEI-GO nanosheets; (e-h) element mappings of the corresponding C, O, and N of PEI-GO; (i) comparison between the dispersion behavior of GO and PEI-GO nanosheets.

Figure 5(a) depicts the TGA curves of PEI, GO, and PEI-GO nanosheets. The successful modification was further confirmed, which also quantified the PEI loading amount on GO nanosheets. The mass of both GO and PEI-GO nanosheets decreased with increasing temperature. GO nanosheets experienced noticeable weight losses between 50°C and 200°C, primarily caused by the release of physically bound water and the breakdown of oxygen-based functional groups on their surfaces. The slight weight loss of PEI between 60°C and 150°C is likely due to the release of trapped moisture as the temperature increases. Additionally, PEI exhibited a distinct thermal degradation of non-aromatic moieties at approximately 330°C [34]. It is notable that PEI-GO nanosheets exhibited remarkably higher weight loss than GO nanosheets, ranging from 250°C to 420°C. The variation in mass reduction observed between GO and PEI-GO within this temperature interval is primarily due to the degradation of PEI chains chemically bonded to the GO surface in the PEI-GO nanosheets, which was calculated to be 29.61% [35]. By 600°C, GO retained 42.18% of its initial weight, while PEI-GO retained only 17.51%.

(a) TGA curves and (b) XRD patterns of PEI, GO and PEI-GO nanosheets.
Figure 5.
(a) TGA curves and (b) XRD patterns of PEI, GO and PEI-GO nanosheets.

XRD spectra of PEI, GO, and PEI-GO nanosheets are presented in Figure 5(b). It can be observed that the neat PEI showed a wide diffraction peak at approximately 18.72°, aligning with the characteristics of an amorphous structure [36]. In the case of GO, a prominent diffraction peak appeared at 10.27°. This characteristic peak arises from the oxygen-based functional groups, including hydroxyl and epoxy moieties on the surface of GO [37]. After the modification of PEI onto GO nanosheets, the peak at 10.27° almost disappeared, signifying successful covalent modification and potential stripping of the GO structure. The appearance of a new broad peak at around 19.17° suggests a partial restoration of graphitic structure.

3.2. Characterization of the PEI-GO/silane films

The morphologies of neat silane film, GO/silane film, and PEI-GO/silane film were assessed by SEM imaging. In Figures 6(a,b), the neat silane film exhibited a rough surface with numerous dispersed silica particles adhering to it, and the water contact angle (WCA) was 151.5°. The gaps and voids among these microscopic structures can effectively capture a large amount of air, enabling water repellency and providing a protective physical barrier for sintered NdFeB substrate [38]. In Figures 6(c-f), GO and PEI-GO nanosheets possessed characteristic crinkles and folds within the silane films, with the nanosheets stacked on the film surface. It is worth noting that the PEI-GO/silane films presented significant integration without noticeable cracks. The distinctive layered structure of these nanosheets facilitates the anchoring of silanol groups, which effectively inhibits crack formation and propagation, thereby enhancing corrosion resistance. The WCA of the GO/silane and PEI-GO/silane films were found to be 146.0° and 148.5°, respectively. The existence of hydrophilic groups on the GO layer contributed to a lower contact angle for the prepared PEI-GO/silane composite films [39]. However, grafting PEI onto GO sheets partially eliminates certain oxygen-based groups, resulting in a modest rise in the WCA to 148.5°. Figures 6(g-j) shows the EDS result of the prepared PEI-GO/silane film, which mainly contains C, O, Si, and N elements. The presence of nitrogen confirms the successful incorporation of PEI-GO into the hybrid silane film. Element mapping of the PEI-GO/silane film revealed a uniform and continuous structure, verifying the successful adhesion of PEI-GO to the film surface.

(a, b) SEM images of neat silane film, (c, d) GO/silane film, (e, f) PEI-GO/silane film, (g-j) EDS and element mapping of PEI-GO/silane film.
Figure 6.
(a, b) SEM images of neat silane film, (c, d) GO/silane film, (e, f) PEI-GO/silane film, (g-j) EDS and element mapping of PEI-GO/silane film.

Figure 7 presents the profile images of sintered NdFeB, neat silane film, GO/silane film, and PEI-GO/silane film. Table 1 provides the roughness and thickness values of these samples. According to the profile outcomes, the NdFeB surface presented minimal roughness due to the pre-treatment of polishing (Figure 7a), with an average roughness (Ra) of 0.098 μm. Figure 7(b) indicates that the neat silane film possessed a high surface roughness with the Ra value of 0.872 μm and a thickness of 9.35 μm. From Figures 7(c and d), incorporating GO and PEI-GO nanosheets into the sol-gel system slightly decreased the surface roughness of prepared samples while increasing the film thickness. The Ra of the GO/silane and PEI-GO/silane films were measured to be 0.597 µm and 0.638 µm, respectively. The composite silane film, which is formed by organosilane bonds and silica microspheres, integrates elevated surface roughness with reduced surface energy, producing a superhydrophobic layer that prevents direct interaction between the substrate and water or other aggressive ions [40].

KLA-Tencor stylus profiler(P-6) images of (a) sintered NdFeB, (b) neat silane film, (c) GO/silane film, and (d) PEI-GO/silane film.
Figure 7.
KLA-Tencor stylus profiler(P-6) images of (a) sintered NdFeB, (b) neat silane film, (c) GO/silane film, and (d) PEI-GO/silane film.
Table 1. Surface roughness of samples and thickness of the films on the substrate.
Sample Ra (μm) Thickness (μm)
Sintered NdFeB 0.098±0.017 /
Neat silane film 0.872±0.038 9.35±0.74
GO/silane film 0.597±0.042 15.11±0.63
PEI-GO/silane film 0.638±0.035 13.82±0.43

±: Standard deviation

The chemical composition of the fabricated neat silane, GO/silane film, and PEI-GO/silane film samples was examined through FTIR analysis in Figure 8(a). The wide absorption band near 3450 cm-1 is associated with the vibration of hydroxyl groups, whereas the two prominent peaks at around 2925 and 2850 cm-1 arise from the C-H stretching vibrational modes from DTMS [41]. The absorption band observed at 1465 cm-1 was assigned to the -CH2 group within the alkyl chains. Additionally, a distinct C-N bond was detected at 1355 cm-1, while another prominent peak near 1620 cm-1 was associated with C=C stretching groups in PEI-GO [42], suggesting that PEI-GO sheets were successfully incorporated into the silane film. After the implantation of PEI-GO, the intensity corresponding to the vibration of hydroxyl groups reduced, potentially resulting from greater cross-linking interactions among silanol groups. The broad absorption feature near 782 cm-1 is linked to the vibrational stretching of Si-O-Fe bonds. The absorption band observed at 1060 cm-1 is assigned to the vibrational mode of the Si-O-Si bond [43], which stemmed from the self-condensation of silanol groups in the hydrolyzed TEOS/DTMS. The peak observed at 1031 cm-1 corresponds to the symmetrical stretching vibration of Si-O-C [44]. Oxygen-based functional groups on GO facilitate covalent functionalization through condensation and esterification, linking GO and DTMS. This vibration confirms the successful grafting of PEI-GO sheets onto the silane film, matching well with the EDS result.

(a) FTIR of neat silane, GO/silane and PEI-GO/silane film, (b) XPS survey scan of PEI-GO/silane film, high resolution spectrum of (c) O 1s, (d) Si 2p, and (e) N 1s.
Figure 8.
(a) FTIR of neat silane, GO/silane and PEI-GO/silane film, (b) XPS survey scan of PEI-GO/silane film, high resolution spectrum of (c) O 1s, (d) Si 2p, and (e) N 1s.

Figures 8(b-e) displays the XPS spectra of the synthesized PEI-GO/silane film. The survey indicates the presence of C, N, O, and Si elements. The O 1s spectrum reveals five distinct oxygen types within the PEI-GO/silane film. Specifically, peaks located at 533.1, 532.7, 531.8, 531.2, and 533.1 eV are associated with C-O-C, Si-O-Si, C-O-Si, C=O, and O-C=O functional groups, respectively (Figure 8c) [45]. The Si 2p spectrum exhibits three distinguishable peaks, detailed in Figure 8(d). The peaks at 102.4 eV and 102.8 eV refer to the Si-O-Si and Si-OH bond, whereas the peak at 103.6 eV is associated with the Si-O-C bond [46]. During the hydrolysis process of siloxane, a significant number of hydroxyl functional groups were generated. A portion of these groups interacted with the surface hydroxyls of PEI-GO, leading to the formation of Si-O-C linkages, whereas the remaining groups participated in self-condensation reactions to create Si-O-Si bonds [47]. The presence of the Si-O-C bond confirms the successful covalent functionalization of PEI-GO with silane. Additionally, Figure 8(e) shows that the peaks traced to N-H and C-N bonds are located at binding energies of 399.7 eV and 401.4 eV in the N 1s XPS spectrum [48]. These results collectively validate the fabrication of the PEI-GO implanted silane film on the NdFeB surface, which was consistent with the FTIR result.

3.3. Corrosion resistance measurements

Electrochemical tests were conducted to evaluate the corrosion protection property of various films on the NdFeB substrate in a 3.5 wt. % NaCl solution. Figure 9 illustrates the potentiodynamic polarization curve for sintered NdFeB, as well as those coated with neat silane, GO/silane, and PEI-GO/silane films. The sample coated with PEI-GO/silane exhibits a reduced anodic current density, suggesting efficient suppression of the anodic dissolution rate of the NdFeB sample, as shown in Eq. (1).

(1)
F e → 2 e − +   F e 2 + ⋯

Tafel plots of sintered NdFeB, neat silane film, GO/silane film, and PEI-GO/silane film-coated sample immersed in a 3.5 wt.% NaCl solution.
Figure 9.
Tafel plots of sintered NdFeB, neat silane film, GO/silane film, and PEI-GO/silane film-coated sample immersed in a 3.5 wt.% NaCl solution.

Additionally, PEI-GO/silane-coated specimen exhibits a markedly reduced cathodic current density compared to the other tested samples, indicating effective retardation of the oxygen reduction reaction rate, as presented in Eq. (2).

(2)
O 2 + 2 H 2 O + 4 e − → 4 O H −

These findings suggest that the silane layer formed on the NdFeB substrate through polycondensation effectively inhibits dissolution at anodic sites. The incorporation of PEI-GO nanosheets into the silane sol-gel system enhances the film’s anti-corrosion properties, resulting in reduced anodic and cathodic current densities [49]. Moreover, the PEI-GO/silane layer diminishes the available surface area of the metal that interacts with the electrolyte, primarily owing to the film’s physical barrier effect.

Tafel slopes for both anodic and cathodic reactions (βa and βc), along with the corrosion current density (Icorr) and corrosion potential (Ecorr), were determined through the conventional extrapolation method (Table 2). The protective efficiency (η) of NdFeB with different film samples was determined, as detailed in Table 2. The η is utilized for quantitative evaluation of the film’s anticorrosive property with Eq. (3) [50].

(3)
η = I c o r r 0 − I c o r r 1 / I c o r r 0 × 100 %

Table 2. Fitting parameters of sintered NdFeB, neat silane film, GO/silane film, and PEI-GO/silane samples from Tafel curves.
Sample

Ecorr

(V)

Icorr

(A·cm-2)

-βc

(mV·dec-1)

βa

(mV·dec-1)

η

(%)
Sintered NdFeB -0.776 7.496×10-6 246 52.0 /
NdFeB with neat silane film -0.768 1.073×10-6 200 87.2 85.7
NdFeB with GO/silane film -0.745 6.526×10-7 155 91.5 91.3
NdFeB with PEI-GO/silane film -0.663 8.365×10-8 103 98.4 98.8

Where I c o r r 0 and I c o r r 1 represent the Icorr of sintered NdFeB and NdFeB specimens coated with different silane films, respectively.

The Ecorr of the film-coated samples demonstrated a notable shift toward more positive values. The Icorr of NdFeB coated with PEI-GO/silane (8.365×10-8 A·cm-2) was reduced by one to two orders of magnitude relative to other samples, strongly indicating the enhanced protective capabilities of the film. The protective effects of neat silane and GO/silane films were relatively modest, with η values of 85.7% and 91.3%, respectively. The relatively inferior performance of the neat silane film is linked to the formation of diffusion pathways for corrosive medium (O2, H2O, and Cl-) due to defects and gaps in the silane matrix. Conversely, the PEI-GO/silane film demonstrates superior protection of the metal substrate in NaCl solution. This enhanced corrosion resistance results from the synergistic effects of its hydrophobic outer surface, stable siloxane framework, and the strong covalent linkages between implanted PEI-GO composite and silanol groups, collectively forming a robust protective layer that can efficiently block the infiltration and movement of corrosive ions at the interaction area between the corrosive electrolyte and NdFeB substrate [51].

EIS is a robust technique for evaluating anti-corrosion properties. Figures 10(a-c) display Nyquist and Bode curves of sintered NdFeB, neat silane films, GO/silane films, and PEI-GO/silane films in a 3.5 wt.% NaCl solution. The experimental data were analyzed using Z-View software, with the corresponding equivalent circuit schematic depicted in Figures 10(d,e). Here, Rct indicates the charge transfer resistance, while Rf denotes the prepared film resistance. Constant Phase Element (CPE) was employed in place of a perfect capacitor, and the impedance of CPE (ZCPE) can be calculated using the equation provided in Eq. (4) [52].

(4)
Z C P E = 1 / Q j w n

(a) Nyquist plots and (b, c) Bode plots for coated and uncoated NdFeB immersed in a 3.5 wt.% NaCl solution, and (d, e) equivalent circuit diagrams.
Figure 10.
(a) Nyquist plots and (b, c) Bode plots for coated and uncoated NdFeB immersed in a 3.5 wt.% NaCl solution, and (d, e) equivalent circuit diagrams.

where Q represents the modulus, and n denotes the phase shift associate with surface heterogeneity. Total resistance (Rt) that consists of Rf and Rct is utilized to figure out the protection efficiency (PE) as shown in Eq. (5, 6) [53].

(5)
R t = R f + R c t

(6)
P E % = ( R t − R t 0 ) / R t

Where R t and R t 0 represent the total resistance of the sintered NdFeB with and without protective films, respectively. The values of Rt and their associated protection efficiencies have been compiled in Table 3.

Table 3. EIS parameters for NdFeB samples without and with different films.
Sample CPEdl (Ω-1·Sn·cm-2) n1 Rct (kΩ·cm2) CPEf (Ω-1Sn·cm-2) n2 Rf (kΩ·cm2) Rt (kΩ·cm2) PE (%)
Sintered NdFeB 1.09×10-5 0.64 0.68 / / / / /
NdFeB with neat silane film 9.03×10-6 0.56 1.56 1.64×10-5 0.79 11.67 13.23 94.8
NdFeB with GO/silane film 4.13×10-6 0.67 4.75 5.76×10-6 0.81 50.95 55.70 98.8
NdFeB with PEI-GO/silane film 2.99×10-7 0.68 8.26 2.89×10-6 0.76 162.1 170.3 99.6

Figure 10(a) shows that the capacitive arc diameters of neat silane and GO/silane films were smaller than that of PEI-GO/silane film, suggesting superior barrier performance of PEI-GO/silane films [54]. Meanwhile, the film capacitance (CPEf) and electrical double layer capacitance (CPEdl) followed the order as below: PEI-GO/silane film < GO/silane film < neat silane film. This variation trend reveals that the PEI-GO/silane film exhibited the most effective water repellence and the smallest contact area with corrosive solution at the interaction area between the film layer and NdFeB substrate [55]. In general, Rt serves as an indicator of sample corrosion resistance, with greater Rt values corresponding to enhanced anticorrosion performance. The results in Table 3 indicated an Rt of approximately 170.3 kΩ·cm2 for the PEI-GO/silane film-coated sample, the highest among the four tested samples, resulting in a protection efficiency of 99.6% against corrosion in NaCl solution. This clearly demonstrates that the fabricated PEI-GO/silane film possessed the highest level of corrosion resistance, effectively safeguarding the NdFeB substrate. The highly water-repellent characteristic can capture substantial air on the film surface, functioning as an insulating layer that hinders electron movement between the substrate and the electrolyte [56]. Moreover, the PEI-GO nanosheets acted as effective isolating layers, prolonging the diffusion pathway for corrosive agents to access the NdFeB surface owing to their excellent impermeability.

The EIS data for the film-coated samples at varying immersion durations were examined to evaluate their long-term anti-corrosion performance, as depicted in Figure 11. The relevant parameters are detailed in Table 4. Following a 3-day immersion period, a reduction in impedance values was noted across medium and low frequency ranges for all coated specimens. This reduction is likely due to the development and expansion of cracks and micropores within the film structure. With prolonged immersion extending to 7 days, the semicircular diameter of the impedance plot continues to decrease, suggesting that more corrosive media are penetrating the film-substrate interface [57]. Additionally, the PEI-GO/silane film retains significantly higher impedance values, suggesting its sustained protective efficacy. As shown in Figure 11(f), the Rt values diminish progressively with prolonged immersion time for each coated sample. Notably, the decrease in Rt for the neat silane and GO/silane film is more pronounced than that for PEI-GO/silane film, underscoring the PEI-GO/silane film’s superior and stable anticorrosion performance.

EIS plots for different film-coated samples after (a, b) 3 days and (c, d) 7 days immersion in a 3.5 wt.% NaCl solution, (e) Equivalent circuit diagrams, and (f) the Rt values of film-coated samples in different immersion times.
Figure 11.
EIS plots for different film-coated samples after (a, b) 3 days and (c, d) 7 days immersion in a 3.5 wt.% NaCl solution, (e) Equivalent circuit diagrams, and (f) the Rt values of film-coated samples in different immersion times.
Table 4. EIS parameters of film-coated samples after immersion.
Sample CPEdl (Ω-1·Sn·cm-2) n1 Rct (kΩ·cm2) CPEf (Ω-1Sn·cm-2) n2 Rf (kΩ·cm2) Rt (kΩ·cm2)
NdFeB with neat silane film-3 days 4.35×10-6 0.63 0.44 2.21×10-4 0.54 10.82 11.26
NdFeB with GO/silane film-3 days 1.14×10-6 0.53 0.48 1.18×10-5 0.61 20.56 21.04
NdFeB with PEI-GO/silane film-3 days 1.97×10-8 0.72 3.10 4.08×10-6 0.59 82.63 85.73
NdFeB with neat silane film-7 days 3.68×10-5 0.65 0.35 1.51×10-4 0.51 4.23 4.58
NdFeB with GO/silane film-7 days 1.35×10-5 0.58 0.37 2.58×10-4 0.58 5.65 6.02
NdFeB with PEI-GO/silane film-7 days 2.12×10-7 0.70 0.43 1.69×10-5 0.53 10.75 11.18

Figure 12 illustrates the surface morphologies of different film-coated samples after 7 days of immersion in a 3.5 wt.% NaCl solution. Macroscopic observations revealed brown corrosion products and micropores on the neat silane and GO/silane film, while the PEI-GO/silane film maintained a relatively flat surface with no visible corrosion products (Figures 12a, d, g). SEM images indicate that the neat silane film-coated substrate displayed extensive loose corrosion products and cracks, whereas the GO/silane film-coated substrate also possessed evident cracks, suggesting a reduced corrosion resistance (Figures 12b, c, e, f). In contrast, the PEI-GO/silane film-coated sample exhibited significantly fewer corrosion products (Figures 12h, i), which was due to the homogeneous distribution and outstanding protective properties of the PEI-GO nanosheets.

Optical micrographs and SEM morphologies of different film-coated samples after 7 days immersion: (a-c) neat silane film, (d-f) GO/silane film, and (g-i) PEI-GO/silane film.
Figure 12.
Optical micrographs and SEM morphologies of different film-coated samples after 7 days immersion: (a-c) neat silane film, (d-f) GO/silane film, and (g-i) PEI-GO/silane film.

During the deposition process, localized hydroxyl enrichment facilitates the hydroxylation of metal surfaces (Fe-OH) through the applied potential to the cathodic side. Simultaneously, self-condensing reactions among silanol groups occur under alkaline catalysis, leading to the formation of stable networks that contribute to the enhanced protective capabilities of the silane film. Initially, the dispersed Si-OH groups react with Fe-OH groups on the surface of static NdFeB magnets, establishing strong Si-O-Fe bonds at the interaction area, forming a tightly interconnected three-dimensional network that greatly strengthens the film’s adhesion [58]. The hydrolyzed TEOS in the sol-gel system was converted into silica nanoparticles in a highly alkaline environment, promoting the development of a textured and porous morphology [59]. In this study, PEI grafted GO nanosheets can improve their dispersion within the sol-gel precursor, effectively preventing aggregation and thereby enhancing the barrier effect of GO. The co-deposited PEI-GO notably enhances the three-dimensional framework of the composite film, and these PEI-GO nanosheets fill the micropores of the silane film surface, augmenting the “labyrinth effect” during corrosion and consequently inhibiting the cathodic reaction (Figure 13). Therefore, the generation of iron hydroxides was minimized owing to the film’s enhanced resistance to corrosive medium (O2, H2O, and Cl-) [60]. In summary, the uniform integration of PEI-GO nanosheets into the hydrophobic neat silane film creates a more convoluted path for corrosive medium, effectively suppressing local corrosion of the sintered NdFeB magnets.

Schematic diagram of anticorrosion mechanism of PEI-GO/silane films.
Figure 13.
Schematic diagram of anticorrosion mechanism of PEI-GO/silane films.

3.4. Magnetic properties measurements

For comparison, the magnetic behavior of NdFeB magnets was analyzed both with and without film treatment. Table 5 provides a comparison of the magnetic parameters for uncoated and PEI-GO/silane film-coated NdFeB specimens, with the demagnetization curves shown in Figure 14. The values of remanence (Br(T)), coercivity (Hcj), and large maximum energy product ((BH)max) exhibited minimal variation following film deposition with changes of only 0.15% for Br(T), 0.87% for (BH)max, and 0.32% for Hcj. Thus, it can be concluded that the PEI-GO/silane film exerts negligible effects on the magnetism of NdFeB magnets. A summary of findings from prior studies on different sol-gel silane protective films containing GO or its derivatives applied to metal surfaces has been summarized in Table 6 [61-65]. The PEI-GO/silane composite film applied to NdFeB magnets displays exceptional performance. This includes a reduced corrosion current density, superhydrophobicity, and minimal impact on the magnetic behavior.

Table 5. Magnetic parameters of NdFeB with and without PEI-GO/silane film.
Sample Br (T) (BH)max (kJ·m-3) Hcj (kA·m-1)
Sintered NdFeB 1.311 345 2205
NdFeB with PEI-GO/silane film 1.309 342 2198
Typical demagnetization curves of NdFeB with and without PEI-GO/silane film.
Figure 14.
Typical demagnetization curves of NdFeB with and without PEI-GO/silane film.
Table 6. Comparison of various protective sol-gel silane films incorporating GO or modified GO from the literature.
Substrate materials Precursors for sol-gel Corrosion current density (A·cm-2) Resistance of film (kΩ·cm2) Contact angle (°) References
Al 2024 alloy GPTMS/GO 1.7×10-7 8.592 / [61]
Titanium alloy GPTMS/TEOS/GO 2.74×10-8 1.804 [62]
Mild steel MTES/TEOS/GO / 3.78 58.0 [63]
Copper HDTMS/TEOS/GO 3.8×10-8 22.1 146.4 [64]
Mild steel IBTS/TEOS/GO 2.7×10-6 1.476 88.0 [65]
Sintered NdFeB DTMS/TEOS/PEI-GO 8.365×10-8 162.1 147.0 This paper

4. Conclusions

A hydrophobic silane film incorporated with PEI-GO nanosheets was successfully fabricated on sintered NdFeB via an electrochemically assisted sol-gel approach. Comprehensive characterization, including FTIR, XPS, SEM, XRD, and TGA, confirmed the effective grafting of PEI onto GO, which also significantly improved the dispersion of GO within the silane sol-gel matrix. Structural and compositional analyses verified the coherent co-deposition of the silane film and PEI-GO, with SEM and elemental mapping demonstrating a uniform distribution of nanosheets throughout the silane network. The resulting PEI-GO/silane hybrid coating exhibited a water contact angle of 148.5°, revealing superior hydrophobicity.

More importantly, the composite coating significantly enhanced the corrosion protection of the NdFeB substrate, as evidenced by a markedly reduced corrosion current density (Icorr) of 8.365×10-8 A·cm-2 and an inhibition efficiency (η) of 98.8%. This improvement is attributed to the synergistic effect of the hydrophobic surface and the enhanced barrier properties provided by the well-dispersed PEI-GO nanosheets. Notably, the excellent anti-corrosive performance was achieved without compromising the intrinsic magnetic properties of the NdFeB material, highlighting the practical potential of this composite film for protecting high-performance magnetic substrates.

Acknowledgment

This project was supported by the Basic Public Welfare Research Program of Zhejiang Province (LGG22E010002), the Innovation Team Project for Colleges and Universities of Guangdong Province (2023KCXTD030), and National Natural Science Foundation of China (52001300 and 52171083).

CRediT authorship contribution statement

Zhu Tao: Investigation, Visualization, Writing- Original draft; Fan Zhang: Formal analysis, Investigation, Writing - Original draft; Li Jiang: Conceptualization, Methodology, Writing Review & editing, Resources, Supervision, Project administration, Funding acquisition; Siyu Chen: Investigation; Rui Wang: Data curation; Jiao Liu: Funding acquisition; Yumeng Yang: Datacuration, Formal analysis; Guoying Wei: Funding acquisition; Yu Chen: Conceptualization, Methodology, Resources.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Data will be made available on request.

Declaration of generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

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