In Situ Synthesis of Graphene Oxide-Sealed LDHs Coatings: A Novel Approach to Enhancing Corrosion Resistance and Tribological Performance on Magnesium Alloys

Abstract

This study introduces an innovative approach to enhancing the corrosion resistance and tribological performance of magnesium alloys by in situ growing zinc-aluminum layered double hydroxide (ZnAl-LDHs) with graphene oxide (GO) sealing. Traditional LDHs coatings exhibit limitations in corrosion protection due to their porous structure. This paper advances the LDHs coating technology by integrating GO, forming a composite LDHs/GO coating on magnesium alloys. The novel incorporation of GO provides a unique two-layered defense system against corrosion: the GO layer serves as a high-resistance barrier to corrosive agents, while the LDHs layer absorbs NO₃⁻ ions, offering a secondary protection. The coating’s properties were meticulously characterized using techniques such as scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Fourier-transform infrared (FTIR) Raman spectroscopy, electrochemical assays, and friction-wear tests. Experimental findings reveal that the synergistic action between LDHs and GO results in significant improvements in corrosion resistance and friction reduction. Specifically, GO’s adherence to the LDHs coating’s pores and its ability to transfer into the friction layer during wear significantly enhances the coating’s integrity and stability. The successful in situ synthesis of LDHs /GO coatings opens new horizons for composite coatings, with potential implications across various industrial applications.

1. Introduction

Magnesium alloys have emerged as vital materials in defense technology, aerospace, automotive, and biomedical applications, courtesy of their high specific stiffness, strength, and biodegradable attributes [1,2]. However, a significant hindrance to their broader application lies in their poor corrosion resistance [3,4,5]. As a response, surface treatment techniques have been considered one of the foremost strategies to mitigate this corrosion susceptibility, thereby extending the service life of magnesium alloys [6,7,8,9].

Layered double hydroxides (LDHs) have been widely recognized as chemical conversion coatings that can substantially enhance the corrosion resistance of metallic substrates [10]. The conventional synthesis method for LDHs is through ion exchange [11]. However, the rapid initial ion exchange can potentially alter the LDHs’ nanosheet structure, diminishing their crystallinity. Moreover, the dissolution rate of the substrate during LDHs film preparation has a significant impact on the growth of the films [12]. The dynamics between host layers and interlayer anions also play a role as different forces lead to varying ease of interlayer anion entry into LDHs. Consequently, a proper selection of interlayer anions is crucial in the ion exchange reaction [13]. Additionally, the inherent porous and loose structure of LDHs implies that the film formed through ion exchange may not always provide robust corrosion protection.

Recent studies have identified certain deficiencies in the friction and corrosion resistance of pure LDHs coatings. A promising avenue to address these shortcomings has been found in the incorporation of graphene oxide (GO), leveraging the multiple positive charges in LDHs layers and negatively charged functional groups on GO’s surface [14,15,16]. Graphene oxide, being an oxidized derivative of graphene with -COOH and -OH functional groups on its surface [17,18], can self-adsorb on LDHs through electrostatic interactions. Combined with the robust bond between LDHs and the metal substrate, a stable composite coating of LDHs and GO can be formed [19,20]. For example, Shen et al. [14] and Zhang et al. [21] have demonstrated the fabrication of LDHs /GO composite coatings that exhibit remarkable anti-wear and anti-corrosion properties.

Building on these insights, this study focuses on the preparation of an LDHs /GO composite coating through a one-step method, exploring the synergistic influence of graphene oxide on the properties of in situ-grown LDHs film on Mg–Gd–Y alloy surfaces. A comprehensive analysis of the composition, microstructure, corrosion resistance mechanism, and tribological behavior of the coating has been conducted. The findings unequivocally illustrate that graphene oxide plays an indispensable role in augmenting both the corrosion resistance and tribological performance of the LDHs composite coating.

2. Experimental

2.1. Materials

The material used in this study is Mg–Gd–Y alloy, and its chemical composition was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The results are presented in

Table 1. Element composition of Mg–Gd–Y alloy.

Element	Gd	Y	Zr	Zn	Mg
Content (%)	9.0	3.0	0.5	0.6	Bal.

.

First, the Mg–Gd–Y alloy was machined into cylindrical specimens with a diameter of 11.3 mm and a height of 10 mm. Then, the specimens were embedded using the hot mounting method, exposing only one plane, and ground using silicon carbide abrasive paper with a grit size of 2000 mesh. After grinding, the specimens were placed in an alcohol-filled beaker and ultrasonically cleaned for approximately 15 min and then dried with cold air for later use.

2.2. Sample Preparation

First, the reaction solution for preparing the LDHs coating was prepared by mixing 0.05 M/L Zn(NO₃)₂-6H₂O and Al(NO₃)₃-9H₂O and stirring the mixture with a stirrer until homogeneous. NaOH was then added to the mixed solution to adjust the pH to about 9 and continued to be stirred with a magnetic stirrer for 10 min. Then, the Mg–Gd–Y alloy was placed in a reaction vessel and the mixed solution was poured in. The temperature was controlled at 90 °C, and the sample was kept warm for about 18 h, resulting in the LDHs coating sample as shown in Figure 1a. To prepare the LDHs /GO coating sample, 2 mL of anhydrous ethanol was added to the above-mixed solution, and then 0.03 g of graphene oxide was added and stirred with a magnetic stirrer for 2 min. Then, the prepared solution was placed in the reaction vessel and kept warm at 90 °C for about 18 h, as shown in Figure 1b.

2.3. Characterization

The surface morphology of LDHs and LDHs/GO coatings and the morphology of corrosion products were observed by scanning electron microscopy (SEM), and the distribution of elements on the coating surface was characterized by energy dispersive X-ray spectroscopy (EDS). The chemical composition and phase composition of different coatings were detected by X-ray diffraction (XRD) with a scanning angle range of 5°~80° and a scanning speed of 8°/min. The test results were analyzed using Jade 5.0 software to obtain information about the phase structure of the coatings. Raman spectroscopy was used to analyze the composition of LDHs and GO in LDHs and LDHs/GO coatings. Finally, three-dimensional laser confocal microscopy was used to capture the three-dimensional morphological changes of different samples before and after immersion in the corrosive solution.

The frictional behavior of the samples was tested using a friction and wear tester. During the test, the motor speed was set at 200 rpm, the rotation radius was 5 mm, and the applied load was 2 N. GCr15 steel balls were used as the friction pair. In addition, the Princeton 4000 electrochemical workstation was used, and a standard three-electrode system was used for testing. In this system, the sample exposed to a 3.5 wt% NaCl solution served as the working electrode, a platinum electrode served as the auxiliary electrode, and a saturated calomel electrode served as the reference electrode. During the tests, the potential range was set to ±10 V and the current range was set to ±250 mA. The scan rate for polarization curve measurements was set to 2 mV/s, while the scan frequency for impedance measurements ranged from 10 kHz to 100 mHz. Prior to testing, all samples were immersed in the solution for 30 min to ensure stable open circuit potential (OCP) readings. More than three parallel samples were run in all tests.

3. Results and Discussion

Figure 2 shows the process of preparing the LDHs/GO composite coating on the surface of the magnesium alloy. In this process, ZnAl-LDHs grow in situ on the magnesium alloy surface, and the charges on the metal layer of ZnAl-LDHs and the surface of GO are opposite [14]. Therefore, they form the LDHs/GO composite coating by electrostatic interaction.

Figure 3 shows the SEM images of LDHs and LDHs/GO coatings. Both coatings show a typical two-dimensional lamellar structure where dense lamellar structures are roughly arranged in intersecting clusters [14,22]. However, there are noticeable differences in the structure between the coatings prepared from different solutions. Figure 3a–c shows that the LDHs’ nanosheets have a loose arrangement with insufficient interlayer density and numerous pores. In contrast, Figure 3d–f shows that the LDHs/GO’s nanosheets are densely packed with minimal loose pores. In addition, the pores are filled with GO, resulting in a smoother and more uniform surface coating. These observations are consistent with previous reports in the literature [17,23]. The negatively charged surface of GO with functional groups such as -OH and -COOH facilitates its adsorption on the LDHs’ surface by electrostatic attraction, which is consistent with previous research results [19,20].

This difference is attributed to the smooth surface structure and surface tension of GO, which can fill the micro-indents on the surface of LDHs, resulting in a smoother and more uniform coating surface. In addition, the abundant oxygen functional groups in GO can form chemical bonds with the surface of zinc-aluminum hydrotalcite during adsorption, reducing the frictional force through solid solution effects and further reducing surface irregularities.

In Figure 4, the XRD patterns of LDHs and LDHs/GO coatings both show distinct peaks characteristic of LDHs. Reflection peaks appear at 10.8°, 34.3°, and 62.2°, indicating the formation of LDHs with NO₃⁻ interlayers in both LDHs and LDHs/GO coatings [21]. In addition, diffraction peaks of Mg(OH)₂ and Al₂O₃ can be observed in the XRD patterns, indicating the positive effect of Mg(OH)₂ and Al₂O₃ formation on the nucleation and growth of LDHs. This result is in agreement with the study of Liang et al. [24].

Compared to pure LDHs, LDHs/GO shows a stronger reflection intensity. This is because, during the preparation of the LDHs/GO composite coating, GO is present in the solution and interacts with LDHs to promote the crystallization process of LDHs. The presence of GO provides more nucleation sites and acts as a guide for NO₃⁻ to enter LDHs crystals and participate in the formation of the crystal structure, thereby enhancing the XRD reflection intensity. In addition, GO itself has a higher surface area and surface energy, which acts as a good catalyst and adsorbent, further facilitating the growth of LDHs crystals. The mechanism underlying the promoting effect of GO on the crystal growth of LDHs is as follows [14,19,20]: First, the surface of GO contains abundant functional groups such as hydroxyl and carboxyl, which can adsorb the precursors of LDHs such as metal ions. This adsorption provides a favorable platform for LDHs crystal growth and promotes nucleation. Second, the interface between GO and LDHs increases the number of reactive sites, thereby accelerating the growth rate of LDHs crystals. In addition, the two-dimensional structure and high oxygen content of GO increase the contact area for species exchange and diffusion during LDHs’ crystal growth, thereby facilitating the reaction. In addition, GO exhibits excellent conductivity and can serve as a bridge for electron transfer, promoting electron transfer in the LDHs’ crystal growth process. This electron transfer process is crucial for improving the reaction rate and efficiency in LDHs’ crystal growth.

The increased crystallinity of LDHs contributes to its stability. When the crystal structure is more complete, the irregular structures at the grain boundaries are reduced, resulting in better thermal stability and mechanical strength of LDHs. At the same time, the higher crystallinity of LDHs allows for a denser internal structure, stronger particle–particle cohesion, and better resistance to the external environment, resulting in improved stability.

Raman spectroscopy can be used to verify the in situ growth of GO in LDHs. Figure 5 shows the Raman spectroscopy results of LDHs and LDHs/GO composite films. In the Raman spectrum of LDHs, distinct characteristic peaks can be observed at 455 cm⁻¹ and 1061 cm⁻¹, which correspond to the absorption peaks caused by the stretching vibrations of M-O and NO₃⁻, respectively. In the Raman spectrum of LDHs/GO, additional D and G peaks appear at 1317 cm⁻¹ and 1598 cm⁻¹, respectively, which are characteristic of the defect sequence and graphite structure in carbon materials, indicating the successful incorporation of GO into LDHs/GO. Similar results were reported in the literature [25]. Based on the results of X-ray diffraction (XRD) patterns and Raman spectra, it can be confirmed that the coatings have successfully synthesized ZnAl-LDHs. The reaction mechanism can be described by the following Equation (1) [14,26]:

$${\mathrm{A}\mathrm{l}\left(\mathrm{O}\mathrm{H}\right)}_4^{-}+[{\mathrm{Z}\mathrm{n}\left(\mathrm{O}\mathrm{H}\right)}_4{]}^{2-}+{\mathrm{N}\mathrm{O}}_3^{-}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{ZnAl}-\mathrm{LDHs}$$

(1)

To investigate the corrosion resistance of LDHs and LDHs/GO coatings, we conducted three immersion tests on magnesium alloy specimens using the magnesium alloy substrate (Figure 6a), LDHs-coated specimens (Figure 6b), and LDHs/GO-coated specimens (Figure 6c) with a 3.5 wt% NaCl solution. Figure 6a shows the microstructure and energy spectrum of the magnesium substrate, which showed severe surface corrosion and the presence of numerous blocky corrosion patches after 48 h of immersion. Figure 6b shows the microstructure and energy spectrum of the LDHs coating, where the appearance of punctate corrosion pits and coating surface damage can be observed after 48 h of immersion, along with a significant increase in corrosion products. In contrast, Figure 6c shows the microstructure and energy spectrum of the LDHs/GO coating, which maintains a consistent surface morphology after 48 h of immersion, demonstrating excellent protective performance.

GO, as a carbon-based material similar to two-dimensional nanomaterials, has a high specific surface area and excellent dispersibility. It can improve the smoothness of the coating surface and act as an effective barrier material, effectively preventing the infiltration and erosion of corrosive substances from the external medium into the internal materials of the coating. Meanwhile, in the LDHs/GO coating, strong molecular interactions occur between LDHs and GO, resulting in mutual synergistic effects. This synergistic effect can enhance the overall stability and corrosion resistance of the coating by improving its resistance to the external medium.

A three-dimensional laser confocal microscope was used to examine the three-dimensional morphology of the magnesium alloy substrate and LDHs and LDHs/GO coatings after immersion in 3.5 wt% NaCl solution for 24 and 48 h, as shown in Figure 7, to further investigate their corrosion resistance.

From Figure 7a,b, it can be observed that the corrosion resistance of the magnesium alloy substrate is poor, as corrosion grooves appeared after 24 h of immersion. With the prolongation of time, after 48 h of immersion, white corrosion products completely covered the surface of the magnesium alloy, and some of the corrosion products fell off, forming deep corrosion pits. The maximum corrosion pit increased from 650.52 μm after 24 h of immersion to 996.16 μm after 48 h of immersion.

From Figure 7c,d, it can be seen that in the LDHs coating, the coating surface remained intact and smooth after 24 h of immersion. However, after 48 h of immersion, spot corrosion appeared, and the maximum corrosion pit increased from 127.37 μm after 24 h of immersion to 171.41 μm after 48 h of immersion.

From Figure 7e,f, it can be seen that there was no obvious corrosion morphology in the LDHs/GO coating even after 48 h of immersion. The maximum corrosion pit increased from 101.82 μm after 24 h of immersion to 101.92 μm after 48 h of immersion. This result is consistent with the SEM image results in Figure 6b, indicating that the LDHs/GO coating has better corrosion resistance than the LDHs coating.

This is because the strong molecular interactions between LDHs and GO produce synergistic effects in the coating, improving the overall stability and corrosion resistance. In addition, the LDHs/GO composite coating provides more complete and thorough coverage, improving the density of the coating and inhibiting localized corrosion reactions within the coating.

Figure 8a shows the polarization curve of the samples after immersion in 3.5 wt% NaCl solution for 30 min. By comparing and analyzing the electrochemical polarization curve, various parameters can be obtained, including corrosion potential (E_corr) and corrosion current density (i_corr).

Compared with the E_corr value of −1.621 VSCE for the magnesium alloy substrate, the E_corr value for LDHs is −1.603 VSCE, and that for the LDHs/GO coating is −1.524 VSCE. Among them, the LDHs/GO coating shows the largest shift with a forward shift of 0.097 VSCE. This indicates that the LDHs/GO coating has stronger corrosion resistance and is less prone to corrosion reactions. In addition, the LDHs/GO coating had the lowest i_corr value of 6.72 × 10⁻⁵ A/cm², with the i_corr value of the LDHs coating being similar to that of the LDHs/GO coating, while the i_corr value for the magnesium alloy substrate was 2.24 × 10⁻⁴ A/cm². Lower corrosion current density means smaller material defects and less corrosion per unit area in the same time period. The LDHs/GO coating had the highest E_corr and the lowest i_corr, indicating better corrosion resistance.

Based on the results shown in Figure 8b, the EIS curve obtained after 30 min of immersion in 3.5 wt% NaCl solution can be used to evaluate the corrosion resistance of the samples. Since the coating acts as an impedance layer, the corrosion resistance of the material can be evaluated based on the magnitude of the capacitance arc radius.

Based on the capacitance arc radius in Figure 8b, the following conclusions can be drawn: Compared to the magnesium alloy substrate, the corrosion resistance of both LDHs and LDHs/GO coatings is improved, but the LDHs/GO coating exhibits better barrier effectiveness. This is because the GO is attached to the LDHs aggregates, blocking the penetration of the original gaps in the LDHs and preventing the penetration of corrosive media, providing the first line of defense against corrosion. In addition, the NO₃⁻ in the LDHs layer can be exchanged for Cl⁻ in the corrosive media, providing the second line of defense against corrosion. In the structure of the LDHs, the layers are positively charged, allowing the adsorption of negatively charged anions between the layers to maintain charge balance. The interlayers of the LDHs can adsorb not only inorganic anions but also organic anions. Different anions have different affinities for LDHs, and through ion exchange reactions, anions with higher affinity (such as Cl⁻) can replace NO₃⁻ with lower interlayer affinity, so NO₃⁻ is often used as a precursor for ion exchange [27]. Therefore, the LDHs/GO coating provides comprehensive dual barrier protection and exhibits excellent corrosion resistance.

Based on the results of Figure 8a,b, it can be concluded that the LDHs/GO coating has good corrosion resistance. First, LDHs have a layered structure and interlayer ion exchangeability, which can adsorb harmful ions from the environment in their interlayers, preventing the corrosion ions from further penetrating into the coating and substrate, thus preventing the occurrence or development of corrosion. Second, as a structurally stable two-dimensional material, GO can effectively act as a barrier material, suppressing the occurrence of typical electrochemical oxidation reactions on the coating surface. Finally, the strong molecular interactions between LDHs and GO generate synergistic effects in the coating, improving its overall stability and corrosion resistance.

In addition, the LDHs/GO composite coating can provide a more complete and thorough coverage of the material surface, improving the coating density and suppressing the occurrence of localized corrosion reactions within the coating. Therefore, the LDHs/GO coating exhibits good corrosion resistance through multiple shielding effects.

To investigate the frictional properties of the LDHs/GO coating, friction and wear tests were conducted under three sets of 2 N loads, as shown in Figure 9. According to the friction and wear curves of the coating and the magnesium alloy substrate, under a 2 N load, the friction coefficient of the magnesium alloy substrate is about 0.6, the friction coefficient of the LDHs coating is about 0.3, and the friction coefficient of the LDHs/GO coating remains stable below 0.1.

For the LDHs coating, its lubricating effect is not ideal, and the coefficient of friction fluctuates significantly, possibly due to the accumulation of wear debris after the coating is consumed. However, the situation is different for the LDHs/GO coating, which has a lower coefficient of friction and less fluctuation. The lower friction coefficient of the LDHs/GO coating is probably due to the addition of graphene oxide (GO), which has a lubricating effect during the friction process, thus reducing the friction coefficient of the coating.

In addition, the LDHs/GO coating exhibits higher chemical stability, which allows for better corrosion resistance. This allows the coating to exhibit a more stable performance and maintain a lower coefficient of friction during the friction process. In conclusion, the results of the friction and wear tests indicate that the LDHs/GO coating has a significant reduction in friction.

Figure 10 shows the friction and wear morphologies of the samples under a 2 N load. Figure 10a,d shows the friction and wear traces of the magnesium substrate under a 2 N load, with a groove width of 735.515 μm and a groove depth of 31.932 μm. The surface is characterized by the presence of numerous wear debris, indicating severe wear of the magnesium substrate. In comparison, Figure 10b,e shows the friction and wear morphologies of the LDHs coating with a groove width of 619.474 μm and a groove depth of 22.861 μm, showing some improvement compared to the magnesium substrate. In addition, Figure 10c,f shows the friction and wear morphologies of the LDHs/GO coating, showing a groove width of 378.275 μm and a groove depth of 17.405 μm. The grooves show fine ridges and a small amount of wear debris.

Combining the results of Figure 9 and Figure 10, a large amount of wear debris and significant plastic deformation are observed on both the magnesium alloy substrate and the LDHs coating, resulting in higher coefficients of friction. However, on the GO-modified ZnAl-LDHs coating, the friction and wear marks appear smooth and shallow, with no distinct grooves. This is attributed to the significant friction-reducing effect of GO at the ZnAl-LDHs’ interface, which inhibits direct contact between the friction pair and the coating during the friction and wear process, thereby reducing abrasive and adhesive wear.

Based on the above results, the mechanism by which the LDHs/GO coating improves the corrosion resistance and friction reduction performance of the substrate is shown in Figure 11. Figure 11a shows the corrosion protection mechanism of LDHs/GO. Initially, during the immersion process, GO blocks the entry of corrosive Cl⁻ ions. When the coating surface is damaged, the interlayer anions (NO₃⁻) of the ZnAl-LDHs coating capture and replace the Cl⁻ ions, thereby slowing down the penetration of the corrosive medium. The reaction mechanism is represented by Equation (2) [14,16,28].

$$\mathrm{N}{\mathrm{O}}_3-\mathrm{LDHs}+{\mathrm{C}\mathrm{l}}^{-}\to \mathrm{C}\mathrm{l}-\mathrm{LDHs}+\mathrm{N}{\mathrm{O}}_3^{-}$$

(2)

Figure 11b shows the mechanism of friction reduction. During the friction process, the wear of the passive film on the ZnAl-LDHs surface leads to the wear of GO. GO has an extremely smooth surface, which allows relative sliding of its interlayer structure. In addition, GO can be adsorbed on the frictional interface to form a lubricating film-like structure. Moreover, if there are abrasive particles or surface irregularities in the friction interface, GO can disperse the abrasive particles and fill the surface pores, reducing the contact between the abrasive particles and reducing the frictional force [14].

4. Conclusions

This study introduced the concept of graphene oxide (GO) sealing to further enhance the corrosion resistance and friction reduction properties of LDHs coatings, leading to the in situ growth of LDHs/GO coatings on magnesium alloys. A comparative analysis demonstrated that the LDHs/GO coatings surpassed the magnesium alloy substrate and conventional LDHs coatings in terms of both corrosion resistance and friction reduction capabilities. The specific research findings can be summarized as follows:

During the corrosion process in corrosive media, graphene oxide significantly enhances the compactness of the LDHs coating surface and adheres to the pores within the LDHs layer. GO’s unique layered structure furnishes a high-resistance barrier to obstruct the penetration of corrosive agents, thus acting as a first line of defense. Concurrently, LDHs, with their distinctive layered architecture, facilitate the absorption of NO₃⁻ ions, mitigating the corrosive impact of these ions, thereby constituting a second line of defense. The synergistic effect between LDHs coating and graphene oxide (GO) finally endows the LDHs/GO coating with excellent corrosion resistance. Compared with the magnesium substrate, the E_corr value of the LDHs/GO coating is increased by about 6.00%, while the i_corr value is decreased by an order of magnitude.

LDHs/GO coatings can be grown in situ on magnesium alloys, forming a cohesive and sleek deposition on the surface. Graphene oxide’s adherence to the pores of the LDHs coating not only augments the overall coating stability but also plays an integral role during the friction process. Specifically, GO’s transfer into the friction layer mitigates tangential adhesive friction, guaranteeing the stability of the friction interface. The coefficient of friction of the LDHs/GO coating is less than 0.1, which is significantly lower than the 0.6 of the magnesium substrate and the 0.3 of the LDHs coating, which are reduced by about 83.33% and 66.67%, respectively.