Corrosion Resistance of Li-Al LDHs Film Modified by Methionine for 6063 Al Alloy in 3.5 wt.% NaCl Solution

Abstract

Methionine (Met) was introduced to modify the Li-Al layered double hydroxides (LDHs) film prepared on 6063 aluminum alloy by in situ method for the first time. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, Scanning electron microscopy, and X-ray diffraction confirmed the successful insertion of Met into LDHs film and revealed that the introduction of Met could make the LDHs film much denser. Electrochemical tests illustrated that the corrosion rate of the Met modified LDHs film was reduced by more than an order of magnitude compared with the bare Al alloy. Moreover, the corrosion rate of the modified LDHs film after immersion in 3.5 wt.% NaCl solution for 21 days was almost the same as that without immersion, which indicates that the modified film has good corrosion durability. The corrosion resistance of the scratched modified film could recover to the level without a scratch on the 14th day based on the scratch test results, meaning the modified film has a good self-healing property. Finally, the anti-corrosion mechanism of the Met was proved by molecular dynamic simulations and found that the enhanced corrosion resistance may be attributed to the addition of Met that slowed the diffusion of the corrosive medium Cl⁻ and water molecules.

1. Introduction

Aluminum alloy has good mechanical properties, high specific strength, and low density, so it is widely used in shipping, aerospace, and other fields. Unfortunately, the chemical properties of aluminum are very active, which makes it prone to various corrosions in practical applications [1,2]. Therefore, different surface treatment techniques, such as anodic oxidation, micro-arc oxidation, chemical conversion, and electroplating, were used to improve the corrosion resistance of aluminum [3,4]. However, once the pretreated surface film is partially damaged, it needs to be repaired or replaced manually, which is cumbersome, expensive, and dangerous. Consequently, the development of a new type of protective film with self-repairing properties in a mild environment (or with low energy consumption) is beneficial to extend the service life of the aluminum alloy, and therefore broaden its application practically.

Layered double hydroxides (LDHs) have an excellent physical barrier function to prevent the corrosive medium from contacting the substrate [5,6], and their excellent ion exchange function can effectively prevent the corrosion of chloride ions [7,8,9,10]. Generally speaking, in order to improve the corrosion resistance of LDHs, corrosion-resistant ions are often inserted between the LDHs layers [11,12]. However, the traditional preparation process of the LDHs film always needs a high temperature (about 125 °C [13,14]), high pressure, and a long reaction time [15,16]. LDHs prepared under mild conditions have attracted much attention because of low energy consumption, and Li-Al LDHs film is the most typical. It only takes tens of minutes to grow a Li-Al LDHs film in situ on the aluminum alloy, and the required reaction temperature is less than 70 °C [17]. Mata et al. [18] used the in situ formation method to directly prepare Li-Al LDHs on the surface of the anodized AA2024 aluminum alloy at room temperature for sealing, the experiments show that this method can greatly improve the corrosion resistance of the aluminum alloy. Zhang et al. [19] used the in situ formation method to generate Li-Al LDHs modified by aspartic acid (ALCC) on 6N01 aluminum alloy and found that the film layer releases Al³⁺ at the damaged location in the NaCl solution to make the solution biased alkali. The higher concentration of Al³⁺ in the nearby area regenerates LDHs in the corrosive electrolyte to produce self-repair. However, the research does not elucidate the effects of the introduced ALCC on LDHs’ corrosion durability and self-healing performance. Lin et al. [20] condensed vanillin and amine derived from disodium aspartate in absolute ethanol, and then used the solution to treat A6N01-T5 aluminum alloy in situ to generate improved LDHs. Studies showed that the excellent corrosion resistance of VLDH (disodium vanillin aspartic acid-modified LDHs) results from the physical barrier, ion exchange, and the corrosion inhibitory function of vanillin aspartate anion. Whereas, the effect mechanism of additive VLDH on corrosion resistance is still not clearly elucidated in this work. In addition, Li et al. [21] prepared 2-guanidinosuccinic acid with aspartic acid inserted it into Li-Al LDHs and found that the protective film has good self-repairing function and antibacterial properties. The work also broadly illustrated the reasons for the corrosion inhibition and self-repair effects of Li-Al LDHs loaded with corrosion inhibitors. However, the mechanism of the improved corrosion resistance of the modified Li-Al LDHs film is also still not sufficiently explained.

Amino acids are widely used as corrosion inhibitors for high purity at a relatively low price [22]. Methionine (Met) is an amino acid containing two functional groups (-NH₂ and -S-CH₃) in its molecule, and it is also a brilliant corrosion inhibitor [23,24,25]. E.E.Oguzie et al. [26] reported that Met could slow down the corrosion rate of mild steel in sulfuric solution. The proposed mixed inhibition mechanism [26] showed that the electrostatic interaction of the pre-adsorbed I⁻ ions on the steel surface increases the adsorption of Met cations by adding a small amount of KI, resulting in a synergistic effect between KI and Met. Using the sol–gel method, Habib Ashassi-Sorkhabi et al. [27] made a crack-free SiO₂ film on Mg alloy and then studied the protective effect of adding amino acid as a corrosion inhibitor into the film. The results showed that the film added with Met has no cracks and holes in the corrosive medium, improving anti-corrosion performance. Prior to this, they studied the corrosion resistance of amino acids such as methionine to aluminum in acidic solutions [28]. Yan et al. found that the Met released from Mg-Al LDHs can adsorb to the metal surface to protect the substrate from corrosion and achieve a self-repairing effect [29]. In general, Met is a suitable corrosion inhibitor. However, when Met is inserted into LDHs, its influence on the structure of LDHs film and impact on corrosion resistance have not been systematically studied yet.

Clarifying the corrosion mechanism and the nature of corrosion inhibitors in the conversion coating is beneficial for improving the film’s performance. In recent years, a theoretical calculation is always used to simulate the molecular’s structure and the film’s properties. Molecular dynamic simulations can quickly help to construct most materials’ structures and provide many different models to calculate relevant information [30,31,32]. Wei et al. [33] used molecular dynamic simulations to obtain the adsorption of CO₂ in some ZIF-8 analogs. Abdallah and his coworkers also used molecular dynamic simulations to investigate the inhibition performance of Dapxitine on AA6063 Al alloy, and they found that the quantum chemical descriptors are suitable for explaining the title molecule’s anti-corrosion performance [34]. Two-dimensional materials such as LDHs have complex structures, especially loaded with different kinds of corrosion inhibitors. Unfortunately, there are few reports about its theoretical calculations in terms of corrosion resistance.

In this study, it is hoped to prepare corrosion-resistant LDHs with self-healing properties and to study its corrosion-resistant mechanism. Li-Al-NO₃⁻ LDHs (NLDHs) film intercalated with Met was prepared on 6063 Al alloys via an easy in situ method. Then the anti-corrosion effect of NLDHs and Li-Al-NO₃⁻ Met LDHs (MLDHs) films on 6063 Al alloys in 3.5 wt.% NaCl solution was studied by polarization curve and electrochemical impedance spectroscopy (EIS). Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Fourier-transformed infrared spectroscopy (FTIR) were used to study the morphology and composition of LDHs films. Finally, the anti-corrosion progress of Met was simulated by molecular dynamic calculations, and a possible corrosion inhibition mechanism of the MLDHs film on the 6063 Al alloys was proposed.

2. Experimental Section

2.1. Materials and Chemical

The 6063 Al alloy with the chemical composition (in wt.%): 0.2–0.6 Si, 0.35 Fe, 0.1 Cu, 0.1 Mn, 0.45–0.9 Mg, 0.1 Cr, 0.1 Zn, 0.1 Ti, balance Al, was cut into blocks with a size of 30 mm × 30 mm × 0.5 mm for later use. Met (C₅H₁₁O₂NS, 98% purify), lithium nitrate (LiNO₃, 99% purify), and lithium hydroxide monohydrate (LiOH·H₂O, 99% purify) were purchased from Beijing Inno Chem Science & Technology Co., Ltd., Beijing, China. All chemicals were used as received without further treatment. All the solutions were prepared with deionized water.

2.2. Synthesis of NLDHs and MLDHs Films

Firstly, the 6063 Al alloy was abraded with grades 400, 800, and 1200 successively. Then, the abraded Al alloy was etched in 1 M NaOH and pickling liquid (1 M H₂SO₄ + 1 M HNO₃) at around 25 °C for 180 s, respectively, to remove the oxide layer on the Al alloy surface. The obtained aluminum alloy is placed in a desiccator for later use. Secondly, a mixed solution composed of 0.05 M LiNO₃ (99% purify), 0.12 M LiOH (99% purify), and 0.014 M Met (98% purify) was prepared, and the pH of the solution was adjusted to 10 by 1 M HNO₃. Then, put the pre-treated 6063 Al alloy into the solution and react at 60 °C for 15 min. Finally, the treated samples were dried at 90 °C for 15 min. NLDHs are synthesized using the same method as above, except that Met is not added during preparation. The schematic illustration for the preparation of the MLDHs is shown in Figure 1.

2.3. Characterization

Microstructures and surface morphologies of the specimens were investigated by SEM (S-4800, Hitachi, Japan). Crystalline structures of the LDHs film were characterized by XRD (D8 advance, Bruker, Germany) with Cu Kα radiation, and the monochromator operated at 40 kV and 30 mA. The 2-theta ranging from 5° to 85° was used for XRD measurement, the scanning rate was 8° min⁻¹, and the data analysis was done using the Jade software. FT-IR spectra of the LDH films were recorded by Iraffinity-1s (Shimadzu, Japan) in a spectral range of 400–4000 cm⁻¹. X-ray photoelectron spectroscopy (XPS) measurements were performed using an EscaLab Xi⁺ system (Thermo Scientific, Waltham, MA, USA) equipped with a monochromatic Al Kα X-rays source (1486.6 eV) and the analyzed area was about 500 μm in diameter. The wide survey spectra were recorded from 0~1400 eV with an energy step size of 1.0 eV with a passing energy of 100.0 eV. The high-resolution XPS spectra were scanned for two scans with an energy step size of 0.05 eV with a passing energy of 20.0 eV. The binding energy was calibrated using contaminant carbon at B.E. = 284.8 eV, and the spectra of XPS data were assessed using the Avantage software using Smart-type background subtraction, and the fits were performed with Gaussian/Lorentzian mix product function. During the self-healing performance test, the scratches were made by a multi-functional material surface performance tester (MFT-4000, Lanzhou Huahui Instrumental Company, Lanzhou, China), under a fixed load of 20 N, using the tool that comes with the instrument to make a 2 cm long scratch on the samples.

The specimens’ corrosion resistance performance was evaluated using an electrochemical workstation (CS2350H, Wuhan Corrtest Instruments, Wuhan, China) in 3.5 wt.% NaCl solution. A standard three-electrode system consisting of the specimens (working electrode), Pt sheet (counter electrode), and a saturated calomel electrode (reference electrode) were adopted. All samples were tested at room temperature (25 ± 2 °C), and the exposed area was 1 cm². EIS was obtained at the open circuit potential (E_ocp) in a frequency range from 100 kHz to 10 mHz with a potential perturbation of 20 mV. The electrical equivalent circuit model and the values were obtained from EIS plots by using Zview software. A polarization curve was performed using a sweep rate of 10 mV/s in the interval of ±0.5 V vs. E_ocp, the corrosion potential (E_corr), corrosion current density (I_corr), and Tafel slopes were obtained by the Tafel extrapolation method. Before each electrochemical test, the working electrodes were first immersed in a 3.5 wt.% NaCl solution for 30 min to obtain a stable E_ocp. When comparing the corrosion resistance of samples with different immersing times, several identical samples were prepared and immersed in the solution together, and then taken out and tested after different times. Additionally, most of the electrochemical data used in this paper have been verified for their reproducibility by repeating the experiment two or three times.

2.4. Theoretical Simulation

When MLDHs are broken, the Met flows out and adheres to the surface of the membrane. So investigation of the inhibition of Met to corrosive mediums is essential to demonstrate the anti-corrosion performance of the sample. MS software package developed by Accelrys was used to simulate the corrosion resistance of Met. First, a molecular model of Met was constructed and its energy and structure were optimized through the Forcite module. Then, the amorphous cell module was used to construct an amorphous tissue structure containing 80 Met molecules and then structural and molecular dynamics simulations were performed. The Al (111) surface, the aforementioned amorphous structure containing a corrosive medium particle, and the vacuum layer were then combined by the “build layer” command as a simulation system [35]. The thermal vibration of the metal surface at room temperature can be ignored, as all Al atoms are constrained during the calculation [36]. Method Andersen [37] was used to control the temperature, and the summation method of Van der Waals and Electrostatic was calculated by the atom-based method.

3. Results and Discussions

3.1. Characterization of NLDHs and MLDHs Films

SEM, XRD, FT-IR, and XPS characterization were performed to characterize the samples’ surface morphology, composition, and possible bonding modes (Figure 2). The surface morphologies of NLDHs and MLDHs films are shown in Figure 2a,b. It can be seen from Figure 2a that the aluminum alloy substrate is covered with the densely reticular crystal lamellae, showing a typical LDH layered structure [38,39]. After being modified by Met, the crystal flakes that make up the LDH film become smaller, and some gaps between the fragments are entirely covered by the product, resulting in a denser LDH film (Figure 2b). The phenomena indicate that the addition of Met into the conversion solution can change the LDHs’ surface morphology.

To further verify the changes in the LDHs caused by the introduction of Met, XRD and FT-IR were used to analyze the structures and compositions of the NLDHs and MLDHs (Figure 2c–e). The prominent diffraction peaks in Figure 2c are all derived from the Al alloy substrate (JCPDS: 04-0787). While the characteristic diffraction peaks of the LDH film are not obvious, the possible reason is that the LDH film is too thin. In the partially enlarged view of Figure 2c, two new diffraction peaks with diffraction angles at 2θ ≈ 10° and 2θ ≈ 20° appeared in both NLDHs and MLDHs, which are characteristic peaks of LDH film and correspond to (003) and (006) crystal face, respectively, (Figure 2d). Compared with NLDHs, the (003) peak position of MLDHs shifts from 2θ = 11.8° to 2θ = 10.8° because the Met inserted into the MLDHs is of a different size than the NO₃⁻ in NLDHs [40], and adding a corrosion inhibitor would reduce the intensities of 003 and 006 peaks, but the effect on the peak position was only to shift the 003 peak, so that the (006) peak position does not change [19,21,40]. The intensity of the diffraction peak in XRD represents crystallinity, and the grain size can be calculated by the half-width of the diffraction peak based on the Scherer formula [41]. In Figure 2d, the peak intensity of (003) and (006) in NLDHs are both stronger than that in MLDHs, which indicates that the Met introduction makes the crystallinity of the LDH film worse. Meanwhile, the (003) and (006) peaks of MLDHs are flatter and broader than those of NLDHs, proving that the grain size composed of LDHs nanosheets in MLDHs is smaller. Therefore, based on the XRD results, it can be concluded that the addition of Met can change the structure of the film and make the film denser, which is also consistent with the previous conclusion from SEM (Figure 2a,b).

Figure 2e shows the FT-IR spectra of NLDHs and MLDHs. In curve a, a broad peak at around 3437 cm⁻¹ corresponds to the stretching vibration of O-H in LDH layers [40]. The two bands located at about 1356 cm⁻¹ (curve a) and 1381 cm⁻¹ (curve b) are ascribed to NO₃⁻ asymmetric stretching vibration in the LDH interlayer. However, in curve b, the NO₃⁻ peak becomes broader than that in curve a, which may be caused by the introduction of Met leading to the overlap of the peak of COOH symmetric stretching vibration with the NO₃⁻ peak [19]. Combined with curve a, the extra peak located at 1092 cm⁻¹ in curve b corresponds to the C-S stretching vibration [29] of Met. The remaining peaks at 758 cm⁻¹, 662 cm^−1, and 534 cm⁻¹ are ascribed to the MO and M-OH (M = Al, Li) lattice vibrations in LDH layers [20]. The above results indicate that NLDHs were prepared successfully on Al alloy, and Met has intercalated into the interlayer of the LDHs. Figure 2f–l shows the XPS spectra of MLDHs and also proves the formation of MLDHs. The XPS spectra show that the binding energy of Al2p (Figure 2g), O1s (Figure 2h), and Li1s (Figure 2i) located at 74.07, 531.73, and 54.95eV, corresponds to Al-OH, OH⁻ and Li-OH, respectively. The N1s XPS spectrum (Figure 2j) manifested the binding energies of C-N located at 399.58 [20]. The spectrum of C1s (Figure 2k) contains three peaks, i.e., 284.8 eV, 285.75 eV, and 288.65 eV, which correspond to C-C, C-O, and O-C=O, respectively [42,43]. A high-resolution XPS spectrum of S2p is also shown in Figure 2l. Two intense peaks at 163.44 and 164.71 eV are assigned to S2p_1/2 and S2p_3/2 of C-S, and the peak at 167.62 eV is ascribed to SO_x [42,43,44]. In this system, C-S and SO_x can only come from the added Met, showing that Met has been successfully introduced into the LDH film. The FT-IR and the XPS results indicate that NLDHs have been prepared successfully, and Met has intercalated into Li-Al LDHs to form the MLDHs film.

Based on the above analysis, a possible formation mechanism of the MLDHs is proposed. First, the Al phase in Al alloy dissolves into Al³⁺ (Equation (1)), then the obtained Al³⁺ reacts with Met and Li⁺, which from LiNO₃ and LiOH to form MLDHs in conversion solution (Equations (2)–(4)) [45,46]. And the corresponding schematic diagram can also be seen in Figure 1.

$$\mathrm{Al}\to {\mathrm{Al}}^{3+}{+3\mathrm{e}}^{-}$$

(1)

$${\mathrm{LiNO}}_3\to {\mathrm{Li}}^{+}{+\mathrm{NO}}_3^{-}$$

(2)

$$\mathrm{LiOH}\to {\mathrm{Li}}^{+}{+\mathrm{OH}}^{-}$$

(3)

$${\mathrm{Li}}^{+}{+\mathrm{Al}}^{3+}{+\mathrm{Met}}^{-}{+\mathrm{OH}}^{-}\to \mathrm{Li}-\mathrm{Al} \mathrm{Met} \mathrm{LDHs}$$

(4)

3.2. Anti-Corrosion Performance

The corrosion rate is directly proportional to the corrosion current density (I_corr). The larger the I_corr, the greater the corrosion rate, indicating that the material has lower corrosion resistance. A polarization curves test is used to obtain the I_corr of NLDHs and MLDHs in 3.5 wt% NaCl to determine their corrosion resistance (Figure 3a). The corresponding fitting results are listed in

Table 1. Polarization parameters of the Al alloy substrate, NLDHs, and MLDHs in 3.5 wt.% NaCl solution.

Samples	Ecorr (V vs. SCE)	Ba (mV/dec)	Bc (mV/dec)	Icorr (A/cm2)
Bare Al alloy	−0.768	44.80	62.13	6.8 × 10−6
NLDHs	−0.908	237.06	35.57	7.1 × 10−7
MLDHs	−0.972	64.27	24.04	2.8 × 10−7

. Compared with bare Al alloy, the I_corr of NLDHs decreased from 6.8 × 10⁻⁶ to 7.1 × 10⁻⁷ A/cm², which indicates the NLDHs have a specific anti-corrosion effect on the aluminum substrate. When the NLDHs were modified with Met to generate the MLDHs, its I_corr continued to decrease to 2.8 × 10⁻⁷ A/cm², which shows that the addition of Met further improves the corrosion resistance of the film. The improved corrosion resistance of MLDHs may be attributed to the higher compactness of the MLDHs film (Figure 2a–d) and the effect of Met on reducing the corrosion of metals [28,29].

Another parameter obtained by the polarization curve is the corrosion potential, which can be used to evaluate the corrosion tendency of the film. From Table 1, it can be seen that the corrosion potential of NLDHs and MLDHs shows a negative shift, the possible reason is that E_corr decreases with the reduction in the cathode reaction rate. At the same time, NLDHs and MLDHs have a higher anodic Tafel slope than Al alloy; this is one of the main reasons why I_corr becomes smaller, further indicating that the corrosion resistance of the prepared LDHs is achieved by hindering the anodic reaction of corrosion electrochemistry. Compared with NLDHs, MLDHs have a lower I_corr, which means MLDHs have higher corrosion resistance. The improvement in corrosion resistance of MLDHs may result from the introduction of Met, which makes the LDH film denser. Moreover, the unsaturated bonds (e.g., C-S, SO_x) derived from Met are adsorbed on the metal surface, effectively blocking the direct contact between the metal substrate and the corrosive medium and leading to an increased corrosion resistance [24].

The durability of LDH film is an important index to evaluate its overall performance. The polarization curves of MLDHs after immersion in 3.5 wt.% NaCl solution for different times was tested to assess the anti-corrosion durability of the MLDHs (Figure 3b). The corresponding I_corr and corrosion potential of MLDHs obtained from Figure 3b are listed in

Table 2. Polarization parameters of MLDHs for different immersion time.

Time d	Ecorr (V vs. SCE)	Ba (mV/dec)	Bc (mV/dec)	Icorr (A/cm2)
0	−0.972	64.27	24.04	2.8 × 10−7
2	−0.827	31.02	34.28	3.1 × 10−7
7	−0.887	53.51	23.37	2.8 × 10−7
14	−0.949	45.36	19.81	4.7 × 10−7
21	−0.936	41.74	18.86	2.6 × 10−7

.

It can be seen from Figure 3b and Table 2 that after immersing for 2 days, the corrosion current density increased slightly compared with the case without immersion. After extending the immersion time to 7 days, the I_corr dropped back to the situation without immersion. The possible reason for the above phenomenon is that at the initial stage of corrosion (2 days), the corrosion electrolyte solution infiltrates into the LDH film to accelerate the corrosion reaction. As the corrosion progresses, some corrosion products accumulate on the LDH film in the middle stage of the corrosion (7 days), which has a certain repair effect on the damaged LDH film and leads to a decrease in the corrosion rate (I_corr). As the immersion time extends to the middle and late stages of corrosion (from the 14th day to the 21st day), the I_corr rises sharply. It then rapidly drops to the same situation as the uncorroded state. The rapid decrease in the corrosion current density in the late stage of corrosion may be attributed to the integrity of the LDH film and the corrosion inhibitor effect of Met, which can be verified by the SEM (Figure 4a,b) and XPS (Figure 4c–j) results. The surface morphology of the LDH film after immersion for 21 days showed that the LDH film was intact, and only a tiny amount of corrosion products accumulated on the surface (the red circle in Figure 4b). Furthermore, the XPS results of the LDH film before and after corrosion also proved the changes in MLDHs. Figure 4c–i shows the XPS spectra obtained from MLDHs before and after immersing for 21 days. The most crucial difference is the appearance of Cl 2p after immersing in 3.5 wt.% NaCl solution for 21 days (Figure 4c,j), which means MLDHs can capture the Cl⁻ into its structure [19]. The ability of the LDH films to capture Cl⁻ is the main reason for its improved corrosion resistance. It can be seen from Figure 4d,e that there is no change in the peak position and the relative peak area ratio of each bond for Al and O, which indicates that the bonding type and content of Al and O have not changed before and after corrosion. For C1s, which come from Met (Figure 4f), the relative peak area of the C–C bond decreases, and the relative peak area of the C–O/C–N bond increases after corrosion for 21 days. The changes in the peak area may be caused by the more release of Met on the surface of LDH after corrosion has occurred. After corrosion for 21 days, the peak position of Li1s is almost disappeared (Figure 4g). The reason for this result may be part of the Li⁺ decomposed from LDHs after being immersed in NaCl solution. While, the peaks at 163.44, 164.71, and 167.62 eV (Figure 4h), which are corresponding to C-S-C of S2p and SO_x, disappeared after corrosion for 21 days. It might be the Met inside the MLDHs diffuses into the corrosive medium due to exchange with Cl⁻, as the immersion time becomes longer, the amount of Met adsorbed on the surface of the MLDHs increases, hindering the progress of corrosion [47]. Based on the above reasons, the I_corr of MLDHs film after corrosion for 21 days returned to the level before its corrosion.

EIS was used to further verify the corrosion durability of the modified LDHs film to demonstrate its corrosion behavior at different times (Figure 5a–c). It can be seen from Figure 5a that the reaction resistance of the MLDHs before and after immersion for 21 days is at the same level, which means MLDHs have excellent long-term corrosion resistance. All curves in the Bode diagram (Figure 5b) have two peaks in phase angle diagrams (Figure 5b) and also two slopes in the modulus graph (Figure 5c), meaning they have two time constants. Meanwhile, based on the unique layered structure of LDHs, the equivalent circuit model in Figure 5d was used to fit the EIS data. In Figure 5d, R_s, R_cpf, and R_ct represent solution resistance, film resistance, and charge transfer resistance (reaction resistance), CPE_cpf, and CPE_dl represent the capacitance of conversion film and double-layer, respectively.

The EIS fitting results can be seen in

Table 3. EIS fitting parameters of MLDHs.

Time/d	Rs/Ω cm2	CPEcpf		Rcpf/Ω cm2	CPEdl		Rct/Ω cm2	Rtotal/Ω cm2	Ceqcpf/F·cm−2
		(Y)/F·cm−2·sn−1	n		(Y)/F·cm−2·sn−1	n
0	33.11	9.39 × 10−6	0.79	2.7 × 105	2.19 × 10−5	0.88	3.4 × 105	6.1 × 105	1.1 × 10−6
2	32.70	7.54 × 10−6	0.94	1.1 × 105	1.80 × 10−5	0.73	3.0 × 105	4.1 × 105	4.4 × 10−6
7	33.10	6.43 × 10−6	0.94	8.5 × 104	1.46 × 10−5	0.71	6.0 × 105	6.9 × 106	3.7 × 10−6
14	7.07	7.58 × 10−6	0.95	9.7 × 103	1.66 × 10−5	0.87	3.0 × 105	3.1 × 105	4.5 × 10−6
21	7.86	7.35 × 10−6	0.94	9.5 × 103	1.88 × 10−5	0.82	4.9 × 105	5.0 × 105	3.9 × 10−6

. The value of CPE_dl represents the actual corrosion area of the coating [19]. It can be seen from Table 3 that as the immersion time becomes longer, the value of CPE_dl first decreases and then increases, which may indicate that the corroded area of MLDHs first increases and then decreases, implying that MLDHs have an exceptional self-healing performance. At the same time, the change of film resistance R_cpf also follows the trend of first reducing and then increasing with the extension of the corrosion time. It can be seen from the Nyquist plots (Figure 5a) of the EIS diagrams that the Bode plot is not an ideal semicircle, which means the tested system is not an ideal capacitor, so the time constant CPE is used to describe the capacitance of the system and n (0.5 ≤ n ≤ 1) is introduced to calibrate the actual capacitance. n represents the deviation coefficient of the capacitance obtained from the system from the ideal capacitance C. Factor n also represents the frequency power of CPE; n = 1 means that CPE is an ideal capacitor (C). Furthermore, an effective capacitance (Ceq_cpf) can be obtained from the CPE parameters using the following equation [48,49]:

$$Y_{CPE}={\left(Ce{q}_{cpf}\right)}^n{\left[{(R_s)}^{-1}-{(R_{cpf})}^{-1}\right]}^{1-n}$$

(5)

The results are shown in Table 3, and the following equation can denote the Ceq_cpf of the samples:

$$Ce{q}_{cpf}=\frac{{\epsilon }_0{\epsilon }_{r}S}{d}$$

(6)

where ${\epsilon }_0$ is vacuum dielectric constant, ${\epsilon }_r$ is the relative dielectric constant of the samples; S is the actual surface area of the films, and d is the film thickness. Based on the relationship between film capacitance (Ceq_cpf) and film area (S) [45], it can be deduced that the film area (S) increase in the early stage of corrosion may originate from a small number of corrosion products covering the gap of LDHs. After that, S showed a trend of first decreasing and then increasing, the reason for the first decrease may originate from the film’s partial corrosion damage due to the increased corrosion. In the later stage, the film area (S) gradually increases due to self-healing. However, since the corrosion resistance of the MLDHs film cannot be maintained for a long time, the value of S decreases slightly in the final stage. The changing trend of CPE_dl and CPE_cpf also indicates that the Met modified film has good corrosion durability. R_ct and R_cpf can be used to characterize the corrosion rate in the systems with film on the surface. The sum of R_ct and R_cpf is represented by R_total (Equation (7)). It can be seen that the changing trend of R_total is precisely the opposite of the changing direction of the I_corr in the polarization curve (Table 2), which indicates that the R_total can be used to characterize the corrosion resistance. Besides, the impedance value of the curves in Figure 5c remains stable in the high-frequency range (except for the unimmersed sample), indicating that the voids in these samples are microscopic. As the immersion time is prolonged, the MLDHs react with the solution to form a new substance that fills the gaps between the MLDHs sheets, thus having a long-term corrosion resistance effect [50].

$$R_{total}=R_{cpf}+R_{ct}$$

(7)

3.3. Self-Healing Performance

To intuitively explain the MLDHs’ self-healing performance, the MFT-4000 was used to make scratches on the surface of the MLDHs, and then the specimens with scratches were placed into 3.5 wt.% NaCl solution. Figure 6 shows the surface morphology of the scratches with different immersion times. The surface morphology of the artificially scratched MLDHs is demonstrated in Figure 6a. There are no MLDHs at the scratch, but after immersing it in the 3.5 wt.% NaCl solution, MLDHs gradually appear at the scratches (Figure 6b–d). There were irregular flakes of LDHs at the scratches when the immersion time was 8 days, and the scratches have completely re-formed LDHs after the immersion time increased to 21 days. The above phenomenon clearly shows that MLDHs have a good self-healing performance.

Figure 6e shows the polarization curves of the scratched LDH film immersed in 3.5 wt.% NaCl solution for different times and the relevant fitting parameters are listed in

Table 4. Polarization parameter of MLDHs with or without a scratch after immersing for different immersion times.

Immersing Time/d	Ecorr (V vs. SCE)	Ba (mV/dec)	Bc (mV/dec)	Icorr (A/cm2)
Without scratch for 0 d	−0.972	64.65	24.09	2.8 × 10−7
With scratch for 0 d	−1.397	409.92	15.46	9.2 × 10−5
With scratch for 2 d	−1.349	79.35	18.47	3.1 × 10−5
With scratch for 7 d	−1.196	95.80	23.99	5.7 × 10−6
With scratch for 14 d	−0.945	25.72	23.50	3.3 × 10−7
With scratch for 21 d	−1.004	96.30	22.49	6.7 × 10−7
Without scratch for 21 d	−0.936	41.74	18.86	2.6 × 10−7

. It can be seen in Table 4 that the surface integrity of the scratched MLDHs film is damaged, causing the corrosive medium to easily penetrate into Al alloy substrate, resulting in the I_corr (9.2 × 10⁵ A/cm²) that is one order of magnitude higher than that of the sample without scratches (2.8 × 10⁻⁷ A/cm²). I_corr gradually decreased with the extension of the immersion time and reached the lowest value (3.9 × 10⁷ A/cm²) after 14 days of immersion, which is close to the I_corr of the sample without scratches (2.8 × 10⁻⁷ A/cm²), indicating that the corrosion resistance of the LDH film has recovered to the state without scratches. The phenomenon also proves that the MLDHs film has better self-healing properties. However, as the immersion time extended to 21 days, the corrosion current density increased twice as compared with the 14 days of immersion. The reason may be that the unscratched part of the LDH film was corroded during long-term immersion in 3.5 wt.% NaCl solution. Furthermore, compared with the corrosion of the sample without scratches for 21 days, the current density of the scratched sample increased by three times after 21 days’ deterioration. The reason may be that the scratched LDH film has not fully recovered to the unscratched films’ original thickness and density when immersed for 21 days, thus resulting in slight differences in I_corr and corrosion resistance.

3.4. Anti-Corrosion Mechanism of MLDHs

The molecular dynamic simulation was used to simulate the penetrating kinetics of the corrosive species (containing H₂O and Cl⁻) into the thin films composed of Met. The schematic diagram of the simulation system can be seen in Figure 7a,b, and the relative simulation results are shown in Figure 7c,d. Figure 7c,d shows the evolution curve of energy and temperature of the system with time when H₂O molecules diffuse in the film composed of Met. The curve fluctuates smoothly after 1500 ps, which means that the temperature and energy have stabilized, and the system has reached equilibrium. The same goes for other systems. Therefore, some stable data is used to calculate the diffusion coefficient of the corrosive species into the membrane consisting of Met. The penetrating dynamic behavior of pure H₂O medium has also been studied for comparison.

The diffusion coefficient of the corrosive species in different systems can be calculated by Einstein’s relation (Equation (8)) [51], among them, Ri(t₀) and Ri(t) are the positions of the i-th molecule or particle at the initial time and t, respectively, and $n$ is the total number of molecules or particles.

$$D=\frac{1}{6}\underset{t\to \infty }{\lim }\frac{d}{dt}{\displaystyle \sum }_i^n<{\left|R_i\left(t\right)-R_i\left(t_0\right)\right|}^2>$$

(8)

The diffusion coefficient of the corrosive species in each system is shown in

Table 5. Diffusion coefficients of H₂O and Cl⁻ in aqueous phase and Met films.

Membrane	109 D/(m2·s−1)
	H2O	Cl−
H2O	2.185	1.02
Met	0.225	0.123

. It is clearly demonstrated that the diffusion coefficients of H₂O and Cl⁻ in the film composed of Met are 0.225 and 0.123 × 10⁹ m²·s⁻¹, respectively. These diffusion coefficients are much smaller than those in the film without Met, indicating that H₂O and Cl⁻ are more challenging to diffuse in the Met-containing films.

Based on the above analysis, the corrosion protection mechanism of the MLDHs film is proposed in Figure 8. The anti-corrosion mechanism of MLDHs in 3.5 wt.% NaCl solution can be summarized as follows: at the beginning of immersion of MLDHs, corrosive species such as Cl⁻ are blocked on the alloy surface by LDHs, as immersion time goes by, some Cl⁻ gradually pass through the gap of the MLDHs platelets, and make contact with 6063 Al alloy substrate, Al easily acts as an anode in a corrosion galvanic reaction, and the oxygen reduction reaction takes place at the cathode so that Al is prone to anodic oxidation, which accelerates the occurrence of corrosion (Equation (9)). Then, the Al phase of MLDHs and Al alloy dissolves into Al³⁺ through the gap and defect of the MLDHs. Some Al³⁺ react with corrosive species such as Cl⁻ to form corrosion products, and some Al³⁺ react with Li⁺, which is derived from dissolved LDHs to regenerate MLDHs (Equations (10) and (11)) [45,50]. Both corrosion products and regenerated MLDHs can protect Al alloy substrate from further corrosion. Besides, when MLDHs are destroyed, the internal Met enters the solution, preventing the corrosive species from spreading inward.

$${4\mathrm{Al}+6\mathrm{H}}_2{\mathrm{O}+3\mathrm{O}}_2\to {4\mathrm{Al}\left(\mathrm{OH}\right)}_3$$

(9)

$${\mathrm{Al}}^{3+}{+4\mathrm{OH}}^{-}\to {\mathrm{Al}\left(\mathrm{OH}\right)}_4^{-}$$

(10)

$${\mathrm{Li}}^{+}{+\mathrm{Al}\left(\mathrm{OH}\right)}_4^{-}{+\mathrm{R}}^{-}{+\mathrm{OH}}^{-}{+\mathrm{H}}_2\mathrm{O}\to {\mathrm{Li}-\mathrm{Al} \mathrm{Met} \mathrm{LDHs}(\mathrm{R}=\mathrm{Met} \mathrm{or} \mathrm{NO}}_3)$$

(11)

4. Conclusions

The MLDHs were successfully prepared on 6063 Al alloy using the alloy substrate as the internal source of Al cations by the in situ growth method. MLDH nanosheets grow vertically on the surface of the Al substrate and completely cover the entire Al substrate. Compared with the bare LDHs, the MLDH nanosheet is much smaller, and the MLDH film is much denser. The polarization curve and EIS test show that MLDH can increase the corrosion resistance of Al substrate in 3.5 wt.% by at least ten times. Moreover, the corrosion current density and corrosion resistance of the MLDH film remained almost unchanged after being immersed in 3.5 wt.% NaCl solution for 21 days. In addition, the scratch test found that after 14 days of immersion, a layered LDH film will re-grow at the scratched area, and the corrosion current density of the MLDH will return to the level before the scratch. Molecular simulation calculations show that the diffusion coefficient of corrosive medium (Cl⁻ and H₂O) in Met is much smaller than that in deionized water, which may be the reason for the increased corrosion resistance of MLDH.