Synergistic Corrosion Inhibition and UV Protection via TTA-Loaded LDH Nanocontainers in Epoxy Coatings

Abstract

To address the issue of metal corrosion in marine environments, we developed a nanofiller with corrosion resistance and UV absorption capabilities. This nanofiller is prepared using a coprecipitation hydrothermal method and consists of TTA intercalated into an LDH structure with an outer layer containing CeO₂, forming a layered double hydroxide (LDH) sandwich structure nanocontainer. TTA can be successfully released in corrosive environments, and the filler exhibits excellent corrosion inhibition and interlayer ion exchange properties. Polarization curve analysis shows that the corrosion inhibition efficiency of MgAl-TTA LDH@CeO₂ reaches 89.87%. After immersion in a corrosion solution for 60 days, the EP/MgAl-TTA LDH@CeO₂ coating maintains a high impedance of 3.88 × 10⁸ Ω·cm² in the low-frequency region, which is 166 times that of the pure EP coating. Even after 240 h of UV aging, the impedance of the EP/MgAl-TTA LDH@CeO₂ coating remains high at 3.10 × 10⁸ Ω·cm² (20,000 times higher than the pure EP coating). This significantly enhances the coating’s anti-aging and corrosion resistance, providing a feasible method for creating new long-lasting corrosion-resistant coatings in challenging environments.

1. Introduction

Metal materials are susceptible to a variety of external environmental influences and corrosion in the process of use, which will reduce the service life of metal materials; serious corrosion can even cause huge economic losses and fatal disasters [1,2,3]. However, organic coatings are susceptible to the generation of free radicals due to microporous defects during the shrinkage of the cured molecular chain and solvent evaporation stages, as well as unsaturated bond breakage in the coating due to light under outdoor conditions [4]. During photoaging, these free radicals form small molecules such as ketones, alcohols, and carboxylic acids [5]. As the coating continues to thin and shrink, it can lead to surface embrittlement and cracking, resulting in a coating that does not provide an effective barrier to corrosive media. Therefore, it is necessary to design multifunctional coatings prepared with corrosion-resistant and ageing-resistant fillers.

Recent studies have shown that smart micron/nano-scale corrosion inhibitor carriers (CISCs) can release the active substances they carry in response to environmental stimuli. This characteristic has attracted significant attention for its potential applications in corrosion protection [6]. The addition of CISCs with responsive irritation properties to organic coatings is a very promising approach to develop coatings with active and passive protective properties. Currently, mesoporous SiO₂ particles [7,8], mesoporous TiO₂ particles [9], eclogite nanotubes, and layered double hydroxide (LDH) [10] are widely used as inorganic nanocontainers.

LDH is known as a hydrotalcite-like compound and has a two-dimensional lamellar structure as well as interlayer ion tunable variability, which can give it the advantage of being an intercalation guest. Among them, MgAl-LDH is widely used in industrial fillers because it is cheap and can be produced in large quantities [11]. It is evident that such technologies have a very broad application potential in the research of corrosion-resistant materials.

Researchers have attempted to encapsulate 2-mercaptobenzothiazole (MBT) [12], benzotriazole (BTA) [13], 8-hydroxyquinoline (8-HQ) [14], and imidazolium-based ionic liquids [15] as organic corrosion inhibitors into LDH interlayers. The results show that LDH with an added corrosion inhibitor has a better corrosion protection effect on the metal substrate. The hydrophobicity of the methyl-branched chain attached to the benzene ring on TTA can enhance its corrosion inhibition on the substrate, and the corrosion inhibition efficiency can reach up to 91% for carbon steel in acidic media. However, TTA is rarely used for altering MgAl-LDH. Other benefits of TTA include low toxicity and low cost [16].

It should be noted that during the light aging process, solar UV rays have the biggest effect on surface coatings. UV absorbers, such as CeO₂, are typically added to coatings to enhance their anti-aging qualities and prolong their service life [4,5]. An et al. [17] added modified CeO₂ nanoparticles into an epoxy resin, and after UV aging for 240 h, the coating was still well protected, indicating that CeO₂ has good UV resistance, which offers an appealing method to increase the potential applications of UV aging-resistant coatings. However, prior research has only addressed the aging or corrosion resistance of coatings and has not yet enhanced these two properties simultaneously.

The novelty of this study lies in the use of LDH nanocontainers encapsulating TTA corrosion inhibitors as fillers for the coating, with the LDH being modified by CeO₂ nanoparticles that possess ultraviolet absorption and reflection properties. This modification imparts long-term corrosion resistance and anti-aging performance to the coating. The approach offers a novel strategy for the development of high-performance anti-corrosion coatings and provides valuable guidance for the preparation of coatings with both active and passive protective functions, as well as enhanced corrosion and aging resistance, specifically for marine environments.

2. Experiments

2.1. Materials

The chemicals included magnesium nitrate hexahydrate (Mg(NO₃)₂·6H₂O), aluminum nitrate nonahydrate (Al(NO₃)₃·9H₂O), sodium carbonate (Na₂CO₃), sodium hydroxide (NaOH), and sodium chloride (NaCl) were purchased from Tianjin Damao Chemical Reagent Factory (Tianjin, China), and cerium dioxide (CeO₂ 20–50 nm) and methylbenzotriazole (TTA) were purchased from Aladdin (Shanghai, China). All reagents were used directly without treatment. Epoxy resin (E44) produced by Shandong Tianmao New Material Technology Co. (Jinan, China) and curing agent (polyamide 650) produced by Jiangxi Yichun Xiangyu Adhesive Co. (Yichun, China) were used. X100 pipeline steel was used as the substrate with a size of 30 × 50 × 5 mm. The steel samples were mechanically polished with SiC sandpaper to 1500 grit. Then, the samples were cleaned with deionized water and ethanol and dried with hot air.

2.2. MgAl-TTA LDH Preparation

The co-precipitation hydrothermal method was used to synthesize MgAl-TTA LDH, and it primarily involved two solutions, A and B. Solution A consisted of 100 mL of distilled water with 7.69 g of Mg(NO₃)₂·6H₂O and 3.75 g of Al(NO₃)₃·9H₂O. Solution B consisted of 50 mL of distilled water with 4 g NaOH and 2 g TTA. Solution B was added dropwise to solution A with vigorous stirring under a nitrogen atmosphere and continuous stirring at 40 °C. The prepared suspension’s final pH was kept at roughly 10 and continuously stirred for 1h. The suspension was then allowed to react for 48 h at 140 °C in a stainless-steel autoclave. Finally, after cooling to room temperature, it was filtered and washed four times in a centrifuge at 4000 r/min and then dried in an oven at 60 °C. To prepare control MgAl-CO₃ LDH, TTA in solution B was replaced with 2 g Na₂CO₃ under an open atmosphere.

2.3. MgAl-TTA LDH@CeO₂ Preparation

Initially, solution A was mixed with 0.5 g of CeO₂ and ultrasonically agitated for 30 min to evenly distribute CeO₂. After 30 min of constant stirring of solution A, the dropwise addition of solution B began. The same procedure used to prepare MgAl-TTA LDH was then used to prepare MgAl-TTA LDH@CeO₂.

2.4. Epoxy-Based MgAl-TTA LDH and MgAl-TTA LDH@CeO₂ Composite Coating Preparation

The following technique was used to create composite coatings based on epoxy (EP). Initially, 4.0 g of anhydrous ethanol was mixed with 0.5% (in relation to the epoxy resin and curing agent) of MgAl-TTA LDH and MgAl-TTA LDH@CeO₂ powders, and the mixture was sonicated for 30 min. After 20 min of stirring with 2.0 g of epoxy resin, the hardener was added and quickly mixed for 10 min. Lastly, an applicator was used to apply the coating on the X100 steel surface. As a control, an epoxy coating devoid of filler was made. EP, EP/MgAl-TTA LDH, and EP/MgAl-TTA LDH@CeO₂ composite coatings were the names given to the prepared samples (Figure 1). They were cured for 48 h at room temperature and had a coating thickness of 80 ± 5 μm.

2.5. Characterization

The chemical structures of the synthesized MgAl-TTA LDH and MgAl-TTA LDH@CeO₂ were characterized by a Fourier Transform Infrared Spectrometer (FTIR, IRPrestige-21, Shimadzu Corporation, Kyoto, Japan) in the range of 400–4000 cm⁻¹. The prepared samples were characterized by an X-ray diffractometer (XRD, Bruker D8, Bruker Corporation, Billerica, MA, USA) using Cu Kα radiation (λ = 0.154 nm) at an operating voltage of 40 kV and 2θ angle of 5–80°. The embedding of corrosion inhibitors between LDH layers was investigated using a UV–vis spectrophotometer (UV–vis, UV-2450, Shimadzu Corporation, Kyoto, Japan) in the wavelength range of 190–400 nm. UV–vis was used to determine the release rate of the corrosion inhibitor at different times, and 10 mg of the sample was dispersed in 10 mL of 3.5 wt% NaCl solution and stirred for a certain period of time before the test. Then, the suspension was centrifuged, and the supernatant was taken for the test. The samples were analyzed morphologically using a field emission scanning electron microscope (SEM, GeminiSEM500, Carl Zeiss, Oberkochen, Germany). UV–vis diffuse reflectance spectra (UV–vis DRS) were measured using a UV–vis Diffuse Reflectance Spectrophotometer (Lambda 850+, PerkinElmer, Waltham, MA, USA).

2.6. Evaluation of Corrosive Properties

An electrochemical workstation (CS350, CorrTest, Wuhan KST Instruments Co., Wuhan, China) was used to measure electrochemical impedance spectroscopy (EIS) and kinetic potential polarization tests in a three-electrode system. The reference electrode and counter electrode were a saturated calomel electrode (SCE) and a high-purity graphite electrode, respectively. The working electrodes were coated steel substrates and bare steel samples with a test area of 1 cm². The UV aging of the composite coatings was carried out in a fluorescent UV chamber. The samples were aged in a series of cycles that included 20 h of UV light exposure and 4 h of immersion in a 3.5 weight percent NaCl solution, until the total UV aging time was 240 h. In accordance with GB/T 10125-2021 [18], the scratch coating’s resistance to corrosion was assessed in a salt spray chamber. According to the GB/T 9286-2021 standard [19], the adhesion of the coating to the substrate was evaluated using the cross-hatch test.

3. Results and Discussion

3.1. Characterization

The structural details and crystalline phase of MgAl-CO₃ LDH, MgAl-TTA LDH, and MgAl-TTA LDH@CeO₂ were analyzed using XRD, and the findings are displayed in Figure 2a. MgAl-CO₃ LDH showed characteristic diffraction peaks from the crystal faces of (003), (006), and (009) of the hydrotalcite layered structure, corresponding to 2θ at 11.3°, 22.8°, and 34.4°, respectively, that are compatible with the standard card (PDF#22-0700), which reflects the fact that the product has a complete layered structure and a good crystal structure. MgAl-TTA LDH@CeO₂ showed CeO₂ diffraction peaks, which coincided with the diffraction peaks of the standard card (PDF#43-1002), indicating that CeO₂ was effectively deposited onto MgAl-TTA LDH nanosheets.

The IR spectra of MgAl-CO₃ LDH, MgAl-TTA LDH, and MgAl-TTA LDH@CeO₂ are shown in Figure 2b. The IR spectra of MgAl-TTA LDH show that the absorption peaks detected at 3450 cm⁻¹ and 1390 cm⁻¹ correspond to the stretching vibrations of N-H and C-N bonds, respectively [12,20]. The IR spectra of MgAl-TTA LDH@CeO₂ similarly showed N-H and C-N bond absorption peaks. This confirms the successful embedding of TTA into MgAl-TTA LDH and MgAl-TTA LDH@CeO₂ interlayers.

Figure 2c shows the UV diffuse reflectance spectra of LDHs (converting diffuse reflectance to Kulbeka–Munk (K–M) values, which are equivalent to absorption values) [21]. According to Figure 2c, when CeO₂ is loaded onto the surface of MgAl-TTA LDH, its absorption energy for mid-wave and long-wave UV increases significantly compared with that of MgAl-TTA LDH. As shown in Figure 2d, the forbidden bandwidth shows a gradual decrease from 3.77 eV to 2.97 eV.

Thus, according to the results of Figure 2, the successful embedding of TTA into the LDH interlayers and the successful loading of CeO₂ onto the LDH surface were confirmed.

The SEM morphology of MgAl-CO₃ LDH, MgAl-TTA LDH, and MgAl-TTA LDH@CeO₂ is shown in Figure 3. After the introduction of TTA, the LDH remained lamellar compared to MgAl-CO₃ LDH, and the lamellar structures of MgAl-TTA LDH and MgAl-TTA LDH@CeO₂ were smaller than those of MgAl-CO₃ LDH, which may be due to the difficulty of TTA ion exchange between the layers of the LDH, restricting the growth of the crystals.

3.2. Effects of Hydrothermal Conditions

Figure 4a shows that under different hydrothermal temperature conditions, MgAl-TTA LDH exhibits characteristic diffraction peaks of hydrotalcite-layered structures at crystal planes (003), (006), (009), and (015). When the hydrothermal temperature is between 100 and 140 °C, the diffraction peak intensity of MgAl-TTA LDH continuously increases with the rise in temperature, and the 2θ angle shifts toward lower angles. The interlayer spacing of the (003) plane increases from 0.78 nm to 0.82 nm, indicating that TTA has successfully intercalated into the LDH layers. However, when the hydrothermal temperature reaches 160 °C, although the diffraction peak intensity significantly increases, the 2θ angle does not shift toward lower angles, and the interlayer spacing does not increase, suggesting that TTA has not intercalated into the LDH layers. Therefore, a maximum hydrothermal temperature of 140 °C is selected.

Figure 4b shows that under different hydrothermal time conditions, MgAl-TTA LDH exhibits characteristic diffraction peaks of hydrotalcite-layered structures at crystal planes (003), (006), (009), and (015). All the diffraction peaks of MgAl-TTA LDH shift toward lower angles, and the interlayer spacing of the (003) plane increases from 0.78 nm to 0.82 nm, indicating that TTA has successfully intercalated into the LDH layers. The change in hydrothermal time does not affect the intercalation efficiency of MgAl-TTA LDH. As the hydrothermal time is extended, the diffraction peak intensity of MgAl-TTA LDH increases, indicating enhanced crystallinity, suggesting that prolonging the hydrothermal time can promote the full growth of MgAl-TTA LDH crystals.

3.3. Corrosion Inhibitor Release Kinetics

The release kinetics of TTA from MgAl-TTA LDH@CeO₂ were ascertained by UV–vis spectroscopy in order to further explore the anticorrosion mechanism, as illustrated in Figure 5. The TTA standard curve graph in Figure 5a, where y and x stand for absorbance and concentration, was used to measure absorbance using 3.5 weight percent NaCl solutions with varying concentrations of TTA corrosion inhibitor. The slope–intercept equation was derived using linear fitting. The results showed that the concentration of corrosion inhibitor in the 3.5 wt% NaCl solution increased with time, reaching a maximum value at 50 min. This confirms that MgAl-TTA LDH@CeO₂ is able to successfully release corrosion inhibitor anions from the interlayer in solution.

3.4. Dynamic Polarization Curve

In order to investigate the corrosion inhibition efficiency of MgAl-TTA LDH@CeO₂, dynamic polarization tests were performed on X100 steel, and the polarization curve analysis results are shown in Figure 6. The Tafel fitting of the results in Figure 6 is listed in

Table 1. Polarization curve fitting parameters of X100 steel in different solutions.

Solution	Ecorr (V)	Icorr (A·cm−2)	vcorr (mm·a−1)	η (%)
3.5 wt.% NaCl	−0.6239	3.6358 × 10−5	0.4265	/
MgAl-TTA LDH 3.5 wt.% NaCl	−0.2509	9.6451 × 10−6	0.1132	73.47%

.

In order to investigate the corrosion inhibition efficiency of MgAl-TTA LDH@CeO₂, dynamic polarization curves of X100 steel in different solutions were analyzed, and the results are shown in Figure 6. The relevant electrochemical parameters of the dynamic polarization curves are listed in Table 1, including corrosion potential ($E_{corr}$), corrosion current density ($I_{corr}$), corrosion rate ($v_{corr}$), and corrosion inhibition efficiency (η). The corrosion rate is calculated using the following equation:

$$v_{corr}={\displaystyle \frac{I_{corr}\times M\times t}{\rho \times n\times F\times 3600}}$$

(1)

In the equation, the corrosion rate $v_{corr}$ is expressed in mm·a⁻¹; the corrosion current density $I_{corr}$ is in A·cm⁻²; M is the anodic metal chemical equivalent in g·mol⁻¹; t is time in s·a⁻¹; ρ is the material density in g·cm⁻³; n is the number of electrons transferred in the metal anodic dissolution; and F is the Faraday constant in Ah·mol⁻¹. For X100 steel, Equation (1) can be simplified as

$$v_{corr}=\mathrm{11,730}\times I_{corr}$$

(2)

The corrosion inhibition efficiency η is calculated using the following equation:

$$\eta (\%)=\left(1-{\displaystyle \frac{I_{corr}^{\prime }}{I_{corr}^0}}\right)\times 100\%$$

(3)

In Equation (3), $I_{corr}^0$ and $I_{corr}^{\prime }$ represent the corrosion current densities of X100 steel in 3.5 wt% NaCl and 3.5 wt% NaCl solution containing MgAl-TTA LDH@CeO₂, respectively. Generally, the smaller the $I_{corr}$, the better the material’s corrosion resistance [22].

It is noteworthy that, compared to X100 steel immersed in 3.5 wt% NaCl solution, the addition of MgAl-TTA LDH@CeO₂ reduces $I_{corr}$ from 3.6358 × 10⁻¹ to 3.6815 × 10⁻⁶ A·cm⁻², with a corrosion inhibition efficiency of 89.87%. This indicates that MgAl-TTA LDH@CeO₂ exhibits good corrosion inhibition, suggesting that the TTA inhibitor can be successfully released from the LDH interlayer and significantly reduce the corrosion reaction rate.

3.5. Effect of the MgAl-TTA LDH@CeO₂ Addition Amount

The EIS of epoxy coatings with different addition amounts of EP/MgAl-TTA LDH@CeO₂ is shown in Figure 7.

The EIS technique was used to evaluate the effect of the addition amount of MgAl-TTA LDH@CeO₂ on the corrosion resistance of the coatings. The resulting Nyquist and Bode plots are shown in Figure 7, and the fitting data are listed in

Table 2. EIS fitting parameters of epoxy coatings with different additions of EP/MgAl-TTA LDH@CeO₂.

Addition Amount	Rs (Ω·cm2)	CPEc (S·sn·cm−2)		Rp (Ω·cm2)
		Y0	n
0	1.08 × 10−6	3.59 × 10−9	0.80	7.56 × 108
0.5%	3.13 × 10−4	2.27 × 10−9	0.66	3.52 × 109
1%	1.02 × 10−4	1.48 × 10−9	0.90	5.86 × 108
3%	1.03 × 10−4	1.50 × 10−9	0.95	1.53 × 108
5%	3.20 × 10−4	2.64 × 10−9	0.82	1.20 × 108

. In the EP/MgAl-TTA LDH@CeO₂ system, the Nyquist plot shows a semicircular arc, where the coating with a 0.5% addition of MgAl-TTA LDH@CeO₂ exhibits the largest capacitive arc radius, with Rp of 3.52 × 10⁹ Ω·cm², which is more than three times the impedance value of the EP/MgAl-TTA LDH coating. Compared to EP/MgAl-TTA LDH, the EP/MgAl-TTA LDH@CeO₂ coating shows significantly improved corrosion resistance. However, when the addition amount exceeds 0.5%, the capacitive arc radius decreases as the addition amount increases, suggesting that an excessively high addition amount reduces the corrosion resistance of the coating. As shown in Figure 7b, the Bode modulus plot shows that the impedance modulus is highest at 1.79 × 10⁹ Ω·cm² when the addition amount is 0.5%. When the addition amount exceeds 0.5%, the impedance modulus continuously decreases. Furthermore, the Bode phase angle plot also shows that the phase angle is closest to −90° in the high-frequency range at the 0.5% addition amount, where fb is the lowest. Therefore, the optimal addition amount of MgAl-TTA LDH@CeO₂ in the EP coating is 0.5%.

3.6. Coating Durability Testing

The EIS method was used to assess the coatings’ long-term durability and corrosion resistance in corrosive solution simulations. The coatings’ EIS patterns at various immersion times are displayed in Figure 8. In the Nyquist plot, R is typically used to show the capacitance arc radius; the greater the value of R, the more corrosion-resistant the material is [23]. The coating’s resistance to corrosion improves with increasing impedance modulus |Z| 0.01 Hz in the low frequency range. Moreover, the breakpoint frequency (f_b), or the frequency with a phase angle of −45°, in the Bode phase angle diagram can be used to reflect the delamination region of the substrate-coating interface. Since the interaction between the coating and the substrate is significantly impacted by the delamination state, fb is also a crucial parameter for assessing the coating’s resistance to corrosion [24]. Generally speaking, the coating’s resistance to corrosion decreases with increasing f_b.

To better understand the corrosion characteristics, the EIS data were fitted using an equivalent circuit model. The equivalent circuit model is shown in Figure 9, where R_s, R_p, and R_ct represent the solution resistance, coating resistance, and charge transfer resistance, respectively, and CPE_c, CPE_dl, and W₀ represent the coating capacitance, double layer capacitance, and Warburg impedance, respectively. During the initial stage of corrosion and aging, water can penetrate into the coating through micropores caused by curing shrinkage and solvent evaporation, but it does not penetrate to the steel substrate surface. At this stage, the EIS data are fitted using the equivalent circuit shown in Figure 9a. As aging and corrosion time increase, the corrosive medium penetrates to the interface between the coating and substrate, forming a corrosion micro-battery near the steel substrate. The EIS data are fitted using the equivalent circuit shown in Figure 9b. As corrosion products continuously accumulate, the corrosion reaction rate is controlled by the diffusion process of the corrosion products at the coating–substrate interface. Therefore, in the later stages of immersion, the coating’s resistance to aging and corrosion is weaker, and the equivalent circuit in Figure 9c is used, introducing Warburg impedance to fit and analyze the EIS data.

As can be seen in Figure 8(a₁), the pure EP coating exhibits a high impedance value of 7.56 × 10⁸ Ω·cm² at the beginning of immersion. Additionally, after 45 days of immersion, Warburg impedance appears in the Nyquist plot, indicating that the coating has been damaged. The pure EP coating’s impedance dropped by two orders of magnitude from the initial stage to just 3.32 × 10⁶ Ω·cm² after 60 days of immersion. As shown in the Bode modulus plot in Figure 8(a₂), the impedance modulus of the pure EP coating at a frequency of 0.01 Hz also decreases with immersion time. Similarly, the phase angle in the high-frequency region decreases with immersion time and f_b increases, both of which indicate that the pure EP coating is broken and the corrosion resistance deteriorates after 60 days of immersion.

However, the impedance value of EP/MgAl-TTA LDH coating is 1.07 × 10⁹ Ω·cm² at the beginning of immersion, the impedance is still 2.08 × 10⁸ Ω·cm² after 60 days of immersion, and the Nyquist plots of the low-frequency region are all single capacitance arcs, which indicates that the substrate/coating interface has not yet been damaged after 60 days of immersion. Bode modulus plots also indicate that the impedance modulus decreases from 9.93 × 10⁸ Ω·cm² to 2.12 × 10⁸ Ω·cm² after 60 days of immersion. Additionally, the Bode modulus plot indicates that the impedance modulus drops from 9.93 × 10⁸ Ω·cm² to 2.12 × 10⁸ Ω·cm² following 60 days of immersion. According to the Bode phase angle plot, the phase angles of the coatings in the high-frequency region after 60 days of immersion are close to −90°, and the f_b increases with the increase in immersion time. This indicates that the EP/MgAl-TTA LDH coatings decreased the corrosion resistance of the coatings after 60 days of immersion in the simulated solution but still maintained the surface integrity.

Furthermore, compared to the pure EP and EP/MgAl-TTA LDH coatings, the impedance value of the EP/MgAl-TTA LDH@CeO₂ composite coating has the highest value at 3.52 × 10⁹ Ω·cm² at the start of the immersion and 3.88 × 10⁸ Ω·cm² at the end of 60 days of immersion. The capacitance arcs in the low-frequency region of the Nyquist plots indicate that no damage to the substrate/coating interface has occurred. The Bode modulus plot shows that the impedance modulus decreases from 1.79 × 10⁹ Ω·cm² to 3.75 × 10⁸ Ω·cm² after 60 days of immersion. The Bode phase angle plot shows that the phase angle in the high-frequency region is close to −90°, with the smallest increment in fb in the immersion time range of 60 days. Long-term durability experiments in corrosive solutions show that the EP/MgAl-TTA LDH@CeO₂ composite coatings have the best corrosion resistance and still have a good barrier effect against corrosive media after 60 days of immersion in simulated corrosive solutions [25].

3.7. Anti-Aging Testing

The anti-aging performance of different coatings under UV light was evaluated using the EIS method, and the coatings were subjected to EIS tests after UV aging for 20, 120, and 240 h, respectively. As shown in Figure 10(a₁), the pure EP coating has only one semicircular arc at the early stage of aging, indicating that the coating has integrity, and the semicircular arc impedance in the low-frequency region is only 3.18 × 10⁶ Ω·cm². The phase angle in the high-frequency region is near −90°, and the impedance modulus at 0.01 Hz is 2.78 × 10⁶ Ω·cm², as illustrated in Figure 10(a₂,a₃). The impedance of the semicircular arcs in the low-frequency region and the impedance modulus of the Bode plot both drop by an order of magnitude in comparison to the early stages of the aging period when the UV aging time reaches 120 h, and two capacitive arcs show up in the Nyquist plot. The fact that two time constants show up in the Bode-phase angle plot indicates that the coating’s surface has already sustained damage. When the coating is exposed to UV light for 240 h of aging, the semicircular arcs in the low-frequency region become smaller, with an impedance of only 1.46 × 10⁴ Ω·cm² and an impedance modulus of only 3.2 × 10⁴ Ω·cm², which are reduced by two orders of magnitude with respect to the early stage of aging. The Bode-phase-angle plot shows two time constants, and the high-frequency region’s phase angle drops to 67.88°. Therefore, the pure EP coating has undergone serious breakage after 240 h of UV irradiation and has poor anti-aging performance.

As shown in Figure 10b, the EP/MgAl-TTA LDH coating exhibits good corrosion resistance at the early stage of aging. After aging with UV light for 240 h, the Nyquist plot semicircular arc impedance decreases from 7.35 × 10⁷ to 5.49 × 10⁶ Ω·cm², and the impedance modulus decreases from 7.42 × 10⁷ to 7.03 × 10⁶ Ω·cm². It is noteworthy that Warburg impedance appears in the Nyquist plot after 240 h of aging, indicating that the coating is broken at this point. The phase angle in the high-frequency region also decreases with aging time, and although it decreases from 90.50° to 85.50°, it is still higher than that of the pure EP coating.

However, the EP/MgAl-TTA LDH@CeO₂ coating showed excellent aging resistance. In the low-frequency region of the Nyquist plot, the semicircular arc impedance drops from 6.26 × 10⁸ to 3.10 × 10⁸ Ω·cm² and the impedance modulus drops from 6.13 × 10⁸ to 3.13 × 10⁸ Ω·cm², a decrease of only 0.3 orders of magnitude, following 240 h of UV aging. The phase angle plots of the Bode demonstrate that, following UV aging, the phase angles in the high frequency region of the EP/MgAl-TTA LDH@CeO₂ coatings are all near −90°, with only a slight increase in f_b occurring. In addition, the Nyquist plot has only one semicircular arc, as well as the Bode modulus plot, which has a diagonal distribution, and the Bode phase angle plot has a phase angle close to −90° across a broad frequency range. These all indicate that the EP/MgAl-TTA LDH@CeO₂ coating still has surface integrity and excellent corrosion resistance after 240 h UV aging. This depends on the absorption and reflection of UV light by MgAl-TTA LDH@CeO₂.

3.8. Salt Spray Resistance Testing

The prepared coatings with artificial defects were put in a salt spray chamber to test their ability to withstand corrosion. The digital photos of the coatings taken after 96 h of exposure are shown in Figure 11, where it is evident that the scratches of the pure EP coatings are covered in rust stains, while the scratches of the EP/MgAl-TTA LDH coatings have fewer corrosion products and the surface of the EP/MgAl-TTA LDH@CeO₂ coatings has no visible corrosion spots. This indicates that the presence of MgAl-TTA LDH can somewhat improve the coating’s anti-corrosion performance.

3.9. Contact Aangle Testing

The increase in the hydrophilicity of the coating surface is due to the rupture of unsaturated bonds during aging, which produces a large number of hydrophilic groups, such as hydroxyl and carboxyl groups. The formation of these groups enhances the interaction between the coating surface and water, leading to a decrease in the contact angle. By monitoring the changes in contact angle, the chemical changes and anti-aging ability of the coating during the aging process can be effectively evaluated. If the coating maintains a high contact angle after aging, it indicates that the coating has good UV resistance and stability; conversely, a significant decrease in the contact angle suggests poor anti-aging performance of the coating. In summary, the evaluation of contact angle changes provides an effective method for studying the anti-aging performance of coatings [4]. Thus, the coating surface contact angle was used to assess the coating surface’s wettability both before and after UV aging. The water contact angle changes in various coatings before and after UV aging are contrasted in Figure 12. The findings demonstrated that the EP/MgAl-TTA LDH coating had the largest water contact angle, the EP/MgAl-TTA LDH@CeO₂ coating had the largest water contact angle, and the pure EP coating had the smallest water contact angle prior to UV aging. All three coating surfaces have a decreasing water contact angle following UV aging. The reductions in water contact angle for pure EP, EP/MgAl-TTA LDH, and EP/MgAl-TTA LDH@CeO₂ coatings were 37.16%, 23.45%, and 18.94%, respectively. These findings imply that MgAl-TTA LDH and MgAl-TTA LDH@CeO₂ can improve the wetting stability of epoxy coatings by delaying their aging.

3.10. Adhesion Strength Testing

The adhesion strength test results of the EP/MgAl-TTA LDH@CeO₂ epoxy coating with an addition amount of 0.5% are shown in Figure 13. The adhesion strength test grading standard is listed in

Table 3. Grading standard of test results.

Grading	0	1	2	3	4	5
Appearance						The peeling condition exceeds grade 4.

, which includes levels 0, 1, 2, 3, 4, and 5, with level 0 representing the highest adhesion strength to the substrate. Based on the adhesion strength test results, the coatings are graded. After the 3M tape peeling test, the adhesion strength of the EP coating is graded as level 0, indicating good adhesion. After the 3M tape peeling test, the adhesion strength of the EP/MgAl-TTA LDH@CeO₂ coating is also graded as level 0, which indicates that the prepared EP/MgAl-TTA LDH@CeO₂ coating exhibits excellent adhesion to the substrate.

3.11. Mechanisms

The negatively charged triazole group of TTA adsorbs to the LDH laminate, and its neighboring benzene ring maintains the stable structure of LDH through the π-π interaction. The nanosheets of LDH are stacked layer by layer in the coating to fill the voids and form a labyrinth effect, which prevents the diffusion of corrosive media and improves the water barrier of the coating, which is the first line of defense. In addition, MgAl-TTA LDH@CeO₂ acts as a UV absorber, converting UV rays into non-destructive visible light and heat, which is the second line of defense. When the coating is damaged, MgAl-TTA LDH is able to capture Cl⁻ and release the TTA corrosion inhibitor, which chelates with the local breakage and slows down the corrosion process, which is the third line of defense. Therefore, it is of great significance to extend the life of steel structures in harsh climatic conditions by constructing functional coatings with a triple line of defense to achieve good water insulation, ageing resistance, and corrosion resistance.

4. Conclusions

In summary, based on the coprecipitation hydrothermal method and the electrostatic adsorption of CeO₂ onto LDH, a sandwich structure nanocontainer was successfully constructed with CeO₂ nanoparticles loaded on the outer layer and corrosion inhibitor intercalated within the LDH layers.

(1)

Characterization using techniques such as XRD, SEM, XPS, UV–vis DRS, and FT-IR confirmed that TTA was successfully intercalated into the LDH layers, and CeO₂ was successfully loaded onto the LDH surface. The preparation process of MgAl-TTA LDH@CeO₂ was optimized through XRD, with the optimal process parameters identified as a hydrothermal temperature of 140 °C and a hydrothermal time of 48 h;

(2)

The release results of the corrosion inhibitor indicated that the MgAl-TTA LDH@CeO₂ system has a fast TTA release rate during the early stage of immersion in NaCl solution, reaching a peak at around 50 min and reaching dynamic equilibrium after 60 min;

(3)

The optimal addition amount of MgAl-TTA LDH@CeO₂ in the coating is 0.5%. At this optimal addition amount, the EP/MgAl-TTA LDH@CeO₂ coating exhibits good adhesion. After 240 h of UV aging, the interface impedance of the EP/MgAl-TTA LDH@CeO₂ coating is 3.10 × 10⁸ Ω·cm², which is four orders of magnitude higher than that of the EP coating. Compared to the EP/MgAl-TTA LDH coating, the EP/MgAl-TTA LDH@CeO₂ coating has a higher water contact angle and better hydrophobicity. After immersion in corrosion solution for 60 days, the impedance of the EP/MgAl-TTA LDH@CeO₂ coating is 3.88 × 10⁸ Ω·cm², which is two orders of magnitude higher than that of the EP coating. After 96 h of salt spray testing, the corrosion products at the scratch sites of the EP/MgAl-TTA LDH@CeO₂ coating are significantly fewer than those on the EP/MgAl-TTA LDH coating.

The above results demonstrate that the prepared EP/MgAl-TTA LDH@CeO₂ coating possesses excellent corrosion resistance, durability, and anti-aging properties. This provides a potential strategy for the preparation of coatings with excellent long-term corrosion resistance and anti-aging effects in harsh environments and also expands the range of applications for epoxy products.