Research on Corrosion Protection of TETA-Modified Li–Al LDHs for AZ31 Magnesium Alloy in Simulated Seawater

Abstract

Magnesium alloys are lightweight metals but suffer from high corrosion susceptibility due to their chemical reactivity, limiting their large-scale applications. To enhance corrosion resistance, this work combines Li–Al layered double hydroxides (LDHs) with triethylenetetramine (TETA) inhibitors to form an efficient corrosion protection system. Electrochemical tests, SEM, FT-IR, XPS, and 3D depth-of-field microscopy were employed to evaluate TETA-modified Li–Al LDH coatings at varying concentrations. Among them, the Li–Al LDHs without the addition of a TETA corrosion inhibitor decreased significantly at |Z|_{0.01 Hz} after immersion for 4 h. However, the Li–Al LDHs coating of 23.5 mM TETA experienced a sudden drop at |Z|_{0.01 Hz} after holding for about 60 h, and the Li–Al LDHs coating of 70.5 mM TETA also experienced a sudden drop at |Z|_{0.01 Hz} after holding for about 132 h. By contrast, at the optimal concentration (47 mM), after 24 h of immersion, the maximum |Z|_{0.01 Hz} reached 7.56 × 10⁵ Ω∙cm²—three orders of magnitude higher than pure Li–Al LDH coated AZ31 (2.55 × 10² Ω∙cm²). After 300 h of immersion, the low-frequency impedance remained above 10⁵ Ω∙cm², demonstrating superior long-term protection. TETA modification significantly improved the durability of Li–Al LDHs coatings, addressing the short-term protection limitation of standalone Li–Al LDHs. Li–Al LDHs themselves have a layered structure and effectively capture corrosive Cl⁻ ions in the environment through ion exchange capacity, reducing the corrosion of the interface. Furthermore, TETA exhibits strong adsorption on Li–Al LDHs layers, particularly at coating defects, enabling rapid barrier formation. This inorganic–organic hybrid design achieves defect compensation and enhanced protective barriers.

1. Introduction

As lightweight materials, magnesium alloys face application limitations in aerospace and biomedical fields due to corrosion failures caused by high chemical activity [1]. Effective corrosion protection is needed.

Among various anti-corrosion approaches, surface treatments based on layered double hydroxides (LDHs) have gained significant attention [2]. LDHs are ideal candidates for next-generation smart coatings owing to their unique 2D layered structure and anion exchange capacity [3]. Their corrosion protection mechanism involves physical barrier effects from the layered structure and Cl⁻ trapping via ion exchange, reducing chloride concentration near the coating surface [4]. Additionally, LDHs can store corrosion inhibitors that release upon anion exchange, providing localized protection [5]. Chen et al. [6] prepared a Co–Fe LDHs coating on magnesium alloy AZ31 treated with micro-arc oxidation (MAO) by the hydrothermal method. On the electrochemical test surface, no matter how long the sample was immersed in the NaCl solution (30 min, 7 days, or 14 days), the Nyquist diameter of AZ31 always remained the smallest. Secondly, the MAO coating had the largest LDHs, which indicates that the LDHs coating has the strongest protective effect on Mg alloys and the lowest corrosion rate. Zeng et al. [7] developed nanoscale Mg–Al LDH coatings on AZ31 magnesium alloy via a one-step urea hydrolysis process. The dense, plate-like nanostructure and ion exchange capability enabled effective protection against chloride-induced decomposition, reducing corrosion current density by an order of magnitude as evidenced by potentiodynamic polarization and electrochemical impedance spectroscopy. Although LDH coatings demonstrate significant corrosion protection for magnesium alloys, their long-term effectiveness remains limited. The porous structure of single LDH coatings tends to dissolve in aqueous solutions, while decomposition-induced microcracks may form conductive pathways for corrosive electrolytes, leading to localized corrosion [8]. To enhance durability, inhibitor-LDHs functional composites offer an effective strategy for smart self-healing protection. Chen et al. [9] incorporated aspartic acid (ASP) into Mg–Al LDHs on AZ31 magnesium alloy through one-step hydrothermal synthesis. The resulting 3D rosette-like nanostructure with high surface coverage achieved an ultra-low corrosion current density of 0.022 μA/cm² (1/1200 of the substrate). After 10 days of immersion in the scratched area, the charge transfer resistance recovered to 1.0 × 10¹⁶ Ω·cm² (close to 4.53 × 10¹⁷ Ω·cm² of the original coating), which indicates that the metal ion recrystallization induced by ASP release can self-repair the damage. Combined with the morphology regulation of LDHs, it shows excellent corrosion resistance and self-repairing ability. Jiang et al. [10] developed imidazole-based dicationic ionic liquid-modified Mg–Al LDHs coatings on AZ31B magnesium alloy. The synergistic effect between dense surface films and LDHs substrates reduced corrosion current density by four orders of magnitude, maintaining structural integrity after 168 h immersion. Anjum et al. [11] investigated 8-hydroxyquinoline (8HQ)-incorporated Mg–Al LDH coatings on AZ31 alloy. The coating maintained a high impedance of 3.93 kΩ·cm² after a 7-day immersion period due to Mg(HQ)₂ chelate formation dynamically sealing micro-defects. Wen et al. [12] developed phenylphosphonic acid (PPA)-intercalated Mg–Al LDH coatings via in situ growth, achieving a low corrosion current density of 2.47 × 10⁻⁹ A/cm², lower than the MgAl-NO₃ LDH coating without PPA (8.93 × 10⁻⁹ A/cm²), which was attributed to Cl⁻ capture capability and metal ion–PPA coordination forming decomposition-resistant barriers. These experimental findings suggest that LDHs combined with organic inhibitors can create synergistic “1 + 1 > 2” effects for advanced corrosion protection.

Based on preliminary studies, it has been revealed that the electron-rich amino functional groups in triethylenetetramine (TETA) molecules can form dense adsorption films on magnesium alloy surfaces through chelation effects, significantly inhibiting anodic dissolution reactions [13]. Inspired by this discovery, this study proposes the construction of a composite protective system combining LDHs with TETA: By leveraging the adsorption effects of TETA molecules on Li–Al LDHs coating surfaces, we aim to establish a long-lasting protective system. Through controlled TETA concentration gradients (23.5–70.5 mM), we comprehensively employed electrochemical characterization techniques, including electrochemical impedance spectroscopy (EIS) and polarization curve analysis, combined with field-emission scanning electron microscopy (FE−SEM), Fourier transform infrared spectroscopy (FT−IR), X-ray photoelectron spectroscopy (XPS), and 3D depth optical microscopy. This multi-method approach systematically elucidated the adsorption kinetics of corrosion inhibitors at LDHs interfaces, the film formation mechanism, and their regulation mechanisms on electrochemical corrosion pathways.

2. Experimental and Materials

This study employed AZ31 magnesium alloy (composition wt%: Mg 95, Al 3.1, Zn 0.73, Mn 0.25, Si 0.02, Fe 0.005, Cu < 0.001, Ni < 0.001, Zr < 0.001, other < 0.30) with dimensions of 50 mm × 40 mm × 3 mm as the substrate material. The AZ31 samples were polished sequentially using 600-, 1200-, and 2000-grit sandpapers under ethanol lubrication, followed by ultrasonic cleaning in ethanol for 3 min and then drying. The control group consisted of AZ31 samples coated with Li–Al LDHs, prepared according to a previously reported method [14]. TETA (Technical Grade) was purchased from Fisher Chemical (Waltham, MA, USA) with a CAS # of 112-24-3. It was in a liquid form with purity of 98%. The TETA inhibitor solution was prepared by dissolving an appropriate mass of TETA in 3.5 wt% NaCl solution. Based on prior optimization [13], TETA inhibitor solutions with concentrations of 23.5, 47, and 70.5 mM were employed.

Electrochemical measurements were performed using a Gamry 1010E (Gamry Instruments, Philadelphia, PA, USA) or Bio-Logic SAS VSP-300 electrochemical workstation in a standard three-electrode configuration. A working electrode with an exposed area of 1 cm², a saturated calomel reference electrode (SCE), and a carbon rod counter electrode comprised the testing circuit. Corrosion performance was evaluated in 3.5 wt% NaCl solution. EIS measurements were conducted with a 10 mV AC perturbation amplitude over a frequency range of 1 MHz to 0.01 Hz. Potentiodynamic polarization tests were initiated after 30 min of open-circuit potential (OCP) stabilization, with a scan rate of 1 mV/s across a potential range of −0.1 to +0.8 V vs. OCP. Corrosion current densities and rates were calculated via Tafel extrapolation.

FT-IR was utilized to identify surface functional groups and chemical bonds on Li–Al LDHs-coated samples after 24 h immersion. XPS analysis was performed using an Al-Ka X-ray source (Kratos Analytical AXIS HSi 165 spectrometer, Shimadzu, Kyoto, Japan) with a spotsize of 400 μm, a step size of 0.05 eV, and 1–15 scans per region. Spectral data were processed using Avantage v5.9925 software. Surface morphologies were characterized by scanning electron microscopy (SEM, Zeiss SUPRA 55, Oberkochen, Germany). Post-immersion sample topography was further analyzed via 3D depth optical microscopy (LEICA DVM6 Leica microsystems, Weizla, Germany).

3. Results and Discussion

3.1. Electrochemical Analysis

Figure 1 presents the Bode plots and Nyquist diagrams of samples under different inhibitor concentrations and immersion durations in 3.5 wt% NaCl. The control group comprised AZ31 magnesium alloy substrates coated with Li–Al LDHs. EIS data were analyzed using the equivalent circuit shown in Figure 2, where R_s: solution resistance from the electrolyte; R_ct: charge transfer resistance at the film/substrate interface; R_f: resistance of the protective film on the alloy substrate; Q_f: constant phase element (CPE) associated with the surface film; and Q_dl: double-layer CPE [15]. L is an inductive element, often indicating corrosion-induced damage to the passive or protective film on AZ-series Mg alloys [16,17]. From the fitting results, it is generally considered that for magnesium alloy systems with localized corrosion, the equivalent circuit shown in Figure 2b is applied, where the inductive element (L) is typically associated with the rupture of passive films or protective coatings on the magnesium alloy surface.

The summarized low-frequency impedance modulus (|Z|_{0.01 Hz}) derived from the Bode plots of EIS is presented in Figure 3. At the initial immersion stage, immersing from 0 h to 12 h, the |Z|_{0.01 Hz} reached a maximum value of 4.47 × 10⁵ Ω·cm² for the sample with 47.0 mM TETA, representing an increase of one order of magnitude compared to Li–Al LDHs coatings without inhibitors. After 24 h of immersion, corrosion was observed in the inhibitor-free Li–Al LDHs coatings, while those with TETA exhibited no significant corrosion, and their low-frequency impedance values further increased. This suggests that the adsorption of TETA onto the Li–Al LDHs coatings requires a specific time-dependent process. At that moment, the highest |Z|_{0.01 Hz} at 47.0 mM TETA was 7.56 × 10⁵ Ω·cm². It was improved by three orders of magnitude compared with the simple use of a Li–Al LDHs coating (2.55 × 10² Ω∙cm²). The Nyquist plots in Figure 1c,f predominantly displayed single capacitive loops, which were fitted using the equivalent circuit in Figure 2a. However, after 24 h of immersion in inhibitor-free 3.5 wt% NaCl solution, localized galvanic corrosion occurred in the Li–Al LDHs coatings, necessitating the incorporation of an inductive element for fitting. The fitted parameters are listed in

Table 1. The corrosion potential and corrosion current density of different concentrations of TETA at varying immersion times in 3.5wt% NaCl solution.

Time	Concentration mol/L	Ecorr VSCE	icorr A/cm2
0 h	0	−1.549	1.294 × 10−6
	0.0235	−1.532 ± 0.0254	1.294 × 10−6 ± 3.641 × 10−7
	0.0470	−1.399 ± 0.0481	2.506 × 10−7 ± 1.288 × 10−7
	0.0705	−1.425 ± 0.0332	4.814 × 10−7 ± 1.518 × 10−7
24 h	0	−1.402	1.812 × 10−6
	0.0235	−1.506 ± 0.0566	3.321 × 10−4 ± 1.518 × 10−4
	0.047	−1.349 ± 0.0246	1.254 × 10−8 ± 3.335 × 10−9
	0.0705	−1.457 ± 0.0551	2.637 × 10−7 ± 3.451 × 10−8

.

From Figure 3, the |Z|_{0.01 Hz} of TETA-modified coatings in 3.5 wt% NaCl solution generally decreased over time, likely due to the adsorption dynamics of TETA’s polar groups, which are both time- and concentration-dependent. Notably, the Li–Al LDHs coating with 47.0 mM TETA maintained stability for ~300 h, and the magnitude of the low-frequency impedance value was still above 10⁵ Ω∙cm², whereas those with 23.5 mM and 70.5 mM TETA showed abrupt impedance drops after ~60 h and ~132 h, respectively, indicating corrosion onset and the termination of measurements. In contrast, the inhibitor-free Li–Al LDHs coating exhibited a significant impedance decline within 4 h, confirming its limited short-term corrosion resistance. The TETA-modified coatings demonstrated progressively enhanced protective efficacy and long-term durability, highlighting the critical role of TETA adsorption in improving corrosion resistance.

The results of the potentiodynamic polarization curves show that for the AZ31 magnesium alloy coated with Li-Al LDHs without TETA as the control group immersed in 3.5 wt% Nacl, from Figure 4, the corrosion potential E_corr and corrosion current density i_corr of the control group were −1.402 V and 1.812 × 10⁻⁶ A·cm⁻² after 24 hours of immersion, respectively. In contrast, with the addition of 47.0 mM TETA, the E_corr shifted positively to −1.349 V, and the i_corr decreased significantly to 1.254 × 10⁻⁸ A·cm⁻², indicating a reduction of two orders of magnitude in anodic current density and a positive shift in corrosion potential of +0.053V. Compared to the blank sample, the corrosion potential E_corr of the TETA-added sample shifted positively, indicating that it primarily inhibited the anodic reaction process of magnesium substrate dissolution. In contrast, the corrosion potentials E_corr of the TETA-modified Li-Al LDHs samples shifted to varying degrees of negativity, demonstrating that these coatings acted as corrosion inhibitors predominantly suppressing the anodic reaction.

Typically, a higher E_corr in a Tafel analysis corresponds to a lower i_corr, reflecting stronger interfacial interactions between the coating and substrate and enhanced corrosion resistance [18]. After 24 h, the TETA-incorporated Li–Al LDHs coatings retained superior corrosion resistance, while the inhibitor-free coatings were degraded, consistent with the observations in Figure 1. These results confirm that the inhibition efficiency of TETA is concentration-dependent. The correlation between TETA concentration and inhibition efficiency aligns closely with findings from prior experiments. The adsorption of TETA on the Li–Al LDHs coating surface forms a dense protective film, effectively mitigating corrosion and improving durability.

3.2. Analysis of Characterization Test Results

Figure 5 shows the FT-IR spectra of TETA and the Li–Al LDHs coating composite for corrosion protection. Through analysis of characteristic peaks, the successful adsorption of TETA molecules on the Li–Al LDHs surface and the enhancement mechanism of the coating’s protective performance could be confirmed. The characteristic peak that appeared in the Li–Al LDHs coating 643 cm⁻¹ arose from the lattice vibration of Al–OH in the LDH coating. Distinct Mg(OH)₂ peaks were observed, originating from the LDH layers. For the TETA-modified Li–Al LDHs, the characteristic peaks of TETA were retained. The modified coating exhibited prominent absorption peaks at 2845 cm⁻¹ and 2940 cm⁻¹, corresponding to the symmetric and asymmetric C–H stretching vibrations of the –CH₂- groups in TETA molecules [19]. Those peaks could be found in both TETA and TETA modified Li–Al LDHs coatings. A characteristic peak for N−H in-plane bending vibration was detected at 1573 cm⁻¹, which was red-shifted compared to that of free TETA molecules (1542 cm⁻¹), indicating chemical coordination between the amino groups of TETA and the LDHs surface [20]. Furthermore, the characteristic peaks of the Li–Al LDHs coating showed minimal overlap with those of the TETA-modified Li–Al LDHs coating. These results confirm the successful adsorption of TETA on Li–Al LDHs; the adsorbed layer is consistent with the formation of a protective barrier to enhance the material’s corrosion resistance. These findings align with the electrochemical test results.

Figure 6 presents the XPS analysis of the Li–Al LDHs coating loaded with the TETA inhibitor 47 mM TETA concentration after immersion in 3.5 wt% NaCl solution for 24 h on an AZ31 magnesium alloy substrate. The detected elements included Mg, Al, N, C, and O. As shown in Figure 6b, the Mg 2p spectrum exhibited four fitted peaks at 50.21 eV (Mg(OH)₂), 49.68 eV (MgO), 49.27 eV (Mg–N), and 48.61 eV (Mg), indicating contributions from both the Li–Al LDHs coating and the reaction between TETA nitrogen and the Mg substrate. In the Al 2p spectrum (Figure 6c), the peak at 74.26 eV corresponded to Al-OH in Li–Al LDHs, but its intensity was notably reduced compared to that of the as-synthesized Li–Al LDHs, consistent with the FT-IR results. The N 1s spectrum (Figure 6d) was fitted into two peaks at 398.50 eV (–NH/NH₂) and 399.45 eV (C–N), with amine-type nitrogen (−NH/NH₂) dominating the nitrogen content. The C 1s spectrum (Figure 6e) was deconvoluted into five peaks at 283.35 eV (C–C), 284.65 eV (C–N), 283.76 eV (C−H), 287.33 eV (O–C–O), and 284.16 eV (C–O). The O 1s spectrum (Figure 6f) showed two peaks at 530.5 eV (–OH) and 531.54 eV (C–O). Notably, no Li–OH peak from Li–Al LDHs was detected, likely due to the formation of a denser surface film by TETA adsorption, which hinders Li–OH detection. This observation aligns with the reduced Al–OH content and corroborates the electrochemical and FT-IR results.

To confirm the previously observed corrosion protection results of the TETA-modified Li–Al LDHs coating composite, a comparative analysis of the SEM images of Li–Al LDHs substrates with and without TETA (Figure 7) was conducted. After immersion in the TETA inhibitor for 24 h, the lamellar structure of the Li–Al LDHs remained but was significantly reduced, with a higher degree of particle separation. This indicates that the basic structure of the LDHs was partially preserved under the experimental conditions, while the majority of the Li–Al LDHs coating underwent adsorption of polar groups from TETA, forming an additional film layer that covered the Li–Al LDHs coating. The modified surface appeared more uniform and exhibited enhanced barrier properties. Based on variations in low-frequency impedance values, it can be inferred that this adsorption process continued even after 24 h and had not ceased. As shown in Figure 7b,c, non-uniform film layers with varying thicknesses were observed, suggesting incomplete coverage over the Li–Al LDHs coating surface. In contrast, the Li–Al LDHs-coated magnesium alloy without TETA immersion for 24 h tended to form a corrosion product film in the corrosive environment, which not only accumulated corrosion products but also exhibited localized corrosion pits and cracks. However, no pits were observed on the TETA-modified Li–Al LDHs coating’s surface. The TETA-modified Li–Al LDHs coating demonstrated optimal corrosion inhibition performance in neutral chloride solutions. These microstructural observations are consistent with the electrochemical data, further validating the enhanced corrosion resistance achieved through TETA modification.

Three-dimensional depth-of-field microscopy characterization (Figure 8) revealed that the Li–Al LDHs coating grown on the magnesium alloy surface exhibited a significant reduction in surface roughness after the addition of TETA, with the peak-to-valley height difference sharply, decreasing from approximately 310 μm to 40 μm, demonstrating a uniform and compact microstructure. This phenomenon can be attributed to the adsorption behavior of TETA molecules on the surface of LDHs nanosheets, which optimizes the surface morphology through the following mechanisms.

The TETA molecule (C₆H₁₈N₄) possesses four amine groups (−NH₂). Its electron-rich nature enables adsorption onto the Li–Al LDHs surface via coordinative interactions and hydrogen bonding: the lone pair electrons in the amine groups form coordinative bonds with the vacant orbitals of Al³⁺ at the edges of LDHs layers, preferentially occupying active sites for crystal growth. Simultaneously, non-coordinated amine groups establish hydrogen-bonding networks with hydroxyl groups (–OH) on the LDHs surface, enhancing molecular stability at the interface [21].

3.3. Discussion and Analysis

Based on experimental results, electrochemical tests demonstrated that TETA-modified Li–Al LDHs exhibited superior protective capabilities, with extended protection durations ranging from 1 to 10 days compared to untreated counterparts, indicating significantly enhanced corrosion resistance,. SEM imaging further revealed that the intercalation of corrosion inhibitors altered the structure of LDHs, resulting in a horizontally aligned lamellar configuration that better shielded the Mg alloy substrate, thereby improving corrosion resistance, the mechanism of TETA modifying Li-Al LDHs is shown in Figure 9.

In prior work [22], standalone LDHs provided short-term corrosion resistance but suffered from rapid performance degradation during prolonged immersion. This limitation likely arises from the finite protective efficacy of LDHs coatings formed under ambient conditions and the inherent reactivity of Mg alloys. Once defects develop in LDHs coatings during long-term immersion, accelerated corrosion of the Mg substrate occurs. To address this, defect sites in the coating require timely repair. Previous studies confirm that TETA enhances corrosion resistance by forming an adsorbed barrier layer. Further investigations demonstrate TETA’s adsorption affinity for multiple interfaces, including Mg alloy surfaces and Li–Al LDHs coatings [23]. Combined with potentiodynamic polarization curves and EIS results from this study, the application of TETA onto Li–Al LDHs-coated Mg alloys significantly improves overall corrosion resistance. Synergistic effects—termed a “1 + 1 > 2” phenomenon—are observed when integrating findings from previous chapters. The composite protection mechanism of TETA and Li–Al LDHs primarily stems from the LDHs coating, which provides stable protection for the complete area, and TETA is successfully adsorbed on the surface of the Li–Al LDHs layer, particularly at coating defects, where rapid adsorption forms an effective barrier. This inorganic–organic hybrid protection design enables complementary defect passivation, forming a more effective protective barrier.

4. Conclusions

The experimental studies in this article fully demonstrate the synergistic protective effect between TETA and Li–Al LDHs, which significantly enhances the long-term corrosion resistance of magnesium alloys. The main conclusions are as follows:

• At the optimal TETA concentration of 47 mM, the maximum |Z|_0.01Hz value reached 7.56 × 10⁵ Ω∙cm² after 24 h of immersion, demonstrating three orders of magnitude of improvement compared to the Li–Al LDHs blank sample without an inhibitor (2.55 × 10² Ω∙cm²). Moreover, after 300 h of immersion, the low-frequency impedance value remained above 10⁵ Ω∙cm², indicating excellent sustained corrosion protection performance.
• The Li–Al LDHs coating modified with TETA exhibited substantially enhanced long-term corrosion resistance, effectively addressing the limitation of standalone LDHs coatings that only provide short-term corrosion protection.
• The adsorption effect of TETA on Li–Al LDHs layers, particularly through rapid adsorption and protection at coating defects, creates effective shielding. This inorganic-organic composite protection design enables defect compensation and forms superior protective barriers through complementary mechanisms.