In Situ-Grown LDH Coatings Intercalated with Organic Inhibitors on AA2024 Aluminum Alloy: Synergistic Corrosion Inhibition and DFT-Based Insights

Abstract

In this study, ZnAl layered double hydroxide (LDH) nanocontainers were grown in situ on AA2024 aluminum alloy through a controlled hydrothermal process, using the native oxide layer as an internal aluminum source, and succinic acid (SA) and tartaric acid (TA) inhibitor anions were subsequently intercalated into the LDH interlayers via an optimized ion-exchange treatment performed at 60 °C under ambient pressure, with exposure times from 15 min to 5 h to evaluate the effect of intercalation duration on inhibitor uptake and film stability. Structural and morphological analyses by X-ray diffraction (XRD) and scanning electron microscopy (SEM) confirmed the formation of well-crystallized LDH films and the successful incorporation of the organic inhibitors. Corrosion resistance in 3.5% NaCl, assessed through immersion tests, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization (PDP), showed that the SA-intercalated coating achieved an inhibition efficiency of 95.1% after 48 h, significantly higher than the 62.5% obtained for the TA-based coating, while the pristine LDH layer provided only short-term barrier protection. The superior performance of LDH_SA is attributed to the synergistic effects of chloride ion trapping within the LDH matrix, the formation of a hydrophobic protective layer generated by released SA, and a self-healing response driven by ion-exchange mechanisms. Density functional theory (DFT) and Monte Carlo (MC) simulations further revealed strong physisorption and chemisorption interactions between the inhibitors and both the LDH surface and the aluminum substrate, and the combined experimental–theoretical results demonstrate that LDH_SA constitutes a robust, environmentally friendly, and highly efficient corrosion-mitigation coating for aluminum alloys in chloride-rich environments.

Introduction

Aluminum alloy 2024 (AA2024) is widely used in aerospace, transportation, and structural applications due to its high strength-to-weight ratio, excellent machinability, and good fatigue resistance. However, the high copper content that imparts strength to AA2024 also makes it particularly susceptible to localized corrosion, especially pitting and intergranular corrosion in chloride-rich environments. This vulnerability severely compromises its long-term durability and limits its use in aggressive service conditions. To overcome these limitations, surface modification via functional conversion coatings has emerged as a promising approach to enhance corrosion resistance and prolong component lifespan. , Various surface treatment methods such as chemical conversion coatings, anodizing, sol–gel coatings, and organic–inorganic hybrid systems have been investigated for aluminum corrosion protection.

Among these, layered double hydroxide (LDH)-based conversion coatings have gained increasing attention due to their dual-action protective mechanism: they serve as passive physical barriers and can actively release corrosion inhibitors embedded within their layered structure. ,, LDHs are a class anionic clays with the general formula [M²⁺ _1–x M³⁺ _x (OH)₂] ^x+[A ^n– _x/n ·yH₂O] ^x−, where M²⁺ and M³⁺ represent divalent and trivalent metal cations, respectively (e.g., Zn²⁺, Mg²⁺, Al³⁺), A ^n– is an interlayer anion such as NO₃ ^–, CO₃ ^2–, or functional corrosion inhibitors, and x represents the molar fraction of trivalent cations. , The structure consists of positively charged brucite-like layers interleaved with galleries containing charge-balancing anions and water molecules. This unique lamellar structure imparts LDHs with high anion-exchange capacity, chemical tunability, and the ability to selectively trap aggressive ions such as chloride ions, making them ideal candidates for smart, self-healing protective coatings. ,

When grown in situ on AA2024 substrates, LDH films form conformal, chemically bonded layers that provide synergistic protection: they physically block external corrosive agents while also enabling stimuli-responsive release of intercalated inhibitors upon environmental triggers such as pH shifts or chloride intrusion. , This self-triggered release mechanism allows for localized corrosion suppression at defect sites, substantially improving coating durability and performance. In addition, the environmentally friendly nature of LDH systems makes them attractive for sustainable corrosion protection of high-performance aluminum alloys like AA2024.

To further improve these smart coatings, organic corrosion inhibitors are increasingly explored as intercalated species within LDH matrices. , These molecules form protective films by adsorbing onto metal surfaces, thereby limiting contact with corrosive agents. Their inhibition performance strongly depends on molecular structure, particularly the presence of electron-rich functional groups that can interact with metal sites via coordination bonds or hydrogen bonding. Among such candidates, succinic acid (SA) and tartaric acid (TA), two bioderived dicarboxylic acids that can be obtained from renewable sources such as plant sugars and fermentation processes, offer promising features including multiple polar groups, strong complexation ability, and environmental compatibility. , In this work, both SA and TA are explored as green intercalated inhibitors in in situ-grown ZnAl LDH films on AA2024 alloy substrates. Succinic acid, a linear dicarboxylic acid, facilitates uniform surface coverage through electrostatic or coordination interactions, whereas tartaric acid, containing both vicinal hydroxyl and carboxyl groups, achieves enhanced adsorption via hydrogen bonding and metal chelation, leading to more compact and protective films. Additionally, their biodegradability, low toxicity, affordability, and inherent affinity for coordinating metal ions make them well suited for sustainable, green corrosion-inhibiting technologies.

Recent advances have demonstrated the efficacy of in situ-grown LDH films intercalated with inorganic inhibitors for corrosion protection of aluminum, magnesium, and steel alloys. ,, These systems harness the high anion-exchange capacity of LDHs to immobilize and release inhibitors in response to corrosion triggers. For example, Farshbaf et al. reported that vanadate-intercalated LDH films grown on anodized AA2024 achieved an inhibition efficiency of approximately 92% after 48 h of immersion in 3.5% NaCl, highlighting the synergistic effect of passive barrier formation and active vanadate release. Beyond inorganic inhibitors, organic anions from the dicarboxylic acid family have also shown effectiveness. Oxalate, a short-chain dicarboxylate, was intercalated in situ into MgAl LDH films grown directly on AZ91 magnesium alloy, providing ∼80% corrosion inhibition after 24 h immersion, through a combination of metal complexation and chloride exchange. Similarly, gluconate, a hydroxycarboxylic acid structurally related to tartaric acid, was in situ intercalated into CaAl LDH films on steel, enhancing coating integrity and triggering inhibitor release under corrosive conditions, resulting in polarization resistance values exceeding 5 kΩ cm² and long-term inhibition efficiencies above 80% over 72 h. Moreover, p-aminobenzoate (pAB), an aromatic monocarboxylate, intercalated into Mg–Fe LDH conversion films on steel, exhibited a high inhibition efficiency of approximately 68% in 3.5% NaCl after 3 h of immersion, demonstrating strong protective capability even under severe chloride exposure.

Despite these promising results, most prior studies have focused on short-chain organic acids or inorganic species, with limited exploration of renewable, biobased dicarboxylates such as SA and TA. Moreover, previous work rarely integrates molecular modeling to probe inhibitor–LDH interactions at the atomic scale. In this context, the present study investigates in situ-grown ZnAl LDH films intercalated with SA and TA directly on AA2024, providing both quantitative corrosion performance and mechanistic insights. To the best of our knowledge, this work presents the first successful in situ intercalation of SA and TA anions into LDH films on AA2024 aluminum alloy, filling a clear gap in the literature where most studies mainly rely on ex situ LDH powders, , postsynthesis soaking treatments, or inorganic inhibitors. Overall, the findings show that renewable bioderived dicarboxylates can be effectively incorporated during LDH film growth, broadening the design of environmentally benign organic–inorganic hybrid anticorrosion coatings with inhibition efficiencies comparable to or superior to previously reported systems.

The resulting LDH films were thoroughly characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM) to investigate crystallographic structure, interlayer chemistry, and surface morphology, while their corrosion resistance was evaluated using immersion testing, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) in chloride-containing environments. Furthermore, unlike most earlier studies, this work integrates density functional theory (DFT) simulations to elucidate the adsorption behavior, stabilization mechanisms, and release dynamics of SA and TA within the LDH host layers and on the aluminum substrate. This dual experimental–computational approach provides deeper insights into inhibitor–LDH–substrate interactions, release behavior, and self-healing pathways, thereby establishing ZnAl LDH films intercalated with renewable dicarboxylate inhibitors as promising, sustainable, and high-performance coatings for advanced aluminum alloy applications.

Experimental Section

Chemicals and Reagents

LDH films were synthesized in situ on AA2024 aluminum alloy substrates. This alloy, widely used in aerospace applications, contains a complex composition including copper (3.7–4.8 wt %), magnesium (1.1–1.7 wt %), manganese (0.2–0.8 wt %), iron (≤0.5 wt %), zinc (0.2 wt %), silicon (≤0.5 wt %), and titanium (0.15 wt %), with aluminum as the balance. Square specimens (15 × 15 × 4 mm) were mechanically prepared by sequential grinding with 800- and 1200-grit SiC papers, followed by rinsing with deionized water and drying under compressed air prior to LDH film growth via hydrothermal treatment.

The following analytical-grade reagents were used for LDH synthesis and inhibitor intercalation: zinc nitrate hexahydrate (Zn­(NO₃)₂·6H₂O, 98%, Alfa Aesar, Germany), sodium nitrate (NaNO₃, ≥99%, Carl Roth GmbH, Germany), sodium hydroxide (NaOH, ≥99%, Merck KGaA, Germany), dl-tartaric acid (C₄H₆O₆, 99%, Alfa Aesar, Germany), succinic acid (C₄H₆O₄, ≥99%, Sigma-Aldrich GmbH, Germany), and sodium chloride (NaCl, ≥99.5%, Fisher Chemical, Germany). These chemicals were used without further purification.

Synthesis of Parental LDH

The synthesis of LDH films on the surface of AA2024 aluminum alloy was carried out via a hydrothermal treatment method. , An aqueous solution containing 0.5 M Zn­(NO₃)₂ and 3 M NaNO₃ was prepared and thoroughly mixed. The pH of the solution was carefully adjusted to 8.00 ± 0.05 by the gradual addition of 2 M NaOH under continuous stirring to ensure homogeneous alkalinity. The pretreated AA2024 specimens were then immersed in the treatment solution, which was preheated and maintained at 90 °C. The hydrothermal synthesis was conducted at this temperature for 4 h, starting from the point of immersion, with constant agitation to promote uniform nucleation and growth of the LDH layer on the alloy surface. Afterward, the samples were removed from the solution, thoroughly rinsed with deionized water to eliminate any loosely bound residues or unreacted precursors, and subsequently dried under ambient air. These specimens, containing ZnAl LDH films with nitrate anions in the interlayer region, were then labeled as parental LDH films and used as the baseline for subsequent inhibitor intercalation steps.

In Situ Intercalation of Organic Inhibitors into LDH Films

The parental ZnAl LDH films were subjected to anion-exchange treatments in aqueous solutions containing either SA or TA to achieve organic inhibitor intercalation. For SA intercalation, the LDH-coated AA2024 specimens were immersed in a 0.1 M SA solution, with the pH carefully adjusted to 8.0 ± 0.2 using 2 M NaOH. The treatment was performed at 60 °C under continuous stirring, and samples were retrieved at different time intervals (15 min, 30 min, 1, 2, 3, and 5 h) to monitor intercalation kinetics. After treatment, all specimens were thoroughly rinsed with deionized water and air-dried. The resulting samples were designated as LDH_SA.

A similar intercalation procedure was applied for TA. The LDH films were immersed in a 0.1 M aqueous solution of TA, with the pH adjusted to 7.8 ± 0.1. As with SA, the intercalation was conducted at 60 °C under constant agitation for durations ranging from 15 min to 5 h (15 min, 30 min, 1, 2, 3, and 5 h). After the exchange process, the samples were rinsed and dried under ambient conditions. These specimens were referred to as LDH_TA.

In both cases, the intercalation was carried out in approximately 100 mL of solution per specimen to ensure sufficient volume for ion exchange. The efficiency of inhibitor incorporation into the LDH galleries was estimated using X-ray diffraction (XRD) by comparing the integrated areas of the basal reflection peaks before and after intercalation. A summary of the intercalation parameters for both inhibitors is provided in Table .

1. Conditions for SA and TA Intercalation Into ZnAl LDH Films.

inhibitor	concentration	pH	temperature	immersion time
succinic acid	0.1 M	8.0 ± 0.2	60 °C	15 min, 30 min, 1 h, 2 h, 3 h, 5 h
tartaric acid	0.1 M	7.8 ± 0.1	60 °C	15 min, 30 min, 1 h, 2 h, 3 h, 5 h

Characterization Instruments

Phase crystallinity of the LDH films was analyzed using a Bruker D8 Advance X-ray diffractometer (Bruker, Karlsruhe, Germany) equipped with a Cu (Kα₁, Kα₂) radiation source (λ = 1.5406 Å), operating at 40 kV and 40 mA. Diffraction patterns were collected in the 2θ range of 5° to 30°, allowing identification of the characteristic LDH basal reflections, particularly the (003) and (006) planes. The scan step size was set to 0.02°, with a counting time of 1 s per step to ensure sufficient signal intensity.

Surface morphology and microstructural features were examined using a TESCAN VEGA3 scanning electron microscope (Tescan, Brno, Czech Republic). Prior to analysis, all specimens were sputter-coated with a thin gold layer to enhance conductivity and prevent surface charging during imaging.

Immersion tests were performed to visually assess the long-term corrosion behavior of the samples. Each specimen was immersed in a separate beaker containing 3.5 wt % NaCl aqueous solution at ambient temperature. Three replicates were used per condition to ensure reproducibility. The specimens were periodically inspected for visible signs of corrosion at predefined time intervals: prior to immersion, and after 1, 3, 6, 24, 48, 72, 120, and 168 h of exposure. Macroscopic surface changes were documented using a Leica D-LUX 3 digital camera (Leica Camera AG, Kyoto, Japan) mounted on a Wild M3C stereomicroscope (Wild Heerbrugg, Switzerland).

Electrochemical impedance spectroscopy (EIS) was employed to assess the corrosion protection performance of the LDH-coated and uncoated AA2024 samples. Measurements were performed using a Gamry Interface 1000 potentiostat (Gamry Instruments, Warminster, USA) in a conventional three-electrode electrochemical cell. The working electrode was either bare or LDH-modified AA2024 (exposed surface area = 0.5 cm2), with a platinum wire as the counter electrode and an Ag/AgCl electrode as the reference. EIS spectra were recorded at open-circuit potential (OCP) in a 3.5 wt % NaCl solution, using a 10 mV sinusoidal perturbation across a frequency range from 100 kHz to 1 Hz, with 9 points per frequency decade. All measurements were conducted at room temperature inside a Faraday cage to minimize external noise. Impedance tests were repeated at specific time intervals (after 30 min, and subsequently at 1, 3, 6, 12, 18, 24, and 48 h of immersion) to monitor the temporal evolution of corrosion resistance.

Potentiodynamic polarization measurements were carried out using the same three-electrode configuration described above, connected to a VoltaLab potentiostat. Polarization scans were conducted in 3.5 wt % NaCl solution following stabilization of the OCP. A linear scan rate of 1 mV/s was applied, with the potential swept from −1 V to +1 V vs OCP to capture both cathodic and anodic behaviors. These measurements provided quantitative insights into corrosion kinetics, including corrosion potential (E _corr) and current density (i _corr).

DFT Study

Computational Approach

To gain a molecular-level understanding of the interactions between ZnAl LDHs, the Al(111) aluminum surface, and organic corrosion inhibitors, namely SA and TA, DFT calculations were performed using the BIOVIA Materials Studio 2017 software. , The study aimed to provide the structural, electronic, and energetic changes associated with the intercalation of inhibitors into the LDH interlayers and their adsorption onto metallic surfaces. These insights are crucial for elucidating the bonding mechanisms and stability of both the inhibitor–host coatings and the inhibitor–metal interfaces, thereby advancing the design of more effective corrosion protection strategies.

Model Construction and Geometry Optimization

A ZnAl LDH structure with a Zn/Al atomic ratio of 2 was constructed using the modeling tools available in BIOVIA Materials Studio 2017, based on the crystallographic data of hydrotalcite (space group R3̅m; lattice parameters: a = b = 3.0729 Å, c = 23.326 Å). , The unit cell was expanded into a 3 × 3 × 1 supercell along the X, Y, and Z directions to better represent the bulk material. Two representative surfaces were modeled for simulation: the (003) basal plane, characteristic of the layered LDH structure, and the (010) edge surface, representing cleaved or defect sites. Each surface model was embedded in a simulation box containing a 20 Å vacuum layer along the z-axis to prevent interaction between periodic images. Geometry optimizations of the LDH surfaces, the Al(111) metallic substrate, and the organic inhibitors (SA and TA) were performed using the Forcite module, applying the Universal force field along with Qeq charge equilibration. , Nonbonded interactions, including van der Waals and electrostatic forces, were treated using an atom-based summation method to obtain energetically stable configurations suitable for subsequent adsorption and electronic structure analyses. ,

Monte Carlo Simulation of Adsorption Behavior

The interaction and adsorption behaviors of SA and TA molecules on the ZnAl LDH and Al(111) surfaces were systematically investigated using the adsorption locator module within the BIOVIA Materials Studio environment. Simulations were carried out using the Universal force field in combination with Qeq charge equilibration. Electrostatic interactions were computed using the Ewald summation method, while van der Waals forces were modeled atomistically with an atom-based approach. To ensure adequate sampling of configurational space and energy convergence, a simulated annealing protocol comprising 10 cycles with 100,000 steps per cycle was employed. The adsorption energy (E _ads) was calculated to quantify the strength of interaction between each inhibitor and the substrate (either the LDH layer or the Al surface), using the following expression

$$E_{\mathrm{a}\mathrm{d}\mathrm{s}}=E_{\mathrm{s}\mathrm{u}\mathrm{b}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}\text{\_}\mathrm{i}\mathrm{n}\mathrm{h}}-(E_{\mathrm{s}\mathrm{u}\mathrm{b}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}}+E_{\mathrm{i}\mathrm{n}\mathrm{h}})$$ 1

where E _ads is the adsorption energy, E _{substrate_inh} is the total energy of the optimized inhibitor–substrate complex (either ZnAl LDH or Al(111)), E _substrate is the energy of the isolated substrate (ZnAl LDH or Al(111)), and E _inh is the energy of the isolated inhibitor molecule (SA or TA). A more negative E _ads value indicates stronger adsorption and a more stable inhibitor–surface interaction.

Electronic Structure and Reactivity Analysis via DFT

The electronic properties of the inhibitors and their interactions with the host layers and the metal surface were analyzed using the DMol3 module. Geometry optimizations and electronic property calculations were carried out using the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) exchange-correlation functional. − A double-numerical basis set with polarization functions (DNP) was employed, with an all-electron treatment of core electrons. Solvation effects were modeled using the COSMO continuum solvent model, with water as the solvent.

Key quantum chemical descriptors were extracted to assess inhibitor reactivity and adsorption propensity, including: the energy gap (ΔE _gap), ionization potential (I) electron affinity (A), global hardness (η), softness (σ), absolute electronegativity (χ), electronic chemical potential (μ), and back-donation energy (ΔE _{back‑donation}) were derived. These parameters include the energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), which represent the molecule’s ability to donate and accept electrons, respectively. The descriptors were calculated using the following standard theoretical relationships ,

$$\mathrm{\Delta }{E}_{\mathrm{g}\mathrm{a}\mathrm{p}}=E_{\mathrm{L}\mathrm{U}\mathrm{M}\mathrm{O}}-E_{\mathrm{H}\mathrm{O}\mathrm{M}\mathrm{O}}$$ 2

$$I=-E_{\mathrm{H}\mathrm{O}\mathrm{M}\mathrm{O}}$$ 3

$$A=-E_{\mathrm{L}\mathrm{U}\mathrm{M}\mathrm{O}}$$ 4

$$\eta =\frac{E_{\mathrm{L}\mathrm{U}\mathrm{M}\mathrm{O}}-E_{\mathrm{H}\mathrm{O}\mathrm{M}\mathrm{O}}}{2}$$ 5

$$\sigma =\frac{1}{\eta }$$ 6

$$\chi =-\frac{(E_{\mathrm{L}\mathrm{U}\mathrm{M}\mathrm{O}}+E_{\mathrm{H}\mathrm{O}\mathrm{M}\mathrm{O}})}{2}$$ 7

$$\mu =-\chi$$ 8

$$\mathrm{\Delta }{E}_{\mathrm{b}\mathrm{a}\mathrm{c}\mathrm{k}‐\mathrm{d}\mathrm{o}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}=\frac{E_{\mathrm{H}\mathrm{O}\mathrm{M}\mathrm{O}}-E_{\mathrm{L}\mathrm{U}\mathrm{M}\mathrm{O}}}{8}$$ 9

These descriptors provide insights into the inhibitors’ electron-donating and -accepting capabilities, their chemical stability, and their affinity for adsorption onto host surfaces. In particular, a smaller ΔE _gap suggests higher chemical reactivity, while a more negative chemical potential (μ) and higher softness (σ) imply stronger interactions with LDH or metallic surfaces. ,

Additionally, Mulliken atomic charge distributions and Fukui functions analyses were performed on the optimized structures to identify electron-donating and electron-accepting sites, enabling the prediction of potential reactive centers. These complementary electronic descriptors support the localization of probable donor–acceptor interactions between the inhibitor molecules and the LDH host or aluminum substrate. ,

Results and Discussion

Structural and Morphological Characterizations

The successful intercalation of SA and TA into ZnAl LDH at various treatment times was confirmed by X-ray diffraction (XRD) analysis, as shown in Figure a,b, respectively. The pristine ZnAl-LDH exhibited sharp and symmetric basal reflections, particularly the (003) and (006) planes, characteristic of a well-ordered lamellar structure indexed to the rhombohedral space group R3̅m. , Upon intercalation, a pronounced shift of the (003) and (006) peaks toward lower 2θ angles was observed, reflecting an expansion of the interlayer spacing due to the successful insertion of organic anions into the gallery space. This structural change was accompanied by a reduction in peak intensity and broadening, likely caused by partial loss of crystallinity and increased interlayer disorder resulting from the presence of bulky biomolecules. The detailed evolution of interlayer spacing (d ₀₀₃), crystallite size, and microstrain as a function of intercalation time for both LDH_SA and LDH_TA coatings is summarized in Table , providing quantitative insight into the structural changes induced by organic anion insertion. The crystallite size (D) was calculated using the well-known Scherrer equation

$$D=\frac{K\lambda }{\beta \cos \theta }$$ 10

where K is the shape factor (typically 0.9), λ is the X-ray wavelength (1.5406 Å), β is the full width at half-maximum (fwhm) of the (003) diffraction peak in radians, and θ is the corresponding Bragg angle.

1.

XRD patterns of LDH samples intercalated with SA (a) and TA (b) at various treatment times, compared to the parental LDH.

2. Evolution of Interlayer Distance, Crystallite Size, and Microstrain of LDH_SA and LDH_TA as a Function of Treatment Time.

samples		d003 (Å)	crystallite size (nm)	microstrain (ε)
LDH_SA	parental LDH	8.89	20.16	0.019
	15 min	12.02	16.87	0.032
	30 min	12.12	16.87	0.032
	1 h	12.03	22.49	0.024
	2 h	11.95	22.49	0.023
	3 h	12.03	25.35	0.021
	5 h	12.06	40.62	0.013

samples		d 003 (Å)	crystallite size (nm)	microstrain (ε)
LDH_TA	parental LDH	8.91	28.99	0.014
	15 min	13.64	40.59	0.015
	30 min	13.53	28.93	0.021
	1 h	13.90	20.24	0.031
	2 h	13.58	12.65	0.048
	3 h	13.60	20.24	0.030
	5 h	13.73	25.34	0.024

Microstrain (ε), representing lattice distortion, was estimated using the Williamson–Hall relation in its simplified form

$$\epsilon =\frac{\beta }{4tg\theta }$$ 11

For LDH_SA, a similar trend was observed in the XRD patterns starting from 2 h, but the 5 h treatment emerged as the most structurally favorable condition. This is evidenced by an expanded interlayer spacing (d ₀₀₃ = 12.06 Å), confirming the effective insertion of SA compared to the parental LDH (8.89 Å). While comparable or slightly greater interlayer distances were observed at earlier treatment times (e.g., 12.12 Å at 30 min), only the 5 h sample exhibited a simultaneous and substantial increase in crystallite size (40.62 nm), indicating improved long-range ordering and better-aligned stacking of the lamellar sheets. Furthermore, this time treatment also yielded the lowest microstrain value (0.013), indicating a relaxed and defect-minimized lattice, in contrast to the higher strain values (0.032) observed at shorter durations. This unique combination of maximal crystallinity, minimal internal distortion, and consistent interlayer expansion suggests that extended intercalation time facilitates not only diffusion but also the optimal orientation and accommodation of SA molecules within the LDH galleries. Conversely, for the LDH_TA coating, the 30 min treatment time offers the most balanced and structurally reliable condition. The basal spacing (d ₀₀₃ = 13.53 Å) at this time point reflects a significant expansion of the interlayer region compared to the parental LDH (8.91 Å), confirming successful incorporation of TA within the layered galleries. Although the 15 min treatment initially appears promising due to its slightly higher d ₀₀₃ value (13.64 Å), the largest crystallite size (40.59 nm), and low microstrain (0.015), a closer examination of the XRD pattern reveals an asymmetric (003) peak, suggesting phase coexistence and incomplete intercalation. In contrast, the XRD pattern at 30 min displays a sharper and more symmetric (003) reflection, reflecting a more homogeneous and single-phase intercalated structure. Although the crystallite size at this point is moderately lower (28.93 nm), it is still close to that of the parental LDH, and the microstrain is slightly higher (0.021); these values remain within acceptable ranges and are offset by the improved phase purity and structural uniformity. At longer treatment durations (1–5 h), the coating exhibits increased microstrain and decreased crystallite size, suggesting growing lattice distortion and loss of structural order. These findings support that while TA intercalation occurs rapidly, a minimum of 30 min is required to achieve a structurally stable and well-defined intercalated phase. Accordingly, 5 h was chosen for LDH_SA to ensure complete intercalation, enhanced crystallinity, and minimal lattice strain, while 30 min was selected for LDH_TA to balance interlayer expansion with structural integrity and phase homogeneity. Both durations represent the optimal intercalation times from both structural and kinetic standpoints and are thus suitable for advancing the performance of these hybrid LDH materials, particularly in applications such as anticorrosion coatings.

The surface morphologies of AA2024 substrates after the formation of the parental LDH (Figure , top), and following the optimized intercalation of SA (Figure , middle) and TA (Figure , bottom), were investigated using SEM. In all cases, the formation of a characteristic flake-like LDH structure is evident, with nanosheets organized into interconnected networks forming bulge-like aggregates. , This morphology closely resembles that observed in in situ LDH growth processes, where the layered flakes nucleate and coalesce across the metal surface. Such distributions may be influenced by local compositional heterogeneities or microstructural features of the AA2024 alloy, particularly the spatial variation of alloying elements like Cu or Fe near grain boundaries and intermetallics, which can modulate dissolution and precipitation dynamics during LDH growth. ,, Although the morphological differences between the pristine LDH and the SA- or TA-intercalated LDH coatings appear subtle, they likely reflect a partial recrystallization phenomenon induced by the structural reorganization required to host the bulky organic anions within the interlayer galleries, while maintaining the overall lamellar integrity. Such recrystallization may enhance layer alignment or alter interflake interactions, thereby contributing to the improved structural order and reduced microstrain observed in the XRD data. These results underscore the close interplay between interlayer chemistry and surface morphology, which is crucial for tailoring LDH-based coatings for corrosion protection.

2.

Surface morphologies of AA2024 substrates coated with parental LDH (top), and after optimized intercalation of SA (middle) and TA (bottom).

Corrosion Performance Assays

The corrosion resistance of the different AA2024 samples was initially assessed through immersion testing under ambient conditions. The test compared the uncoated AA2024 alloy with surfaces modified by the parental LDH and its derivatives intercalated with SA and TA (Figure ). The bare AA2024 substrate exhibited rapid degradation, with pronounced corrosion features, especially localized pitting, becoming evident within the first 6 h of exposure. This severe degradation highlights the alloy’s inherent susceptibility to chloride-induced corrosion in the absence of surface protection. In contrast, the samples coated with parental LDH and LDH_TA showed intermediate corrosion resistance. Visual signs of corrosion initiation, including darkening and pitting, appeared on the LDH_TA-coated surface after approximately 24 h of immersion, and after 72 h for the parental LDH coating. This limited protective effect may result from partial breakdown of the LDH film or insufficient inhibition provided by the intercalated species. Notably, however, the LDH_SA-coated sample demonstrated significantly improved resistance to corrosion, with initial degradation features only observed after 120 h of immersion, indicating a substantial delay in the onset of corrosion phenomena. Even after prolonged exposure, this surface remained free from pitting, exhibiting only a slight darkening suggestive of superficial changes rather than active corrosion. This remarkable enhancement in corrosion protection can be attributed to the efficient intercalation of the SA anions into the LDH structure, which likely stabilizes the layered structure and provides active inhibition through anion exchange or surface passivation mechanisms. These findings are consistent with previously reported observations for vanadate- or molybdate-intercalated LDHs on Mg-based substrates, where the presence of functional anions not only improves film integrity but also introduces ion-responsive protection mechanisms. Furthermore, the superior behavior of LDH_SA may be partially linked to the structural reorganization inferred from XRD analysis, where enhanced crystallinity and reduced microstrain suggest a more ordered and robust lamellar network. This synergistic interplay between improved structural properties and electrochemical performance underscores the role of intercalated organic species in tailoring LDH coatings for advanced corrosion-resistant applications.

3.

Optical images of bare AA2024, parental LDH-coated, and LDH films intercalated with SA and TA after immersion in 3.5 wt % NaCl aqueous solution for various exposure durations.

To elucidate the corrosion inhibition processes, EIS was conducted on bare AA2024 and AA2024 coated with parental LDH, LDH_SA, and LDH_TA. The corresponding Bode plots (impedance modulus Z _mod and phase angle) and Nyquist plots are presented in Figure , while the evolution of the Z _mod at 0.1 Hz over immersion time is depicted in Figure . The EIS spectra clearly highlight differences in the electrochemical behavior between the bare and coated specimens. As shown in Figure , the impedance modulus at 0.1 Hz, which is commonly associated with charge transfer resistance and long-term barrier properties, clearly differentiates the protective behavior of the tested coatings over immersion time. The bare AA2024 substrate exhibits the lowest Z _mod values across all time points, decreasing steadily from 17,670 Ω cm² at 0.5 h to 4086 Ω cm² at 48 h, indicating rapid degradation and poor corrosion resistance. In contrast, the parental LDH coating provides the highest Z _mod values throughout the entire immersion period, peaking at 56,400 Ω cm² at 30 min and remaining high at 35,140 Ω cm² after 48 h, confirming its robust and sustained barrier effect. The LDH_SA coating demonstrates initially high Z _mod values, starting at 38,100 Ω cm² at 30 min, with relatively stable behavior throughout immersion, maintaining values above 30,000 Ω cm² across most time points. Notably, from 12 to 48 h, LDH_SA consistently outperforms the parental LDH, culminating in a higher Z _mod at 48 h (39,900 vs 35,140 Ω cm²), highlighting its improved long-term corrosion resistance. The superior late-stage performance of LDH_SA can be attributed to the intercalation of SA molecules, which enhance the structural compactness and hydrophobic character of the LDH layers, thereby reducing electrolyte ingress and slowing degradation processes. LDH_TA, although initially comparable to AA2024 (17,000 Ω cm² at 30 min), exhibits a progressive decline in Z_mod with immersion, falling below 15,000 Ω cm² after 6 h and reaching only 14,190 Ω cm² at 48 h. This suggests a less stable barrier effect compared to both LDH_SA and parental LDH. Overall, these results highlight the more durable and effective protective action offered by LDH_SA compared to both the parental LDH and LDH_TA, in full agreement with immersion test observations.

4.

Nyquist and Bode plots (Z _mod and phase angle) of bare AA2024 (a–c) and AA2024 coated with parental LDH (d–f), LDH_SA (g–i), and LDH_TA (j–l).

5.

Impedance modulus (Z _mod) at 0.1 Hz for bare AA2024 and AA2024 coated with parental LDH, LDH_SA, and LDH_TA as a function of immersion time.

A detailed analysis of the EIS data in Figure , including Bode modulus, phase angle, and Nyquist plots, provides insight into the protective mechanisms of LDH-based coatings. Across all samples, three characteristic frequency regions can be identified, each reflecting distinct electrochemical processes at the interface:

• High-frequency region: the impedance modulus remains relatively constant, and the phase angle tends toward 0°, a behavior typically attributed to the uncompensated solution resistance (R _s). This region is mainly influenced by the electrolyte conductivity and test setup and is generally unaffected by surface modification. ,

• •Midfrequency region: this region reveals substantial differences among the coatings. For all coated samples, the phase angle exhibits a pronounced peak (up to −80°), and the Z _mod varies nearly linearly with frequency on a logarithmic scale. This behavior is characteristic of a capacitive response, associated with the passive layer or double-layer capacitance (C _dl). , The bare AA2024 (Figure a–c) shows a broader, less distinct capacitive arc in the Nyquist plots, indicating a thinner or nonuniform protective oxide layer and poor corrosion protection. This behavior is consistent with the progressive decline in phase angle and the steady decline in the Z _mod slope over time in the Bode plots, reflecting the gradual breakdown of the surface barrier and increased charge transfer at the metal–electrolyte interface. The parental LDH coating (Figure d–f) initially displays well-defined capacitive behavior, characterized by a pronounced arc and high Z _mod values, indicating good early stage barrier properties. However, over time, a gradual decline in Z _mod, along with a broadening and slight flattening of the phase angle profile, suggest a loss of capacitive integrity and a progressive degradation of the protective coating. This decline highlights the limited long-term stability of the barrier layer in the absence of intercalated organic inhibitors, which are essential for maintaining structural cohesion and resisting electrolyte penetration during prolonged exposure. In contrast, LDH_SA (Figure g–i) exhibits a sharper and more pronounced capacitive arc in the Nyquist plots, along with stable, well-defined phase angle peaks in the Bode plots. These futures suggest the formation of a compact and homogeneous barrier layer with low-defect density, effectively impeding electrolyte ingress. The persistence of a nearly linear Z _mod slope over time further confirms the durability of the capacitive behavior and the sustained corrosion protection offered by LDH_SA. Finally, LDH_TA (Figure j–l) exhibits intermediate performance better than bare AA2024 but inferior to LDH_SA, with reduced Z _mod values and less pronounced phase angle maxima over time. The progressive decrease in Z _mod and the fading of the capacitive arc suggest a gradual loss of barrier efficiency. This may result from lower coating compactness, less efficient intercalation, or differing release kinetics or inhibition efficiency of TA compared to SA, leading to a less stable and protective interfacial layer.

• •Low-frequency region: the impedance modulus plateaus and the phase angle again approaches 0°, indicating that charge-transfer resistance (R _ct) becomes the dominant electrochemical process. , This frequency range is crucial for evaluating corrosion resistance, as it reflects the barrier properties against electrolyte penetration and corrosion kinetics at the metal/coating interface. Among all samples, the LDH_SA sample exhibits the most stable Z _mod (Figure g–i) throughout the immersion period, as also evidenced at 0.1 Hz (Figure ), indicating a superior and sustained R _ct. This suggests that LDH_SA provides the most effective corrosion protection, likely due to the hydrophobic nature and efficient surface-blocking effect of SA anions intercalated within the LDH galleries. The parental LDH shows initially high impedance values, but a progressive decline in Z _mod over time (Figure d–f) suggests structural degradation or nitrate anion leaching, reducing long-term protection. The LDH_TA coating shows better performance than bare AA2024, with moderately high Z _mod values, but remains inferior to parental LDH and LDH_SA. This may result from lower intercalation density, higher solubility of TA, or a less compact barrier layer.

The Nyquist plots further complement the Bode analysis by revealing the impedance behavior in the complex plane. The bare AA2024 (Figure c) displays a small, depressed semicircle, indicative of low R _ct and fast corrosion kinetics, accompanied by a low-frequency tail characteristic of Warburg-type impedance, reflecting diffusion-controlled processes. In contrast, LDH_SA (Figure i) presents the largest semicircle diameter, confirming the highest R _ct and most effective corrosion inhibition. The parental LDH (Figure f) shows smaller semicircles than LDH_SA, indicating moderate protection, particularly at prolonged immersion times. LDH_TA (Figure l) exhibits intermediate semicircles, along with low-frequency tailing, again indicative of Warburg-type behavior. This implies that corrosion in this coating may involve mass transport phenomena, such as the diffusion of chloride ions or dissolution products through partially degraded or porous coating layers.

To extract quantitative parameters, the EIS spectra were fitted using a classical {R _s(C _dl R _ct)} equivalent circuit, where R _s represents the solution resistance, R _ct the charge transfer resistance at the metal/electrolyte interface, and C _dl a constant phase element reflecting the nonideal capacitance of the rough metal surface, , as illustrated in Figure . The fitting results, including capacitance (C _dl), resistance (R _ct), and inhibition efficiency (IE %), are summarized in Table . The inhibition efficiency of each sample (IE %) was calculated according to the following equation

$$\mathrm{I}\mathrm{E}\%=\frac{R_{\mathrm{c}\mathrm{t}}-R_{\mathrm{c}\mathrm{t}}^{\mathrm{b}\mathrm{l}\mathrm{a}\mathrm{n}\mathrm{k}}}{R_{\mathrm{c}\mathrm{t}}}\times 100$$ 12

6.

Electrical equivalent circuit model used for fitting the EIS spectra.

3. Fitting Parameters of the EIS Spectra for Bare AA2024 and AA2024 Coated with Parental LDH, LDH_SA, and LDH_TA.

specimen	immersion time	C dl (μF/cm2)	R ct (kΩ/cm2)	IE (%)
bare AA2024	0 min	20.57	2.63	-------
	30 min	21.97	7.99	67.04
	1 h	31.59	8.12	67.50
	3 h	39.56	6.28	58.09
	6 h	39.37	5.92	55.55
	12 h	40.44	4.94	46.70
	18 h	42.74	3.21	18.12
	24 h	42.79	3.22	18.35
	48 h	53.49	3.27	19.67
parental LDH	30 min	8.89	69.17	88.44
	1 h	8.72	46.81	82.65
	3 h	10.87	50.31	87.51
	6 h	13.44	32.89	82.00
	12 h	12.83	26.92	81.64
	18 h	12.08	25.63	87.47
	24 h	12.40	34.29	90.60
	48 h	12.81	33.44	90.22
LDH_SA	30 min	9.49	34.05	79.47
	1 h	14.16	41.01	80.19
	3 h	16.82	33.18	81.07
	6 h	18.18	33.87	82.52
	12 h	19.79	37.16	86.70
	18 h	20.79	47.97	93.30
	24 h	21.79	49.25	93.46
	48 h	22.34	66.79	95.10
LDH_TA	30 min	2.72	10.68	25.18
	1 h	3.54	10.44	22.22
	3 h	6.09	9.86	36.30
	6 h	8.41	9.98	40.68
	12 h	11.69	10.61	53.44
	18 h	13.92	10.86	70.44
	24 h	15.87	10.15	68.27
	48 h	21.45	8.72	62.50

A clear improvement in corrosion resistance is evidenced by the inhibition efficiencies (IE %) after 48 h of immersion. The bare AA2024 substrate exhibits a significant drop in R _ct, with an IE of only 19.67%, confirming its high susceptibility to chloride-induced corrosion. In contrast, samples coated with parental LDH, LDH_SA, and LDH_TA achieve significantly higher IE values of 90.60%, 95.10%, and 62.50%, respectively, after 48 h of immersion, indicating enhanced protective performance, particularly for the LDH_SA coating.

The evolution of the C _dl values over time offers further insight into the interfacial behavior of these coatings. For bare AA2024, a continuous increase in C _dl from 20.57 to 53.49 μF/cm² indicates progressive degradation of the native oxide layer and increased electrolyte access to the metal surface. In contrast, the parental LDH maintains relatively stable and much lower C _dl values (ranging from 8.72 to 13.44 μF/cm²), reflecting a more persistent barrier effect despite the absence of intercalated inhibitors. LDH_SA exhibits a gradual increase in C _dl from 9.49 to 22.34 μF/cm², suggesting delayed electrolyte infiltration possibly due to controlled release of the intercalated SA and the progressive expansion of the electroactive interface. Nevertheless, the consistently high R _ct values and inhibition efficiency confirm that its protective properties remain effective over time. For LDH_TA, a sharper rise in C _dl (from 2.72 to 21.45 μF/cm²) points to more significant electrolyte penetration and lower barrier integrity, which is consistent with its reduced inhibition efficiency. This may be attributed to differences in intercalation density, release kinetics, or molecular interactions between the TA anions and the LDH host layers.

These results are in line with previous findings on LDH-coated systems. For instance, in the study of AZ91 Mg alloy coated with parental LDH and intercalated vanadate/oxalate, similar trends were observed: while the parental LDH layer initially exhibited high impedance, it degraded rapidly due to poor long-term stability. In that case, vanadate intercalation successfully stabilized the coating, whereas oxalate failed to provide long-term protection. Analogously, the high and sustained impedance in LDH_SA coatings on AA2024 suggests the formation of a more compact and stable barrier, likely due to strong interactions between SA anions and the LDH host layers, leading to improved inhibition. Moreover, the improved impedance responses over time for LDH_SA (and to a lesser extent, LDH_TA) could reflect an initial self-healing or passivation effect, commonly attributed to the slow release of organic inhibitors from the LDH galleries, contributing to the regeneration of a protective surface layer. This behavior aligns with earlier studies on LDH intercalation systems, where such inhibitors delayed electrolyte penetration and mitigated substrate degradation. ,,

To further assess the corrosion behavior of the AA2024 substrate and the protective performance of the various LDH-based coatings, PDP measurements were carried out after different immersion times in 3.5 wt.% NaCl solution. This electrochemical technique enables the extraction of key kinetic parameters, such as the corrosion current density (i _corr), corrosion potential (E _corr), Tafel slopes (β_a and β_c), and polarization resistance (R _p), which collectively provide insight into the anodic and cathodic reaction mechanisms and the overall corrosion rate. The evolution of these parameters was monitored over 48 h for the uncoated AA2024 alloy and samples coated with parental LDH, LDH_SA, and LDH_TA. This time-resolved approach allows for a comparative evaluation of the coatings’ ability to delay corrosion initiation, reduce the corrosion rate, and maintain electrochemical stability under prolonged chloride exposure. The obtained polarization curves and fitting parameters are presented in Figure and Table , respectively, and are discussed in correlation with the impedance results to provide a comprehensive understanding of the inhibition mechanisms.

7.

Potentiodynamic polarization (PDP) curves of bare AA2024 (a), AA2024 coated with parental LDH (b), LDH_SA (c), and LDH_TA (d).

4. Fitting Parameters of the PDP Curves for Bare AA2024 and AA2024 Coated with Parental LDH, LDH_SA, and LDH_TA .

specimen	time	i corr (mA.cm–2)	–E corr (mV)	βa (mV/dec)	–βc (mV/dec)	R p (Ω/cm2)
bare AA2024	0 min	0.019	572.1	23.4	1201.7	5.21
	30 min	0.990	935.6	134.1	61.3	14.37
	1 h	1.854	929.8	166.6	78.3	9.52
	3 h	0.969	897.7	248.5	102.8	29.09
	6 h	0.985	877.3	359.0	117.5	36.88
	12 h	0.531	873.1	382.6	112.2	70.55
	18 h	0.608	827.9	673.8	126.9	82.52
	24 h	0.469	827.6	743.6	125.3	108.95
	48 h	0.227	806.1	619.9	119.9	208.42
parental LDH	30 min	1.041	938.9	135.7	57.2	13.07
	1 h	6.017	943.7	3755.9	91.7	7.98
	3 h	0.909	844.3	243.5	108.8	33.80
	6 h	0.628	812.9	357.2	134.5	79.09
	12 h	0.676	828.9	393.8	125.2	65.73
	18 h	0.496	814.1	336.2	129.0	94.77
	24 h	1.388	817.5	352.1	139.1	27.62
	48 h	0.151	776.8	591.8	129.7	349.48
LDH_SA	30 min	0.657	932.1	166.1	61.5	23.59
	1 h	1.150	913.3	206.0	88.3	18.24
	3 h	0.677	885.1	268.4	111.4	46.08
	6 h	0.686	877.8	310.8	115.1	48.17
	12 h	0.521	870.1	328.5	116.6	68.41
	18 h	0.466	868.1	400.8	115.9	79.09
	24 h	0.168	855.1	289.8	107.7	205.59
	48 h	0.078	803.2	417.2	115.1	473.87
LDH_TA	30 min	2.114	953.2	2005.6	62.2	14.37
	1 h	0.793	914.5	199.4	87.0	27.26
	3 h	0.412	883.9	309.7	105.9	77.91
	6 h	0.890	879.6	402.6	116.1	35.65
	12 h	0.688	855.9	383.8	126.5	56.20
	18 h	0.505	856.2	428.2	125.3	86.50
	24 h	0.186	831.2	336.4	118.6	217.67
	48 h	0.097	812.1	466.0	116.1	401.41

^aDFT insights into LDH–inhibitor–metal interfaces.

The PDP fitting parameters reveal a clear evolution of corrosion behavior across the different samples, in coherence with the EIS data. For bare AA2024, the corrosion current density (i _corr) increases sharply from 0.019 mA cm^–2 at 0 min to a peak of 1.854 mA cm^–2 at 1 h, confirming rapid corrosion initiation, followed by a gradual decline to 0.227 mA cm^–2 at 48 h, likely due to the accumulation of corrosion products, with the polarization resistance (R _p) increasing from 5.21 to 208.42 Ω cm². The parental LDH initially offers moderate protection, with relatively high i _corr values up to 6.017 mA cm^–2 at 1 h, possibly reflecting surface defects or inhomogeneities, despite its strong early stage impedance response. However, it shows delayed improvement, dropping to 0.151 mA cm^–2 and R _p = 349.48 Ω cm² at 48 h, indicating a progressive reorganization or compaction of the LDH structure. The LDH_SA sample exhibits significantly better protection throughout the immersion period, with i _corr decreasing steadily from 0.657 to 0.078 mA cm^–2 and R _p increasing from 23.59 to 473.87 Ω cm², confirming the effective barrier role of SA, likely through carboxylate-metal coordination and hydrogen bonding with the LDH matrix. Meanwhile, LDH_TA shows an initially high i _corr of 2.114 mA cm^–2 at 30 min, but this decreases progressively to 0.097 mA cm^–2 at 48 h, with R _p rising to 401.41 Ω cm², suggesting a time-dependent improvement in inhibition, likely through passivation from released TA and its ability to form multidentate complexes with metal cations via its hydroxyl and carboxyl groups. However, this trend contrasts somewhat with EIS results, which indicate a gradual decline in long-term barrier integrity, potentially due to coating porosity or lower structural cohesion. These results clearly demonstrate the time-dependent enhancement in corrosion protection, especially for LDH_SA, in agreement with the increasing Z _mod and R _ct values observed in the EIS study.

Adsorption Behavior from MC Simulations

To elucidate the adsorption behavior and confirm the favorable interactions of organic inhibitors, namely SA and TA, with both ZnAl LDH and aluminum surfaces, MC simulations were performed. The study investigates the adsorption configurations and energetics of SA and TA on ZnAl LDH (Figure a) and Al(111) surfaces (Figure b). The optimized geometries reveal short intermolecular distances between the inhibitors and both the LDH layer (∼2.4 Å) and the Al(111) surface (∼2.66 Å), indicating strong interactions and efficient adsorption. According to the data presented in Table , all calculated adsorption energies are negative, confirming the spontaneous, exothermic, and thermodynamically favorable nature of the adsorption processes. , On the ZnAl LDH layer, SA exhibits a more favorable adsorption energy (−37.42 kJ mol^–1) compared to TA (−31.26 kJ mol^–1), suggesting a slightly stronger interaction of SA with the LDH matrix. This observation quantitatively correlates with its higher dicarboxylate affinity and enhanced inhibitor loading capacity. In contrast, adsorption on the Al(111) surface yields significantly more negative energies: −227.89 kJ mol^–1 for SA and −203.42 kJ mol^–1 for TA, reflecting stronger chemisorption interactions, which are critical for surface passivation and barrier formation. The substantial difference between LDH and Al(111) adsorption energies highlights the dual-functionality of SA and TA as inhibitors: moderate, reversible interactions with LDH for controlled release, and strong, stable anchoring on aluminum surfaces.

8.

Optimized adsorption configurations of SA and TA on (a) ZnAl LDH, and (b) the Al(111) surface.

5. Calculated Adsorption Energies of the Most Stable Configurations of SA and TA on ZnAl-LDH and Al(111) Surfaces.

surface	inhibitor	adsorption energy (kJ/mol)
ZnAl LDH	SA	–37.42
	TA	–31.26
Al(111)	SA	–227.89
	TA	–203.42

MC simulations also provide in-depth insights into molecular-level adsorption mechanisms. On the ZnAl LDH surface, the adsorption of SA and TA is primarily governed by physisorption, involving hydrogen bonding and electrostatic interactions (Figure ). The carboxylic (–COOH) and hydroxyl (–OH) functional groups of the inhibitors establish hydrogen bonds with surface hydroxyl groups of the LDH layers. Electrostatic attractions between negatively charged oxygen atoms of the inhibitors and positively charged (Zn²⁺, Al³⁺) centers further stabilize the complexes, forming energetically favorable ZnAl_SA and ZnAl_TA adducts. In contrast, on the Al(111) surface, adsorption shifts toward chemisorption, with oxygen atoms from –COOH and –OH groups directly coordinating with surface aluminum atoms, forming covalent or dative bonds. This stronger interaction is consistent with the lower Al(111) adsorption energies and explains the superior inhibition efficiency of SA observed experimentally. Furthermore, π-metal interactions may occur between the delocalized π-electrons of the inhibitors (particularly from any conjugated regions) and the electron-rich metallic surface. In addition, the presence of adsorbed water molecules on the Al(111) surface can introduce hydroxyl groups (Al–OH), allowing the formation of secondary hydrogen bonds with the inhibitors’ polar functional groups, further enhancing the adsorption stability. The combined contribution of coordination bonding, hydrogen bonding, and π-metal interactions on Al(111) leads to the formation of a dense and adherent organic layer, that impedes Cl^– penetration. Overall, the MC results highlight distinct substrate-dependent adsorption modes and quantitatively rationalize the dual role of LDH–inhibitor–metal interfaces.

9.

Hydrogen bonding and electrostatic interactions between the ZnAl LDH layer and organic inhibitors (SA and TA).

Electronic Structure and Reactivity Parameters of SA and TA Inhibitors

To complement the adsorption analysis derived from MC simulations, DFT calculations were employed to evaluate the electronic structure and global reactivity descriptors of the organic inhibitors SA and TA (Figure ). These quantum chemical parameters are crucial for understanding the intrinsic reactivity, electron transfer capabilities, and ultimately the inhibition efficiency of the molecules. Among the primary descriptors, the energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) provide essential insights into the charge transfer behavior. A higher E _HOMO reflects a greater ability to donate electrons, while a lower E _LUMO indicates a stronger tendency to accept electrons. , Thus, molecules with high E _HOMO and low E _LUMO values are expected to interact more efficiently with metallic surfaces through donor–acceptor mechanisms. The energy gap (ΔE _gap) is widely used as an indicator of molecular reactivity, where a smaller gap typically correlates with higher chemical reactivity and stronger adsorption potential. ,, In the present study, both SA and TA exhibit low energy gaps, with SA having a slightly smaller value (ΔE = 0.187 eV) compared to TA (ΔE = 0.194 eV), suggesting that SA is electronically more reactive and more likely to participate in charge transfer with the substrate. In addition to Frontier orbital energies, other global reactivity indices were computed to further assess the inhibitors. The electron affinity (A) and ionization potential (I) describe the molecule’s tendency to accept or donate electrons, respectively. From these values, further descriptors were derived: electronegativity (χ), which quantifies the molecule’s tendency to attract electrons; chemical hardness (η) and softness (σ), which describe the resistance or ease with which the electron density can be deformed; and chemical potential (μ), which reflects the overall tendency of the molecule to exchange electrons with its environment. A more negative μ value typically implies a stronger driving force for electron exchange with metallic surfaces, thereby enhancing the adsorption propensity. − As summarized in Table , SA shows a slightly higher electronegativity (χ = 0.142 eV) and a higher softness value (σ = 10.752 eV) than TA (χ = 0.147 eV; σ = 10.309 eV), suggesting that SA is more polarizable and more responsive to external electric fields. Its lower hardness value further supports a greater likelihood of undergoing electron redistribution upon interaction with a metal surface. Collectively, these quantum chemical descriptors confirm that SA exhibits higher intrinsic reactivity, a stronger ability to engage in donor–acceptor interactions, and greater sensitivity to surface-induced electronic perturbations than TA. Such electronic features help explain the experimentally observed superior inhibition efficiency of SA, providing a clear molecular-level rationale for its enhanced protective behavior. These insights are fully consistent with the MC simulations and electrochemical results, which likewise demonstrate the more favorable adsorption characteristics and overall inhibition performance of SA.

10.

HOMO and LUMO maps of SA and TA.

6. Quantum Chemical Reactivity Parameters of SA and TA (in eV).

Inh	E HOMO	E LOMO	ΔE gap	A	I	χ	η	σ	μ
SA	–0.236	–0.049	0.187	0.049	0.236	0.142	0.093	10.752	–0.147
TA	–0.244	–0.050	0.194	0.050	0.244	0.147	0.097	10.309	–0.142

To further elucidate the electronic behavior of the SA and TA inhibitors and support the interpretation of their reactivity, a combined Mulliken atomic charge and Fukui function analysis was performed. Mulliken charge distribution provides a first insight into the localization of electron density within each molecule and enables the identification of atoms most capable of participating in nucleophilic (electron-donating) or electrophilic (electron-accepting) interactions. , As shown in Figure , the oxygen heteroatoms in both SA and TA exhibit the most negative partial charges, confirming their role as the primary nucleophilic centers and indicating a strong propensity to interact with positively charged sites on the Al(111) surface. This electron-donating behavior is fully consistent with the donor–acceptor mechanism inferred from the HOMO–LUMO analysis.

11.

Mulliken atomic charge distribution of the SA and TA molecules.

To complement this charge-based analysis and more precisely identify the selective reactive sites, Fukui functions were evaluated. These quantum descriptors quantify how the electron density of a molecule responds to the addition or removal of electrons, thereby providing a more rigorous assessment of the regions most susceptible to electrophilic (f _A ^– ) or nucleophilic (f _A ⁺ ) attack. Their interpretation was refined using the dual descriptor (Δf _A), where negative values (Δf _A < 0) denote nucleophilic (electron-donating) sites and positive values (Δf _A > 0) identify electrophilic (electron-accepting) centers. , The most prominent isosurfaces of f _A ^– and f _A ⁺ for both inhibitors are presented in Figure , and the corresponding condensed Fukui indices are summarized in Table .

12.

Fukui function isosurfaces showing electrophilic and nucleophilic reactive sites in SA and TA.

7. Local Condensed Fukui Indices (f _A ^–, f _A ⁺, Δf _A) for SA and TA.

succinic acid				tartaric acid
atoms	f A –	f A +	Δf A	atoms	f A –	f A +	Δf A
O (1)	0.068	0.067	–0.001	O (1)	0.050	0.030	–0.020
O (2)	0.068	0.067	–0.001	O (2)	0.057	0.025	–0.032
O (3)	0.235	0.126	–0.109	O (3)	0.063	0.071	0.008
O (4)	0.235	0.126	–0.109	O (4)	0.066	0.069	0.003
C (5)	–0.018	–0.034	–0.016	O (5)	0.204	0.143	–0.061
C (6)	–0.018	–0.034	–0.016	O (6)	0.215	0.138	–0.077
C (7)	0.073	0.161	0.088	C (7)	–0.007	–0.015	–0.008
C (8)	0.073	0.161	0.088	C (8)	–0.005	–0.014	–0.009
H (9)	0.051	0.072	0.021	C (9)	0.060	0.153	0.093
H (10)	0.051	0.072	0.021	C (10)	0.063	0.146	0.083
H (11)	0.051	0.072	0.021	H (11)	0.050	0.069	0.019
H (12)	0.051	0.072	0.021	H (12)	0.061	0.072	0.011
H (13)	0.041	0.036	–0.050	H (13)	0.022	0.018	–0.004
H (14)	0.041	0.036	–0.050	H (14)	0.026	0.020	–0.006
-	-	-	-	H (15)	0.037	0.038	0.001
-	-	-	-	H (16)	0.039	0.038	–0.001

For SA, oxygen atoms O(3) and O(4) exhibit the highest electrophilic reactivity (f _A ^– ≈ 0.235) and negative dual descriptor values, confirming their strong electron-donor character. The carbonyl carbons C(7) and C(8) present the largest nucleophilic indices (f _A ⁺ ≈ 0.161), indicating their ability to accept electron density in complementary interactions. Similarly, in TA, oxygen atoms O(5) and O(6) show the highest f _A ^– values (0.204–0.215), while carbons C(9) and C(10) exhibit the highest f _A ⁺ values (0.146–0.153). Due to its additional hydroxyl groups, TA displays a more distributed reactivity pattern, suggesting multiple potential adsorption configurations. The consistent identification of oxygen atoms as the most negative Mulliken sites and as the centers with the largest |f _A ^–| and negative Δf _A values supports their decisive role in coordinating with electrophilic Al surface atoms. This combined Mulliken–Fukui analysis demonstrates that adsorption on Al(111) is primarily driven by strong interactions between electron-rich oxygen atoms and electron-deficient surface aluminum sites. SA is therefore expected to adsorb primarily through a bidentate mode involving both carbonyl oxygens, while TA owing to its additional hydroxyl groups can establish multipoint interactions combining coordination and hydrogen bonding. Although TA offers a richer interaction network, the higher electronic reactivity of SA, reflected by its more negative oxygen charges, large Fukui electrophilic indices, and smaller ΔE _gap, enables stronger donor–acceptor interactions with surface Al atoms. This electronic advantage explains the experimentally observed superior inhibition efficiency of SA, as it promotes the formation of a more stable and strongly bound protective layer despite its structurally simpler coordination mode.

Corrosion Mechanism of Bare AA2024 and Inhibition by LDH_SA or _TA Coating

In Situ Growth of ZnAl–NO₃ LDH on AA2024 Surface

The ZnAl–NO₃ LDH is synthesized directly on the AA2024 aluminum alloy through an in situ hydrothermal process that leverages the native oxide layer as a source of aluminum. Under alkaline conditions, the surface Al₂O₃ dissolves to release aluminate species via: Al₂O₃ + 3H₂O + 2OH^– → 2Al­(OH)₄ ^–. Simultaneously, zinc ions in the solution form hydrolyzed complexes: Zn²⁺ + OH^– → Zn­(OH)⁺. These species then coprecipitate in the presence of nitrate anions to form the LDH phase: Zn­(OH)⁺ + Al­(OH)₄ ^– + NO₃ ^–+ H₂O → ZnAl–NO₃ LDH. The result is a compact and adherent LDH film uniformly anchored to the alloy surface, ensuring enhanced surface stability and providing a foundation for effective corrosion protection.

Corrosion Mechanism of Bare AA2024

In aggressive media such as 3.5 wt % NaCl, the AA2024 alloy undergoes localized corrosion due to its heterogeneous microstructure, which contains intermetallic particles (e.g., Al–Cu–Fe–Mn) that act as cathodic sites, accelerating anodic dissolution of the surrounding aluminum matrix. This galvanic coupling leads to the oxidation of aluminum: Al → Al³⁺ + 3e^–. At the same time, the cathodic reduction of dissolved oxygen occurs on the intermetallic sites: O₂ + 2H₂O + 4e^– → 4OH^–. Chloride ions (Cl^–) penetrate surface defects and disrupt the protective oxide layer, promoting pit initiation and growth. The resulting corrosion products, mainly Al­(OH)₃, form porous and loosely adherent deposits that offer limited protection, allowing corrosion to propagate further.

Enhanced Corrosion Protection by LDH_SA or _TA Coating

The resulting LDH_SA or _TA hybrid coating provides a dual-function inhibition mechanism (Figure ):

• i.Passive barrier function: the lamellar LDH structure acts as a physical barrier, impeding the diffusion of aggressive species such as Cl^– to the metal interface.
• ii.Controlled release and self-healing: when exposed to corrosive environments, Cl^– ions penetrate the LDH matrix and trigger the anion-exchange release of intercalated SA anions: LDH_C₄H₄O₄ ^2– or C₄H₄O₆ ^2– + 2Cl^– → LDH-2Cl^– + C₄H₄O₄ ^2– or C₄H₄O₆ ^2–. The released SA or TA species rapidly adsorb onto the exposed or defect sites of the AA2024 surface, forming a hydrophobic monolayer that reduces wettability and further inhibits electrolyte access.
• iii.Synergistic inhibition mechanism: beyond physical blocking and self-triggered release, the corrosion protection offered by the LDH_SA and the LDH_TA coatings is further enhanced by the chemical interaction of released succinate (SA^2–) or tartrate (TA^2–) ions with corrosion products such as Al³⁺ or Zn²⁺. These interactions lead to the formation of insoluble carboxylate complexes, which effectively seal active sites and inhibit further metal dissolution. This chemical passivation, combined with the coating’s ion-exchange-driven self-healing behavior, ensures the sustained suppression of pitting and stabilizes the protective layer over time. ,,

13.

Schematic of corrosion inhibition mechanism of LDH_SA coating on AA2024 alloy.

Conclusions

This work demonstrates the successful in situ growth of LDH coatings on AA2024 aluminum alloy with the direct intercalation of the organic inhibitors succinic acid (SA) and tartaric acid (TA). XRD analyses confirmed optimized intercalation and effective incorporation of both acids. Corrosion tests, including immersion tests, EIS, and PDP, showed that while the parental LDH provides only short-term protection, LDH intercalated with SA delivers sustained and markedly improved corrosion resistance, reaching inhibition efficiencies (IE %) of up to 95.1% after 48 h compared with 62.5% for TA. This enhancement is attributed to superior surface coverage and formation of a more stable, compact, and hydrophobic protective layer in the SA-modified coating. DFT calculations revealed strong adsorption through hydrogen bonding, electrostatic interactions, and coordination with Zn²⁺ and Al³⁺ centers, with SA exhibiting higher electronic reactivity and more favorable adsorption energetics, providing a molecular-level rationale for its superior inhibition efficiency. Overall, SA is identified as a highly promising green inhibitor, and this study highlights the synergistic value of combined experimental and theoretical approaches to elucidate inhibition mechanisms. Future work will focus on applying LDH films intercalated with bioderived organic inhibitors via plasma-assisted deposition on Al and Mg surfaces and performing comparative studies to develop smart, self-healing, robust and sustainable anticorrosion coatings.

The authors confirm that the data supporting the findings of this study are provided within the article.

Amal Abdouli: Conceptualization, methodology development, experimental preparation, formal analysis, data collection, DFT analysis, and writing of the original draft. Mohamed Amine Djebbi: Data curation, supervision, writing original draft, review, and editing. Abdesslem Ben Haj Amara: Provision of resources, funding acquisition, and validation. Hafsia Ben Rhaiem: Provision of resources. All authors have read and approved the final version of the manuscript.

The authors declare no competing financial interest.