Enhanced Corrosion Resistance of Carbon Steel Rebar in Chloride-Containing Water Solutions: The Role of Lotus Extract in Corrosion Inhibition

Abstract

Corrosion inhibitors play a crucial role in the corrosion protection of rebars in reinforced concrete structures under harsh service conditions. However, conventional corrosion inhibitors often suffer from low efficiency and environmental concerns. This study investigates a low-cost and environmentally friendly lotus leaf extract (LLE) as a corrosion inhibitor and examines its effects on carbon steel rebar corrosion under various conditions. The structure and composition of LLE were characterized using SEM, FTIR, and LC-MS. The effects of LLE on rebar corrosion behavior under different environmental conditions were investigated using electrochemical tests, Mott–Schottky analysis, and XPS. The main findings indicate that LLE is rich in polar chemical bonds and functional groups, which facilitate adsorption and film formation on the rebar surface. In a 3.5% NaCl solution, rebar corrosion is primarily influenced by the solution pH, and low concentrations of LLE exhibit effective corrosion inhibition. In a simulated concrete pore solution, higher concentrations of LLE promote the formation of a passivation film in a chloride-alkaline environment. Studies on pre-passivated rebar indicate that LLE effectively protects the passivation film, with the optimal LLE concentration for passivation film protection and adsorption film quality being 0.5 wt%. This study contributes to the application and development of novel LLE-based corrosion inhibition technology for carbon steel rebar.

1. Introduction

In harsh environments, corrosion-induced expansion and cracking of reinforced concrete can lead to reduced structural durability and a shortened service life, resulting in commercial losses and safety threats [1,2,3,4]. To effectively mitigate rebar corrosion, various protective and repair measures have been employed, including enhancing concrete density, applying cathodic protection [5,6], electrochemical chloride removal, using corrosion-resistant steel [7,8], and incorporating corrosion inhibitors [9,10,11,12,13]. Corrosion inhibitors can be either incorporated into concrete or applied to its surface, effectively preventing or slowing down rebar corrosion. These inhibitors are extensively employed owing to their straightforward application, economic viability, and superior efficacy [14]. However, conventional corrosion inhibitors suffer from limitations in long-term effectiveness and environmental sustainability. Due to their significant environmental impact, their use has been increasingly restricted. As a result, green corrosion inhibitors have attracted growing attention in the industry, and the development of novel inhibitors is expected to focus on eco-friendliness, long-term efficiency, and high corrosion resistance [15].

Currently, environmentally friendly high-performance corrosion inhibitors have made significant progress [16,17,18]. However, their development still faces challenges [19,20,21,22,23]. At present, green corrosion inhibitors are primarily represented by plant-based, pharmaceutical, and biopolymer-based organic inhibitors. These inhibitors are well-suited for reinforced concrete structures subjected to diverse corrosive conditions and varying protection needs. However, they also exhibit limitations, such as insufficient corrosion inhibition efficiency and poor stability. Therefore, the development of novel, eco-friendly corrosion inhibitors capable of providing durable corrosion protection is imperative to withstand aggressive service conditions.

Recent research has seen a growing emphasis on utilizing natural plant extracts as innovative corrosion inhibitors for rebar in concrete structures. Typical examples include zein-based corrosion inhibitors and green tea extract-based inhibitors [24]. These green inhibitors are derived from extracts of natural plants or plant waste and offer advantages such as low cost, ease of production, non-toxicity, biodegradability, and excellent environmental biocompatibility. They have demonstrated promising corrosion inhibition potential [25,26,27,28]. Lotus leaves are widely distributed and recognized as both medicinal and edible plants. Lotus leaf extract (LLE) is a class of natural compounds extracted from lotus leaves [29,30]. It has been extensively applied in the pharmaceutical and health sectors and exhibits significant potential for corrosion protection. LLE possesses abundant polar functional groups that facilitate adsorption onto rebar substrates, subsequently forming an inhibitive film through coordinate bonding, making it a promising candidate for use as a steel corrosion inhibitor. Recent studies have demonstrated that LLE possesses excellent corrosion inhibition properties [31,32]. At present, there are research reports on the application of lotus leaf extract for the corrosion inhibition performance of copper surfaces [33,34]. Studies on the corrosion inhibition of lotus leaf extract on carbon steel mainly focus on acidic environments [31]. Additionally, several numerical simulation studies have been conducted on the corrosion inhibition mechanisms of lotus-leaf-inspired inhibitors [32,35]. However, research reports focusing on alkaline environments relevant to reinforced concrete applications remain scarce, and the relevant mechanisms remain unclear.

This study systematically investigates the effects of LLE on the corrosion characteristics of carbon steel rebar. The structure, composition, and film-forming properties of LLE on the rebar surface are characterized. Electrochemical characterization, coupled with other analytical techniques, was systematically conducted to evaluate LLE’s corrosion inhibition performance on rebar substrates exposed to both 3.5 wt% NaCl solution and simulated concrete pore solution. Furthermore, the inhibition mechanism of LLE in mitigating rebar corrosion under various conditions is explored.

2. Materials and Methods

2.1. Materials

The rebar used was carbon steel (HRB400) with a diameter of 16 mm. Its composition is presented in

Table 1. Chemical composition of rebars (wt%).

Chemical Composition	Fe	C	Si	Mn	S	P	Carbon Equivalent
HRB400	Balance	0.25	0.80	1.60	0.045	0.045	0.54

. The lotus leaf extract (LLE) used in the experiments was supplied by Guosheng Biotechnology Co., Ltd. (Ankang, China).

2.2. Analysis and Testing Methods

2.2.1. Composition, Structure, and Performance Analysis

Scanning electron microscopy (SEM) utilizes an electron beam that interacts with the sample surface to generate signals, such as secondary electrons and backscattered electrons. These signals are detected and used to produce images of the sample surface, providing information about its morphology and composition. The morphological characteristics of different samples were observed using a GeminiSEM 300 SEM (ZEISS, Oberkochen, Germany). Elemental qualitative and quantitative analyses of the surface corrosion products were conducted in conjunction with energy-dispersive X-ray spectroscopy (EDS).

Fourier-transform infrared spectroscopy (FTIR) is based on the principle of molecular absorption of infrared radiation. In the infrared spectral region, when chemical bonds in molecules vibrate, they absorb specific wavelengths of infrared radiation. These absorption bands can be used to determine the molecular structure and chemical composition of compounds. The functional groups and molecular structure of LLE were analyzed using a Nicolet iS20 FTIR spectrometer (Thermo Scientific, Waltham, MA, USA).

Liquid chromatography–mass spectrometry (LC-MS), also known as LC-MS coupling technology, employs liquid chromatography to introduce samples into the mass spectrometry system. The components of the sample are first separated by a chromatographic column and then directly ionized and analyzed for mass in the mass spectrometer. The main chemical components of LLE were qualitatively analyzed using an 1290 UPLC-QTOF 6550 liquid chromatography–mass spectrometer (LC-MS, Agilent, Santa Clara, CA, USA). The chromatographic separation was performed on a Waters BEH C18 column (2.1 mm× 100 mm, 1.7 μm particle size) using a binary mobile phase system: (A) 0.1% (v/v) aqueous formic acid and (B) methanol. The flow rate was maintained at 0.3 mL/min with a 5 μL injection volume. The mass spectrometry (MS) scan range was set to 50–1000 m/z in the first stage, and the electrospray ionization (ESI+) mode was applied at a voltage of 4000 V.

X-ray photoelectron spectroscopy (XPS) is based on the emission of photoelectrons from atoms in a sample when it is irradiated with X-rays. By measuring the kinetic energy and number of emitted photoelectrons, information about the elemental composition and chemical states of the material’s surface can be obtained. XPS analysis was conducted using a K-Alpha spectrometer (Thermo Scientific, Waltham, MA, USA) equipped with a monochromatic Al Kα radiation source (1486.6 eV) to investigate the chemical composition and oxidation states of elements within the passive film formed on the rebar surface. Data analysis and peak fitting were conducted using Avantage software 6.6.

2.2.2. Neutral Chloride Salt Corrosion System

A test was performed in a 3.5% NaCl solution to simulate the chloride-induced corrosion environment. Five different concentrations of LLE (0.1, 0.2, 0.3, 0.4, and 0.5 wt%) were added to the NaCl solution to establish the corrosion system, with a blank control group included for comparison. The chloride corrosion resistance of LLE at different concentrations was analyzed.

2.2.3. Simulated Concrete Pore Solution Corrosion System

The alkaline pore solution of concrete was simulated by preparing a saturated calcium hydroxide solution. This solution was prepared by mixing calcium hydroxide powder with deionized water (pH ≈ 12.5). After preparation, the solution was subjected to 24 h of sedimentation to allow the undissolved calcium hydroxide particles to settle. The upper clear liquid was then used for the experiment. Five different concentrations (0.1, 0.2, 0.3, 0.4, and 0.5 wt%) of LLE were added to the solution to form the corrosion system, with 3.5% NaCl providing the corrosive chloride medium.

2.2.4. Electrochemical Testing

The electrochemical workstation used in this experiment is the CHI600E, manufactured by Shanghai Chenhua Instruments (Shanghai, China). A three-electrode system was used for all experiments, with the rebar, SCE, and platinum acting as the working electrode, reference electrode, and auxiliary electrode, respectively, in the corrosion simulation tests. Each test was conducted with no fewer than three replicate samples. The temperature during the tests was maintained at 25 ± 2 °C.

Electrochemical impedance spectroscopy (EIS) is a non-destructive technique used to investigate electrochemical corrosion behavior. It applies an AC signal to the electrochemical system and analyzes the impedance response to characterize its electrochemical properties. The impedance spectrum is represented in complex form, comprising a real part and an imaginary part. The real part reflects resistive behavior, while the imaginary part reflects capacitive or inductive characteristics. In this study, EIS measurements were conducted at the open circuit potential, with a frequency range of 100 kHz to 10 mHz and a sinusoidal perturbation amplitude of ±5 mV. The acquired EIS data were fitted using ZSimpWin 3.6 software.

Potentiodynamic polarization testing involves applying a varying potential to the working electrode while recording the resulting current to analyze the corrosion behavior of the metal. In this study, the PDP tests were conducted by scanning the potential from 0.25 V below the open circuit potential (OCP) to 1 V at a rate of 1 mV/s. Corrosion current density and other electrochemical parameters were obtained using the Tafel extrapolation method.

2.2.5. Mott–Schottky Test

A passive film forms on the surface of carbon steel rebar in the highly alkaline environment (pH > 12.5) provided by the concrete pore solution. The semiconductor properties of the passive film are strongly correlated with its corrosion behavior. The semiconductor characteristics of the passive film formed on rebar under passivation conditions containing various concentrations of lotus leaf-based corrosion inhibitor were investigated using Mott–Schottky (M-S) analysis. This technique is based on the Mott–Schottky equations (see Equations (1) and (2)) and involves measuring the capacitance of the electrochemical system as a function of the applied potential. From the resulting M-S plots, key electrical properties of the semiconductor can be derived.

$${\displaystyle \frac{1}{{\mathrm{C}}_{\mathrm{S}\mathrm{C}}^2}}={\displaystyle \frac{2}{\mathrm{e}{\mathrm{N}}_{\mathrm{D}}{\mathrm{\epsilon }}_0\mathrm{\epsilon }}}({\mathrm{E}}_{\mathrm{m}}-{\mathrm{E}}_{\mathrm{f}\mathrm{b}}-{\displaystyle \frac{\mathrm{k}\mathrm{T}}{\mathrm{e}}})(\mathrm{n-type}\mathrm{semiconductor})$$

(1)

$${\displaystyle \frac{1}{{\mathrm{C}}_{\mathrm{S}\mathrm{C}}^2}}={\displaystyle \frac{2}{\mathrm{e}{\mathrm{N}}_{\mathrm{A}}{\mathrm{\epsilon }}_0\mathrm{\epsilon }}}({\mathrm{E}}_{\mathrm{m}}-{\mathrm{E}}_{\mathrm{f}\mathrm{b}}-{\displaystyle \frac{\mathrm{k}\mathrm{T}}{\mathrm{e}}})(\mathrm{p-type}\mathrm{semiconductor})$$

(2)

In the equations, ε represents the dielectric constant of the passive film on the metal surface, ε₀ is the permittivity of free space (8.854 × 10⁻¹⁴ F·cm⁻¹), and e is the elementary charge (1.602 × 10⁻¹⁹ C). N_D and N_A denote donor and acceptor densities, respectively. E_fb is the flat band potential, k is the Boltzmann constant (1.38 × 10⁻²³ J·K⁻¹), and T is the absolute temperature. The term kT/e can be neglected, as it accounts for only about 25 mV at room temperature. N_D and N_A can be determined from the slope of the Mott–Schottky plot. The flat band potential (E_fb) can be estimated by linear extrapolation, while the dielectric constant (ε) is derived from the plot parameters. The validity of the Mott–Schottky analysis is based on the assumption that the space charge layer capacitance is significantly smaller than the double-layer (Helmholtz) capacitance. It is generally accepted that the passive film formed on carbon steel exhibits n-type semiconducting behavior in the passivation region. In this study, Mott–Schottky analysis was performed at a fixed frequency of 1 kHz with an amplitude of 10 mV. The potential ranged from −1.5 to +1.5 V, at a rate of 50 mV/s.

3. Results

3.1. Structure and Composition Analysis of Lotus Leaf Extract

Lotus leaf extract (LLE) is rich in polar groups and cyclic structures, which have a strong chemisorption affinity for rebar surfaces, forming an organic protective film that impedes the intrusion of corrosive media and effectively suppresses both the initiation and propagation of rebar corrosion (Figure 1a). Figure 1b shows the SEM image and elemental surface scan results of LLE. As observed, LLE consists of uneven spherical particles, primarily composed of C, O, and K elements. The FTIR spectrum of LLE is presented in Figure 1c. The absorption bands at 1026 cm⁻¹, 1420 cm⁻¹, 1640 cm⁻¹, 2926 cm⁻¹, and 3415 cm⁻¹ are associated with the C-O stretching vibration, the C-H bending vibration of alkyl groups, the N-H and C=O stretching vibrations, the stretching vibration of saturated C-H, and the O-H stretching vibration in water molecules, respectively. The analysis indicates that LLE is rich in O-H, N-H, and C-O. These polar functional groups can effectively adsorb onto the rebar surface, inhibiting the occurrence and progression of corrosion.

Figure 2 shows the chromatogram and mass spectrum of the main components of LLE. As shown in Figure 2a, the mass spectrum exhibits four peaks with mass-to-charge ratios of 282.1491, 283.1526, 284.1543, and 299.1823. The difference between the first and fourth peaks is 17.0332, which corresponds to the precise molecular mass difference of NH4+ and H+, measured at 17.0265. Therefore, it is determined that 282.1491 corresponds to the protonated compound peak, and 299.1823 corresponds to the ammonium adduct peak. A molecular formula search for the peak at 282.1491 suggests the molecular formula C₁₈H₁₉NO₂, which corresponds to de-methylated lotusine. Based on the analysis of Figure 2b–f, the main chemical components of LLE are de-methylated lotusine, quercetin, hypericin, kaempferol, lotusine, and isoquercitrin. The heteroatoms, such as N and O, in the chemical composition of LLE, as well as its heterocyclic structure, can serve as adsorption centers. Electron-rich heteroatoms form coordinate covalent bonds by donating lone pair electrons into the unoccupied 3D orbitals of surface iron atoms, establishing stable Fe(II/III)-ligand complexes and forming an adsorptive film on the rebar surface.

3.2. Anti-Corrosion Properties of LLE on Rebars in Neutral Chloride Salt Solutions

To analyze the anti-chloride ion corrosion ability of LLE, EIS was used to study the electrochemical processes at the rebar/corrosion solution interface in 3.5% NaCl solutions with different LLE concentrations. The EIS data obtained after soaking the rebar in solutions for 1 day are shown in Figure 3a. From the Nyquist plot (Figure 3(a1)), it can be observed that all LLE concentrations show a single high-frequency capacitive arc. These arcs are not perfect semicircles due to frequency dispersion caused by the roughness or non-uniformity of the electrode surface. The Bode impedance modulus plot (Figure 3(a2)) shows that the impedance modulus of the samples with LLE is higher than that of the samples without LLE, indicating that LLE is adsorbed on the rebar surface, thereby resisting charge transfer. The singular peak observed in the Bode phase plot (Figure 3(a3)) corresponds to a unified electrochemical process, demonstrating that interfacial corrosion kinetics are predominantly governed by electron transfer mechanisms. The addition of LLE did not change the shape of the arc but only increased the diameter of the semicircle. Optimum corrosion inhibition performance was achieved at a 0.1 wt% inhibitor concentration, as evidenced by the maximum diameter observed in the Nyquist plots, indicating that this concentration provides the best anti-corrosion effect after 1 day of soaking. This is due to the limited soaking time, where the dominant factor affecting rebar corrosion is the solution’s pH value. Since LLE is weakly acidic (the pH values measured for 0–0.5 wt% LLE were 6.21, 5.75, 5.43, 5.15, 5.05, and 5.04, respectively), the pH value gradually decreases with increasing LLE concentration, resulting in a reduction in the protective efficacy.

As the soaking time extended to 3 days (Figure 3b), the impedance arc radius of the samples with added LLE increased to varying degrees. This increase is due to the prolonged soaking time facilitating progressive LLE adsorption, ultimately yielding a more robust protective film. Under the combined effects of LLE adsorption and solution pH, the increase in the diameter of the impedance semicircle was greatest for the 0.2 wt% LLE sample. Similarly, the samples with added LLE exhibited higher low-frequency modulus values compared to the samples without LLE (Figure 3(b2)), and phase angle values in the mid-frequency region also increased (Figure 3(b3)). This indicates that the reduction in the local dielectric constant and the increased LLE adsorption on the electrode surface decreased the capacitance of the rebar surface, thereby increasing the resistance to electrochemical reactions.

After 7 days of soaking (Figure 3c), optimal electrochemical protection was achieved at a 0.2 wt% LLE concentration, as evidenced by the maximum values in both the impedance arc radius and impedance modulus. However, with the extended soaking time, the corrosion inhibition effect of the 0.1 wt% LLE system no longer increased. Figure 3d presents the EIS results after 14 days of soaking, showing that the 0.2 wt% LLE system still exhibits the largest radius of impedance arc (Figure 3(d1)) and low-frequency impedance modulus (Figure 3(d2)). Simultaneously, the impedance arc radius of the 0.3 wt% LLE corrosion inhibitor increased, demonstrating the progressive adsorption of inhibitor molecules onto the rebar surface from concentrated LLE solutions, which enhances the corrosion inhibition effect and demonstrates the long-term corrosion inhibition potential of the LLE inhibitor. The corrosion inhibition capacity of the 0.1 wt% LLE has significantly deteriorated. This phenomenon stems from the weaker interfacial adhesion of the protective layer generated by dilute LLE solutions, which has been degraded by Cl⁻ corrosion.

The impedance spectrum was fitted with the R_s(Q_f(R_f(Q_dlR_ct))) equivalent circuit. Here, R_s, R_f, and R_ct represent the resistance of the solution, film, and the rebar/solution interface, respectively. Q or CPE represent the capacitance, where Q_f is the film capacitance and Q_dl is the capacitance of the rebar/solution interface. The CPE is represented by the constants Y₀ and n₀. The parameter Y₀ reflects the magnitude of pseudo-capacitance relative to an ideal capacitor, whereas n₀ serves as a dimensionless index quantifying the departure from purely capacitive behavior, with values ranging from −1 < n₀ < 1. Specifically, n₀ = −1, 0, and 1 correspond to the definitions of inductance, resistance, and ideal capacitance, respectively. The parameter values obtained from the EIS fitting of the samples soaked for 14 days are shown in

Table 2. Impedance spectral electrochemical parameters of rebars in neutral chloride salt solutions containing different concentrations of corrosion inhibitors.

LLE Concentrations	Rs /(Ω·cm2)	Yf × 10−3 /(Ω−1·sn·cm−2)	n1	Rf /(Ω·cm2)	Ydl × 10−3 /(Ω−1·sn·cm−2)	n2	Rct /(Ω·cm2)
0 wt%	7.49	2.33	0.78	11.24	0.40	0.94	1379
0.1 wt%	9.33	1.88	0.81	3.86	2.15	0.80	2757
0.2 wt%	9.26	0.07	0.59	4.76	0.46	0.89	4603
0.3 wt%	8.35	0.05	0.59	17.75	0.60	0.85	3720
0.4 wt%	7.96	0.55	0.91	1626	1.37	0.84	681.9
0.5 wt%	7.38	0.60	0.90	1600	2.91	0.98	481.1

. As shown in the table, with increasing LLE concentration, the R_ct value initially increases and then decreases, reaching its highest value at 0.2 wt% LLE. These results demonstrate enhanced charge transfer resistance at this specific concentration, resulting from the development of an LLE-derived protective barrier on the rebar substrate that physically isolates the metal from corrosive species, consequently retarding the electrochemical degradation process.

The total polarization resistance (R_p) and corrosion inhibition efficiency (η_EIS) of each corrosion system were calculated using Equations (3) and (4). As defined in reference [36], the total polarization resistance consists of both film transfer resistance (R_f) and charge transfer resistance (R_ct). The results presented in

Table 3. Total polarization resistance and rust inhibition efficiency.

LLE Concentrations	Rp/(Ω·cm2)	ηEIS/(%)	RSD
0 wt%	1390.24	--	--
0.1 wt%	2760.86	49.64	2.0
0.2 wt%	4607.76	69.83	2.3
0.3 wt%	3737.75	62.81	2.2
0.4 wt%	2307.90	39.76	1.9
0.5 wt%	2081.10	33.20	1.9

demonstrate that the R_p values of all systems followed a normal distribution relative to the dosage of the LLE corrosion inhibitor. The maximum polarization resistance (corresponding to a peak inhibition efficiency of 69.8%) was achieved at an optimal inhibitor concentration of 0.2 wt%.

$$R_p=R_{ct}+R_f$$

(3)

$${\eta }_{EIS}(\%)={\displaystyle \frac{R_P^{inh}-R_P^0}{R_P^{inh}}}\times 100\%$$

(4)

In the equation, $R_P^0$ and $R_P^{inh}$ represent the total polarization resistance of the control group (without corrosion inhibitor) and the experimental groups (with different inhibitor concentrations), respectively.

Figure 4 presents the Tafel curves for specimens immersed in a 3.5% NaCl solution with varying LLE concentrations for 14 days. The corresponding polarization parameters are summarized in

Table 4. Dynamic potential polarization curve parameters and corrosion inhibition efficiency.

LLE concentrations	Ecorr /(V)	icorr × 10−3 /(μA/cm2)	IE/(%)	RSD
0 wt%	−0.70	5.76	--	--
0.1 wt%	−0.70	2.88	50.00	2.2
0.2 wt%	−0.80	2.00	65.28	2.1
0.3 wt%	−0.81	3.52	38.89	2.3
0.4 wt%	−0.82	1.55	73.09	2.4
0.5 wt%	−0.83	2.36	59.03	2.2

. The corrosion inhibition efficiency (η) for different inhibitor concentrations is calculated using the following equation [37]:

$$IE\left(\%\right)=(1-{\displaystyle \frac{i_{corr}}{i_{corr}^0}})\times 100\%$$

(5)

In the equation, $i_{corr}^0$ and $i_{corr}$ represent the corrosion current density values of the specimens after immersion in corrosive media without and with the LLE corrosion inhibitor, respectively.

LLE adsorption on the rebar surface influences the corrosion process by either diminishing the active area for corrosion reactions or modifying the activation energy of these reactions. The observed negative shift in corrosion potential (E_corr) across LLE concentrations may result from organic molecule adsorption at cathodic sites or from solution pH effects. Compared to the samples without LLE, the samples with added LLE show lower corrosion current density. As the concentration of the LLE corrosion inhibitor increases, the corrosion current density decreases while the inhibition efficiency improves, reaching a peak efficiency of 73.09% at an optimal concentration of 0.4 wt%. The inconsistency between the Tafel polarization curve results and the EIS results is attributed to the low concentration of the corrosion inhibitor used in the tests. Since the Tafel polarization curve method is tested under relatively strong electric fields and determines corrosion potential and corrosion current through extrapolation, it has relatively large errors. In contrast, EIS is tested near the open circuit potential with minimal perturbation, keeping the system in a near-equilibrium state. Moreover, EIS is highly sensitive to minute changes on the electrode surface. Therefore, when the corrosion inhibitor concentration is low, especially when the adsorption effect on the electrode surface is weak, polarization curves can hardly distinguish such differences, whereas EIS can. Nevertheless, the electrochemical test results still demonstrate that the LLE corrosion inhibitor has excellent chloride ion erosion resistance.

Figure 5 shows the SEM and EDS analysis of the corrosion morphology of the rebar samples soaked for 7 days in a solution without LLE, as shown in Figure 5a1,a2. The rebar surface is fully corroded, with a corrosion product layer covering most of the area; this layer is loose, porous, and exhibits a cluster-like irregular morphology. EDS analysis indicates that the corrosion products primarily consist of Fe and O. Figure 5b shows the corrosion morphology and composition of the rebar samples soaked for 7 days in a solution containing 0.1 wt% LLE. The specimen surfaces exhibit uniformly dispersed cluster-type formations with dimensional variations, with the primary elements being C, O, and Fe. The Fe element mainly originates from the rebar matrix not covered by the cluster-like substances. This suggests that the LLE molecules cover the rebar surface, forming a relatively uniform film with cracks, which inhibits the formation of corrosion products. Figure 5c shows the corrosion morphology and composition of the rebar samples after soaking for 7 days in a solution containing 0.5 wt% LLE. It can be observed that a more uniform flake-like film structure forms on the rebar surface, with the Fe element predominantly distributed in the rebar matrix beneath the film. This indicates that as the LLE concentration increases, the protective layer formed on the rebar substrate exhibits enhanced characteristics, leading to better uniformity and a larger coverage area.

3.3. Anti-Corrosion Properties of LLE on Rebars in a Simulated Concrete Pore Solution

The alkaline conditions in reinforced concrete promote passive film formation on rebar surfaces, providing corrosion protection. Chloride ions, however, can degrade this passive layer, initiating pitting corrosion. This study examines how varying LLE inhibitor concentrations influence passive film development and prevent depassivation in a simulated concrete pore solution by characterizing the surface films and corrosion performance of both non-passivated and pre-passivated rebar specimens with/without LLE addition.

Initially, unpassivated rebar were immersed in 3.5% NaCl simulated concrete pore solutions, with and without various concentrations of LLE, and the EIS impedance test results after 1 day of immersion are displayed in Figure 6a. It shows that when no LLE is added or when low concentrations of LLE (0.1 wt%) are used, the corrosion inhibitor molecules are unable to effectively form an adsorption film to protect the rebar surface, resulting in a weak adsorption film with a relatively small impedance arc radius, impedance modulus, and phase angle. When the LLE corrosion inhibitor concentration is 0.2 wt% or higher, the impedance arc radius, impedance modulus, and phase angle are larger, indicating that the rebar can effectively passivate during the initial immersion stage, and the LLE inhibitor demonstrates strong resistance to Cl⁻ corrosion. In solutions containing 0.2 wt% to 0.5 wt% LLE, the impedance arc radius, impedance modulus, phase angle, and phase angle peak width in the low to medium frequency range are similar, as the immersion time is relatively short and has not reached the corrosion threshold, resulting in similar corrosion inhibition performance.

Following 3 days of immersion in simulated concrete pore solution (Figure 6b), varying corrosion inhibition efficacy was noted among the higher LLE concentrations. The phase angle spectra (Figure 6b2) demonstrate enhanced maximum values and broader peaks at elevated concentrations, corresponding to improved polarization resistance and capacitive properties. The 0.2 wt% LLE system exhibited a substantial reduction in Nyquist semicircle diameter, accompanied by decreased impedance magnitude and narrowed phase angle peaks in the Bode representation, signaling diminished protective capability. Concurrently, specimens in 0.1 wt% LLE displayed aggravated corrosion damage.

As the immersion time increased to 7 days (Figure 6c), the impedance arc radius in all concentration systems decreased to varying degrees. At this point, the 0.4 wt% and 0.5 wt% LLE corrosion inhibition systems maintained good corrosion protection performance. This is because chloride-induced corrosion mainly leads to pitting, and in systems with higher concentrations of corrosion inhibitors, the corrosion inhibitors achieve complete surface coverage on the rebar, blocking active sites and forming a composite film layer with high corrosion resistance. In the 0.2 wt% LLE system, the protective efficacy of the 0.3 wt% LLE system substantially declined, accompanied by accelerated surface deterioration, as evidenced by the decrease in impedance arc radius, impedance modulus, and phase angle peak width. The high phase angles and wide phase angle peaks in the 0.4 wt% and 0.5 wt% LLE systems indicate a smooth surface of the samples with minimal corrosion damage.

As the immersion time was extended to 14 days (Figure 6d), the rebar in all concentration systems continued to corrode, exhibiting varying degrees of corrosion damage, as reflected by the decrease in the impedance arc radius. When the LLE concentration was 0.4 wt%, the rebar exhibited the best corrosion resistance. The sudden decline in corrosion inhibition at 0.5 wt% LLE may be attributed to inhibitor desorption.

The equivalent circuit R(Q(R(QR))) modeling results for the 14-day immersion EIS measurements are summarized in

Table 5. Electrochemical parameters of unprepassivated rebars in simulated concrete pore solutions containing varying concentrations of corrosion inhibitors.

LLE Concentrations	Rs /(Ω·cm2)	Yf × 10−3 /(Ω−1·sn·cm−2)	n1	Rf /(Ω·cm2)	Ydl × 10−3 /(Ω−1·sn·cm−2)	n2	Rct /(Ω·cm2)
0 wt%	7.80	0.14	0.86	132	2.81	0.53	3805
0.1 wt%	8.42	0.10	0.84	630	0.98	0.58	3560
0.2 wt%	7.89	0.07	0.86	845	0.82	0.64	4886
0.3 wt%	8.70	0.08	0.86	1925	0.59	0.55	15,360
0.4 wt%	8.80	0.23	0.84	8035	0.28	0.65	39,000
0.5 wt%	7.23	0.07	0.88	4888	0.23	0.49	33,640

. As shown in the table, the introduction of LLE in the simulated concrete pore solution increases the R_ct value, indicating that the adsorbed film on the rebar surface enhances the resistance to charge transfer. The fact that R_f << R_ct indicates that LLE adsorption on the rebar surface does not form a thick film, and the electrochemical reaction at higher LLE concentrations is still controlled by charge transfer resistance. Moreover, with increasing LLE concentration, the uniformity parameter (n) increases, suggesting a reduction in surface irregularities and roughness.

The total polarization resistance (R_p) and corrosion inhibition efficiency values are presented in

Table 6. Total polarization resistance and rust inhibition efficiency.

LLE Concentrations	Rp/(Ω·cm2)	ηEIS/(%)	RSD
0 wt%	3937	--	--
0.1 wt%	4190	6.04	2.2
0.2 wt%	5731	31.31	2.5
0.3 wt%	17,285	77.22	2.4
0.4 wt%	47,035	91.63	2.6
0.5 wt%	38,528	89.78	2.7

. The variation trend of R_p in the table similarly reflects the corrosion inhibition effectiveness of the LLE inhibitor. This demonstrates that the organic components in the plant extract can enhance the electrolyte/steel rebar interface through surface adsorption. The increasing R_p values during immersion indicate that higher inhibitor concentrations lead to a greater accumulation of organic inhibitory substances in the simulated solution, thereby strengthening the suppression of corrosion processes on the carbon steel surface. When the LLE concentration reached 0.4 wt%, the maximum inhibition efficiency of 91.63% was achieved.

The potentiodynamic polarization behavior of rebars in chloride-containing simulated concrete pore solutions with varying LLE concentrations is illustrated in Figure 7, while the corresponding derived corrosion parameters are tabulated in

Table 7. Results of the kinetic potential polarization curve analysis of rebars in different concentrations of the LLE corrosion inhibitor.

LLE Concentrations	Ecorr /(V)	icorr × 10−3 /(μA/cm2)	IE/(%)	RSD
0 wt%	−0.75	32.49	--	--
0.1 wt%	−0.69	10.02	69.16	2.5
0.2 wt%	−0.66	3.67	88.71	2.4
0.3 wt%	−0.67	3.89	88.02	2.6
0.4 wt%	−0.56	0.99	96.96	2.7
0.5 wt%	−0.61	1.04	96.80	2.6

. As shown in the figure, the addition of LLE causes a downward shift in the Tafel polarization curve, reflecting a decrease in corrosion current and demonstrating LLE’s dual functionality in inhibiting both anodic dissolution and cathodic reduction reactions. Moreover, with increasing LLE dosage, the Tafel curve shifts to positive potentials, with the most significant shift occurring at 0.4 wt% LLE, where the corrosion potential rises to approximately −0.56 V, indicating a high corrosion inhibition capability. This suggests that organic molecules of the corrosion inhibitor containing O and N heteroatoms can enhance the surface passivation film by adsorbing on active sites and limiting anodic and cathodic reactions. Increasing LLE concentration in the simulated pore solution induces a progressive anodic shift in corrosion potential, accompanied by a systematic reduction in corrosion current density (3.249 × 10⁻² to 0.99 × 10⁻³ μA/cm²), corroborating the EIS-derived observations. The corrosion inhibition efficiency increased significantly from 69.16% for the 0.1 wt% LLE system to 96.96% for the 0.4 wt% LLE system.

Mott–Schottky analysis was performed to assess LLE’s impact on passivation films (Figure 8). Figure 8a shows the Mott–Schottky (M-S) curves illustrating the influence of different inhibitor concentrations on the passive film of steel rebar, where all curves display two characteristic linear regions. The initial positive slope indicates n-type semiconductor behavior of the carbon steel passive film, while the subsequent negative slope at higher potentials reveals film breakdown due to predominant iron ion vacancies. The charge carrier density (Nd) was calculated using Equation (6).

$$N_d={\displaystyle \frac{2}{{\mathrm{\epsilon }\mathrm{\epsilon }}_{0}qs}}$$

(6)

Here, ε represents the relative dielectric constant (taken as 12 for steel materials), ε₀ denotes the vacuum permittivity (typically 8.55 × 10⁻¹⁴ F·cm⁻¹), q is the elementary charge (1.602 × 10⁻¹⁹ C), and s stands for the slope of the Mott–Schottky curve.

As shown in Figure 8b, in the blank system, the Mott–Schottky (M-S) plot shows a relatively low slope and a higher charge carrier density (2.65 × 10²¹ cm⁻³), indicating a thin space charge layer. In contrast, the addition of LLE increased the slopes while significantly reducing N_d to 1.78 × 10²¹ cm⁻³ at 0.3 wt% and 1.82 × 10²¹ cm⁻³ at 0.4 wt%, demonstrating enhanced film compactness, with the best corrosion protection achieved for the 0.3 wt% and 0.4 wt% LLE. This improvement is attributed to the inhibitor’s ability to modify the passive film’s iron oxide conductivity and improve its protective properties. These results are consistent with the EIS and polarization curve.

Electrochemical impedance characterization (Figure 9a) of pre-passivated rebars after 1 day of immersion reveals a concentration-dependent enhancement: LLE incorporation elevates polarization resistance, with systematic increases in arc radius, impedance modulus, and phase angle corresponding to improved inhibition efficacy. Optimal performance occurs at 0.3 wt%, where all three parameters are maximized.

Prolonged immersion for 3 days (Figure 9b) resulted in substantial expansion of the capacitive loop diameter in systems with elevated inhibitor concentrations. In the Bode plot, the low-frequency impedance and mid-frequency phase angle were higher, indicating an increased protective effect, with the rebar electrode remaining in the passivated state. LLE enhanced the passivation layer at the alkaline solution/rebar interface. However, the Nyquist semicircle diameter substantially contracted in the 0.1 wt% LLE system, potentially attributable to either chloride ion adsorption at the metal-electrolyte interface or water molecule penetration through the passive layer. This led to a reduction in the film capacitance and a decrease in the dissolution resistance of the passivation layer, resulting in the failure of LLE protection on the passivation film.

After 7 days of immersion (Figure 9c), although the charge transfer resistance radius showed some contraction compared to day 3, the higher concentration LLE systems still exhibited good corrosion inhibition performance. The increase in LLE concentration raised the impedance modulus and phase angle in the Bode plot, and the degree of decline in corrosion inhibition performance decreased with the increase in LLE concentration. The 0.3 wt% LLE formulation demonstrated excellent initial-stage corrosion inhibition efficacy for rebar; however, the formed adsorption film lacked sufficient compactness, preventing it from achieving long-term corrosion inhibition. The phase angle and phase angle peak width of the sample gradually decreased with prolonged immersion, indicating ongoing corrosion of the rebar.

The results after 14 days of immersion (Figure 9d) indicate that the total impedance of all corrosion inhibitor systems decreased over time, suggesting the activation of corrosion phenomena on the rebar surface. The passive layer experienced activation-depassivation transitions, resulting in a deteriorated electrochemical impedance response. Prolonged immersion caused a reduction in both the magnitude and breadth of phase angle peaks, evidencing enhanced surface roughness and the initiation of localized corrosion. Although the chloride resistance of all samples decreased over the immersion period, the simulated solution with 0.5 wt% LLE maintained its superiority, indicating that the dosage of the LLE corrosion inhibitor is crucial.

The equivalent circuit modeling results (R(Q(R(QR)))) for the impedance data (Figure 9d) are compiled in

Table 8. Electrochemical parameters of pre-passivated rebars in simulated concrete pore solutions containing different concentrations of corrosion inhibitors.

LLE Concentrations	Rs /(Ω·cm2)	Yf × 10−5 /(Ω−1·sn·cm−2)	n1	Rf /(Ω·cm2)	Ydl × 10−3 /(Ω−1·sn·cm−2)	n2	Rct × 104 /(Ω·cm2)
0 wt%	8.04	7.99	0.89	219	1.42	0.60	0.51
0.1 wt%	8.20	0.89	0.85	645	1.07	0.45	0.80
0.2 wt%	7.19	5.81	0.90	3488	0.28	0.56	1.03
0.3 wt%	6.88	8.96	0.89	3073	0.41	0.62	1.11
0.4 wt%	8.23	5.90	0.89	5411	0.23	0.52	2.23
0.5 wt%	8.64	6.93	0.88	9093	0.13	0.47	4.34

. The progressive enhancement of charge transfer resistance (R_ct) with elevated LLE concentrations in corrosive media confirms the concentration-dependent improvement in corrosion inhibition efficacy.

The calculated total polarization resistance (R_p) and corrosion inhibition efficiency for different corrosion systems are shown in

Table 9. Polarization resistance and rust resistance efficiency.

LLE Concentrations	Rp/(Ω·cm2)	ηEIS/(%)	RSD
0 wt%	5324	--	--
0.1 wt%	8666	38.56	2.3
0.2 wt%	13,828	61.50	2.5
0.3 wt%	14,133	62.33	2.7
0.4 wt%	27,701	80.78	2.8
0.5 wt%	52,443	89.85	3.1

. The results indicate that the variation trend in polarization resistance (R_p) is consistent with the charge transfer resistance (R_ct). As the concentration of the corrosion inhibitor increases, the polarization resistance correspondingly increases. When the concentration of the LLE corrosion inhibitor reaches 0.5 wt%, the polarization resistance reaches its maximum value of 52,443 Ω·cm², and the corresponding corrosion inhibition efficiency also reaches its highest value of 89.85%. These results demonstrate that during the corrosion process of steel rebar, higher concentrations of corrosion inhibitor molecules cover a considerable surface area through strong binding with the metal surface, thereby significantly improving the corrosion resistance of the steel rebar.

Figure 10a shows the Tafel curves of rebar samples in systems with varying LLE concentrations. The positive shift in corrosion potential upon LLE incorporation demonstrates reduced thermodynamic corrosion susceptibility, with this protective effect intensifying proportionally with concentration. The concurrent downward displacement of the polarization curves confirms the systematic decrease in corrosion current densities.

Table 10. Analytical results of polarization curves for pre-passivated rebars in different concentrations of LLE corrosion inhibitors.

LLE Concentrations	Ecorr /(V)	icorr × 10−3 /(μA/cm2)	IE/(%)	RSD
0 wt%	−0.71	5.65	--	--
0.1 wt%	−0.74	6.32	--	--
0.2 wt%	−0.70	3.76	33.45	2.7
0.3 wt%	−0.67	1.64	70.97	2.9
0.4 wt%	−0.68	1.47	73.98	3.2
0.5 wt%	−0.63	0.55	90.27	3.1

presents the fitted polarization curve parameters. The tabulated data demonstrate a concentration-dependent reduction in corrosion current density, with optimal inhibition attained at 0.5 wt% LLE, where i_corr diminishes to 5.527 × 10⁻⁷ A·cm⁻². After the addition of LLE, the overall change in Ecorr in the polarization direction is not significant, with a difference of less than 85 mV relative to SCE. These findings confirm LLE’s dual functionality in retarding both half-cell reactions, characterizing it as a mixed-type inhibitor. The protective mechanism involves LLE adsorption at cathodic and anodic sites, creating an interfacial barrier that modulates mass transport. Concentration-dependent variations in both β_c and β_a values verify the simultaneous suppression of metallic dissolution and oxygen reduction processes.

Figure 10b–d show the fine XPS spectra of the rebar after 7 days of immersion in a 0.5 wt% LLE simulated concrete pore solution. The binding energy peaks of the Fe2p spectrum are 711.54, 710.00, and 706.89 eV, corresponding to Fe₂O₃, FeO, and Fe, respectively. These results confirm the formation of passivation film on the rebar surface. Deconvolution of the C1s spectrum reveals three components: C-C/C-H (284.57 eV) and C-O functionalities (286.22 eV and 288.23 eV), with relative abundances of 49.44% and 50.56%, respectively. This may be due to the formation of a carbon-containing organic film on the surface of the rebar. The O1s spectrum is also divided into three peaks: the peak at 532.70 eV corresponds to O-H bonds, with a content of 24.54%; the peak at 531.66 eV corresponds to C-O bonds, with a content of 50.79%; and the peak at 530.27 eV corresponds to FeO/Fe₂O₃, with a content of 24.68%. The results of the XPS experiment further confirm the formation of an iron oxide passivation layer and a carbon-containing organic film on the rebar electrode surface.

Figure 10e–h compares the surface morphology of rebar specimens following 14 days of immersion in simulated pore solutions with and without 0.5 wt% LLE corrosion inhibitor. As seen in the images, the surface of the rebar sample without the corrosion inhibitor is severely corroded by chloride salts. The surface is covered with a large amount of irregular, granular corrosion products that aggregate into lumps. In contrast, the surface of the rebar with 0.5 wt% LLE corrosion inhibitor shows a significant reduction in corrosion products. The corrosion products are granular, resembling rice grains, with a loose overall distribution, and the corrosion is notably less severe. These findings provide additional evidence for the robust corrosion protection afforded by the LLE inhibitor system.

We compared the corrosion inhibition performance of lotus leaf extract obtained in this study with results reported in other studies, as summarized in

Table 11. Comparative analysis of corrosion inhibition efficiency between the present study and the other literature.

Plant Extract Source	Corrosive Medium	ηEIS/(%)	IE/(%)	Reference
Urtica dioica leaf	0.3 M KOH + 0.1 M NaOH in saturated Ca(OH)2 solution containing 1 wt% NaCl	77.00%	-	[38]
Phragmites australis leaf	Concrete specimens immersed in 3% NaCl electrolyte	-	76.98%	[27]
Damask rose leaf	0.5 M Ca(OH)2 + 0.5 M KOH + 0.1 M NaOH + 0.5 M NaCl	81.90%	81.60%	[16]
lotus leaf	3.5% NaCl electrolyte	69.83%	73.09%	This paper
lotus leaf	Non-passivated steel rebar in saturated Ca(OH)2 solution	91.63%	96.96%	This paper
lotus leaf	Pre-passivated steel rebar in saturated Ca(OH)2 solution	89.85%	90.27%	This paper

. The data clearly demonstrate that our lotus leaf extract inhibitor exhibits superior corrosion inhibition performance in alkaline chloride-containing environments, achieving an inhibition efficiency exceeding 90%. This outstanding performance in simulated concrete pore solution demonstrates exceptional corrosion protection capability, providing crucial durability assurance for reinforced concrete structures. Furthermore, in non-alkaline chloride environments, the lotus leaf extract still maintains a relatively high inhibition efficiency (around 70%), indicating good resistance to chloride ion attack. Compared with other plant-derived inhibitors reported in the literature, lotus leaf extract also possesses significant advantages in industrial applications due to its wide availability, easy accessibility, and simple, mild extraction process, which offer notable cost benefits and environmental friendliness. These findings provide a new approach for developing next-generation green and high-performance protection technologies for reinforced concrete structures.

3.4. Anti-Corrosion Mechanism of LLE on Rebars in Neutral Chloride or Alkaline Chloride Solutions

In the absence of LLE in neutral chloride or alkaline chloride environments, chloride ions attack the carbon steel surface (or pre-passivated film), leading to the depassivation of the surface passive layer and the initiation of corrosion reactions that generate corrosion products, as shown in Figure 11a. After adding the LLE corrosion inhibitor, the inhibitor can form an adsorbed film layer on the carbon steel and the passivated surface. In neutral chloride solutions, when the inhibitor concentration is too low (0.1 wt%), a stable protective film cannot form, making it susceptible to Cl⁻ attack and ultimately leading to rusting. Conversely, at excessively high concentrations, the solution pH decreases, which is unfavorable for steel corrosion inhibition. Therefore, in neutral chloride corrosion environments, the corrosion inhibition behavior of LLE is jointly regulated by both inhibitor concentration and pH, with an optimal concentration range (approximately 0.2 wt%) for achieving the best corrosion inhibition effect on steel, as shown in Figure 11b.

In simulated concrete pore solutions (alkaline chloride environments, Figure 11c), under non-pre-passivated carbon steel conditions, the LLE inhibitor promotes the formation of a complete passive film on the steel surface and enhances corrosion resistance by forming a composite passive layer through molecular adsorption. The inhibition effect initially improves and then declines with increasing inhibitor concentration, with 0.4 wt% LLE demonstrating the optimal enhancement effect on steel passivation and corrosion resistance in alkaline chloride environments. Furthermore, for pre-passivated carbon steel, the LLE inhibitor provides excellent protection to the steel’s passive film, with the depassivation inhibition effect strengthening as the inhibitor concentration increases. The best corrosion inhibition performance is achieved at a concentration of 0.5 wt%.

4. Conclusions

This work presents a comprehensive investigation into how the LLE corrosion inhibitor influences carbon steel rebar performance, examining its molecular architecture, film formation characteristics, and concentration-dependent anti-corrosion behavior across diverse electrolyte environments (3.5% NaCl solution system and simulated concrete pore solution) using SEM, EDS, FTIR, LC-MS, and electrochemical testing. The mechanism of LLE in inhibiting rebar corrosion under different conditions was revealed. The conclusions are as follows:

(1)

Lotus leaf extract (LLE) is rich in polar functional groups (e.g., O-H, N-H, C=O) and heterocyclic structures, which form a protective film on the steel surface through physical adsorption and chemical chelation. Fourier-transform infrared spectroscopy (FTIR) and liquid chromatography–mass spectrometry (LC-MS) analyses revealed that LLE contains abundant alkaloids and flavonoids, which can coordinate with the 3D orbitals of iron atoms via lone pair electrons, forming a dense adsorption layer that hinders the penetration of corrosive agents (e.g., Cl⁻). Scanning electron microscopy (SEM) tests demonstrated that LLE forms an adsorption film on the steel surface, with higher concentrations resulting in more uniform and compact layers.

(2)

Electrochemical impedance spectroscopy (EIS) measurements in a 3.5% NaCl solution confirmed that the adsorption film significantly increases charge transfer resistance (R_ct). The corrosion inhibition performance of LLE exhibits concentration dependence, with 0.2 wt% being the optimal concentration. Although higher concentrations (e.g., 0.5 wt%) form more uniform films, the further reduction in solution pH (pH ≈ 5.04) weakens the corrosion inhibition effect, indicating that the efficiency in neutral environments is jointly regulated by pH and the quality of the adsorption film.

(3)

In an alkaline environment (saturated Ca(OH)₂ + 3.5% NaCl), LLE inhibits corrosion by promoting passive film formation and suppressing Cl⁻ attack. In the unprepassivated rebar system, 0.4 wt% LLE exhibited the best performance, achieving an R_ct of 3.9 × 10⁴ Ω·cm² and reducing i_corr to 0.99 × 10⁻³ μA/cm². Mott–Schottky (M-S) tests showed that LLE reduces the carrier density of the passive film, enhancing its n-type semiconductor properties and delaying film breakdown. In the pre-passivated system, 0.5 wt% LLE achieved the highest R_ct (4.34 × 10⁴ Ω·cm²), demonstrating its long-term protective effect on the passive film. X-ray photoelectron spectroscopy (XPS) confirmed that 0.5 wt% LLE treatment led to the formation of an Fe₂O₃/FeO passive film and an organic layer containing C-O bonds, significantly improving corrosion resistance.

In summary, for neutral environments (e.g., marine or de-icing salt exposure), a lower LLE concentration (0.2 wt%) is recommended, whereas in alkaline chloride-containing environments (e.g., concrete pore solution), a concentration of 0.4–0.5 wt% LLE is more effective.