Cinnamon (Cinnamomum verum) Mediated Silver Nanoparticles as Green Corrosion Inhibitors for C45 Steel in Hydrochloric Acid

INTRODUCTION

Corrosion is an inevitable electrochemical process that result from oxidation-reduction reactions occurring at the interface between metallic materials and their surrounding environment. The rate and severity of corrosion are strongly influenced by environmental parameters such as temperature, pH, dissolved oxygen, humidity, electrolyte composition, and the presence of aggressive ions [1]. Under acidic conditions, particularly in hydrochloric acid (HCl) solution, the corrosion of metallic materials is significantly accelerated, leading to considerable material degradation and economic losses [2]. Carbon steel remains one of the most extensively utilized engineering materials in the petroleum and petrochemical industries because of its low cost, excellent mechanical strength, ease of fabrication, and widespread availability [3]. Despite these advantages, its relatively poor corrosion resistance in acidic environments restricts its long-term durability. During oil well acidizing, hydrochloric acid is routinely injected to dissolve carbonate scales and mineral deposits, thereby improving reservoir permeability and enhancing hydrocarbon production. However, this process simultaneously exposes steel pipelines, tubing and processing equipment to highly aggressive acidic media, substantially increasing corrosion rates and reducing service life [4,5]. Consequently, the development of efficient and economically viable corrosion protection strategies remains an important industrial and scientific objective.

Various approaches have been employed to mitigate metallic corrosion, including protective coatings, cathodic protection, alloy modification, and the addition of corrosion inhibitors [6]. Among these methods, corrosion inhibitors are considered one of the most practical and cost effective solution because only small concentration are required to substantially reduce metal dissolution. These compounds suppress anodic and cathodic electrochemical reaction by adsorbing onto the metal surface, thereby creating an adsorbed barrier that reduces the interaction between the substrate and the corrosive environment [7-10]. Depending on their chemical nature, corrosion inhibitors are generally classified into inorganic and organic compounds, the latter including both synthetic chemicals and naturally derived plant extracts [11,12].

Growing environmental concerns and increasingly stringent regulations have stimulated the search for sustainable alternatives to conventional synthetic inhibitors. Plant-derived corrosion inhibitors have attracted considerable attention because they are renewable, biodegradable, inexpensive, and generally exhibit low toxicity [13]. Their inhibition performance originates primarily from adsorption of naturally occurring phytochemical containing heteroatoms, aromatic ring, and π-electron systems onto metallic surface, resulting in the formation of adsorbed protective layers that reduce the corrosion reaction [14.15]. Nevertheless, although numerous plant extracts have demonstrated promising inhibition efficiencies, their practical performance is often limited by inadequate surface coverage, insufficient stability, and relatively weak adsorption under highly aggressive acidic conditions. These limitations have encouraged researchers to integrate green corrosion inhibition with nanotechnology to enhance adsorption efficiency, increase active surface area, and improve the durability of the protective layer.

Among the various plant resources investigated for green nanotechnology, Cinnamomum verum (cinnamon), a member of the Lauraceae family, has attracted considerable scientific interest because of its abundance of bioactive phytochemical and remarkable biological activities [16-18]. The bark of Cinnamomum verum is particularly rich in polyphenols, flavonoids, terpenoids, alcohols, esters, organic acids, and phenolic compounds such as eugenol and pyrogallol, which exhibit excellent antioxidant and reducing capabilities [19-21]. These naturally occurring phytochemical not only contribute to the medicinal value of cinnamon but also serve as efficient reducing and stabilizing agents during the green synthesis of metallic nanoparticles [22-24]. Consequently, plant-mediated synthesis has emerged as an environmentally sustainable alternative to conventional chemical methods by eliminating the need for toxic reducing agents while producing stable nanostructures.

Among metallic nanomaterials, silver nanoparticles AgNPs have received particular attention because of their high chemical stability, excellent surface reactivity, and exceptionally large surface area to volume ratio [25-27]. These characteristics enhance their adsorption capability and facilitate the development of an adsorbed surface layer on metallic surfaces, making them promising candidates for corrosion inhibition applications [28]. Furthermore, combining AgNPs with phytochemical-rich plant extract is expected to generate a synergistic effect that improves nanoparticle stability, increases adsorption efficiency, and enhances corrosion protection under aggressive acidic conditions.

Although several plant extract have been investigated as green corrosion inhibitors, studies concerning green-synthesized silver nanoparticles derived from Cinnamomum verum for protecting carbon steel in hydrochloric acid remain limited. In particular, the relationship between nanoparticle physicochemical characteristics, electrochemical, behavior, adsorption performance, and inhibition mechanism has not yet been comprehensively established. Addressing these aspects is essential for understanding the protective action of green nanomaterials and expanding their practical applications in industrial corrosion control.

Accordingly, the present study aims to synthesize environmentally friendly silver nanoparticles using Cinnamomum verum extract AgNPs.Cin and to evaluate their corrosion inhibition performance for carbon steel in 1 M HCl solution. The synthesized nanoparticles were characterized using uv-Visible spectroscopy, FTIR, FESEM, and TEM analyses, while their corrosion inhibition performance was systematically investigated through potentiodynamic polarization and electrochemical impedance spectroscopy at different inhibitor concentration and temperatures. In addition, adsorption behavior, thermodynamic parameters, and kinetic characteristics were analyzed using the Langmuir adsorption isotherm, Arrhenius equation, and transition-state theory to elucidate the inhibition mechanism.

The novelty of this work lies in the development and evaluation of green synthesis AgNPs.Cin derived from Cinnamomum verum extract as a green corrosion inhibitor for C45 carbon steel in 1.0 M HCl. This study investigates the corrosion inhibition performance of this green-synthesized nanocomposite through physicochemical characterization, electrochemical evaluation, adsorption modeling, and thermodynamic and kinetic analysis. The obtained result provide a comprehensive evaluation of the corrosion inhibition performance of the green-synthesized AgNPs.Cin system and demonstrate its potential as a sustainable green corrosion inhibitor in acidic environments.

MATERIALS AND METHODS

The materials used in this research included dried cinnamon (Cinnamomum verum( silver nitrate (AgNO₃) (purchased from Merck, Germany), 1 M hydrochloric acid (HCl), and distilled water.

Plant extract preparation [29]

Dried cinnamon (Cinnamomum verum) was purchased from a local herbal market and authenticated based on its morphological characteristics. The dried cinnamon was finely ground, and 10 g of the powder was mixed with 100 ml of distilled water in a 250 ml conical flask. The mixture was heated at 80°C for 2 h and then filtered using filter paper. The obtain extract was stored 4°C and used immediately for the synthesis of silver nanoparticles.

Synthesis of silver nanoparticles (AgNPs.Cin)[30]

A 1 mM AgNO₃ solution was prepared by dissolving 0.1699 g of silver nitrite in 100 ml of distilled water. Subsequently, 50 ml of the prepared AgNO₃ solution was added dropwise to 5 ml of the cinnamon extract under magnetic stirring for 20 min. The volume ratio of AgNO₃ solution to cinnamon extract was 10:1 (v/v). The reaction temperature was maintained between 45°C and 50°C until the solution changed to a brown color, indicating the formation of silver nanoparticles AgNPs.Cin.

Scanning Electron Microscopy (FESEM)

FESEM pictures were obtained using a Field Emission Scanning Electron Microscopy, MIRA III, Tescan, Czech. at various magnification levels.

UV-Visible spectra

Ultraviolet Visible, 1900, Sgimadzu, Jaban was used to analyze the UV-visible spectra of silver nanoparticle citral 1mM. This indicates that it uses light in the visible and adjacent (near-UV and NIR) spectrums. Our perception of color is directly impacted by the absorption of molecules in the visible spectrum. Molecules go through electrical changes in this area of the visual spectrum.

Fourier Transform Infrared Spectroscopy (FT-IR)

FT-IR was used to identify the chemical bonds and functional groups of AgNPs. In a hydraulic press, pellets were made by combining KBr and AgNPs.Cin. It was examined in the 400–4000 cm^-1 range using the Shimadzu 8400S Jaban FT-IR spectrometer.

Corrosion inhibition study

AgNPs.Cin synthesis using Cinnamomum verum (cinnamon) extract were utilized as the corrosion inhibitor. The reported concentration 100, 300, and 500 ppm refer to the synthesized AgNPs.Cin colloidal suspension used directly after the green synthesis process. The nanoparticles were neither isolated nor dried before preparing the test solutions. The blank solution consisted 50 ml of 1 M HCl. Carbon steel specimens were immersed in 1 M HCl containing (100, 300, 500) ppm of the AgNPs.Cin colloidal suspension for 7 hours temperature ranging from 298 to 318 K. Electrochemical measurements were carried out within a potential range of ± 200 mV.

Corrosion measurement

The potentiostat set up includes a Host computer, thermostat, magnetic stirrer, (EmStat 4s, Palm Sens, Holland) potentiostat, and galvanostat. The cell is (250) ml capacity made of Pyrex consist of internal and external bowls. The electrochemical corrosion cell is three electrodes. carbon steel as a working electrode used to determine the potential of it according to the reference electrode, an auxiliary electrode is a platinum with length (10)cm and reference electrode a saturated calomel (Hg/Hg₂Cl₂ sat.KCl). The working electrode was immersed in the test solution for 15 min to establish steady state open circuit potential (Eocp), then electrochemical measurements were performed in a potential range of (± 200) mV.

Polarization Curves

The corrosion current density (icorr) and corrosion potential (Ecorr) were obtained by the extrapolation of the cathodic and anodic Tafel in absence and presence the inhibitors molecules in HCl solution. Tafel plot reveals that Ecorr for C.S in the presence the inhibitors shifts to a higher (noble) position compared with blank solution, implying that the protection acts as an anodic protection. The inhibition efficiency (%IE) was calculated by the following equation [31,32].

Where (I_corr)_ois the corrosion current density in the absence of inhibitors, (I_corr) is the corrosion current density in the presence of inhibitors [33].

Chemical Composition of Carbon Steel C45

The chemical composition of the carbon steel C45 specimen employed in this work is summarized in Table 1. As can be seen, iron (Fe) represent the major component, while other elements such as carbon (C), silicon (Si), manganese (Mn), sulfur (S), phosphorus (P), copper (Cu), nickel (Ni), and chromium (Cr) are present in minor proportions, which may influence the corrosion behavior.

RESULTS AND DISCUSSION

The UV-visible spectra

Fig. 2 present the UV-visible spectrum of green-synthesized silver nanoparticles AgNPs.Cin prepared using Cinnamomum extract. The reaction mixture changed from pale yellow to dark brown after the addition of AgNO₃, indicating the reduction of Ag⁺ ions and the formation of AgNPs.Cin. The spectrum exhibited a strong absorption band at 287 nm, attributed to the bioactive phytochemicals of the extract that acted as reducing and stabilizing agents. A broad absorption band at 465 nm corresponds to the localized surface Plasmon resonance (LSPR) of silver nanoparticles, confirming their successful formation [34].

Overall, the Uv-Visible results confirm are consistent with the successful green-synthesis of AgNPs.Cin using Cinnamomum extract, where the plant phytochemical promoted the reaction of A⁺ ions to metallic Ag⁰ while simultaneously stabilizing the formed nanoparticles. These findings are in good agreement with the results reported by Vijayan et al. [35], who also attributed the formation of silver nanoparticles to the reduction of Ag⁺ ions by plant-derived phytochemical during green synthesis.

FTIR spectrum

The FTIR spectrum of the AgNPs.Cin (Fig. 3) exhibits broad absorption band at 3425.58 cm^-1, Corresponding to O-H stretching vibrations of hydroxyl groups presnt in phenolic compounds. The bands at 2920.23 cm^-1 and 2889.37 cm^-1 were assigned to C-H stretching vibrations of aliphatic groups. The absorption bands observed at 1773-1707 cm^-1 were attributed to C=O stretching vibrations of carbonyl-containing compounds, while the band at 1622 cm^-1 was associated with C=C stretching vibrations of aromatic rings and conjugated carbonyl groups. The bands at 1354 and 1321 cm^-1 were assigned to C-N and O-H bending vibrations, whereas the band at 1072 and 1042 cm^-1corresponded to C-O stretching vibrations of alcohols, ethers, and phenolic compounds. These spectral features are consistend with the presence of phytochemical constituents from the cinnamon extract and suggest the possible involvement of hydroxyl, carbonyl, carboxylate, and related oxygen-containing functional groups in the reduction of Ag⁺ ions [36,37]. However, because FTIR analysis was performed only on the finl AgNPs.Cin product without direct comparison with the original cinnamon extract, washed nanoparticles, or reference compounds, these assignments should be regarded as supportive rather than conclusive evidence of their role in the green-synthesis process.

FE-SEM analysis

Surface Morphology and Structural Features Analysis

The surface topography, microstructure, and morphological characteristics of the green-synthesized AgNPs.Cin were comprehensively scrutinized using Field Emission Scanning Electron Microscopy (FE-SEM). The obtained FE-SEM micrographs at different magnifications provide crucial insights into the morphological characteristics and growth mechanism of the green-synthesized AgNPs.Cin, as illustrated in Fig. 4.

At a moderate magnification of 15.00 KX Fig. 4a. the FE-SEM micrograph unveils a highly continuous, interconnected, and porous network. The topography exhibits a homogenous and uniform distribution of the synthesized nanomaterials over a wide scanned area, implying the efficacy of the green synthesis approach in producing a well-dispersed matrix [38]. The observed micro-porosity and rough surface architecture are highly advantageous for various technological, as they significantly increase the effective surface-area-to-volume ratio [39].

To gain a deeper understanding of the distinct nanostructures, a high-magnification FE-SEM characterizations was performed at 80.00 KX Fig. 4b. The high-resolution image indicates that the fabricated AgNPs.Cin possess a predominantly spherical to quasi-spherical morphology. This spherical geometry is thermodynamically favored during the nucleation and growth stages of silver ions (Ag⁺ to Ag⁰), as it minimizes the surface free energy [40].

The FE-SEM images reveal nanoscale features with apparent particle diameters ranging from approximately 58.80 nm to 133.3 nm. However, some large structures reaching approximately 194.2 nm are observed, which are mainly attributed to nanoparticle aggregation. This agglomeration is a characteristic phenomenon in green synthesis protocols [41]. It can be scientifically attributed to the presence of secondary metabolites and bioactive macromolecules within the cinnamon extract, such as cinnamaldehyde, eugenol, and polyphenols. These biomolecules act not only as potent reducing agents but also as capping and stabilizing agent (Capping ligands). The bio molecular coating forms a surrounding organic matrix that bridges the adjacent nanoparticles, resulting in the observed branch-like cluster morphology [42].

The structural features revealed by this FE-SEM study indicate Cinnamomum verum extract provides an effective, eco-friendly biogenic route for the controlled synthesis of silver nanoparticles [43].

Elemental Composition Analysis (EDX)

The EDX spectrum of the spectrum of the synthesized AgNPs.Cin is shown in Fig. 5. A prominent peak is observed at approximately 3 KeV, corresponding to the characteristic AgLα emission of metallic silver, confirming the presence of silver in the synthesized sample [44]. Additional Ag peaks with lower intensities are also observed, supporting the identification of silver [45]. A low-intensity oxygen (O) peak is detected at lower energy. The EDX spectrum confirms the elemental composition of the synthesized AgNPs.Cin [46,47].

Transmission Electron Microscopy (TEM) Analysis

The TEM micrograph is consistent with the successful green synthesis of silver nanoparticles AgNPs.Cin using cinnamon extract. The nanoparticles exhibit predominantly spherical to quasi-spherical morphologies, and are distributed within the nanoscale rang Fig. 6A. The strong electron-dense contrast is consistent with presence of silver nanoparticles. Noticeable nanoparticle agglomeration is also observed, which is commonly reported for plant-mediated AgNPs and is mainly attributed to the high surface energy of silver nanoparticles, antiparticle attractive forces, drying effects during TEM sample preparation, and interactions between, phytochemical capping molecules [48]. The corresponding particle-size distribution histogram Fig. 6B shows that most nanoparticles are within the 30-50 nm range, with the highest frequency observed between 40 and 45 nm, while fewer particles are found at smaller and larger size ranges. Overall the observed morphology, particle-size distribution, and aggregation behavior are consistent with previous reports on green-synthesized AgNPs.Cin and indicate the successful formation of nano sized silver particles with characteristics that may be favorable for corrosion inhibition [49].

Description of corrosion system

Potentiodynamic Polarization (PDP) measurements

Potentiodynamic Polarization (PDP) measurements were carried out using a conventional three-electrode electrochemical cell connected to a potentiostat/galvanostat system. The electrochemical cell was made of Pyrex glass with a total capacity of 250 ml and consisted of inner and outer compartments. A carbon steel specimen with an exposed surface area of 1 cm² was used as the working electrode. Reference electrode a saturated calomel electrode (SCE, Hg/Hg₂Cl₂/saturated KCl), positioned approximately 2mm from the working electrode minimize the ohmic potential drop, while platinum rod (10 cm) served as the counter electrode. The experimental temperature was maintained using a thermostatic cooling-heating circulating water bath throughout the measurement.

Prior to each experiment, the exposed surface of the carbon steel specimen was polished using silicon carbide (SiC) abrasive paper, degreased with acetone, rinsed twice with distilled water, and allowed to try at room temperature. The working electrode was then immersed in the test solution for 15 min to allow the open circuit potential (OCP) to reach a steady before polarization measurements. PDP measurements were subsequently performed over a potential rang of ±200 mV relative to the OCP at a scan rate of 1.0 mV/s in 1.0 M HCl solution containing different concentration of AgNPs. All electrochemical measurements were conducted in triplicate under identical experimental conditions, and the reported values represent the average of three independent measurements. Fig. 7A.

Electrochemical Impedance Spectroscopy (EIS) Measurements

Electrochemical impedance spectroscopy (EIS) measurements were carried out using a computer-controlled potentiostat/galvanostat system coupled with a conventional three-electrode Pyrex electrochemical cell (100 ml capacity) as shown in Fig. 7B. The C45 carbon steel specimen served as the working electrode with an exposed surface area of 1 cm², while a saturated calomel electrode (SCE, Hg/Hg₂Cl₂/saturated KCl) and a high-purity platinum electrode (1 cm²) were used as the reference and counter electrodes, respectively.

Prior to each experiment, the working electrode was sequentially with 200, 600, and 1200 grit silicon abrasive papers, rinsed thoroughly with deionized water, washed with ethanol, and dried before immersed in the test solution. The working electrode was immersed in the electrolyte for 15 min to allow the open-circuit potential (OCP) to reach a steady state before each measurement. EIS measurement were carried out by applying a sinusoidal AC perturbation with an amplitude of 10 mV over a frequency rang of 0.1 Hz to 100 Hz. The impedance spectra were analyzed using PSTrace 5.12 soft were and fitted with the equivalent electrical circuit Rs-(QǁRct) to obtain the solution resistance (Rs), charge transfer resistance (Rct), and constant phase element (CPE) (Fig. 7B).

Potentiodynamic Polarization Analysis (PDP) Tafel

Potentiodynamic polarization (PDP) measurements were performed to evaluate the corrosion inhibition performance of the green-synthesized AgNPs.Cin toward C45 carbon steel 1M HCl solution. The electrochemical parameters obtained from the polarization curves, including the corrosion potential (E_corr), corrosion current density (I_corr), anodic and cathodic Tafel slopes (β_a and β_c), polarization resistance (Rp), corrosion rate (CR), and inhibition efficiency (IE%), are summarized in Table 3, while the corresponding polarization curves are presented in Fig. 8. The addition of AgNPs.Cin markedly altered the polarization behavior of C45 carbon steel compared with the uninhibited solution. The corrosion current density decreased substantially from 493.9 μA/cm² to 46.62 μA/cm² demonstrating a significant reduction in the electrochemical corrosion reaction. Simultaneously, the polarization resistance increased remarkably from 70.31 Ω to 2217 Ω, suggesting the formation of an adsorbed protective layer on the steel surface, which is consistent with hindered charge transfer across the metal/electrolyte interface. Consequently, the corrosion rate decreased from 5.744 mm/y to 0.458 mm/y, resulting in an inhibition efficiency of approximately 91%, which demonstrates the excellent protective performance of the synthesized AgNPs.Cin.

The displacement of the corrosion potential E_corr provides useful information regarding the influence of the inhibitor on the anodic and cathodic reaction. Generally, inhibitors are classified as anodic, cathodic, or mixed-type according to the magnitude of the E_corr shift in comparison with the uninhibited solution [50-53]. As illustrated in Fig. 7A, the presence of AgNPs.Cin reduced both anodic metal dissolution and the cathodic hydrogen evolution reaction, indicating that the inhibitor effectively suppressed the overall corrosion process through simultaneous modification of both electrochemical reactions [54,55]. These findings, together with the pronounced decrease in corrosion current density and the significant increase in polarization resistance, suggest that the corrosion inhibition performance is associated with adsorption of AgNPs.Cin on the carbon steel surface, resulting in electrochemical behavior consistent with the formation of an adsorbed barrier in acidic media. E corrosion, I corrosion, μA, Polarization Resistance, Ω, Anodic β Tafel constant, V/decade, Cathodic β Tafel constant, V/decade, Corrosion rate, mm/year, IE% inhibition efficiency.

To calculate the degree of surface covering(Ө) by the inhibitor, use the following equation:

Icorr.uninh = Corrosion current density without inhibitor, Icorr.inh = Corrosion current density in the presence of inhibitor.

By applying the above equation to the current values, we obtain the area of the part covered by the inhibitor, which is equal to θ = 0.91.

Effect of inhibitor concentration and temperature on the corrosion inhibition performance

The influence of AgNPs.Cin concentration and temperature on the corrosion behavior of carbon steel in 1M HCl was evaluated using potentiodynamic polarization measurements, and the obtained electrochemical parameters are summarized in Table 4. As the inhibitor concentration increased from 100 to 500 ppm, the corrosion current density I_corr decreased markedly at both investigated temperatures 298 and 308 K, indicating progressive suppression of the electrochemical corrosion process. This behavior was accompanied by a continuous increase in surface coverage (θ) and inhibition efficiency (IE%), confirming the concentration-dependent inhibition performance of AgNPs.Cin Figs. 9-12.

At 298 K, increasing the inhibitor concentration from 100-500 ppm reduced I_corr from 49.49 μA/cm² to 31.07 μA/cm², while the inhibition efficiency increased from 90% to 94%, with the surface coverage rising from 0.90 to 0.94. A similar trend was observed at 308 K, although the inhibition efficiency was slightly lower, reaching 93% at the highest concentration. Simultaneously, the corrosion rate decreased markedly in the presence of the inhibitor, demonstrating the effective corrosion inhibition performance of the synthesized nanomaterial compared with the uninhibited solution.

The enhanced inhibition performance at higher inhibitor concentrations may be attributed to the increased adsorption of AgNPs.Cin species onto the carbon steel surface, leading to greater inferred surface coverage and more effective blocking of the active corrosion sites [56-58]. Consequently, the corrosion current density (I_corr) decreased progressively with increasing inhibitor concentration, while the inhibition efficiency (%IE) and surface coverage (θ) increased accordingly. These results indicate that increasing the inhibitor concentration improves the protection of carbon steel against the aggressive hydrochloric acid medium[59-60].

Increasing the solution temperature from 298 to 308 K resulted in a slight increase in the corrosion current density and corrosion rate, accompanied by a marginal decrease in inhibition efficiency and surface coverage. Nevertheless, AgNPs.Cin maintained high inhibition efficiency even at 308 K, demonstrating its effective corrosion protection over the investigated temperature range [61-63].

The polarization curves presented in Figs. 13 and 14 further support these findings. The addition of AgNPs.Cin significantly reduced the corrosion current density I_corr, confirming its effective corrosion inhibition performance. Both the anodic and cathodic polarization branches exhibited lower current densities in the presence of the inhibitor, indicating that the inhibitor influenced both electrochemical reactions and significantly reduced the overall corrosion rate [64].

Study of the Adsorption Isotherm of AgNPs.Cin on Carbon Steel Surface

The corrosion inhibition performance of AgNPs.Cin is primarily attributed to the adsorption of inhibitor molecules onto the carbon steel surface, leading to the proposed formation of an adsorbed protective layer that reduces the interaction between the metal surface and the aggressive acidic environment. During the adsorption process, inhibitor molecules progressively replace the water molecules initially adsorbed on the metal surface through a substitution mechanism, which can be represented by the following equilibrium [65].

Inhibitor_sol + xH₂O_ads ⇌ Inhibitor_ads + xH₂O_sol

Adsorption isotherm analysis provides valuable information regarding the interaction between inhibitor molecules and the metal surface, allowing the adsorption mechanism to be evaluated as physical, chemical, or mixed. Among the various adsorption models examined, the Langmuir adsorption isotherm provided the best fit for the experimental data at both 298 and 308 K, according to the following equation [66,67].

The Langmuir plots shown in Fig. 15a-b exhibit excellent linear relationships with correlation coefficients of 1.0000 and 0.9998 at 298 and 308 K respectively. While the calculated slopes (1.052 at 298 K 1.0663 at 308 K) are very close to unity. These results demonstrate that the adsorption behavior of AgNPs.Cin is well described by the Langmuir adsorption model, suggesting adsorption consistent with a predominantly monolayer coverage on energetically equivalent active sites of the carbon steel surface [68].

The adsorption equilibrium constants K_ads calculated from the intercept values of the Langmuir plots were 0.1596 and 0.1601 at 298K and 308 K, respectively Table 5. The corresponding standard free energies of adsorption ∆G_ads^° were -29.68 and -30.69 KJ/mol, respectively. The negative values ∆G_ads^° indicate that the adsorption proceeds is spontaneous. In addition, the magnitude of ∆G_ads^° is consistent with a mixed adsorption mechanism, involving both physical and chemical interactions between the inhibitor species and the carbon steel surface.

Based on the parameters of the equilibrium constant of the K_ads process, we can compute the free energy ∆G_ads^° of compression using equation No. 5 [69] Table 5. Here, C_w represents the concentration of water, which was taken as 10⁶ mg/L.

Electrochemical Impedance Spectroscopy (EIS)

Nyquist plot and electrochemical

The Nyquist plots obtained at 298, 308 and 318 K Fig. 16A-C, exhibit a single depressed capacitive semicircle for both the uninhibited solution and all AgNPs.Cin concentrations, indicating that the corrosion process is predominantly governed by a single charge-transfer mechanism. Accordingly, the experimental data were satisfactorily fitted using the equivalent circuit shown in Fig. 16D. The depressed nature of the semicircles reflects surface heterogeneity and non-ideal capacitive behavior arising from surface roughness and frequency dispersion, therefore, a constant phase element (CPE) was employed instead of an ideal capacitor, as commonly reported for corroding steel surfaces [70].

The addition of AgNPs.Cin markedly increased the diameter of the capacitive semicircle, and this enlargement became more pronounced as the inhibitor concentration increased from 100 to 500 ppm, demonstrating a progressive increase in charge-transfer resistance, consistent with adsorption of the nanoparticle-based inhibitor on the steel surface [71,72]. In contrast, increasing the temperature from 298 to 318 K reduced the semicircle diameter for all system, reflecting the acceleration of corrosion kinetic at elevated temperature [73]. Nevertheless, the inhibited solution retained significantly larger semicircles than the blank, indicating that AgNPs.Cin remained effective even at 318 K.

Charge-transfer resistance and inhibition efficiency

The electrochemical parameters summarized in Table 6 support the Nyquist observation. At 298 K, the charge-transfer resistance (Rct) increased from 180 Ω.cm² for the blank solution to 642.86, 857.14 and 1200 Ω.cm² after the addition of 100, 300 and 500 ppm AgNPs.Cin, corresponding to inhibition efficiencies of 72, 79 and 85%, respectively. The continuous increase in R_ct is consistent with increased surface coverage by the adsorbed inhibitor, leading to more effective blocking of active corrosion sites and suppression of both iron dissolution and hydrogen evolution reactions [74]. Increasing the temperature led to a gradual decrease in both R_ct and inhibition efficiency. At 500 ppm, the inhibition efficiency decreased from 85% (298 K) to 80% (308K) and 72% (318K), while the blank R_ct decreased from 180 to 95 Ω.cm². This behavior indicates that higher temperature facilitate charge-transfer reactions and suggesting partial desorption adsorbed inhibitor at elevated temperature [75].

Constant phase element and double-layer capacitance

Additional evidence for inhibitor adsorption is provided by the CPE exponent (n) and the calculated double-layer capacitance (Cdl). The n values increased from 0.87-0.90 to 0.92-0.96 after inhibition addition, indicating a more homogeneous metal/solution interface, consistent with adsorption of the inhibitor on the steel surface [76,77]. Meanwhile, Cdl decreased progressively as the inhibitor concentration increased. At 298 K, Cdl decreased from 60.45 μF/cm² for the blank solution to 55.12, 50.53 and 44.47 μF/cm² at 100, 300, and 500 ppm, respectively. Although Cdl increased with increasing temperature for all tested systems, it remained consistently lower than that of the uninhibited solution [78]. This reduction in Cdl attributed to the displacement of adsorbed water molecules and chloride ions by the phytochemical constituents of the cinnamon-derived silver nanoparticles, together with an increase in the thickness of the electrical double layer [79]. These finding are consistent with increased surface coverage by the adsorbed inhibitor, as reflected the increase in R_ct and the corresponding improvement in inhibition efficiency.

Results and Discussion Arrhenius Analysis of the Corrosion Process

As shown in Fig. 17, because EIS measurement were performed at three temperature, 1/R_ct was used as a surrogate apparent electrochemical kinetic parameter because it is inversely related to the charge-transfer resistance rather than being treated as an exact corrosion rate. Consequently, the calculated activation energies represent apparent electrochemical activation energies. Accordingly, the following apparent Arrhenius relationship was used [80,81]:

ln(1/ R_ct) = ln(A) - Ea/RT

where R_ct is the charge-transfer resistance, A is the frequency factor, Ea is the apparent activation energy, R is the gas constant, and T is the absolute temperature.

Which was used to construct the Arrhenius plots. Ea values obtained this way are therefore apparent activation energies, but the relative trend across concentrations remains valid for mechanistic interpretation, as commonly practiced in EIS-based corrosion studies [82].

Where 1/ R_ct was used as a surrogate for the apparent electrochemical kinetic parameter rather than as an exact corrosion rate, consistent with common practice in EIS-based corrosion studies. The resulting plot of ln(1/ R_ct) versus 1/T Fig. 15 gave excellent straight-line fits for all four systems R² = 0.9987-0.9999 from which the apparent activation energies were extracted from the slope [83].

As shown in Table 7, the apparent activation energy Ea increased steadily with inhibitor concentration, from 25.2 KJ/mol in the blank solution to 49.7 KJ/mol at 500 ppm. An increase in the apparent activation energy Ea upon inhibitor addition suggests that the inhibitor increases the apparent energy barrier for the electrochemical process, most likely because adsorption progressively block active surface sites [84,85]. Although the apparent activation energy values are consistent with adsorption involving predominantly physical interactions, the Ea values alone cannot be considered conclusive evidence for distinguishing between physisorption and chemisorption [86].

Activation Thermodynamics (Eyring Analysis)

The Eyring transition-state equation was applied to complement the Arrhenius treatment:

Ln(1/R_ct.T) = [ln(R/Nh) + ∆S/R] - ∆H/RT

Plots of ln(1/R_ct.T) versus 1/T were linear (R² > 0.998) for all systems Fig. 18, yielding ∆H* and ∆S* from the slop and intercept, respectively [87] (Table 8).

The apparent activation enthalpy ∆H* increased with inhibitor concentration following the general tread as the apparent activation energy Ea consistent with the relation Ea ≈ ∆H* + RT [88]. The more diagnostic result here is ∆S* its large negative values indicate that the activated complex is more ordered than the ground-state reactants, meaning the rate-determining step proceeds via an associative rather than a dissociative pathway consistent with formation of a compact Fe-H₂O/Cl^- transition complex at the metal surface [89]. As the concentration increased, the apparent activation entropy ∆S* become progressively less negative, suggesting that the adsorbed inhibitor raises the enthalpic barrier to this step without imposing additional conformational restriction on the transition complex itself. These observations suggest that the inhibitor primarily increase the apparent energy barrier of the electrochemical process rather than acting as a structural constraint on the activated state [90].

Comparative of the Present Green Inhibitor with Previously Published Eco-Friendly Inhibitors

To place the present findings in context, the performance of the proposed inhibitor was qualitatively compared with previously reported plant-based silver nanoparticle corrosion inhibitors. Because the published studies were conducted under different experimental conditions, including inhibitor concentration, temperature, steel type, and testing methodology, the comparison is intended only to provide a general qualitative perspective rather than a direct ranking of inhibition performance. The relevant studies are summarized in Table 9.

The suggested adsorption mechanism

Although the formulated green inhibitor comprises a complex mixture of diverse phytochemicals, its efficiency on the C45 carbon steel surface is primarily governed by the synergistic action between the biogenic silver nanoparticles AgNPs.Cin and the capping organic constituents (mainly Eugenol and Pyrogallol) [95]. The synthesized AgNPs.Cin nano complex features an abundance of heteroatoms (such as oxygen) with high electronegativity, alongside dense π-electron clouds residing within the aromatic ring and conjugated double bonds, include aldehydes, alcohols, esters, acids, monoterpenes, diterpenes, sesqui terpenes, benzopyrenes, hydrocarbons, phenolic compounds (Eugenol and pyrogallol), favonoids (procya nidin dimers type A and B), and phenolic compounds. Based on the combined electrochemical measurement (potentiodynamic polarization and electrochemical impedance spectroscopy), adsorption isotherm analysis, and the calculated thermodynamic and kinetic parameters, a plausible adsorption mechanism is proposed. The protective adsorbed layer illustrated in Fig. 19 is therefore inferred from this complementary result rather than directly confirmed by post-immersion surface characterization. The proposed inhibition process can be described through two distinct yet interconnected adsorption pathways [96-99].

Physisorption and Synergistic Molecular Displacement

Prior to chemical bonding, preliminary physical adsorption takes place via electrostatic interactions, In aggressive acidic or saline mediums, In aggressive acidic or chloride-containing environments, chloride ions (Cl⁻) are specifically adsorbed onto the carbon steel surface, forming an anion-rich interfacial layer that promotes the subsequent adsorption of positively charged inhibitor species [100]. Concurrently, the heteroatoms of the AgNPs.Cin complex or the nanoparticle core itself can undergo protonation, acquiring a net positive charge. This induces a long-range electrostatic attraction (physisorption) between the protonated nano-inhibitor species and the Cl^--conditioned steel surface [101] As the AgNPs.Cin complex approaches the interface, it drives a thermodynamic displacement process, thermodynamically displaces the weakly adsorbed water molecules occupying the active sites, according to the substitution equilibrium [102]:

Org_(sol)+xH₂O_(ads)⇌ Org_(ads)+ xH₂O_(sol)

The replacement of interfacial water molecules by inhibitor species is expected to increases the surface coverage and enables. A more compact arrangement of the hydrophobic phytochemical constituents surrounding the AgNPs.Cin core [103]. This cooperative molecular packing is proposed to promote the development of an adsorbed hydrophobic barrier, which may reduce the transport of dissolved oxygen, chloride ions, and other aggressive electrolyte species toward the active corrosion sites, thereby contributing to lower anodic metal dissolution and cathodic reduction reactions [104]. Fig. 20a.

Chemisorption via Coordinate and Back-Donation Bonding

This pathway may involve the direct chemical interaction between the frontier orbitals of the inhibitor molecules and the steel substrate, as tentatively proposed in Fig. 20b. The lone pairs of electrons on the oxygen atoms of the hydroxyl (-OH) and methoxy (-OCH₃) groups may interact with the surface Fe atoms through coordinate bonding. Likewise, the π-electrons of the aromatic rings and the propene chain (-CH=CH-CH₃) may contribute to the adsorption process through possible π interactions with the metal surface. However, these interactions are tentatively proposed and were not directly confirmed in the present study [105]. A possible metal-to-ligand back-donation (π→π^* retro-donation) interaction has been proposed in the literature for similar inhibitor systems. However, such an interaction cannot be confirmed from the present experimental data because no surface spectroscopic or theoretical investigations were performed.

If such electronic interactions occur, they could contribute to the chemisorption of the mixed adsorption mechanism, complementing the initial physisorption process and enhancing the stability of the adsorbed inhibitor species on the steel surface [106]. Although this adsorption model is consistent with the electrochemical measurements, adsorption isotherm analysis, and the calculated thermodynamic, and kinetic parameters, the proposed molecular interactions remain hypothetical and cannot be directly confirmed because no surface spectroscopic characterization (e.g., XPS) or theoretical calculations were performed in the present study.

Study limitation

The present study was designed to evaluate the corrosion inhibition performance of the final biosynthesized AgNPs.Cin nanocomposite. Therefore, the electrochemical experiments were performed by comparing the blank solution with the final inhibitor formulation. The individual contributions of cinnamon extract and silver nanoparticles were not investigated separately. Such control experiments would provide further insight into the specific role of each component and will be considered in future studies. In addition long-term corrosion performance was beyond the scope of the present study and will be investigated in future work. Furthermore, DLS and zeta-potential measurement were not performed, representing a limitation in assessing the colloidal stability of the AgNPs.Cin suspension and its effect on corrosion inhibition.

CONCLUSION

A green nano-inhibitor AgNPs.Cin was successfully synthesized using Cinnamomum verum extract. FESEM and TEM analyses confirmed the formation of predominantly spherical to semi-spherical nanoparticles, while EDX analysis verified the presence of silver. Electrochemical measurements (PDP and EIS) demonstrated that AgNPs.Cin effectively inhibited the corrosion of C45 carbon steel in 1.0 M HCl solution, with inhibition efficiency increasing as the inhibitor concentration increased and decreasing with increasing temperature. The adsorption behavior followed the Langmuir adsorption isotherm, whereas adsorption behavior followed the Langmuir adsorption isotherm, where the thermodynamic and kinetic analyses were consistent with a spontaneous adsorption process involving both physisorption and chemisorption. Overall, the green-synthesized AgNPs.Cin demonstrated promising potential as an environmentally friendly nano-inhibitor for the protection of C45 carbon steel in acidic media.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.