Dracaena fragrans Extract as a Corrosion Inhibitor for SAE 1025 Steel Used in Aircrafts

Abstract

This study evaluated the corrosion inhibition performance of Dracaena fragrans extract for SAE 1025 steel in artificial seawater. Inhibition efficiency was assessed using weight loss measurements, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). The results showed that inhibition efficiency increased with higher extract concentrations, reaching a maximum of 97% at 500 ppm. Potentiodynamic polarization measurements indicated that the extract acts as a mixed-type inhibitor, affecting both anodic and cathodic reactions. EIS analysis revealed an increase in charge transfer resistance and a decrease in double-layer capacitance, confirming the formation of a protective adsorbed film on the steel surface. Adsorption studies indicated that the process followed the Frumkin isotherm and was predominantly governed by physisorption, with a standard Gibbs free energy of adsorption $∆G_{ads}^{°}$ of approximately −12.33 kJ mol⁻¹. Surface analyses confirmed enhanced protection of the steel substrate in the presence of the extract. Moreover, toxicity tests yielded a germination index (GI) of 35.1% and a relative germination (RG) of 45.6% at 500 ppm. These findings demonstrate the potential of Dracaena fragrans extract as an environmentally friendly corrosion inhibitor for steel exposed to chloride-containing environments in marine and aeronautical applications.

1. Introduction

Corrosion of metallic materials is an inevitable phenomenon that becomes critically important in the aeronautical sector. The progressive degradation of metallic components not only affects the service life of aircraft, but also increases maintenance costs and puts operational safety at risk. For this reason, corrosion control continues to be a topic of great interest for both the industry and the scientific community [1,2].

A wide variety of materials are used in the aeronautical industry. Low-carbon steels, such as SAE 1025 steel, continue to play a relevant role, although they are not used as primary structural materials. They are commonly employed in secondary structural components, fastening elements, shafts, bolts, and supports, where a balanced combination of mechanical strength, toughness, good machinability, and weldability is required. Furthermore, their compatibility with heat treatments and manufacturing processes commonly used in the aeronautical industry makes them a reliable and economically viable option [3,4].

However, these mechanical advantages contrast with a significant limitation: their high susceptibility to corrosion when exposed to aggressive environmental conditions. Common conditions during aircraft operation, storage, and maintenance include humid environments, saline atmospheres, thermal fluctuations, and chemical contaminants, all of which promote the corrosive deterioration of SAE 1025 steel. Over time, this may lead to premature component failure, increasing the frequency of inspections, repairs, and component replacement, thereby generating economic and operational impacts [5].

This issue is particularly relevant in aircrafts operating in coastal regions, offshore platforms, and naval aviation environments. In such locations, chloride-rich marine aerosols are continuously deposited on exposed metallic surfaces, including fastening systems, support structures, landing gear accessories, and other secondary steel components. The combination of high humidity, salt deposition, and cyclic wetting–drying conditions promotes the formation of thin electrolyte films that accelerate both uniform and localized corrosion processes. Several studies have identified marine atmospheres as among the most aggressive environments affecting the durability and reliability of metallic materials used in aeronautical applications [6,7,8].

Although these components are generally protected by coatings and preventive maintenance programs, prolonged exposure to marine atmospheres can result in the accumulation of hygroscopic salts and moisture condensation on metallic surfaces. These deposits create localized chloride-rich electrolytes capable of destabilizing passive films and promoting corrosion initiation. Consequently, understanding the corrosion behavior of aeronautically relevant steels under chloride-containing environments remains an important step in the development of effective protection strategies.

To reproduce the corrosive action of these marine environments under controlled and reproducible laboratory conditions, corrosion studies commonly employ artificial seawater prepared according to ASTM D1141 [9]. While this electrolyte does not reproduce all operational variables encountered during aircraft service, it provides a standardized multi-salt medium that simulates severe chloride exposure and allows for the comparative evaluation of corrosion processes and mitigation strategies. Therefore, artificial seawater is widely accepted as a representative testing environment for the preliminary assessment of materials and corrosion inhibitors intended for applications exposed to coastal and naval atmospheric conditions [10,11].

To reduce the corrosion rate, various protection strategies are employed, among which the use of corrosion inhibitors stands out. However, many of the conventional inhibitors currently used are based on synthetic organic compounds such as amides, amines, azoles, and others, which present problems related to toxicity and environmental persistence. This has led to increasingly strict regulatory restrictions, particularly in the aeronautical sector [12,13]. This scenario has driven the search for safer and more sustainable alternatives.

In this context, plant-based corrosion inhibitors have emerged as an environmentally friendly alternative. Numerous studies have demonstrated that plant extracts are effective corrosion inhibitors in saline environments due to their content of organic compounds such as flavonoids, tannins, phenolics, and alkaloids, which are capable of interacting with the metallic surface and forming a protective barrier that reduces the corrosion rate [14,15].

Examples of natural inhibitors that have shown high performance include Tamarix aphylla extract, which demonstrated an efficiency greater than 90% for carbon steel [16]; Morinda citrifolia, exhibiting an efficiency higher than 70% for low-carbon steel [17]; Turnip peel, with a 91% inhibition rate [18]; rice husk, yielding an efficiency of 82% [19]; Levisticum officinale, with efficiencies reaching up to 88% [20]; and Trifolium repens, which achieved a maximum efficiency of 98% [21]. Due to their biodegradability, wide availability, and low environmental impact, these natural inhibitors have significant potential for industrial applications.

Dracaena fragrans (D. fragrans), a plant belonging to the Asparagaceae family and native to tropical Africa, is notable for its richness in secondary metabolites with antioxidant activity, such as flavonoids, phenols, alkaloids, and saponins [22,23], which suggests a possible inhibitory effect against corrosion [24]. However, there is still limited information regarding its performance as a green inhibitor for carbon steels of aeronautical interest, such as SAE 1025 steel. Evaluating its effectiveness under controlled conditions may contribute to the development of more sustainable solutions without compromising the technical requirements of the aeronautical sector.

Therefore, the objective of this study is to evaluate the corrosion inhibition efficiency of D. fragrans extract on SAE 1025 steel in artificial seawater through gravimetric and electrochemical techniques. The use of artificial seawater as the testing medium provides a controlled representation of chloride-rich marine exposure conditions relevant to coastal and naval aeronautical environments, allowing for the preliminary assessment of the inhibitor’s potential for protecting steel components subjected to such service conditions. The results obtained may contribute to the development of environmentally friendly corrosion protection strategies compatible with the safety, reliability, and sustainability requirements of the aeronautical industry.

2. Materials and Methods

2.1. Metal

SAE 1025 steel specimens were used, with a chemical composition corresponding to carbon (C) between 0.22 and 0.28%, manganese (Mn) between 0.60 and 0.90%, phosphorus (P) ≤ 0.040%, sulfur (S) ≤ 0.050%, and Fe as the balance. For the gravimetric measurements, cylindrical samples with a height of 2 cm and a diameter of 9.5 mm were cut. The metal surface was prepared through polishing with silicon carbide sandpaper in progressive order from 100 to 600 grit to ensure a homogeneous surface. Subsequently, the samples were cleaned in an ultrasonic bath with distilled water, degreased with acetone, and completely dried.

For the electrochemical evaluations, cylindrical samples measuring 3 cm in length and 1.27 cm in diameter were encapsulated in epoxy resin, leaving a cross-sectional exposed surface area of 1.27 cm².

2.2. Preparation of the Extract

The natural extract of Dracaena fragrans was obtained from plant material collected from nurseries in Pachuca, Hidalgo. The botanical samples were dried at room temperature, ground, and subjected to a three-day methanolic extraction via maceration. Afterwards, the mixture was filtered, and the solvent was removed using rotary evaporation to yield the crude D. fragrans extract.

2.3. Preparation of Artificial Seawater

Artificial seawater was prepared according to the ASTM D1141-98 standard, the Standard Practice for the Preparation of Substitute Ocean Water [9], which is commonly used for corrosion testing of materials, ensuring controlled and reproducible conditions comparable to those of real seawater.

2.4. Gravimetric Method

Weight loss measurements were carried out according to the ASTM G31 standard for immersion corrosion testing [25] under static conditions. The initial weight of the SAE 1025 steel samples was recorded, and subsequently, the samples were suspended in 100 mL of solution in triplicate to evaluate their behavior in artificial seawater without the presence of the inhibitor and with different concentrations of the inhibitor during immersion times of 24, 48, and 72 h.

After the exposure periods, the samples were removed from the solution, and the surface corrosion products were removed via gentle manual brushing with a fine-bristled plastic brush, followed by ultrasonic cleaning. The samples were then rinsed with acetone, dried with hot air, and stored in a desiccator. Finally, the dry weight of each sample was recorded. The corrosion rate (C_R) was calculated according to the following equation:

$$C_R={\displaystyle \frac{K·W}{A·\rho ·t}}$$

(1)

where W is the mass loss (g), A is the exposed area (cm²), t is the exposure time (h), $\rho$ is the density of SAE 1025 steel (7.85 g/cm³), and K is the constant 8.76 × 10⁴ used to obtain the corrosion rate (C_R) in mm/year.

Meanwhile, the inhibition efficiency $({\eta }_w\%)$ was calculated using the following equation:

$${\eta }_w\%={\displaystyle \frac{C_{R2}-C_{R1}}{C_{R2}}}\ast 100$$

(2)

where C_R2 is the corrosion rate in the absence of the inhibitor and $C_{R1}$ is the corrosion rate of the steel in the presence of the inhibitor [26,27].

2.5. Electrochemical Measurements

The electrochemical tests were performed using a three-electrode cell consisting of an Ag/AgCl reference electrode (saturated KCl, +0.197 V vs. SHE at 25 °C), a graphite counter electrode, and a SAE 1025 steel working electrode immersed in naturally aerated artificial seawater prepared according to ASTM D1141. All experiments were conducted under static conditions without mechanical stirring or forced aeration. A PGSTAT302N potentiostat/galvanostat (Metrohm Autolab, Utrecht, The Netherlands) was utilized, and each experiment was conducted in triplicate to ensure the reproducibility of the results. Before starting the measurements, the working electrode was immersed in the test solution until the open-circuit potential reached a steady state (E_OCP). Polarization curves were recorded within a potential range of ±250 mV with respect to E_OCP, using a scan rate of 1.0 mV/s. Different parameters were evaluated, such as the corrosion current density (i_corr) and the corrosion potential (E_corr), and extracted via Tafel extrapolation software (Nova 2.0.4) for the uninhibited (blank) and inhibited systems. The inhibition efficiency of the D. fragrans extract ${(\eta }_P\%)$ was determined using the following equation:

$${\eta }_P\%=1-{\displaystyle \frac{i_{corr2}}{i_{corr1}}}\ast 100$$

(3)

where $i_{corr1}$ is the corrosion current density of the steel electrode in the absence of the inhibitor, and $i_{corr2}$ is the corrosion current density of the steel in the presence of the inhibitor [28,29].

Electrochemical impedance spectroscopy (EIS) measurements were carried out using a frequency range from 0.05 to 10,000 Hz with a signal amplitude of 10 Mv.

The inhibition efficiency obtained from the EIS measurements ${\eta }_{EIS}$ was determined using the following equation:

$${\eta }_{EIS}\%=1-{\displaystyle \frac{R_{ct\left(0\right)}}{R_{ct\left(inh\right)}}}\ast 100$$

(4)

where $R_{ct\left(0\right)}$ is the charge transfer resistance in the absence of the inhibitor, and $R_{ct\left(inh\right)}$ is the charge transfer resistance in the presence of the inhibitor [29].

2.6. Quantification of Total Phenols and Flavonoids

The Folin–Ciocalteu reagent (Sigma-Aldrich, St. Louis, MO, USA) method was used to quantify the total phenolic content of the methanolic D. fragrans extract, measuring the reducing capacity of the organic extract [30]. The extract was dissolved in double-distilled water (ddW) to a concentration of 5 mg/mL. A 50 µL aliquot of the extract solution was placed into each well of a 96-well plate, followed by the addition of 50 µL of Folin reagent. The blank was prepared using 50 µL of ddW and 50 µL of Folin reagent. The reaction mixtures were incubated for five minutes, after which 100 µL of 0.35 M NaOH was added to each well. The plates were then incubated for 20 min in the dark, and their absorbance was measured at 734 nm using a spectrophotometer. The total phenol concentration was calculated using a gallic acid (0.30–2000 µg/mL) reference curve and reported as mg GAE/g extract.

The aluminum chloride (AlCl₃) method was employed to quantify flavonoids, following the procedure suggested by [31] with minor modifications. Briefly, 50 µL of the extract solution (5 mg/mL) was added to a 96-well plate, followed by 10 µL of a 10% AlCl₃ solution. The blank was prepared using 50 µL of ddW and 10 µL of the 10% AlCl₃ solution. The plate was incubated for six minutes, after which 150 µL of 95% ethanol was added and the plate was shaken. Finally, 10 µL of 1 M CH₃COONa was added, and the plate was incubated for 40 min in the dark. Subsequently, the absorbance was measured at 415 nm using a spectrophotometer. The quantification of total flavonoids was determined using a chrysin (0.78–2500 µg/mL) reference curve, and the flavonoid content was reported as mg ChE/g extract.

2.7. Toxicity Evaluation

The toxicity of the extract was evaluated through a seed germination and seedling growth bioassay using Lactuca sativa seeds, following the methodology employed in plant ecotoxicity studies [32]. Ten seeds were placed in sterile Petri dishes (9 cm in diameter) containing filter paper moistened with 10 mL of the extract solutions at various concentrations. A 0.2 M ZnSO₄ solution was used as the positive control, while mineral water was employed as the negative control.

All treatments were carried out in triplicate. The Petri dishes were incubated in darkness at 25 ± 2 °C for 5 days. At the end of the incubation period, the germination percentage was recorded, and the lengths of the hypocotyl and radicle were measured using a digital caliper.

The phytotoxic effect was expressed in terms of relative root growth (%RRG), relative germination (%RG), and germination index (GI), calculated using the following equations:

$$\%RRG={\displaystyle \frac{Radicle elongation in the extract}{Control radicle elongation}}\times 100$$

(5)

$$\%RG={\displaystyle \frac{Number of germinated seeds with extract}{Number of seeds germinated in the control}} \times 100$$

(6)

$$GI={\displaystyle \frac{RG\ast RRG}{100}}$$

(7)

3. Results and Discussion

3.1. Gravimetric Method

The weight loss results demonstrate the inhibitory effect of D. fragrans extract on the corrosion behavior of SAE 1025 steel in artificial seawater. As shown in Figure 1, the corrosion rate decreased progressively with increasing inhibitor concentration across all evaluated immersion times (24, 48, and 72 h) at 25 ± 2 °C. This behavior confirms that the extract effectively mitigates the corrosion process in chloride-containing media. The highest inhibition efficiencies were obtained at 500 ppm, reaching 88%, 82%, and 79% after 24, 48, and 72 h of immersion, respectively.

The observed decrease in corrosion rate with increasing inhibitor concentration is characteristic of adsorption-type organic inhibitors, which reduce the interaction between the metallic surface and the corrosive environment through the formation of a protective adsorbed layer [33,34]. The phytochemical constituents present in D. fragrans, particularly compounds containing heteroatoms and π-electron systems, may interact with the steel surface through adsorption processes, thereby limiting charge transfer and reducing the corrosion rate.

As shown in Figure 1, the corrosion rate of the uninhibited solution increased from 4.114 to 4.298 mmpy when the immersion time increased from 24 to 48 h, indicating the continued progression of the corrosion process. However, after 72 h of exposure, the corrosion rate decreased to 2.930 mmpy. Although the cumulative weight loss continued to increase with immersion time, as shown in

Table 1. Weight loss results and corrosion parameters of SAE 1025 steel in artificial seawater in the presence of Dracaena fragrans extract at different immersion times.

Time (h)	Cinh (ppm)	Wl (mg cm−2)	σ	Vcorr (mmpy)	θ	ηw (%)
24	0	8.904	0.322	4.114	0	0
	200	3.875	0.325	1.790	0.56	56
	300	2.096	0.371	0.969	0.76	76
	400	1.293	0.226	0.598	0.85	85
	500	1.096	0.031	0.506	0.88	88
48	0	18.605	0.139	4.298	0	0
	200	11.256	0.125	2.600	0.40	40
	300	7.657	0.112	1.769	0.59	59
	400	3.569	0.059	0.824	0.81	81
	500	3.312	0.038	0.765	0.82	82
72	0	19.0250	0.285	2.930	0	0
	200	14.4560	0.185	2.226	0.24	24
	300	8.8570	0.155	1.364	0.53	53
	400	5.3650	0.255	0.826	0.72	72
	500	3.9560	0.102	0.609	0.79	79

, the lower average corrosion rate observed after 72 h may be associated with changes occurring at the steel surface during prolonged exposure. The formation and accumulation of corrosion products may partially hinder oxygen diffusion and reduce the number of active sites available for further metal dissolution. In addition, because the experiments were conducted under static conditions, local oxygen depletion and the accumulation of corrosion products near the metal/electrolyte interface may contribute to a decrease in corrosion kinetics at longer exposure times [35,36].

Table 1 further shows that the inhibition efficiency decreased slightly with increasing immersion time. At 500 ppm, the inhibition efficiency decreased from 88% after 24 h to 79% after 72 h. This behavior may be related to the gradual desorption of adsorbed species, partial degradation of active constituents present in the extract, and the continuous action of aggressive chloride ions, which can weaken the protective layer formed on the steel surface. Similar trends have been reported for several plant-based corrosion inhibitors, where prolonged exposure results in a gradual reduction in surface coverage and protective performance [35,36].

Despite this slight decrease in inhibition efficiency, the extract maintained a significant protective effect throughout the entire exposure period. In particular, at concentrations of 400 and 500 ppm, the corrosion rates remained substantially lower than those measured in the uninhibited solution, indicating that the adsorbed phytochemical layer retained its protective properties even after prolonged immersion.

3.2. Adsorption Isotherm Analysis

The adsorption behavior of Dracaena fragrans on the surface of SAE 1025 steel was evaluated using the data obtained from the weight loss tests, fitted to the Langmuir, Temkin, Frumkin, and Flory–Huggins models. This type of analysis allows for the determination of the nature of the adsorption process, as well as the interactions between the inhibitor molecules and the metallic surface [37,38].

The degree of surface coverage (θ) was calculated from the inhibition efficiency using the following equation:

$$\theta ={\displaystyle \frac{{\eta }_w\%}{100}}$$

(8)

Figure 2 shows the linear fitting of each adsorption model. For the Langmuir model (Figure 2a), the fitting obtained was R² ≈ 0.98 suggesting that the adsorption process can be approximated as monolayer adsorption; however, the observed deviations indicate that the steel surface is not completely homogeneous, which is consistent with systems involving complex organic inhibitors [39]. For the Temkin model (Figure 2b), the fitting was R² ≈ 0.98, indicating that the adsorption process is influenced by lateral interactions between inhibitor molecules, which is typical of plant extracts containing multiple active compounds with different affinities for the metallic surface [40]. In the Flory–Huggins model (Figure 2c), the lowest fitting was obtained, with R² ≈ 0.95, indicating that the inhibitor exhibits characteristics of relatively large molecules capable of displacing multiple water molecules upon adsorption onto the metallic surface. This behavior is consistent with recent studies on green inhibitors, where the formation of protective layers through the displacement of water molecules has been observed [41,42]. The Frumkin model (Figure 2d) showed the best fitting, with R² ≈ 0.99, This model is described by the following equation:

$$ln\left[\left({\displaystyle \frac{\theta }{1-\theta }}\right)\ast {\displaystyle \frac{1}{C_{inh}}}\right]=-ln{K}_{ads}+2\alpha \theta$$

(9)

where $K_{ads}$ is the Frumkin adsorption constant, related to the adsorption energy and the affinity of the inhibitor for the surface; $C_{inh}$ is the concentration of D. fragrans; and $\alpha$ is the lateral interaction parameter between the adsorbed molecules.

The Frumkin model suggests that the adsorption of the inhibitor is strongly influenced by attractive interactions between the adsorbed molecules, which favors the formation of a compact protective film. This behavior has recently been reported in organic inhibitor systems in chloride-containing media [43]. Based on this model, the Gibbs free energy was calculated using the following equation:

$$∆G_{ads}=-RTln\left(55.5K_{ads}\right)$$

(10)

where R is the universal gas constant, T is the temperature, 55.5 is the concentration of water, and $K_{ads}$ is the Frumkin adsorption constant.

The calculated value of $∆G_{ads}$ was approximately −12.33 kJ/mol, indicating that the adsorption process is spontaneous due to its negative value [44,45]. This parameter is fundamental for determining the adsorption mechanism. In general, values around −20 kJ/mol or less are associated with physisorption processes, characterized by weak electrostatic interactions, whereas values close to or greater than −40 kJ/mol indicate chemisorption, where chemical bonds are formed between the inhibitor and the metallic surface [46,47]. Therefore, the obtained value of −12.33 kJ/mol suggests that the adsorption mechanism is predominantly governed by physisorption, implying that the inhibitor molecules are adsorbed onto the steel surface through electrostatic forces, dipole–metal interactions, and possible π–metal interactions. This type of behavior is typical of green inhibitors derived from plant extracts, which contain organic compounds with heteroatoms such as oxygen and nitrogen capable of interacting with the metallic surface without forming strong chemical bonds [48].

3.3. Potentiodynamic Polarization Curves

The results obtained from the potentiodynamic polarization curves for the corrosion of SAE 1025 steel in seawater with different concentrations of the D. fragrans inhibitor are shown in Figure 3. Displacement of both the anodic and cathodic branches toward lower current density values can be observed as the inhibitor concentration increases. This indicates that the inhibitor decreases the rate at which the anodic reaction occurs, Fe → Fe²⁺ + 2e⁻, thereby simultaneously reducing metal dissolution and the predominant cathodic reaction in aerated seawater, corresponding to oxygen reduction, O₂ + 2H₂O + 4e⁻ → 4OH⁻. This type of behavior has previously been reported for organic inhibitors in chloride-rich media [49].

Table 2. The electrochemical parameters obtained from potentiodynamic polarization curves for SAE 1025 steel in seawater in the presence of the inhibitor.

Cinh (ppm)	Ecorr (V)	icorr (A/cm2)	βa (V/dec)	βc (V/dec)	ηp (%)
0	−0.473	7.54 × 10−6	0.0249	−0.0412	0
300	−0.484	5.75 × 10−6	0.0297	−0.0351	24
400	−0.496	2.05 × 10−6	0.0279	−0.0372	73
500	−0.504	2.05 × 10−7	0.0263	−0.0314	97

shows that as the inhibitor concentration increases, there is a significant decrease in the corrosion current density (i_corr). The most significant variation was obtained when a concentration of 500 ppm was used, where the corrosion current density decreased from 7.54 × 10⁻⁶ A/cm² to 2.05 × 10⁻⁷ A/cm² compared to the blank solution; this corresponds to an inhibition efficiency of 97%. This behavior supports the effectiveness of the inhibitor in reducing the corrosion rate of steel in seawater. The corrosion potential (E_corr) shifted slightly toward more negative values in the presence of the inhibitor. When the change in E_corr does not exceed ±85 mV, the inhibitor is considered to be of the mixed type, meaning that it acts on both anodic and cathodic reactions [50].

On the other hand, the Tafel slopes (βa y βc) did not show significant changes in the presence of the inhibitor. This behavior has been observed in previous studies, where the low variability in the Tafel slopes suggests that the corrosion mechanism is not modified, since the inhibitor acts mainly through adsorption onto the metallic surface, blocking the active sites [51].

At a concentration of 300 ppm, the inhibition efficiency reached only 24%, which may indicate partial coverage of the metallic surface. However, when the inhibitor concentration increased to 400 ppm, the inhibition efficiency increased to 73%, and a maximum value of 97% was obtained at 500 ppm. This behavior indicates progressive surface coverage by adsorbed inhibitor molecules onto the steel, leading to the formation of a protective barrier that limits charge transfer and reduces the corrosion rate [52].

3.4. Electrochemical Impedance Spectroscopy

Figure 4a shows the Nyquist diagram for the corrosion of SAE 1025 steel in the saline medium with different concentrations of the inhibitor. An increase in the semicircle diameter can be observed with rising D. fragrans content, which is characteristic of a system where the corrosion mechanism is controlled by charge transfer. This behavior agrees with the $R_{ct}$ values presented in

Table 3. The electrochemical parameters obtained from electrochemical impedance spectroscopy for SAE 1025 steel in seawater in the presence of the inhibitor.

Cinh (ppm)	Rs (Ω/cm2)	Rct (Ω/cm2)	Cdl (µF/cm2)	n	ηEIS (%)
0	40.1	1059	37.8	0.96	0
300	14.45	2695	0.235	0.97	61
400	76.98	4636	0.273	0.97	77
500	75.98	6598	0.121	0.97	84

, where a progressive rise in resistance is observed as the inhibitor concentration increases. The absence of a well-defined Warburg impedance indicates that diffusional processes are not dominant, confirming that the cathodic reaction is limited by charge transfer kinetics rather than by mass transport [53].

The Bode phase angle plot (Figure 4b) corroborates the behavior observed in the Nyquist diagrams, displaying an increase in the maximum phase angle and a widening of the frequency range in which the system exhibits capacitive behavior at higher inhibitor concentrations [54]. Although the 300 ppm solution exhibited slightly higher phase angle values over part of the frequency range, the overall corrosion protection cannot be evaluated solely on the basis of the phase angle magnitude. The charge transfer resistance obtained from the impedance fitting increased progressively from 2695 Ω·cm² at 300 ppm to 6598 Ω·cm² at 500 ppm, confirming that the highest corrosion resistance was achieved at the highest inhibitor concentration.

The Bode phase angle plots (Figure 4b) confirm the behavior observed in the Nyquist diagrams through the increase in the maximum phase angle and the widening of the frequency range in which the system exhibits capacitive behavior as the inhibitor concentration increases [54]. Although the 300 ppm solution exhibited slightly higher phase angle values over part of the frequency range, the overall corrosion protection cannot be evaluated solely on the basis of the phase angle magnitude. The charge transfer resistance obtained from the impedance fitting increased progressively from 2695 Ω·cm² at 300 ppm to 6598 Ω·cm² at 500 ppm, indicating that the highest corrosion resistance was achieved at the highest inhibitor concentration.

The Bode impedance modulus plots (Figure 4c) further support this interpretation. The impedance magnitude increased progressively with inhibitor concentration over the entire frequency range, with the highest |Z| values being obtained at 500 ppm. This behavior is indicative of enhanced corrosion resistance and confirms the formation of a protective adsorbed layer on the steel surface [54].

In particular, at 500 ppm, the phase angle reaches values close to −60°, indicating a more capacitive response and a more stable interface. This behavior can be corroborated by the values of the CPE exponent n reported in Table 3, which approach unity (≈0.97), indicating a more homogeneous surface and a well-defined protective film [55].

The effective double-layer capacitance (Cdl) was estimated from the characteristic frequency corresponding to the maximum phase angle according to the following relationship:

$$C_{dl}={\displaystyle \frac{1}{2\pi f_{max}{R}_{ct}}}$$

(11)

where $f_{max}$ is the frequency at the maximum phase angle and $R_{CT}$ is the charge transfer resistance. The values reported in Table 3 therefore correspond to effective double-layer capacitances derived from the impedance spectra and are not identical to the fitted CPE parameters.

As shown in Table 3, the addition of D. fragrans extract produces a marked reduction in Cdl compared with the uninhibited solution. The Cdl value decreases from 37.8 µF cm⁻² in the blank solution to values below 0.3 µF cm⁻² in the presence of the inhibitor, reaching its minimum value of 0.121 µF cm⁻² at 500 ppm. This behavior indicates significant modification of the steel/electrolyte interface due to the adsorption of inhibitor molecules. The reduction in Cdl is commonly associated with an increase in the thickness of the interfacial layer and/or a decrease in the local dielectric constant as water molecules are progressively displaced by adsorbed organic species [56,57]. The lowest Cdl value obtained at 500 ppm suggests the formation of a more compact and protective adsorbed film, which effectively hinders charge transfer processes and enhances corrosion resistance [58].

The increase in inhibition efficiency with concentration, observed in both polarization and EIS measurements, suggests a progressive adsorption process on the steel surface. At low concentrations (300 ppm), the surface coverage is partial, whereas at 500 ppm a more compact protective layer is formed. This is reflected by the increase in Rct, the decrease in icorr, and the lower Cdl values. These results are consistent with the Frumkin adsorption isotherm obtained in this study, which provided the best fit among the adsorption models evaluated [59,60].

According to the results obtained from EIS, a modified Randles-type equivalent circuit is proposed for the behavior of SAE 1025 steel in seawater in the presence of low concentrations of D. fragrans extract. The circuit consists of the solution resistance (Rs) in series with a parallel arrangement formed by the charge transfer resistance (Rct) and a constant phase element (CPE), as represented in Figure 5:

This model is widely used to describe corrosion systems where the dominant process is charge transfer at the metal and electrolyte interface [61,62]. The presence of the CPE, instead of an ideal capacitor, is justified by the deviation from ideal capacitive behavior observed in the Bode diagrams, where the phase angle does not reach −90°. This behavior is associated with the heterogeneity of the metallic surface, surface roughness, the non-uniform distribution of active sites, and the adsorption of the inhibitor onto the surface [63,64,65].

Although broadening of the phase angle peak is observed at higher inhibitor concentrations, the Bode plots do not show a clearly resolved second maximum that would unequivocally indicate the presence of a separate time constant. Therefore, a single modified Randles equivalent circuit was considered sufficient to describe the electrochemical behavior of the system over the entire concentration range investigated.

Consequently, no additional film resistance or film capacitance elements were included in the fitting procedure, since the experimental spectra can be adequately interpreted using a single charge transfer process associated with inhibitor adsorption at the steel/electrolyte interface.

3.5. Total Polyphenol and Flavonoid Content Determination

Phytochemical analysis of the methanolic D. fragrans extract revealed a total phenolic content of 48.27 ± 3.82 mg GAE/g extract and a total flavonoid content of 2.74 ± 0.80 mg ChE/g extract. The marked richness in phenolic compounds suggests that they play a predominant role in the corrosion inhibition efficiency observed for SAE 1025 steel. The molecular structure of phenols is characterized by multiple hydroxyl (-OH) groups, which act as active adsorption centers capable of interacting with the metal surface to form a protective barrier. This aligns with previous studies where plant extracts rich in phenolic compounds have been successfully used as green corrosion inhibitors for metals in aggressive environments [66,67].

3.6. Corrosion Mechanism Proposal

The electrochemical results suggest that Dracaena fragrans extract acts as a mixed-type corrosion inhibitor for SAE 1025 steel in a saline medium, simultaneously shifting both anodic iron dissolution and cathodic oxygen reduction reactions. This behavior can be attributed to the adsorption of secondary metabolites from the plant extract onto the metal surface, particularly flavonoids, phenolic compounds, alkaloids, and saponins. These phytochemicals contain heteroatoms (such as oxygen and nitrogen) and electron-rich $\pi$ aromatic systems capable of interacting with the metal surface [68]. In the absence of the inhibitor, SAE 1025 steel exposed to the saline medium undergoes anodic iron oxidation:

$$Fe\to {Fe}^{2+}+2e^{-}$$

(12)

Meanwhile, the predominant cathodic reaction corresponds to the reduction of dissolved oxygen:

$$O_2+2H_{2}O+4e^{-}\to 4O{H}^{-}$$

(13)

The presence of chloride ions promotes the destabilization of the surface passive film and accelerates charge transfer at the metal/electrolyte interface, accelerating the formation of corrosion products such as iron hydroxides and oxyhydroxides [69,70]. Conversely, the high inhibition efficiency observed for D. fragrans suggests that the phytochemical compounds adsorb onto the metal surface through a mechanism initially dominated by physisorption. In saline media, the prior adsorption of ${Cl}^{-}$ ions onto the steel surface enhances the electrostatic attraction of protonated organic species from the plant extract, thereby displacing adsorbed water molecules and promoting the formation of a protective surface film:

$${Org}_{\left(sol\right)}+x{H}_2{O}_{\left(ads\right)}\to {Org}_{\left(ads\right)}+x{H}_2{O}_{\left(sol\right)}$$

(14)

This behavior is consistent with the moderately negative adsorption free energy values reported for green inhibitors, which indicate a predominance of physical interactions associated with electrostatic forces and hydrogen bonding. However, in addition to physical adsorption, certain phenolic metabolites and alkaloids are likely to undergo partial chemisorption. This occurs through the interaction of lone electron pairs from oxygen and nitrogen atoms with the vacant d orbitals of the surface iron. Specifically, flavonoids and phenolic compounds possess aromatic hydroxyl groups capable of forming Fe-O surface complexes, whereas alkaloids can interact via Fe-N coordinate bonds [71]. These interactions can be represented as follows:

$${Fe}^{2+}+ :O-R \to Fe(O-R{)}_{\left(ads\right)}$$

(15)

$${Fe}^{2+}+:N-R \to Fe(N-R{)}_{\left(ads\right)}$$

(16)

Additionally, the conjugated aromatic systems in flavonoids and tannins can interact with the vacant d-orbitals of iron through $\pi$-d interactions, increasing the stability of the adsorbed film and blocking active corrosion sites [72]. The formation of this compact organic film restricts the access of aggressive species such as ${Cl}^{-}$ and $O_2$ to the metal surface, thereby reducing both the iron dissolution rate and the cathodic oxygen reduction reaction. Consequently, the charge transfer resistance increases while the double-layer capacitance decreases, a behavior characteristic of adsorption-type inhibitors on metal surfaces [73].

Therefore, it is proposed that the inhibition mechanism of Dracaena fragrans occurs through a combined process of dominant physisorption and partial chemisorption of phenolic and nitrogenous compounds, followed by the formation of a hydrophobic and electrochemically stable protective film. This adsorbed layer acts as a physical and electronic barrier, simultaneously mitigating the anodic and cathodic reactions responsible for the corrosion process in saline environments.

3.7. Toxicity Test

The results of the toxic effect analysis of D. fragrans extract at different concentrations, evaluated using L. sativa seeds, are presented in

Table 4. Effect of Dracaena fragrans extract on germination and growth of Lactuca sativa.

Cinh (ppm)	RRG%	RG %	GI
Control	97	92	89.2
200	95.3	83.45	79.3
300	89.2	78.9	70.4
400	82.5	68.5	56.5
500	76.9	45.6	35.1
600	64.3	41.8	26.9

. It can be observed that, as the extract concentration increases, both the germination percentage and relative radicle growth decrease. This behavior suggests that certain compounds present in the D. fragrans extract exert an inhibitory effect on the germination and early seedling development processes. This type of response has also been reported for other plant extracts and has been associated with an effect dependent on the dose or concentration [74].

Figure 6 presents the graph corresponding to the relative germination percentage (RG) and germination index (%GI) of Lactuca sativa seeds exposed to different concentrations of D. fragrans extract. A progressive decrease in the germination index can be observed as the extract concentration increases. The GI decreased from 89.2% in the control treatment to 56.5% at 400 ppm, 35.1% at 500 ppm, and 26.9% at 600 ppm, indicating a concentration-dependent phytotoxic effect of the extract.

However, only at concentrations of 500 ppm were values below 50% recorded. According to toxicological classification criteria based on the median inhibitory concentration (IC₅₀), this behavior suggests that the evaluated extract can be considered moderately toxic, since significant inhibition of the germination process occurs only at high concentrations [75]. Nevertheless, at concentrations effective for corrosion inhibition, the environmental impact is limited, suggesting that the extract may represent a viable alternative to traditional synthetic inhibitors.

3.8. Macroscopic Surface Examination

Figure 7 shows the evolution of the SAE 1025 steel surface after exposure to artificial seawater in the absence and presence of Dracaena fragrans extract at different immersion times (24, 48, and 72 h) and inhibitor concentrations (0, 400, and 500 ppm). The images correspond to macroscopic photographs acquired after the corrosion tests and were used for qualitative visual assessment of the surface condition of the specimens. In the absence of the inhibitor, progressive deterioration of the metallic surface can be observed with increasing exposure time. At 24 h, the surface shows initial signs of corrosion, characterized by the appearance of dispersed corrosion products and slightly roughened areas. At 48 h, a more uniform layer of corrosion products is evident, with yellowish and greenish tones typical of iron oxides and hydroxides formed in chloride-containing media. Finally, at 72 h, the surface exhibits severe deterioration, with heterogeneous deposits, localized attack regions, and possible pit formation. This behavior is consistent with the corrosion of steel in saline media, where chloride ions facilitate the breakdown of the passive layer and promote localized corrosion mechanisms [1,76].

In the presence of 400 ppm of the extract, a significant improvement in the surface integrity of the steel is observed. At 24 h, the surface exhibits a more homogeneous appearance with less evidence of corrosion products, suggesting partial protection. At 48 h, although some corrosion products appear, they are less abundant and more uniformly distributed compared to the system without inhibitors. At 72 h, deposits are still observed but in a lower proportion, indicating that the inhibitor continues to exert a protective effect, although reduced. This behavior suggests the formation of an adsorbed film acting as a barrier between the metal and the aggressive medium, limiting the access of chloride ions to the metallic surface [77].

At a concentration of 500 ppm, the steel surface shows more effective protection. At 24 h, the surface is noticeably smoother and more uniform, with an almost complete absence of visible corrosion products. At 48 h, the surface remains relatively homogeneous, with minimal deposit formation. Even at 72 h, the surface exhibits less deterioration compared to the other conditions, demonstrating the formation of a more stable and compact protective film. This behavior confirms that increasing the inhibitor concentration promotes surface coverage and the formation of a more effective barrier against corrosion.

This phenomenon has been widely reported in systems where organic inhibitors form adsorbed layers that reduce the interaction between the metal and the corrosive medium [78,79]. Although no microscopic characterization was performed in the present study, the macroscopic observations provide visual evidence consistent with the corrosion inhibition behavior determined by gravimetric and electrochemical techniques.

Therefore, the photographs should be interpreted as qualitative indicators of surface condition rather than as microstructural or morphological characterization of the corrosion products. The improvement in surface integrity with increasing inhibitor concentration confirms that the inhibition mechanism is based on the adsorption of organic molecules onto the steel surface.

The progressive degradation observed at prolonged exposure times (72 h) suggests that the protective film may undergo desorption or deterioration processes, which agrees with the decrease in inhibition efficiency observed in the weight loss tests. This behavior is characteristic of green inhibitors, whose stability may be affected by the composition of the medium and the exposure time [80].

4. Conclusions

Dracaena fragrans extract proved to be an effective green corrosion inhibitor for SAE 1025 steel in artificial seawater, achieving inhibition efficiencies of up to 97% through electrochemical techniques. This inhibition mechanism is predominantly governed by a physical adsorption process that forms a protective film on the metallic surface, thereby reducing the interaction between the steel and the corrosive medium. While the effectiveness of the inhibitor increased with concentration, its stability decreased over longer exposure times due to desorption processes occurring in the chloride-rich environment. Ultimately, the strong correlation between gravimetric techniques, electrochemical measurements, and surface analysis validates the reliability of these results, supporting the extract’s potential for industrial and aeronautical applications due to its moderate environmental impact.