Extraction of Vegetable oil from Acrocomia aculeata Kernels for the Development of Corrosion Inhibitors

Abstract

This study presents the extraction of vegetable oil from Acrocomia aculeata kernels. That oil was characterized by infrared spectroscopy and gas chromatographic methods. It was found that oil is composed of medium-fatty methyl esters. The aim is to synthesize imidazoline compounds from A. aculeata oil and evaluate their effectiveness as corrosion inhibitors. Static corrosion experiments were carried out to study the effect of the synthesized imidazolines on the CO₂ corrosion of the AISI-1810 carbon steel. The influence of synthesized imidazolines on the corrosion processes was examined by open circuit potential, linear polarization resistance, potentiodynamic polarization, and electrochemical impedance spectroscopy. The results showed that the corrosion rate decreases from 8.31 to 1.0 mm•y^–1 with 100 ppm A. aculeata imidazoline (IM-AA). However, the inhibition efficiency (IE) was below 90%, which was attributed to the composition of the A. aculeata oil. That oil contains predominantly medium-chain fatty acids, and the resulting imidazolines offer limited corrosion protection due to their shorter hydrocarbon chains. Then, IM-AA was mixed with oleic imidazoline (IM-OL) to study the effect of long-chain imidazolines on the inhibition performance. The results showed a significant reduction in the corrosion rate from 8.31 to 0.66 mm•y^–1, achieving an IE greater than 90%. Moreover, the biodegradability of IM-AA and IM-OL ranged between 28 and 36%, indicating that these compounds can be effectively used in combination for the development of corrosion inhibitors. These findings highlight that long-chain imidazolines enhance corrosion inhibition.

1. Introduction

Carbon dioxide (CO₂) corrosion, also known as sweet corrosion, is a major challenge in enhanced oil recovery (EOR) operations where CO₂ is injected into reservoirs to improve oil displacement and production. When CO₂ dissolves in the formation water or produced fluids, it forms carbonic acid, which subsequently decreases the pH of the aqueous phase and promotes a corrosive environment for carbon steel used in pipelines, well tubing, and surface processing facilities.

The mitigation of CO₂ corrosion requires an integrated approach that combines chemical, material, and operational strategies to prevent or control metal degradation induced by carbonic acid. The most widely implemented methods include the injection of corrosion inhibitors, the use of corrosion-resistant alloys or protective coatings, and the optimization of operating parameters such as temperature, flow rate, and CO₂ partial pressure to minimize corrosion risks and extend the service life of production equipment.

Film corrosion inhibitors are continuously or intermittently injected into production or injection streams, which adsorb onto the metal surface to create a hydrophobic barrier that reduces the diffusion of corrosive species and other aggressive ions to the metal interface. Imidazoline compounds are a class of film-forming corrosion inhibitors, and they are widely used for mitigating CO₂ corrosion in the oil and gas industry. These compounds function by anchoring their polar head groups onto the metal surface while orienting their hydrophobic tails outward, creating a dense and cohesive protective film.

Imidazoline compounds are usually synthesized from fatty acids such as lauric acid, oleic acid, and linoleic acid. − These fatty acids are typically derived from vegetable oils or animal fats. Common sources of fatty acids include olive oil, soybean oil, palm oil, coconut oil, sunflower oil, and canola oil. For example, Yoo et al. reported the synthesis of imidazoline derivatives from palm oil, soybean oil, and castor oil. Corrosion inhibitors derived from vegetable oils are gaining increasing attention due to their inherent biodegradability, low toxicity, and environmental compatibility, positioning them as sustainable alternatives to conventional synthetic inhibitors.

Acrocomia aculeata, commonly known as macaúba palm, coyol palm, or macaw palm, is a tropical species native to Central and South America. The oil extracted from A. aculeata kernels has been used in various applications, including the production of biofuels, cosmetics, and food products. , However, there are no studies on the synthesis of corrosion inhibitors using A. aculeata oil. This oil is rich in fatty acid methyl esters, making it a promising and sustainable feedstock for the development of imidazoline derivatives. Additionally, the fruit of A. aculeata offers several advantages, including renewability and wide availability, which support its potential as an alternative source for eco-friendly corrosion inhibitors.

This study presents the extraction of vegetable oil from A. aculeata kernels as well as the synthesis and characterization of imidazoline compounds. The aim is to synthesize imidazoline compounds from A. aculeata oil and evaluate their effectiveness as corrosion inhibitors. The influence of the synthesized imidazolines on the corrosion processes was examined by open circuit potential, linear polarization resistance, potentiodynamic polarization, and electrochemical impedance spectroscopy. Additionally, the biodegradability of the imidazolines was assessed through biochemical oxygen demand measurements.

2. Materials and Methods

2.1. Preparation of A. aculeata Kernels

A. aculeata fruits were collected from Sierra Mariscal in Chiapas, Mexico. These fruits were then dried until the seeds naturally detached from the pulp. The seeds were manually dehulled to extract the kernels, which were subsequently oven-dried to reduce the moisture content. The dried kernels were finely ground, and the resulting powder was oven-dried at 80 °C for 4 h.

2.2. Vegetable oil Extraction

Vegetable oil was extracted from the kernel powder using the Soxhlet extraction method. Kernel powder (50 g) was packaged in a filter paper cartridge and placed in the thimble chamber of the Soxhlet extractor, and 250 mL of n-hexane was added to the boiling flask. The system was heated to boiling point temperature and maintained for 8 h to complete the extraction. Afterward, the solvent–oil mixture was filtered using Whatman filter paper to remove any solid particles. The solvent was then removed under reduced pressure at 50 °C using a rotary evaporator. The resulting extract was analyzed by Fourier transform infrared spectroscopy (FTIR) and stored in a deep freezer at 5 °C until further use.

2.3. Fatty Acid Synthesis

The fatty acids were obtained through a saponification reaction (a schematic representation is provided in Figure ). The reaction was carried out at 80 °C using 15.5 g of vegetable oil (0.023 mol, $\stackrel{\circledR }{\mathrm{M}\mathrm{n}}$ = 667.3 g•mol^–1) and 100 mL of a 2.5% NaOH solution (2.50 g, 0.062 mol, 40 g•mol^–1). The mixture was stirred continuously while maintaining between 80 and 85 °C for approximately 1.5 h. Afterward, the mixture was cooled to room temperature, and 100 mL of a 2% HCl (2.00 g, 0.055 mol) solution was gradually added; the hydrochloric acid neutralized the soap, leading to the separation of fatty acids as an insoluble organic layer. Then, the neutralized mixture was transferred to a decantation funnel and left to settle for 2 h to ensure the complete separation of the aqueous and organic layers. Finally, the organic compounds were recovered, analyzed by FTIR, and stored for further use or processing.

1.

Synthesis scheme of the saponification reaction.

2.4. Chromatographic Analysis

The composition of fatty acids was determined by gas chromatography using a Perkin–Elmer Clarus 500 system equipped with a TRACE TR-WAX capillary column (60 m × 0.25 mm, 0.25 μm film thickness) and a flame ionization detector (FID). The sample injection (1.0 μL) was performed in the split mode (1:5), and nitrogen (30 psi) was used as the carrier gas. The injector and detector temperatures were set at 250 and 280 °C, respectively. The oven temperature was initially held at 100 °C for 3 min and then increased at a rate of 20 °C·min^–1 up to 280 °C. The samples were diluted with 98.5% HPLC-grade hexane before performing chromatographic analysis. Specie quantification was performed by interpolation with a calibration curve for each analyte.

2.5. Imidazoline Synthesis

The fatty acids obtained from A. aculeata kernel oil (10 g, 0.0476 mol, $\stackrel{\circledR }{\mathrm{M}\mathrm{n}}$ = 209.8 g•mol^–1) were used to synthesize A. aculeata imidazoline (IM-AA), while oleic acid (10 g, 0.035 mol, Mn = 282.46 g•mol^–1) was employed for the synthesis of oleic imidazoline (IM-OL). The IM-AA compound was synthesized through the amidation of fatty acids (step 1), followed by the cyclization of the resulting amides (step 2); a schematic representation of the reactions involved in the imidazoline synthesis is provided in Figure . In the first step, N-(2-hydroxyethyl)­ethylenediamine (5 g, 0.048 mol, Mn = 104.15 g•mol^–1) was reacted with the respective fatty acid (1:1 molar ratio) at 130 °C for 4 h, forming fatty amides and water as a byproduct. The reaction continued at an elevated temperature (190 °C) for another 4 h to promote cyclization. A vapor trap was employed during this last step to remove the remaining water, ensuring the completion of the cyclization process. Finally, the products were characterized by FTIR to confirm the disappearance of the characteristic amide carbonyl absorption bands and the appearance of signals corresponding to the imidazoline ring. The isolated product mass was 11.20 g (yield = 85.1%).

2.

Synthesis scheme of imidazoline synthesis.

2.6. Static Corrosion Test

The test was carried out in a 200 mL double-walled cylindrical glass cell. The cell was equipped with an Ag/AgCl (KCl saturated) reference electrode (RE), a graphite counter electrode (CE), and a cylindrical (2.827 cm² exposed area) working electrode (WE) made of AISI-1018 carbon steel. Moreover, the RE was placed inside a Luggin capillary tube and filled with a KCl solution to eliminate the ohmic drop.

The results were obtained by immersing the WE in 180 mL of the test solution under static conditions at atmospheric pressure and a constant temperature (70 °C) for 18 h. This solution was prepared by bubbling CO₂ gas into an aqueous solution composed of 3.0% NaCl. It is important to highlight that the CO₂ gas was bubbled for 1 h before starting the measures, and the gas flow was maintained throughout to prevent oxygen dissolution in the test solution. Moreover, the pH of the test solution was adjusted (pH = 4) by adding a deoxygenated hydrochloric acid solution (HCl).

The surface of the AISI-1018 sample was polished before each measurement. The surface was polished using SiC sandpaper, changing the grit size from 220 to 2000 to ensure a smooth surface. Afterward, it was rinsed with water and acetone and then ultrasonically cleaned for 10 min to remove residual particles from the metal surface. Finally, it was taken out, dried with hot air, and immersed in the test solution.

Two stock solutions were prepared by dissolving the required amounts of the inhibitor in isopropanol to obtain a concentration of 10,000 ppm. The stock solution of IM-AA was prepared by dissolving 1.0 g of IM-AA in isopropanol until a volume of 100 mL was obtained. The IM-MX solution (composed of a mixture of IM-AA and IM-OL) was prepared to investigate the effect of long-chain imidazolines on the inhibition performance of IM-AA. This solution was prepared by dissolving 0.75 g of IM-AA and 0.25 g of IM-OL in isopropanol until a volume of 100 mL was obtained to achieve the same concentration of 10,000 ppm.

Electrochemical methods such as open potential circuit (OCP), linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization scans (PD) were conducted to evaluate the performance of the synthesized imidazolines. These methods were conducted using a Solartron Modulab-XM potentiostat. The OCP was monitored during immersion in the WE. LPR measurements were carried out at ±5 mV vs the OCP using a scan rate of 0.125 mV•s^–1. EIS measurements were conducted by applying an AC amplitude of 10 mV in the frequency range from 100 kHz to 10 mHz at the OCP. The PD were performed in cathodic (−0.3 V vs OCP) and anodic (0.3 V vs OCP) directions using a scan rate of 0.166 mV•s^–1.

The corrosion test was conducted first by immersion of WE in the test solution. Then, the OCP was immediately recorded until stabilization (∼5 min). Subsequently, electrochemical measurements were carried out in the following sequence: OCP monitoring for 29 min, LPR measurement, OCP monitoring for 25 min, and EIS measurement. This sequence was repeated for 18 h. Finally, the WE was polarized in the cathodic direction, followed by a stabilization period of 30 min, after which anodic polarization was applied. The corrosion tests were conducted at different inhibitor concentrations (0, 25, 50, 75, and 100 ppm), and the inhibitor was added to the test solution during the OCP stabilization stage.

2.7. SEM-EDS Analysis

The metal specimens were rinsed with double-distilled water to remove the residual electrolyte and subsequently dried in an oven at 80 °C for 12 h to eliminate any remaining moisture. The surface morphologies of the specimens were then examined by using a JEOL JSM-7800 scanning electron microscope (SEM). The analysis was conducted in secondary electron mode at an accelerating voltage of 25 kV, with a magnification of 500×, and a working distance range of 11–12 mm. In addition, energy dispersive spectroscopy (EDS) was employed to analyze the elemental distribution on the metal surface.

2.8. Biodegradability Analysis

The determination of biodegradability was conducted in accordance with the ISO10708 method. Four stock solutions were prepared, and their composition is provided in Table . For the preparation of 1 L of mineral nutrient medium, 10 mL of stock solution A and 1 mL of each stock solution B–D are added to approximately 800 mL of distilled water, and the final volume is adjusted to 1 L. The resulting medium has a pH between 7.2 and 7.4, ensuring optimal microbial activity. Prior to inoculation, the medium is aerated for at least 20 min to achieve a dissolved oxygen concentration close to saturation (8–9 mg O₂/L at 25 °C). The inoculum was obtained from a secondary domestic wastewater treatment system. Oxygen consumption was measured by using a HANNA model HI6421 dissolved oxygen meter. The measurements were taken at the beginning of the experiment and at 7, 14, 21, and 28 days of incubation. In this test, sodium acetate (30 ppm) was used as a biodegradable control compound.

1. Composition of the Stock Solutions.

solution	compound	conc. (g•L–1)	solution	compound	conc. (g•L–1)
A	KH2PO4	8.5	B	MgSO4•7H2O	22.5
	K2HPO4	21.75	C	CaCl2	27.5
	Na2HPO4•2H2O	33.4	D	FeCl3•6H2O	0.25
	NH4Cl	0.5

The test was performed in airtight glass bottles with a total volume of 60 mL. Each bottle was filled with mineral nutrient medium and 0.1 mL of a microbial consortium. The imidazoline compounds were tested at a concentration of 30 ppm. Bottles are incubated under static, dark conditions for up to 28 days. Each measurement was performed in duplicate or triplicate bottles to avoid oxygen depletion in the same bottle over time.

The percentage of biodegradability was calculated according to eq :

$$\mathrm{B}\mathrm{D}=\frac{\sum \mathrm{B}}{\mathrm{T}\mathrm{h}\mathrm{O}\mathrm{D}}\times 100$$ 1

where ∑B is the sum of oxygen consumption calculated for each compound, and ThOD is the theoretical oxygen demand obtained by the elemental analysis of each compound.

3. Results and Discussion

3.1. Extracted Oil Characterization

The Soxhlet extraction of the A. aculeata kernel powder using n-hexane resulted in a high-quality oil extract characterized by a clear, light-yellow appearance. The extraction yield was found to be 35% w/w, which is consistent with values reported for A. aculeata kernel oil. Figure shows the infrared spectrum of the oil extracted from the A. aculeata kernel (OL-AA). This spectrum exhibits a prominent absorption band at 1744 cm^–1, corresponding to the CO stretching vibration of carbonyl functional groups; the CO groups are characteristic of ester compounds found in vegetable oils. Additionally, the absorption bands observed at 3008 cm^–1 and 1656 cm^–1 are associated with = C–H stretching and CC stretching vibrations, respectively. Those adsorption bands correspond to the olefinic groups of unsaturated hydrocarbon chains.

3.

Infrared spectrum of oil extracted from part A. aculeata kernel (OL-AA) and A. aculeata fatty acids (FA-AA).

Several papers have reported the composition of the oils extracted from A. aculeata fruit. ,− For example, Del Rio et al. reported that the pulp oil of A. aculeata contains a high proportion (∼70%) of unsaturated triglycerides (palmitoylstearoylolein and triolein, principally), whereas the kernel oil is predominantly composed (∼98.6%) of shorter-chain saturated triglycerides, with only trace amounts of free fatty acids (0.9%), sterols (0.2%), and monoglycerides (0.3%). Likewise, Lieb et al. reported that the mesocarp oil has a substantial proportion of di- and triunsaturated triacylglycerols composed of long-chain fatty acids (C16:0, C18:1n9), whereas the kernel oil presents a high proportion (76.2–77.8%) of saturated triacylglycerols with medium-chain fatty acids, with lauric acid (C12:0) being its main constituent (41.6–42.9%).

The saponification process effectively converted the triglycerides present in A. aculeata kernel oil to their corresponding free fatty acids. The yield of fatty acids obtained from 15.5 g of oil was 12.6 g, corresponding to a percent yield of 86.21%. The fatty acid composition obtained after the saponification of A. aculeata kernel oil is presented in Table . The analysis revealed that the saponification reaction produces decanoic acid (30.61%), dodecanoic acid (26.83%), myristic acid (22.72%), palmitoleic acid (16.58%), and a small proportion of oleic acid (3.21%). This composition confirms that the kernel oil of A. aculeata is rich in medium-chain saturated fatty acids. The saponification of A. aculeata kernel oil has been reported in several studies, aiming to obtain its free fatty acids or soaps for oleochemical applications. For example, Ferrari et al. demonstrated that lauric acid (38.7%) is the predominant fatty acid in the kernel oil. Similarly, Coimbra et al. and del Río et al. (2016) characterized the fatty acid profiles of both pulp and kernel oils, confirming that the kernel fraction exhibits a composition dominated by saturated C10–C14 chains. The differences between the results of this study and those reported in the literature could be attributed to factors such as variations in fruit maturity, geographical origin, extraction methods, and analytical procedures.

2. Fatty Acid Profile of A. aculeata Kernel Oil.

compound		average concertation (wt %)
decanoic acid	C10:0	30.61 ± 0.51
dodecanoic acid	C12:0	26.83 ± 0.56
myristic acid	C14:1	22.72 ± 0.13
palmitoleic acid	C16:1	16.58 ± 0.28
oleic acid	C18:1	3.21 ± 0.12

Figure also shows the infrared spectrum of fatty acids (FA-AA) obtained from the kernel oil. This spectrum shows four absorption bands at 1709 cm^–1 (CO stretching vibration), 1282 cm^–1 (C–O stretching vibration), and 941 cm^–1 (O–H vibration out-of-plane). These absorption bands confirm the formation carboxyl (–COOH) functional groups. Several studies have reported the saponification of vegetable oils and the use of FTIR to confirm the chemical transformation from triglycerides to fatty acid salts or free fatty acids. , The characteristic ester carbonyl (CO) band of triglycerides observed around 1740–1750 cm^–1 disappears after saponification, indicating the cleavage of ester bonds. Concurrently, new absorption bands corresponding to the asymmetric and symmetric stretching vibrations of the carboxylate anion (COO^–) emerge at approximately 1550–1600 cm^–1 and 1400–1450 cm^–1, respectively, confirming the formation of soap (fatty acid salts). When the saponified product is acidified, the FTIR spectrum instead exhibits a strong carbonyl absorption near 1700–1715 cm^–1 and a broad O–H stretch between 2500 and 3300 cm^–1, characteristic of free fatty acids. These spectral changes have been consistently reported for a wide variety of vegetable oils, confirming that FTIR analysis is a reliable tool for verifying saponification reactions and assessing the conversion of oils into soaps or fatty acids.

3.2. Imidazoline Characterization

The infrared spectra of IM-AA and IM-OL are shown in Figure . These spectra show absorption bands at 1602 cm^–1 (CN stretching vibration in the imidazoline ring), 1556 cm^–1 (NC–N stretching vibration in the imidazoline ring), 1453 cm^–1 (N–H bending vibration), and 1267 cm^–1 (C–N stretching aliphatic). The synthesis of imidazolines and their derivatives also have been reported by other authors. , For example, Zheng et al. synthesized a mercapto-oleic imidazoline from the reaction between mercaptopropionic acid and oleic imidazoline. They identified absorption peaks at 1655–1652 cm^–1 corresponding to the CN bond of the imidazoline ring. Likewise, Zhang et al. reported the synthesis of symmetric stearic acid imidazolines and observed peaks at 1647 cm^–1 (stretching vibration of CN bond) and 1552–1544 cm^–1 (stretching vibration of C–N bond), confirming the formation of the imidazoline ring. The FTIR spectra confirmed the successful synthesis and high purity (≈98%) of the IM-AA and IM-OL compounds, evidenced by the presence of characteristic absorption bands corresponding to imidazoline ring vibrations.

4.

Infrared spectrum of A. aculeata imidazoline (IM-AA) and oleic imidazoline (IM-OL).

3.3. Inhibition Performance

E_OCP measurements are essential in corrosion analysis because they provide valuable insight into the electrochemical behavior and stability of metals in specific environments. Figure a shows the E _OCP variation of WE immersed in test solutions containing different concentrations of the IM-AA compound. The E _OCP trend in the reference curve shows a shift toward more positive values, which may be attributed to the accumulation of corrosion products on the WE surface. The E _OCP values also increase toward more positive values when the IM-AA compound is added to the test solution. This behavior may be associated with a change in the solution chemistry and/or the formation of a protective film on the WE surface.

5.

Interfacial behavior of the AISI-1018 electrode immersed in the test solution using different concentrations of the IM-AA compound: (a) E _OCP and (b) polarization resistance.

LPR is an electrochemical technique used to measure the polarization resistance (Rp) of a metal surface immersed in a corrosive solution. This resistance is inversely proportional to the corrosion rate, making the LPR a powerful tool for real-time corrosion monitoring. Figure b shows the Rp variation of WE immersed in test solutions containing different concentrations of the IM-AA compound. The Rp value in the absence of the inhibitor decreased slightly (from ∼25.75 Ω•cm² to ∼21.51 Ω•cm²) after 18 h of immersion. In contrast, the Rp values tend to increase with immersion time in the presence of IM-AA. For instance, the Rp value increases from ∼23.81 Ω•cm² to ∼106.04 Ω•cm² when the concentration is 25 ppm. This behavior is associated with the gradual adsorption of imidazoline molecules on the WE surface, which promotes the development of a stable and protective film. On the other hand, the Rp values also increase as the concentration of IM-AA increases, achieving its higher value (∼306.2 Ω•cm²) with 100 ppm after 18 h of immersion. This behavior may be attributed to the increased surface coverage as the IM-AA concentration rises, which decreases the number of active corrosion sites and improves the integrity of the protective film.

The inhibiting action of imidazoline compounds is commonly attributed to their adsorption on the metal surface, where they form a protective film that interferes with the redox reactions involved in corrosion. The adsorption primarily occurs through interaction between the nitrogen atoms of the imidazoline ring and the charged metal surface. Once adsorbed, the molecules form a dense protective layer that prevents direct contact between the corrosive medium and the metal surface. Additionally, the long hydrocarbon tail of the imidazoline molecule provides a hydrophobic barrier that further repels water and corrosive species. ,

Potentiodynamic polarization curves (PPC) for WE immersed in the test solution with and without IM-AA are shown in Figure . In the absence of IM-AA (blank curve), the anodic current density increases as the applied potential becomes more positive, indicating active dissolution behavior. Similarly, the cathodic current density increases slightly with a more negative potential. The polarization of carbon steel immersed in CO₂-satuded brines has been studied and reported. , For example, Liu et al. studied the effect of chloride ions (Cl^–) on the polarization of carbon steel in CO₂-saturated solutions. They found that the anodic current increases with increasing Cl^– content. Likewise, Chen et al. studied the influence of CO₂ partial pressure on the corrosion of carbon steel in 3.5% NaCl at 120 °C. They observed an active-dissolution behavior during anodic polarization, as well as a slight increase of anodic current densities with the increase of CO₂ partial pressure.

6.

Polarization curves of the AISI-1018 electrode immersed in the test solution using different concentrations of the IM-AA compound.

The current densities obtained in the presence of the IM-AA compound are lower than those without the inhibitor. Moreover, the cathodic branches remain almost parallel to each other, suggesting that the IM-AA compound does not alter the mechanism of the hydrogen evolution reaction (HER). In contrast, the anodic branches shift to lower current densities with increasing IM-AA concentration. This behavior suggests that more IM-AA molecules are adsorbed on the anodic sites of the WE, leading to a reduction in the metal dissolution rate as the concentration of IM-AA increases. The influence of imidazoline compounds on the anodic polarization behavior of carbon steel has been previously studied and is well documented in the literature. For example, Fan et al. found that the imidazoline molecules inhibit anodic reactions by absorbing on the metal surface and forming a protective layer. Peng et al. made a similar observation, which suggests that the imidazoline molecules inhibit anodic reactions by blocking the anodic active sites.

The electrochemical parameters obtained from PPC are listed in Table . In the absence of the inhibitor, the Tafel slopes are 0.059 mV•dec^–1 and −0.183 mV•dec^–1 for the anodic (βa) and cathodic (βc) reactions, respectively. The β_a value obtained in the presence of IM-AA is higher than that recorded in its absence, indicating that the imidazoline molecules reduce the rate of the anodic reaction. Additionally, the corrosion potential (E _corr) values were nearly identical to that of the uninhibited solution, suggesting that IM-AA molecules act as a mixed-type inhibitor. On the other hand, the current density of corrosion (i _corr) value calculated in the presence of IM-AA is lower than that recorded in its absence. Moreover, the i _corr values decrease further as the IM-AA concentration increases from 25 to 100 ppm. The progressive reduction in i _corr across the tested concentrations confirms that the inhibition efficiency increases as the inhibitor concentration rises, which can be attributed to the greater surface coverage of the metal by the adsorbed imidazoline molecules.

3. Electrochemical Parameters from Polarization Curves for the AISI-1018 C-Steel Electrode Immersed in Test Solutions Using Different IM-AA Concentrations, after 18 h of Immersion.

Concentration	E corr	βa	βc	i corr	CR	IE
ppm	mV (Ag/AgCl)	mV•dec–1	mV•dec–1	μA•cm–2	mm•y–1	%
blank	–661 ± 33.4	59	–183	a 715 ± 23.7	8.31	–
25	–663 ± 22.1	101	–277	b 307 ± 21.7	3.57	57.07
50	–662 ± 26.9	124	–261	c 208 ± 17.9	2.42	70.91
75	–656 ± 18.8	114	–196	d 131 ± 19.4	1.52	81.68
100	–651 ± 23.7	95	–162	e 86.2 ± 11.3	1.00	87.95

The i _corr data of Table were subjected to the analysis of variance, and the results were presented by regressions and means according to Tukey’s test at 5% probability. This analysis revealed a significant influence of inhibitor concentration on i _corr. The model sum of squares (763,355.62) was notably higher than the error sum of squares (601.04), indicating that differences between treatments explain most of the experimental variability. The F-statistic (3175.14, p < 0.0001) confirms significant differences among inhibitor concentrations, demonstrating its direct effect on corrosion reduction. These results validate the inhibitor’s effectiveness and the goodness of fit of the applied statistical model.

Tukey’s post hoc analysis identified the concentrations that exhibited statistically significant differences. The results indicated that all groups differed significantly (p < 0.05), as reflected by the assignment of different letters (a, b, c, d, e) to each concentration (see Table ). The blank (group a) showed the highest i _corr value of 715 μA•cm^–2, whereas 100 ppm (group e) exhibited the lowest i _corr of 86.2 μA•cm^–2. The sequence of letters and corresponding averages confirms a clear trend of decreasing i _corr with increasing inhibitor concentration, demonstrating a progressive protective effect of the inhibitor.

The corrosion rate (CR) was calculated from the i _corr values using eq .

$$\mathrm{C}\mathrm{R}=K\frac{i_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}}{D}\mathrm{E}\mathrm{W}$$ 2

In this equation, K is a conversion factor (3.27 × 10^–3 mm•g•μA^–1•cm^–1•y^–1), EW is the equivalent weight of the material (27.92 g•eq^–1), and D is the metal density (7.86 g•cm^–3). On the other hand, the inhibition efficiency (IE) was calculated from the corrosion rate values, using eq .

$$\mathrm{I}\mathrm{E}={\frac{\mathrm{C}{\mathrm{R}}^{\mathrm{o}}-\mathrm{C}\mathrm{R}}{\mathrm{C}{\mathrm{R}}^{\mathrm{o}}}}^{*}\times 100$$ 3

where CR° and CR* are the corrosion rates obtained without and with the inhibitor, respectively.

The CR obtained in the presence of IM-AA is lower than that without an inhibitor. Moreover, the CR value decreases with increasing IM-AA concentration. The IE also improved with higher IM concentrations, achieving a maximum of 87.95% at 100 ppm. However, the inhibition efficiency of imidazoline-based corrosion inhibitors typically ranges from 90% to 95%, depending on the specific compound, concentration, and environmental conditions. For instance, a study by G. Huang et al. reported that a novel imidazoline derivative achieved an inhibition efficiency of 98.54% at 300 ppm in a 1 M HCl solution at 30 °C. Similarly, S.Y. Peng et al. found that 2,2′-(carbonothioyldisulfanediyl)­diacetic acid-based oleic imidazoline provided 99.67% inhibition efficiency for carbon steel in supercritical CO₂ environments. Other studies have observed slightly lower efficiencies. For example, Xiong et al. found that the inhibition efficiencies of imidazoline derivatives for Q235 carbon steel were 95.95% and 90.69%, as determined by electrochemical impedance spectroscopy and potentiodynamic polarization, respectively.

3.4. Effect of Oleic Imidazoline

Oleic imidazoline is a type of unsaturated imidazoline synthesized from oleic acid. This imidazoline and its derivatives function as corrosion inhibitors by adsorbing on metal surfaces and forming a protective hydrophobic film that limits contact with corrosive agents. For instance, Farelas et al. studied the inhibition performance of two oleic-based imidazolines: 1-(2-hidroxyethyl)-2­(heptadec-8-enyl)-imidazoline and 1-(2-aminoethyl)-2­(heptadec-8-enyl)-bis-imidazoline. The study showed that both compounds provided inhibition efficiencies of >90%, which was attributed to their ability to form a persistent and protective film on the metal surface. Oleic imidazoline has also been used in combination with other inhibitors. For example, Zhao et al. investigated the synergistic effect between the oleic imidazoline derivative and sodium benzoate. They found that the combined use of these compounds can protect mild steel from CO₂ corrosion more efficiently than either inhibitor alone. Zhu et al. studied the synergistic effect between the oleic imidazoline derivative (OIM) and mercaptoethanol (ME). They found that the corrosion inhibition efficiency rises 96.56% when the ratio of the OIM and ME is 3:1, which is much higher than that of the individual inhibitors.

In order to study the effect of long-chain imidazolines on the inhibition performance of IM-AA, an imidazoline mixture (IM-MX) was prepared (75% IM-AA and 25% IM-OL). Figure a shows the E _OCP variation of WE immersed in test solutions containing different concentrations of the IM-MX compound. It is evident that the addition of IM-MX to the corrosive medium causes a shift in E _OCP toward more positive potentials. Moreover, the E _OCP values for the IM-MX samples are less negative than those of the blank curve, suggesting that the mixture enhances the electrochemical stability of the metal surface. Likewise, this shift indicates a possible reduction in the corrosion tendency.

7.

Interfacial behavior of the AISI-1018 electrode immersed in the test solution using different concentrations of the IM-MX compound: (a) E _OCP and (b) polarization resistance.

Figure b shows the variation of Rp with different concentrations of IM-MX. The results show that the Rp values obtained in the presence of IM-MX are higher than those of the blank. Moreover, the Rp values of IM-MX curves increase with the immersion time. For instance, the Rp value increases from ∼24 to ∼177 Ω•cm² when the concentration is 25 ppm. suggesting the progressive formation of a protective film on the WE surface as immersion time increases. The Rp values also increase with the concentration, reaching a higher value (∼384 Ω•cm²) with 75 ppm after 18 h of immersion. On the other hand, the Rp values obtained in the presence of 100 ppm IM-AA (see Figure b) are lower than those achieved with 75 ppm IM-MX (see Figure b). This suggests that IM-MX forms a more effective protective film at 75 ppm compared to IM-AA alone.

The PPC for WE immersed in the test solutions with and without the IM-MX compound is shown in Figure . The results show that the current densities obtained in the presence of IM-MX are lower than those without the inhibitor. Moreover, the cathodic branches slightly shift toward lower current densities with increasing IM-MX concentration, suggesting a decrease in the rate of the HER. Similarly, the anodic current densities also decrease as the IM-MX concentration increases, indicating that an increase in concentration decreases the rate of anodic dissolution. Additionally, the anodic branches show an infection point at potentials more positive than E _corr. This inflection point may be attributed to the partial desorption of IM-MX molecules from the metal surface during anodic polarization.

8.

Polarization curves of the AISI-1018 electrode immersed in the test solution using different concentrations of the IM-MX compound.

The electrochemical parameters corresponding to the polarization curves are given in Table . It can be observed that βa is primarily affected by the presence of the IM-MX compound. Moreover, the E _corr obtained in the presence of IM-MX is less negative than that of the blank, suggesting that IM-MX acts as an anodic-type inhibitor. On the other hand, the CR is lower in the presence of IM-MX than in its absence (without the inhibitor), and it further decreases with increasing IM-MX concentration. Moreover, the IE reached 92% when the IM-MX concentration was increased to 75 ppm. The increase in the IE with increasing IM-MX concentration can be attributed to the progressive adsorption of the imidazoline molecules on the metal surface, which leads to greater surface coverage and minimizes the interaction between the aggressive solution and the metal substrate.

4. Electrochemical Parameters from Polarization Curves for the AISI-1018 C-Steel Electrode Immersed in Test Solutions Using Different IM-MX Concentrations, after 18h of Immersion.

Concentration	E corr	βa	βc	i corr	CR	IE
ppm	Volts (Ag/AgCl)	Volts•dec–1	Volts•dec–1	μA•cm–2	mm•y–1	%
blank	–661 ± 33.4	59	–183	a 715 ± 23.7	8.31	–
25	–664 ± 23.7	91	–183	b 146 ± 20.3	1.70	79.57
50	–628 ± 17.9	70	–212	c 81.3 ± 17.0	0.94	88.63
75	–621 ± 22.1	78	–143	d 57.2 ± 13.2	0.66	92.00

The i _corr data of Table were subjected to the analysis of variance, and the results were presented by regressions and means according to Tukey’s test at 5%. This analysis revealed a clear influence of inhibitor concentration on i _corr. The model sum of squares (878,017.40) was substantially greater than the error sum of squares (467.78), indicating that variations in concentration accounted for nearly all of the experimental variability. The F-statistic (5005.30, p < 0.0001) confirmed the existence of significant differences among treatments, demonstrating the strong influence of increasing inhibitor concentration on reducing i _corr. These findings confirm the effectiveness of the inhibitor and indicate that the applied statistical model provided an excellent fit, reflecting high consistency among the experimental data.

Tukey’s posthoc analysis identified statistically significant differences among the tested concentrations. All concentrations differed significantly (p < 0.05), as indicated by the assignment of distinct letters (a, b, c, d) to each group (see Table ). The blank (group a) exhibited the highest i _corr of 715 μA•cm^–2, whereas 75 ppm (group d) showed the lowest value of 57.2 μA•cm^–2. The sequence of letters and mean values confirms a consistent and progressive decrease in i _corr with increasing inhibitor concentration, demonstrating a concentration-dependent and efficient protective effect.

Figure shows the PPC obtained in the absence and presence of IM-AA and IM-MX compounds. The values of i _corr are 7.15 × 10^–4 A•cm^–2 in the blank solution, 8.62 × 10^–5 A•cm^–2 in the solution containing 100 ppm IM-AA, and 5.72 × 10^–5 A•cm^–2 in the solution containing 75 ppm IM-MX. These results indicate that IM-MX is more effective in reducing the corrosion rate than IM-AA, even at a lower concentration. The lower i _corr value observed in the presence of IM-MX suggests that it inhibits both anodic and cathodic reactions more efficiently. This improved performance can be attributed to the strong adsorption of IM-OL molecules on the metal surface, effectively blocking active sites and hindering both metal dissolution and hydrogen evolution. Therefore, IM-MX exhibits a higher inhibition efficiency compared with IM-AA under the tested conditions.

9.

Polarization curves of the AISI-1018 electrode using different inhibitors.

The Nyquist and Bode plots of the EIS, obtained with 100 ppm IM-MX are shown in Figure . Nyquist plots (Figure a) show two overlapped depressed semicircles, indicating nonideal electrochemical behavior at the interface between the WE surface and the electrolyte. Moreover, the diameter of these semicircles increases progressively with immersion time, suggesting the gradual adsorption of imidazoline molecules on the surface of WE. The |Z| modulus plot (Figure b) shows that the presence of IM-AA increases the impedance magnitude with time. Additionally, the Bode phase angle plot shows two overlapping peaks in the middle-frequency regions, indicating the presence of two distinct time constants. The second time constant is attributed to the formation of a self-assembled inhibitor layer on the WE surface.

10.

EIS plots of the AISI-1018 electrode immersed in the test solution using 75 ppm IM-MX compound: (a) Nyquist diagram and (b) Bode diagram.

The equivalent circuits depicted in Figure a were used to fit the experimental spectra: Rs is the resistance of the solution; Y _dl is a constant phase element (CPE) representing the double-layer capacitance; R _ct is the charge transfer resistance; Y _fm is a constant phase element (CPE) representing the capacitance of the inhibitor film; and R _po corresponds to the pore resistance of the protective layer formed by the inhibitor. The impedance of the CPE is defined in eq as

$$Z_{\mathrm{C}\mathrm{P}\mathrm{E}}=Y_{\mathrm{o}}^{-1}{(j\omega )}^{-n}$$ 4

In this equation, Z _CPE is the impedance of the CPE, Y _o is a factor proportional to the capacitance, j is $\sqrt{-1}$ , and ω is the angular frequency. The depletion degree of the impedance loops depends on the value of n, which can take values from 0 to 1. The capacitance (C _i) can be calculated from Y _o and R, with eq .

$$C_i={(Y_{\mathrm{o}i}{R}_i^{1-n_i})}^{1/n_i}$$ 5

The parameters derived from the fitting are summarized in Table . It was found that the R _ct increased from 245 to 347 Ω•cm² with 100 ppm IM-MX. In addition, the capacity of the double layer was reduced from 9.54 × 10^–4 to 4.11 × 10^–4 μF•cm^–2. This behavior can be attributed to the progressive adsorption of IM-MX molecules on the WE surface. On the other hand, the R _po increases from 24 to 38 Ω•cm² indicating that the IM-MX molecules form a compact and less porous protective film.

5. EIS Parameters were Obtained by Fitting Experimental Results for 75 ppm IM-MX.

		CPEfm				CPEdl
time (h)	R s (Ω•cm2)	Y fm (Ω–1•sn•cm2)	n	C fm (μF•cm–2)	R po (Ω•cm2)	Y dl (Ω–1•sn•cm2)	n	C dl (μF•cm–2)	R ct (Ω•cm2)	x 2 × 10–3
3	1.02	–	–	–	–	9.54 × 10–4	0.76	604	245	2.25
6	1.11	–	–	–	–	8.26 × 10–4	0.78	570	318	3.34
9	1.2	–	–	–	–	7.33 × 10–4	0.79	510	345	3.12
12	1.28	6.52 × 10–4	0.81	248	24	5.09 × 10–4	0.85	376	336	5.25
15	1.32	6.38 × 10–4	0.80	253	29	4.86 × 10–4	0.87	373	341	4.12
18	1.33	6.21 × 10–4	0.81	261	38	4.11 × 10–4	0.89	322	347	5.41

3.5. Morphological Analysis

The SEM micrographs of carbon steel surfaces are shown in Figure . The surface of corrode sample (see Figure a) presents a porous-like morphology, which is attributed to the preferential dissolution of the ferrite phase. − Farelas et al. studied the corrosion of carbon steel in CO₂-saturated brines. They found that ferrite undergoes preferential dissolution because it is more active than pearlite or cementite (Fe₃C). The elemental analysis reveals the presence of alloying elements such as carbon (C) and iron (Fe), which corresponds to the Fe₃C phase.

11.

SEM-EDS analysis of the AISI-1018 electrode after 18 h of immersion in the test solution: (a) without the inhibitor, (b) with 100 ppm IM-AA, and (c) with 75 ppm IM-MX.

The damage to the carbon steel surface was significantly reduced in the presence of inhibitors (see Figure b,c). The images clearly demonstrate that IM-AA and IM-MX inhibit the CO₂ corrosion of carbon steel. The inhibition effect is attributed to the formation of a protective film on the metal surface, which reduces the level of contact between the steel and the corrosive solution. The elemental analysis revealed the presence of nitrogen (N) and oxygen (O) on the metal surface. These elements are attributed to the imidazoline molecules and confirm the formation of a protective layer on the metal surface.

3.6. Biodegradability Analysis

Biochemical oxygen demand tests are widely applied in the environmental impact assessment of industrial effluents, wastewater, and chemical substances, as it provides a reliable measure of the oxygen demand associated with microbial degradation of organic matter. This parameter serves as a key indicator of the potential pollution load and the biodegradability of chemical compounds in aquatic environments. Figure shows the oxygen consumption during the biodegradation of sodium acetate, A. aculeata imidazoline (IM-AA), and oleic imidazoline (IM-OL). Sodium acetate was used as a biodegradable control compound. This compound exhibited the greatest oxygen consumption after day 7. In contrast, the two imidazoline samples (IM-AA and IM-OL) showed less pronounced oxygen consumption. Nevertheless, both compounds reached high levels of oxygen consumption by the end of the test on day 28.

12.

Oxygen consumption by microorganisms in the stabilized consortium during the biodegradation of sodium acetate, IM-AA, and IM-OL compound.

The elemental analysis data to calculate theoretical oxygen demand (ThOD) and the percentage of biodegradability (BD) are provided in Table . The results show that sodium acetate achieved the highest percentage of biodegradability (>90%), followed in descending order by IM-AA (35.97%) and finally by the IM-OL compound (28.48%). Sodium acetate exhibited the highest biodegradability percentage, confirming its role as the reference biodegradable compound. On the other hand, the biodegradability of IM-OL was lower than that of the IM-AA compound. The IM-OL sample exhibited high oxygen consumption during aerobic degradation (1.43 ppm) and a high theoretical oxygen demand (5.02 ppm), resulting in a relatively low biodegradability percentage of 28.48%. In contrast, IM-AA showed lower oxygen consumption (1.36 ppm) and a lower theoretical oxygen demand (3.78 ppm), leading to a higher biodegradability percentage of 35.97%.

6. Theoretical Oxygen Demand of the Evaluated Compounds and the Result of the Biodegradability Percentage Calculation.

	atomic constituents
compound	C	H	N	Na	O	ThOD (ppm)	∑B (ppm)	BD (%)
sodium acetate	2	3	0	1	2	1.40	1.71	>90%
IM-AA	17	32	2	0	1	3.78	1.36	35.97
IM-OL	22	42	2	0	1	5.02	1.43	28.48

Longer carbon chains correspond to higher theoretical oxygen demands, as a greater amount of oxygen is required by microbial consortia to fully degrade the compound. Each additional methylene (–CH₂–) group increases the degree of reduction, thereby increasing the oxygen needed for microbial mineralization to CO₂ and H₂O. , The degradation of organic compounds generally involves oxygenase enzymes, such as monooxygenases and dioxygenases, which incorporate oxygen atoms into the substrate during the initial oxidation steps, further contributing to oxygen consumption. Therefore, compounds with longer alkyl chains, such as IM-OL, exhibit higher theoretical oxygen demands and consequently lower biodegradability percentages compared with those containing shorter chains, such as IM-AA. On the other hand, long-chain organic compounds tend to be more hydrophobic and less soluble, which limits their bioavailability and slows down biodegradation processes. Then, the biodegradability of IM-AA can be attributed to its relatively medium carbon chain length (C10–C14). This structural characteristic facilitates greater solubility and interaction with aqueous microorganisms, improving the microbial accessibility for enzymatic attack.

Reported data indicate that the biodegradation percentage of imidazoline-based corrosion inhibitors typically ranges between 20% and 40% after 28 days. This relatively low biodegradation is primarily attributed to the hydrophobic nature of imidazoline molecules. The long hydrophobic alkyl chains of these compounds limit their dispersion in water, reducing their availability to aquatic microorganisms and consequently decreasing their biodegradation under typical test conditions. , However, studies have shown that under modified test conditions such as structure modification, extended incubation periods, inclusion of dispersants, or use of adapted microbial consortia, the biodegradability of imidazolines can increase markedly as hydrolysis products become available for microbial oxidation. −

3.7. Potential Application

The oil extracted from A. aculeata is considered to be one of its main products and a key driver for market entry. The kernel oil is rich in medium-chain saturated fatty acids such as decanoic, lauric acid, and myristic acids, making it suitable for oleochemical applications. This oil can be processed into biofuels, lubricants, surfactants, and biodegradable chemicals, including imidazoline-based corrosion inhibitors, offering environmentally friendly alternatives to petroleum-derived products. Additionally, the relatively high oil content (30–35%) ensures economically attractive extraction yields, encouraging the development of commercial plantations and industrial-scale processing. The combination of renewable origin, chemical versatility, and sustainability appeal positions A. aculeata oil as a strategic product for entering emerging markets focused on green chemistry, renewable feedstocks, and biobased industrial materials.

The extraction of oil from A. aculeate fruit has been the subject of many publications. ,, Researchers have investigated several extraction methods, including mechanical pressing, solvent extraction, supercritical CO₂ extraction, and aqueous enzymatic extraction, with each offering distinct advantages in terms of yield, efficiency, and oil quality. Mechanical pressing remains a simple and cost-effective technique, while solvent extraction achieves higher recovery rates but poses environmental and safety concerns. In contrast, supercritical and enzymatic methods have gained increasing attention as green technologies capable of preserving bioactive compounds and reducing solvent use. Nevertheless, the literature reveals a clear need for techno-economic and life-cycle assessments to identify cost-effective and sustainable extraction routes for A. aculeate oil, particularly within integrated biorefinery frameworks that can valorize all components of the fruit.

The production of biodiesel from A. aculeata oil has been widely explored in recent years. ,, Studies have optimized diverse transesterification routesincluding alkaline, acid, enzymatic, and supercritical processesto achieve high conversion efficiencies and biodiesel quality that meets international standards. , The researchers have also explored the integration of A. aculeata oil into biorefinery systems, where multiple components of the fruit (oil, kernel cake, and fibrous residues) are utilized for biofuel, animal feed, and biochemical production. Biodiesel synthesis continues to be the most investigated and industrially significant application, emphasizing the potential of this species as a sustainable source of nonedible oil for renewable energy production. In contrast, the use of A. aculeata oil as a raw material for the synthesis of corrosion inhibitors is an area that remains underexplored.

The synthesis and application of corrosion inhibitors derived from vegetable extracts have been extensively studied and well documented in the literature. , Numerous investigations have demonstrated that plant-based inhibitors can effectively adsorb onto metal surfaces, forming protective films that reduce the interaction between the metal and the corrosive environment. These films act as barriers, thereby minimizing reactions that are responsible for corrosion. Research in this area has focused on optimizing extraction techniques to obtain bioactive compounds, identifying the phytochemical constituents responsible for inhibition, and elucidating the adsorption mechanisms governing their protective behavior. The growing interest in vegetable-derived inhibitors is driven by their biodegradability, low toxicity, renewability, and environmental compatibility, making them promising alternatives to conventional synthetic inhibitors in various industrial applications.

The results presented in this work show that the oil extracted from A. aculeata is composed of fatty acid methyl esters (FAMEs). Moreover, these compounds can be converted into their corresponding fatty acids through saponification. On the other hand, the fatty acids contain carboxylic functional groups that can react with polyamines to form imidazoline compounds. Imidazoline-based inhibitors are widely employed in the petroleum industry for CO₂ corrosion control due to their ability to form stable and adherent protective films on metal surfaces. The results of the corrosion tests demonstrated that the CO₂ corrosion of AISI-1018 steel decreases in the presence of IM-AA. The corrosion rate was reduced from 8.31 to 1.0 mm•y^–1 with the addition of 100 ppm IM-AA, achieving a maximum inhibition efficiency of 87.95%. However, the IE values reported in the literature for imidazoline-based inhibitors are typically higher than 90% when evaluated under similar experimental conditions.

Imidazoline-based corrosion inhibitors composed of long fatty chains exhibit higher inhibition efficiencies than those with medium or short chains because the alkyl chain length strongly influences the structure and stability of the protective film formed on the metal surface. Long-chain imidazolines promote the formation of a dense and cohesive hydrophobic layer that effectively blocks the diffusion of corrosive species to the metal interface. These effects have been demonstrated in several studies. For instance, Peng et al. reported that the inhibition efficiency increased with the chain length up to an optimal limit (around C₁₈–C₂₀), beyond which solubility and aggregation effects could slightly reduce performance.

The maximum IE of the IM-AA compound was 87.95% at a concentration of 100 ppm, whereas the IM-MX compound reached a maximum IE of 92% at 75 ppm. These results indicate that the inhibition efficiency increases when IM-AA is combined with oleic imidazoline. This enhancement can be attributed to the longer fatty chain of oleic imidazoline, which improves the hydrophobic character and the protective film formed on the metal surface. The effect of oleic imidazoline on the performance of IM-AA was evaluated by using a mixture composed of 75% IM-AA and 25% oleic imidazoline. Nevertheless, further studies are required to investigate additional combinations of these compounds to determine the optimal formulation for achieving a higher inhibition efficiency.

The effectiveness of IM-AA was assessed using electrochemical techniques that allow rapid and quantitative measurements of corrosion rates, polarization behavior, and inhibitor efficiency, facilitating comparisons between different concentrations. However, these tests were performed under controlled laboratory conditions that may not fully represent the complexity of real industrial systems, where factors such as temperature fluctuations, flow dynamics, pH variations, and the presence of multiple corrosive species can significantly influence the inhibitor performance. Nevertheless, the results provide valuable insight into the potential of A. aculeata oil as a sustainable feedstock for the development of efficient corrosion inhibitors.

4. Conclusions

The kernel of A. aculeata contains approximately 30–35 wt % vegetable oil, which is characterized by a high proportion (80%) of medium-chain fatty acid methyl esters. Moreover, these FAMEs can be converted into fatty acids through a saponification reaction and subsequently used as precursors for the synthesis of imidazoline compounds.

The molecules of IM-AA form a protective layer that limits the access of corrosive species to the substrate. This layer increases the corrosion resistance of AISI-1018 steel, and the resistance is further enhanced by increasing the IM-AA concentration. Moreover, these molecules block the anodic sites of the material, leading to a reduction in the magnitude of the anodic current during polarization.

The IM-AA compound inhibits the corrosion of AISI 1018 steel. However, it does not provide sufficient protection because it is composed of medium-chain imidazolines. These medium-chain imidazolines are generally considered less effective due to their limited ability to form stable, compact, and hydrophobic protective films.

The combination of IM-AA with oleic imidazoline results in a higher inhibition efficiency. The long-chain imidazolines of the oleic imidazolines enhanced the inhibition efficiency because they have the ability to form hydrophobic monolayers on metal surfaces.

This work has demonstrated that the content of long-chain fatty acid methyl esters is a key factor when selecting a vegetable oil for the synthesis of imidazolines.

A.R.E., V.Y.M.C., and R.H.A. were involved in conceptualization, data acquisition, investigation, formal data analysis, writingoriginal draft, and writingreview and editing; I.E.M., F.A.A.A., and G.P.F. contributed to conceptualization, data acquisition, investigation, visualization, and data analysis; and L.F.H.R., M.A.G.E., and L.R.M.C. assisted in visualization and data analysis. All authors have read and agreed to the published version of the manuscript.

The authors received no external funding for this work.

The authors declare no competing financial interest.