Unveiling green corrosion inhibitor of Aloe vera extracts for API 5L steel in seawater environment

This study evaluated Aloe vera extract as a green inhibitor to prevent corrosion in seawater environments. A. vera extract was produced by maceration with methanol﻿–water at room temperature. Electrochemical techniques were used to evaluate the corrosion inhibitor effectiveness of the A. vera extract. The morphology of the corrosion products was analyzed by FE-SEM equipped with EDS and AFM. FT-IR and LCMS characterized the functional and structural groups in this extract. The electrochemical measurements show that A. vera extract could effectively reduce the corrosion of API 5L steel in seawater environments. Inhibition efficiency (IE) increases with increasing concentration. Optimal corrosion inhibition efficiency of around 83.75% (PDP) and 88.60% (EIS) was obtained by adding 300 mg L−1 of extract at 310 K. Furthermore, the higher the concentration of A. vera extract, the greater the activation energy (Ea), with the highest activation energy being 48.24 kJ mol−1 for the concentration of 300 mg L−1. Conversely, increasing the temperature and exposure duration reduces the corrosion inhibition efficiency (IE) values; the best exposure period was 30 min with 88.34% IE by a concentration of 300 mg L−1 at 300 K. This corrosion inhibition is achieved by the adsorption process of A. vera bioactive on metal surfaces with a mixed inhibitor through a physisorption-chemisorption mechanism. This finding was confirmed by the smoother surface morphology of the steel treated with A. vera extract than without. This unveiling investigation found that A. vera extract has the potential to be an environmentally friendly corrosion inhibitor in the seawater environment.

The electrolyte solution in this work is a synthetic seawater solution made from a mixture of salt powder (Marine Art SF-1, Japan) and distilled water.The solution is made by dissolving 38-g salt powder for every 1000 mL.The main composition of salt powder consists of 22.1 g NaCl, 9.9 g MgCl⋅6H 2 O, 1.5 g CaCl 2 ⋅2H 2 O, 3.9 g Na 2 SO 4 , 0.61 g KCl, and 0.19 g NaHCO 3 .

Environmentally corrosion inhibitor
The green inhibitor used for API 5L steel corrosion is sourced from A. vera extract.The Aloe vera (L.) Burm.f. (Asphodelaceae) was acquired from the Indonesian Medical and Aromatic Crops Research Institute (BAL-ITTRO), Bogor, West Java, and identified in the Botany Laboratory (Herbarium Bogoriense), Directorate of Scientific Collection Management, National Research and Innovation Agency (BRIN) (Letter ID: B-3369/II.6.2/DI.05.07/9/2022).The extraction procedure was carried out following previous work 42 .Briefly, 25 g of dried A. vera was extracted with 200 mL methanol-water (1:1 v/v) for 3 × 24 h at room temperature with a change of new solution every day.Next, it was filtered and evaporated at a temperature of 323 K with a speed of 70 rpm using a vacuum evaporator.The bioactive structure of A. vera extract was characterized using liquid chromatography-mass spectrometry-mass spectrometry (LCMS-MS) and Fourier transform infrared spectrometry (FT-IR, Bruker-Tension).

Electrochemical study
Electrochemical studies were performed with a corrosion measurement system (Gamry, PCI4G750-50090).Before electrochemical measurements, the workpiece specimens were sanded with sandpaper to a size of 1200 mesh and then rinsed with running water and acetone.In this study, measurements were carried out with a conventional three-electrode cell assembly (API 5L steel as the working electrode, graphite as the auxiliary electrode, and calomel as the reference electrode).Before potentiodynamic polarization (PDP) and impedance spectroscopy (EIS) measurements, open circuit potential (OCP) measurements were carried out for 10 min to obtain a stable potential.The potentiodynamic polarization was measured with a scanning rate of 1 mV/s at ± 250 mV www.nature.com/scientificreports/versus OCP, and electrochemical parameters (i corr , E corr , corrosion rate) were determined from the polarization curves using Echem-Analyst software.The inhibition efficiency value (IE, %) was determined using Eq.(1) 43 : where (i corr ) 1 represents the corrosion currents with A. vera extract, while (i corr ) 0 denotes the corrosion currents without extract (mA/cm 2 ).Meanwhile, electrochemical impedance spectroscopy (EIS) analysis was utilized at the frequency level of 0.1 Hz to 0.5 MHz using AC with 10 points/decade of resolution.The inhibitor efficiency value (IE, %) with EIS is determined based on Eq. (2) below 44 R ct1 and R ct0 represent polarization resistance with inhibitor and without the addition of inhibitors (blank), respectively.All measurements and evaluations (OCP, PDP, and EIS) were done at least three times to ensure reproducibility and stability of measurements.

Surface analysis
The steel surface morphology appeared using Field Emission Scanning Electron Microscopy (FE-SEM, Jeol Multibeam JIB-4610F) and energy dispersion spectroscopy (EDS).The surface roughness and morphology of the steel were analyzed using Atomic Force Microscopy (AFM, Park NX10).The presence of active substances in the functional group of A. vera extract is believed to have a role in inhibiting corrosion.The OH functional group in A. vera extract inhibits Al corrosion by 82.35% in acidic media 45 .Another study also reported that the active content in A. vera inhibited steel corrosion in NaCl media 40 .The (1) www.nature.com/scientificreports/dominant compound in the A. vera extract was identified as methyl phaeophorbide (10.93) and kaempferol-3-Orutinoside (2.91), with the structure presented in Fig. 2. The presence of these two molecules is strongly suspected to be responsible for inhibiting corrosion.

Characteristic of A. vera extracts
Other compounds in the resulting A. vera extract are 2"-O-Feruloylaloesin (3.62) and Natsudaidain (3.88).A summary of compound names, chemical formulas, and molecular weights identified in A. vera extracts is tabulated in Table 1.

OCP analysis
Figure 3 illustrates the open circuit potential (OCP) diagram of API 5L steel specimens when exposed to synthetic seawater media, both with and without the presence of A. vera extract at different concentrations.In the absence of A. vera extract, E corr experienced initial fluctuations in the first few minutes, finally stabilizing at a value of − 0.709 V after 600 s.The observed potential decrease is consistent with an active surface, which can be associated with electrode corrosion and the formation of corrosion products that do not adequately protect the metal surface.
When exposed to A. vera extract, the E corr value rises to greater positive levels, as shown in Table 2.The observed displacement could be ascribed to inhibiting the anodic reaction involved in iron release, as demonstrated by prior research 46,47 .The difference in corrosion potential becomes more significant as the extract concentration increases.In adding A. vera extract, the corrosion potential value stabilized at around -0.647 V.The observed potential shifts suggest that the inhibitory action predominantly targets anodic processes.

PDP analysis
Figure 4 illustrates the polarization curve of API 5L steel immersed in seawater media, where different concentrations of A. vera extract were administered at various temperatures.The potentiodynamic polarization-related data  3.It can be demonstrated that the polarization curve shows an apparent decreasing trend in the corrosion current density (i corr ).The corrosion current density in the absence of A. vera is 36.48μA cm −2 .Meanwhile, in the presence of A. vera concentrations ranging from 100 to 300 mg L −1 , the corrosion current density obtained values were very low, namely 8.25 μA cm −2 to 7.14 μA cm −2 at a temperature of 300 K. Similar behaviour was observed at temperatures of 310 and 320 K, where an increase in concentration resulted in a decrease in the corrosion current density.Electrochemical corrosion of steel surfaces in a corrosive media produces Fe 2+ ions as an anodic reaction, followed by water dissociation at the cathodes.The presence of an inhibitor in the test solution medium resulted in a significant decrease in both the cathodic and anodic current curves, as shown in the polarisation curve (Fig. 4).This occurrence depicts the inhibitor absorbing onto the metal surface, covering the active corrosion-prone area 45 .As a result, the rate of electro-corrosive dissolution in steel is lowered.Nonetheless, the drop in cathodic current is substantially more significant than the reduction in anodic current; therefore, the inhibitor's cathodic features take precedence.
In the presence of inhibitors, the corrosion potential of API 5L (E corr ) steel tends to shift towards the cathodic side when it is immersed in artificial seawater.The observed behaviour indicates that the A. vera adsorption on the investigated samples hinders the dissolution of API 5L steel and restricts the movement of reactants between the bulk solution and the metal surface 45 .However, the E corr shift tends towards the anodic area along with the addition of A. vera extract, so it did not characterize the inhibitor's behavior as pure cathodic type and indicated a mixed type of environmentally corrosion inhibitor.Therefore, it can be said that A. vera extract functions as a corrosion inhibitor, which shows mixed corrosion inhibition for API 5L steel in various media, as evidenced by studies 37,38,41,45 .
Furthermore, based on several previous literature reports, if the difference in corr corrosion potential (E corr ) between the absence and presence of a corrosion inhibitor is less than 85 mV, then the inhibitor could be classified as a mixed-type inhibitor 40,41,48 .Otherwise, it will be categorized as either anodic or cathodic, as stated in reference 49,50 .The data in Table 3 and the plot in Fig. 4 indicate that the corrosion potential (E corr ) change is less than 85 mV towards the cathodic area.This data suggests that A. vera is of a mixed type, predisposed towards cathodic behavior at low frequencies and anodic behavior at high frequencies.In other words, A. vera preferentially adsorbs on cathodic sites and greatly influences the dissolution of mild steel at low temperatures.Similarly, in another study, A. vera extract in a 3% NaCl medium was classified as a mixed-type corrosion inhibitor with cathodic tendencies 51 .
Table 3 also shows that as the concentration of the inhibitor studied increases, the i corr value decreases, but the inhibition efficiency (IE) increases.However, with increasing temperature, corrosion inhibition efficiency tends to decrease.This case demonstrates that A. vera extract can successfully decrease API 5L steel corrosion in marine environments at room (300 K).This situation is indicated by the inhibition efficiency value, which tends to rise with increasing inhibitor concentration and decline with rising medium temperature.The efficiency value (IE) of the A. vera inhibitor for API 5L steel is determined from corrosion current density (i corr ) data by referring to Eq. ( 1).The maximum inhibitory power reaching 83.75% was obtained by adding 300 mg L −1 of A. vera extract at a temperature of 310 K.A study on steel in an atmosphere containing hydrochloric acid also revealed this correlation, with the effectiveness of reducing the rate of corrosion of steel decreasing somewhat as temperature increased and being in line with inhibitor concentration 52 .The influence of A. vera concentration on bronze was also investigated in chloride media, and it was discovered that its inhibitory effectiveness increased with raising inhibitor, reaching 86% at 750 ppm 36 .This inhibition is related to the adsorption of active substance electrons, thus blocking the active area of API 5L steel.

EIS analysis
Nyquist and Bode's plots illustrate the behavior of API 5L steel in seawater media with various concentrations of A. vera extract at different temperatures, as presented in Fig. 5. Figures 5a-c display the Nyquist plots acquired for API 5L steel submerged in seawater media, both in the absence and presence of different inhibitor concentrations.The figure illustrates that the Nyquist plot is characterized by slightly depressed semicircles, suggesting that corrosion in API 5L steel primarily happens through a charge transfer mechanism 53 .Another study stated that capacitive arc reactance in mild steel immersed in solution is influenced by charge transfer mechanisms at the interface between the solution and surface metal 1 .Surface inhomogeneities or interface phenomena can contribute to forming curve deviations from the ideal semicircle 26 .In addition, imperfect semicircular capacitive loops can be attributed to dispersion effects caused by the non-uniformity of the metal surface 54 .Figure 5 shows that the diameter of the semicircle expands when the A. vera extracts are added to the synthetic seawater solution.This enlargement of the semicircle continues www.nature.com/scientificreports/as the inhibitor concentration rises.This result clarifies why API 5L steel surfaces develop an inhibitor layer that prevents corrosion.The impedance spectrum of the blank medium displays a modest and incomplete inductive impedance induced by the adsorption-desorption process of the inactive active ingredient on the metal surface.Figure 6a shows the analogous circuit for this circumstance.Another electrical circuit is supplied for the situation by including an inhibitor (Fig. 6b) to produce a better simulation of the low-frequency inductance loop, which has an inductor resistance (R L ) and is in series with the inductor (L) 55 .The relaxation of inhibitors causes the inductive loop seen on the Nyquist plot adsorbed on the metal surface 56 .Due to corrosion-induced surface heterogeneity, the Nyquist plot displays a depressed semicircle rather than an ideal semicircle.To generate an excellent relationship between experimental impedance data and simulated, CPE was utilized instead of ideal double-layer capacitance in the circuit.Singh et al. define CPE impedance as the following equation 57 : Y 0 is the quantity of CPE (constant phase element), j 2 = − 1 is an imaginary integer, ω denotes the angular frequency, and n represents the exponent.The value of n, where − 1 ≥ n ≥ 1, indicates the characteristics of the CPE, which were described in the previous study 57,58 .The double layer capacitance (C dl ) value is determined using the formula Eq. ( 4), shown by the CPE installation results.
The efficiency of A. vera extracts was estimated from R ct data using Eq. ( 2) 44 , and the installed impedance parameters are presented in Table 4.These parameters include charge transfer resistance (R ct ), solution resistance (R s ), and corrosion inhibition efficiency (IE%).Calculations for R ct , inductive reactance (L), and their associated resistance (R L ), as well as the results of installing constant phase elements (CPE), are used to determine these values.The impedance associated with a Constant Phase Element (CPE) can be represented mathematically, as shown in reference 57 .
Bode plots are illustrated in Fig. 5aʹ-cʹ.The Bode plots demonstrate that increasing the concentration of A. vera extract results in an increase in impedance (|Z|) at low frequencies, which implies good corrosion inhibition efficacy 59 .Moreover, the curve of the Bode phase plot indicates that the phase angle increases with increasing A. vera concentration, reaching a maximum of − 49.42 0 , − 47.39 0 , and − 46.28 0 with the addition of 300 mg L −1 (3)    www.nature.com/scientificreports/extracts at 300, 310, and 320 K, respectively.This results in more significant inhibitor adsorption, which is related to improved capacitive performance 57 .Additionally, a comparison was made between the corrosion ability of A. vera and other plant extracts in the existing literature on the behavior of steel in seawater medium.According to the study, the A. vera extracts could reduce steel corrosion by 82.28% when adding 300 mg L −1 of concentration at 300 K of media temperature.In comparison, chamomile flower extract had an inhibitory efficiency of 75.66% at 20 mL/L in artificial seawater 60 , while Acacia Tortilis bark extract showed a slightly lower efficiency, namely 72.9% 15 .Moreover, the maximum iron inhibition efficiency was 86.4%, with a high concentration (3000 ppm) for Terebinth extracts in 3% NaCl media at room temperature 19 .These findings indicate that A. vera is a highly effective corrosion inhibitor.

Adsorption, thermodynamics, and kinetics analysis
To study the mechanism of corrosion inhibition of A. vera on API 5L steel in seawater media, the adsorption properties of A. vera on the steel surface and thermodynamics were analyzed using the information data presented in Tables 3 (PDP) and 4 (EIS).The experimental data show good agreement with the Langmuir isotherm equation (Eq.5), as reported in the previous work 7 .
where C symbolizes the concentration of A. vera extract and is a closed surface taken from PDP and EIS measurements, the results of the fitting process between C/θ versus C for each PDP and EIS data are depicted in Fig. 7a  and b.The equilibrium of the adsorption constant (K ads ) value was determined and presented in Table 5 based on the intersection of the fitted straight lines.The value of K ads showed a negative relationship with temperature, confirming that the adsorption efficiency of A. vera decreases as the temperature increases.The free energy of www.nature.com/scientificreports/adsorption (ΔG ads ) value for A. vera extracts on the API 5L steel surface was determined according to Eq. ( 6), and the adsorption enthalpy value (ΔH ads ) was calculated using Eq. ( 7) as explained in reference 61 .The ΔG ads , ΔH ads , and ΔS ads values were obtained from calculations presented in Table 5. Figure 7c presents a graphical representation of the fitting results for the logarithmic function of K ads and the inverse of temperature (1000/T).The data analysis reveals that the adsorption of A. vera active substances on the steel surface tends to be a physisorptionchemisorption and exothermic process (negative ΔH ads value).
Temperature is a crucial variable that must be evaluated in addition to the adsorption process to understand thermodynamic phenomena and reaction kinetics.A significant increase in temperature can affect various material attributes such as corrosion rate, equilibrium constant, and kinetics.The Arrhenius equation (Eq.8) could be used to study the correlation between temperature factors and corrosion rate, as shown as follows 62 : CR represents the corrosion rate, E a denotes the activation energy, R is the gas constant (J.K −1 mol −1 ), T is the absolute temperature (K), and A is the pre-exponential constant.The activation energy of the reaction could be calculated from the plot of log CR versus 1/T, as illustrated in Fig. 8.The activation energies to every extract concentration as an inhibitor are 0 mg L −1 , 100 mg L −1 , 200 mg L −1 , and 300 mg L −1 are 16.832, 20.738, 38.174, and 48.235 kJ.mol −1 , respectively.A summary of activation energy parameter data at various inhibitor concentrations is tabulated in Table 6.The activation energy is more significant with the addition of A. vera extract than without the addition of the extract because of the adsorption process 53 .The rising E a with increasing temperature indicates the bioactive molecules' adsorption process on the steel surface 63 .Based on several studies of plant extracts, adding extracts could drastically alter the morphology of surface API 5L steel, the chemical characteristics, and surface roughness following exposure 64 .

Effect of temperature and time
The inhibitor performance was evaluated based on media temperature and immersion time to provide an overview of the optimal temperature and injection time in inhibitor applications.www.nature.com/scientificreports/steel based on solution temperature and immersion time with 300 mg L −1 A. vera extract are presented in Fig. 9. Electrochemical parameter data resulting from the influence of temperature and time are tabulated in Table 7 and Table 8.Potentiodynamic polarization and impedance data based on temperature show that with increasing solution temperature, the inhibition efficiency (IE) value decreases.The optimum efficiency value obtained was 78.82% for PDP and 88.34% for EIS.This phenomenon of lower efficiency can be explained by the fact that green inhibitors from plant extracts are readily degraded or decomposed at high temperatures 50 .This finding aligns with previous studies on A. vera extracts as a corrosion inhibitor for mild steel in a 15% HCl, where inhibition efficiency was affected by temperature 65 .Another study states that the efficiency of A. vera extracts reduced at high media temperatures was caused by internal competition in the adsorption and desorption forces of specific inhibitor molecules involved in the corrosion inhibition at active areas on the steel surface 66 .Similar results were also found in other extracts where the inhibition efficiency decreased at higher temperatures 17,27 .
A decrease in efficiency is also seen when the soaking time is extended in both PDP and EIS results.This phenomenon of decreased value is caused by adsorption on the metal surface, and then the concentration decreases with the length of soaking time.As immersion time rises, the corrosion potential slightly moves in the cathodic direction (Fig. 9a), and the diameter of the semicircle of the Nyquist curve increases (Fig. 9b).R ct will decrease over time due to a decrease in the stability of the protective layer because of the diffusion and desorption of bioactive extract molecules toward the electrolyte interface or protective layer.Moreover, in the Bode phase plots, the phase angle decreases with increasing temperature and immersion time (Fig. 9c and cʹ).Soaking times have a significant effect on corrosion phenomena on metal surfaces, and consequently, several studies have evaluated the performance of inhibitors based on exposure time 12,19,40,48 .
Other studies have evaluated the effects of temperature and time, such as the study on the performance of extracts obtained from ginger and methanol as a solvent 67 .According to their investigation, 200 mg L −1 at 298.15 K enhanced inhibition efficiency by up to 94%.Regia fruit peel extract in the 3.5% sodium chloride solutions was also studied as an organic corrosion inhibitor for mild steel and revealed that the inhibitory efficiency increased with soaking times over 48 h 16 .The optimal efficiency was achieved at around 94% by adding 1000 mg L −1 extract.Another study evaluated the corrosion inhibition efficiency of imidazoline on Q235 steel in artificial seawater containing 3.5% NaCl as a corrosive media 68 .The efficiency of inhibition was increased with the increase in temperature and concentration of extract.The optimum corrosion inhibition performance was obtained at approximately 90.69% for the PDP method and 95.95% for the EIS method, respectively 68 .
Furthermore, the performance study of A. vera on low-carbon steel in acid environments (HCl and HNO 3 ) revealed that inhibitory efficiency decreased as temperature and soaking duration increased.The study discovered that A. vera extracts could act as inhibitors, with efficiency of up to 77.32% 40 .The investigation of A. vera at different times (in 3-day intervals) in H 2 SO 4 solution also showed that the weight loss of mild steel decreased as the activity of A. vera decreased over time 48 .

Comparative efficiency and future study prospects
A careful comparative analysis was carried out to provide a comprehensive evaluation of the performance of A. vera as a green inhibitor, as tabulated in Table 9.This analysis was carried out to contextualize the performance of A. vera and highlight the potential of plant extracts as a new and promising biomaterial in inhibiting steel corrosion.Table 9 compares this study's results with several recent studies in the development of A. vera extracts as green corrosion inhibitors (last ten years).In this study, the corrosion rate inhibitor efficiency (IE) was around 83.75% (PDP) and 88.60% (EIS).The findings of this study show extraordinary corrosive inhibition performance, with only a relatively small dose addition (300 mg L −1 ) able to inhibit steel corrosion in a seawater environment above 80%.However, the efficiency value in this study is still relatively small compared to some literature (Table 9).Therefore, extra studies are needed, such as extracting and isolating bioactive ingredients that prevent corrosion, to open future research avenues for developing corrosion inhibitors that are environmentally friendly, efficient, cost-effective, and sustainable in the marine environment.

SEM/EDS analysis
After the experiment, the surface corrosion morphology of the API 5L steel samples was analyzed using scanning electron microscopy (SEM).For this analysis, the API 5L steel was immersed for 72 h.The results obtained are presented in Fig. 10.Morphological results show that the solution without the extract has caused significant surface damage to API 5L steel due to corrosion.On the contrary, the surface of API 5L steel remains smooth in the solution inhibited by the extract, indicating that the use of A. vera extract significantly inhibits the corrosion reaction of API 5L steel in an artificial seawater environment.In solution without the extract, Fig. 10 also shows the elemental composition of the metal surface, which mainly consists of Fe and O.The presence of Fe and O indicates the formation of iron oxide on the surface of the metal.The decrease of O element took place in synthetic seawater by adding the extract.This decrease implies that there is a less dense corrosion product, such as iron oxide, when adding the extract.

AFM analysis
AFM data describe steel's surface morphology and inhibitors' effect on the formation and development of corrosion at the interface between the metal and solution 27 .Three-dimensional (3D) and two-dimensional (2D) AFM morphology profiles for API 5L steel surfaces without and with the presence of A. vera extract soaked in seawater media are shown in Fig. 11, respectively.
In addition to surface morphology, nominal quantitative mean roughness deviation (R a ) and root mean square roughness (R q ) of a metal surface can be obtained from AFM analysis.The R a and R q values for the steel surface without any extract (blank specimen) are 0.424 µm and 0.5579 µm, respectively.The visible roughness on the steel surface without extract is caused by corrosion attack and the absence of surface protection.This data shows that the surface of API 5L steel exposed in a seawater environment without inhibitors (extract) has a rougher surface morphology than the metal surface treated with the inhibitor, indicating that the API 5L steel surface is not protected, resulting in a rougher morphology, as seen in Fig. 11a.In contrast, Fig. 11b shows a smoother surface in seawater media containing 300 mg L −1 A. vera extract.The surface smoothness is generated by forming a compact protective layer of Fe 2+ and the Fe(OH) 2 complex on the metal surface because of the interaction of the extract's active components, which inhibits corrosion on surface steel 75   www.nature.com/scientificreports/API 5L steel surfaces are 0.1104 µm and 0.1429 µm, respectively.The R a and R q values were significantly lower in the extract-impeded environment than in the unconstrained environment.This parameter ensured that the surface was smoother than the steel surface without inhibitors.

Mechanism of corrosion inhibition
The corrosion rate data demonstrate that the 300 mg L −1 A. vera extract formulation achieves 83.75% IE on API 5L steel immersed in seawater.Polarization experiments reveal that the extract components work as mixed inhibitors, with a synergistic relationship between concentration and rate of corrosion inhibition.The AC spectrum impedance also indicates that the protective layer forms on the steel surface.Figure 12 depicts the reaction mechanism for corrosion and its prevention for steel surfaces in a seawater solution, which is explained by the reduction and oxidation reactions listed below.
Iron complexes formed in the solution when the inhibitor (A.vera) was added to a seawater formulation.The formation of complex iron begins with the diffusion of active substances towards the metal surface during exposure to steel in a solution containing the extract.Furthermore, on the steel surface, the active substance reacts with iron to form complex iron.Fe 2+ ions react with OH − to form Fe(OH) 2 as a protective layer in the cathodic area according to reaction Eq.(13) 76 : Therefore, complex Fe 2+ and Fe(OH) 2 produce the protective layer, which has a synergistic effect.Additionally, organic molecules that have heteroatoms, electronegative functional groups, electron double bonds, and aromatic rings that form bonds with metal atoms are typically the ones that prevent corrosion 17,18,28 .It is known that organic inhibitors work by first substituting inhibitor molecules for water molecules, which starts the metal adsorption process.In different ways, sometimes concurrently, organic corrosion inhibitors can slow down corrosion when in contact with corrosive media.Because experimental conditions can alter the mechanism, it is challenging to identify a mechanism precisely.
Corrosion inhibition through combined physisorption-chemisorption involves the simultaneous action of inhibitors that use both methods to protect metal surfaces from corrosion 57 .Physisorption inhibitors can form a protective layer on metal surfaces through low physical interaction like hydrogen bonds or Van der Waals forces 17 .In contrast, chemisorption inhibitors generate stronger chemical interactions with metal surfaces, resulting in a more stable and long-lasting protective layer 77 .Physisorption inhibitors can be adsorbed on metal surfaces initially by adsorbing chloride ions (Cl − ) via the electrostatic relationship with the steel surface (Fe 2+ ).Positive surface charge facilitates anion adsorption, while negative charge enhances cation adsorption.However, organic compounds in solution are quickly protonated and changed to cations.Protonation occurred in both simulated seawater and A. vera.Protonated A. vera cannot directly adhere to the positively charged steel surface due to the repulsive interaction between the active substance and the steel surface, but it can adhere to the steel surface via electrostatic interactions between Cl − and protonated A. vera.Compounds containing nitrogen frequently exhibit physisorption 78 .Some groups in most extracts, such as ester groups, aromatic groups, and -NH, are rapidly protonated in aggressive media 58 .
Chemisorption occurs when electrostatic interactions protonate the Cl − and A. vera ionic interactions.Furthermore, heteroatoms (S, N, and O) and other double bonds in the A. vera molecule can have several free electron pairs in the neutral state, which can cause d-orbital vacancies in Fe metal, resulting in coordination (9)  Fe → Fe 2+ + e anodic process bonds like the Fe-N and Fe-S, which considerably improve chemical absorption.The adsorption of aromatic and ester groups at the anodic area on the surface prevents API 5L steel from dissolution.Furthermore, the inhibitor molecules' P-electrons adhere to the steel's anodic surface, reducing corrosion to the iron metal on the anode area.As a result, the electrochemical corrosion process of API 5L steel was reduced by A. vera extract 48 .When both types of inhibitors are present, they can work synergistically to provide better corrosion protection 28 .

Conclusion
Based on the findings of the analysis and evaluation of the performance of A. vera extracts as green inhibitors on API 5L steel in seawater environment, the conclusion points obtained can be described as follows: The electrochemical analysis indicates that A. vera extract can successfully reduce the corrosion of API 5L steel in seawater environments by adsorbing on metal surfaces.The PDP analysis achieved an optimal corrosion efficiency of around 83.75%, and the EIS analysis achieved an optimum of 88.60% with an inhibitor concentration of 300 mg L −1 at 310 K.
• The analysis results reveal that A. vera extract has a mixed-type inhibitor and tends to be an exothermic reaction.• The higher the concentration of A. vera extract, the greater the activation energy (E a ), with the highest activa- tion energy being 48.24 kJ mol −1 for the concentration of 300 mg L −1 .• Increasing the temperature and exposure time tends to reduce the corrosion inhibition efficiency (IE) values, and the optimum exposure time was obtained at 30 min with 88.34% IE by a concentration of 300 mg L −1 at 300 K. • The surface morphology of the steel treated with A. vera extract was smoother, presenting the efficacy of the active components in the A. vera extract.• The dominant components of active substance in A. vera extract strongly suspected to be responsible for the inhibition based on LCMS analysis are methyl phaeophorbide and kaempferol-3-O-rutinoside. • The corrosion inhibition mechanism by extract was initiated by adsorbing the bioactive component onto the steel surface, forming a protective layer of iron (physisorption-chemisorption process).• This unveiling investigation discovered that A. vera extract has the potential to be a green corrosion inhibitor in seawater environments.

Figure 1
Figure 1 depicts the results of the FT-IR spectrum study conducted on the A. vera extract.The infrared spectrum reveals that A. vera extract possesses many functional groups, enhancing active chemicals' ability to associate with metal surfaces by adsorption.The main absorption peak at 3252.58 cm −1 in A. vera extract suggests the existence of OH or NH bonds, indicating the existence of alcohol compounds or amide group chemicals.At a wavelength of 2925.40 cm −1 , another peak indicates the stretching of C-H bonds, suggesting the presence of hydrocarbon group molecules.Furthermore, the signal peak observed at 1024.97 cm −1 indicates the existence of amine chemicals.The signal observed at 1583.06 cm −1 predicts the presence of the aromatic compound, namely the stretching of the C=O bond.Furthermore, several peaks at 1397.68 cm −1 , 1320.92 cm −1 , and 1256.11cm −1 suggest the potential existence of C-N and C-O functional groups.The presence of active substances in the functional group of A. vera extract is believed to have a role in inhibiting corrosion.The OH functional group in A. vera extract inhibits Al corrosion by 82.35% in acidic media45 .Another study also reported that the active content in A. vera inhibited steel corrosion in NaCl media40 .The

Figure 3 .
Figure 3. Open circuit potential (OCP) curves of API 5L steel specimens in seawater media containing various A. vera extracts.

Figure 5 .
Figure 5. Nyquist and Bode plots of API 5L steel in seawater media with various A. vera concentrations at different temperatures.

Figure 6 .
Figure 6.Model of circuit used for A. vera extract performance.

Figure 7 .
Figure 7. Langmuir isotherm of the adsorption for (a) PDP data, (b) EIS data, and (c) log K ads plots versus solution temperature.

Figure 10 .
Figure 10.SEM/EDS results on the API 5L steel surface after immersion (72 h) in a seawater environment.

( 10 )Figure 12 .
Figure 12.Schematic of (a) active corrosion pit and (b) corrosion inhibition by active extract compounds on steel in seawater environment.

Table 1 .
Compound names, molecular weight, formula, and retention time in A. vera extracts.

Table 2 .
Electrochemical open circuit potential fitting of API 5L steel in seawater media. T

Table 3 .
Electrochemical potentiodynamic polarization results of API 5L steel in seawater medium with various A. vera concentrations for different temperatures.

Table 4 .
Electrochemical impedance fitting results of API 5L steel in seawater medium with various A. vera concentrations for different temperatures.

Table 5 .
Thermodynamics parameters of A. vera extracts in seawater media at different temperatures.

Table 6 .
Energy activation parameters of A. vera inhibition.

Table 9 .
Comparative efficiency of A. vera extracts with several previous studies.
. The average R a and R q values of