EFFECT OF SYZYGIUM CUMINI LEAF EXTRACT AS A GREEN CORROSION INHIBITOR ON API 5L CARBON STEEL IN 1M HC l

leaf extract (SCLE) was used as a new green corrosion inhibitor under acidic conditions. The inhibition properties of the novel cumini extract were thoroughly characterized using potentiodynam-ic polarization (PDP), electrochemical impedance spectroscopy (EIS), Fourier-transform infrared spectroscopy (FTIR), and atomic force microscope (AFM). The results show that the cumini inhibitor has excellent corrosion inhibition with 93 % inhibition efficiency. The adsorption behavior of the inhibitor follows the Langmuir Adsorption Isotherm due to the near-ness of R 2 to unity. The potentiodynam-ic and electrochemical measurements demonstrate the mixed type of corrosion inhibitor. Thermodynamic calculation of Δ Gads is – 18.41 kJ mol -1 showing the physical adsorption process between the inhibitor and metals. Further inspection of Δ Hads at ‒ 58.93 kJ mol -1 considers releasing energy during adsorption. The FTIR results agree with the increased growth of passive layers due to the adsorption of polyphenol and flavonoids on metals. Remarkably, the adsorption peak at 3266.59 cm -1 corresponds to the adsorption of –OH. The peak at 1612.56 and 1698.4 cm -1 is attributed to C=C and C=O functional groups. The above functional groups serve as adsorption centers to reduce the corrosion effect. The surface treatment of AFM indicated a good relationship with the functional group characterization and confirmed the significant corrosion rate reduction. This work can be used as a benchmark to develop a natu-ral plant as a corrosion inhibitor


Introduction
Acids have been used in numerous industrial operations, including refining crude oil, pickling, cleaning, acid scaling, and petrochemical [1,2]. Hydrochloric acid is the most extensively used among commercially available acids in the oil and gas industry, owing to its high solubility in the aqueous phase. It acidifies carbonates and removes scale, rust, and carbonite deposits [3][4][5]. However, hydrochloric acid might cause corrosion in a metallic material and is highly destructive [6,7]. Adding corrosion inhibitors is one of the most practical and convenient ways to protect the metal from corrosion attacks. A corrosion inhibitor is a substance that, when introduced in low quantities to a corrosive environment, effectively reduces or suppresses the corrosion of metal in an acidic medium [8][9][10]. It reduces the rate of metal dissolution by adsorbing ions or molecules to form a protective layer on the metal [11].
Various chromates, silicates, phosphates, and arsenate-based inorganic inhibitors were offered as high-performance inhibitors; nevertheless, their usage is restricted by numerous environmental laws [12]. Synthetic organic inhibitors were also developed in the past; however, their toxicity level and manufacturing cost limit their use [13]. Therefore, researchers focused on developing eco-friendly, inexpensive, and biodegradable corrosion inhibitors [14,15]. In the existing development of inhibitors, various sources generated from biopolymers [16], ionic liquids [17], and plant extract [18] have been invented and developed to suppress aggressive acidic solutions.
With this in mind, the shifting to harness the potential of green inhibitors provides several benefits, such as high accessibility, ease of production, renewability, and effectiveness [19,20]. The inhibitive property of plant extracts is ascribed to active organic compounds containing nitrogen, oxygen, and sulfur heteroatoms, as well as C-C bonds with π electrons that interact with vacant metal d-orbitals [20,21]. This work attempts to resolve the use of leaf extract from Syzygium Cumini, which might be a potential candidate to become a natural corrosion inhibitor due to its biodegradability and high solubility in water. Unfortunately, there is little discussion, such as the procedure to prepare the inhibitor, type of adsorption, and surface modification, which become an obstacle to commercializing the inhibitor. Therefore, the study requires research to unveil the potential of Syzygium Cumini as a green corrosion inhibitor.

Literature review and problem statement
Several publications related to developing a part of a natural plant as an inhibitor have recently provided better insight, including using leaves. The study conducted in [22] shows that the combination between piper beetle and green tea is suitable to interrupt the corrosion rate of API X-52 steel in the aerated 3.5 % NaCl solution. The other publication [23] uses Chrysanthemum Coronarium leaves to increase the evolution thickness of the protective film on the aluminum surface. The film raises the electrostatic interaction between the hydrogen bonds and their combinations of aluminum metals and the inhibitors. In addition, the work [24] provides a clear explanation of how the Date palm leaf extract protects the X60 carbon steel under 15 wt % HCl solution. They reported that the inhibitor depresses the corrosion rate to nearly 82 % at 60 °C. Moreover, the study [25] shows the extract of Pluchea indica to investigate the corrosion resistance of the inhibitor, which appears as a mixed-type inhibitor. The work reveals that the solution effectively protects carbon steel for nearly 220 hours due to the contribution of its functional groups.
It is shown that corrosion involves interaction between the metal, electrolyte, and electrochemical potential, which can be mitigated by controlling one of the above parameters. But the unresolved issue is related to disconnecting the conjunction using a corrosion inhibitor and conducting its corresponding testing as a key to process newly developed corrosion inhibitors. The reason can be determining the corrosion resistance performance of the coated materials by exposing them to a corrosive solution, which makes the corresponding research inexpedient.
An option to overcome the relevant difficulties can be extracting dominant essential molecules from plant leaves to inhibit the electrochemical process. The approach used in the Bauhinia Tomentosa leaf extract [8] is an example of a suitable corrosion inhibitor for steel with inhibition efficiency beyond 90 %. According to their report, an increasing concentration in the temperature range of 308-323 K raises the resistance of charge transfer while depressing the double-layer capacitance. The extract of Centipeda Minima [26] has provided an inhibition property of about 96.2 % using 500 mg/L inhibitor solution. The uniqueness of the prepared inhibitor shows the extraction of facile hot water, and the freeze-drying process has successfully given superior inhibition to protect carbon steel under hydrochloric acid solution. In addition, the study [27] shows that an equal concentration of sodium dichromate and Solanum aethiopicum leaf extract at 0.83 % chemically protected the steel-rebar corrosion activity. In this study, the phytochemical results indicate that the leaf extract fits the green concept of environmentally and eco-friendly inhibitors.
In this work, utilizing the Syzygium cumini leaf extract is an attractive possibility because it would provide a better insight into corrosion mitigation with minimal or no hazardous effect. Specifically, the study attempts to test the inhibition activity to identify the most influential functional groups and their contribution under aggressive acidic solutions. The previous work [28] unveiled that the phenolic content of natural inhibitors is water soluble, and protonated functional groups such as -OH, C-O-C, C=O, and Aryl-CH 3 in their structure give higher solubility in water. The inhibitor adsorption model shows the quantitative and qualitative relationship between the inhibitor molecules and bare metals. From the surface modification point of view, the adsorbed inhibitor smoothens the damaged surface, especially when metal is immersed within a time frame to estimate the corrosion resistance of the prepared inhibitor.
Particularly, this work considers using the Syzygium Cumini leaf extract to protect the API 5L carbon steel related to the corresponding inhibition action, thermodynamics, and surface studies. It includes the effect of inhibitors when the metal was immersed for some time and tested for their inhibitory activity. The latter objective was not detailed in the above publications.
All this allows us to argue that it is appropriate to conduct a study devoted to revealing the effectiveness of Syzygium cumini in controlling corrosion scenarios.

The aim and objectives of the study
The study aims to provide the anticorrosive Syzygium Cumini leaf extract (SCLE) as a new corrosion inhibitor. This will enable injecting the inhibitor into oil pipelines to improve corrosion resistance under low pH environments.
To achieve this aim, the following objectives are accomplished: -to characterize the associated inhibitor functional group in increasing the anti-corrosion activity, including the immersion time of adsorption; -to evaluate the effect of temperature on the considerable adsorption behavior of inhibitors related to their thermal stability; -to compare the surface treatment in the absence and presence of the Syzygium Cumini leaf inhibitor.

1. Object and hypotheses of the study
The object of this study is Syzygium Cumini leaves taken from Jakarta, Indonesia. Before processing the with the addition of various concentrations of SCLE. Fig. 1 shows the Tafel polarization curves of the working electrode immersed in the SCLE inhibitor at concentrations of 100, 200, 300, 400, and 500 ppm, including the blank solution measured at 30-50 °C. The electrochemical parameters entail corrosion current density (i corr ), corrosion potential (E corr ), anodic and cathodic Tafel slope (β a and β c ), and inhibition efficiency (η). inhibitor, the leaves were cleaned, dried, and crushed to increase the extraction surface area. It is assumed that the leaf extract's phenolic and amine compounds increase the inhibitor's thermal stability and reduce the corroded area. It is also hypothesized that the conjugation of C=C, C=O, -OH, -NH, and the inhibitor's heteroatoms groups involves the mitigation of corrosion through the formation of chemical bonding. It is also presumed that the inhibitor concentration affects its effectiveness during the immersion period of metals.

3. Anticorrosion inhibitor test
Electrochemical impedance spectrometry (EIS) and Potentiodynamic polarization measurements were conducted using the Gamry Potentiostat Corrosion Measurement System (CMS). In detail, the API 5L serves as a working electrode, while a platinum wire was used as a counter electrode. In addition, a saturated calomel electrode was selected as a reference electrode. A comparable steady status was attained by immersing the working electrode in the test solution to achieve OCP (open circuit potential) for one hour. The EIS test was executed upon receiving the OCP-tested data. On the other hand, the perturbation signal was set at the selected frequency range of 300,000-0.1 Hz using the amplitude of AC signals at 10 mV. In addition, the potentiodynamic polarization curves were conducted using a scan rate of 5 mV with a potential range of -250 to +250 mV vs. OCP.

Identification of a functional group and surface analysis of inhibitors
Fourier-Transform Infrared (FTIR) spectra were collected using the ASTM E1944 standard to unveil the functional group of the inhibitor extract, including identifying the adsorbed compound over the surface of the working electrode. The spectrum range was 400-4,000 cm -1 with 64 scan numbers. The surface characterization was characterized using Atomic Force Microscope (AFM) by immersing the mild steel with and without a 500 ppm SCLE inhibitor.

1. Anti-corrosion activity of the inhibitor
The potentiodynamic polarization technique was conducted to study the API 5L polarization response in 1 M HCl a b Fig. 1 illustrates that the anodic and cathodic branches of the Tafel curves shift to a lower current density after the addition of inhibitors. In the inhibited working electrode, the corrosion potential, E corr , shifted towards the negative region/branch at all the studied temperatures ( Fig. 1). On the other hand, the corrosion current density (i corr ) shows depression when the inhibitor is added to the test solution. The results of calculated E corr and i corr equally indicate the corrosion protection of the inhibitor. It is also important to note that the shape of the anodic and cathodic regions is similar except for the graph at 50 °C, which exhibits a little deviation at 400 ppm solution. Table 1 shows the results of some electrochemical parameters E corr , i corr , β a , β c and η to obtain more details about the corrosion process, including the calculation of inhibition efficiency, η, using (1) as shown below: In the above equation, 0 corr i and i corr are corrosion current densities in the absence and presence of inhibitors. Table 1 agrees with Fig. 1 to show that the presence of the inhibitor alters the anodic and cathodic curves in lowering the current density values (i corr ). The finding is particularly significant showing that increasing the concentration of inhibitor to 500 ppm improves the API 5L's corrosion resistance by dramatically lowering the i corr value. The highest efficiency values obtained at 30, 40, and 50 °C are 90 %, 68 %, and 64 %, respectively. The results of Table 1 indicated that an increase in temperature raises i corr (0.385 mA/cm 2 , 1.54 mA/cm 2 , and 2.68 mA/cm 2 ) and the corresponding corrosion rate, resulting in a decrease in inhibitor efficiency. It is essential to note from Fig. 2 that single depressed semicircles for all temperatures appear. Careful observation of the graph shows that the diameter of the semicircles increases in the inhibited solution compared to without the inhibitor. Despite a similar trend, the effect of higher temperature limits the inhibitor protection as the diameter of the semicircle decreases. The capacitive loops for all temperatures are identical and slightly concave to show the increasing surface heterogeneity of the working electrode [29]. The imperfect single plot reveals that the inhibition process has one relaxation time constant to a growing frequency dispersion effect [30]. Table 1 Calculated polarization parameters of the test solution in the medium at different inhibitor concentrations and temperatures In the above equation, 0 ct R and R ct are the charge transfer resistance of the bare electrode and the inhibitor. Table 2 shows the electrochemical impedance spectroscopy results of the inhibitor spans from 30-50 °C. Table 2 Electrochemical parameters from the EIS fitting results at different concentrations and temperatures According to the impedance results in Table 2, the charge transfer resistance (R ct ) value rises as the SCLE inhibitor concentration increases. It is noteworthy to compare that the remarkable rise of R ct at 30 °C is smaller than at higher temperatures. The highest protection given by the EIS test was 74.1 % and 61.5 % at 30 and 40 °C. When the experiment continues to reach the temperature of 50 °C, the charge transfer resistance drops with 500 ppm solution. The gradual decrease of R ct was observed at 50 °C to show a downward trend in transferring an electron from the inhibitor to the working electrode. Likewise, as the concentration of SCLE increases, the value of the double-layer capacitance (C dl ) decreases. The lowest value of C dl is 60.90 μF, which refers to the corrosion protection of the inhibitor. It is also reported that at higher temperatures, the inhibitor protection achieves a lower value of η at 39.1 %.
The results of EIS align with the electrical equivalent diagram, as depicted in Fig. 3.  As shown in Fig. 4, only one semicircle Nyquist single depression graph is seen and increases with the immersion time. However, the diameter at higher temperatures is shorter than at lower temperatures. The recorded maximum protection of metal is 30 Ω at 30 °C (Fig. 4, a). It is also revealed that the semi-arc shape of the Nyquist spectra does not change remarkably, and each temperature measurement gives the same shape [32]. It is also obvious in Fig. 4 that the Nyquist plots are somewhat oblate after 60 min immersion time at all measured temperatures, which confirms the result of the electrochemical parameter as depicted in Table 4. Table 4 illustrates the results of electrochemical measurement with 500 ppm inhibitor solution at various immersion times and temperature ranges. It can be reported that the inhibitor achieves maximum protection for 1 hour in all observable measurements. The values of R ct and C dl at 30 °C b are 27.43 Ω and 53.20 μF to give an inhibition efficiency of 54.1 %. Likewise, the inhibitor gradually lost its protection at 50 °C to give lower charge transfer resistance and a capacitive double layer at 8.444 Ω and 113.70. Hence, it can be concluded that a single dose of inhibitor injection into the test solution interrupts the electrochemical activity [33]. The above results correlate with the adsorption of several functional groups from the inhibitor to the steel surface. Fig. 5 provides the results of FTIR to delve into more profound insight into the actual molecules involved in the inhibition reaction. Fig. 5 unveils several absorption peaks related to the increase in the corrosion resistance of inhibitors. The broad absorption peak at 3266.59 cm -1 is attributed to -OH and -NH stretching vibration [34]. The band appears at 2933.85 cm -1 , referring to C-H stretching vibration and >CH 2 associated with saturated aliphatic groups (alkenes). The peak at a wavenumber of 1447.64 cm -1 indicates the C-H bond. The absorption peaks at 1219.06 and 1041.61 are related to C-O stretching vibration. Meanwhile, the absorption peaks at 1612.56 and 1698.4 cm -1 indicate the presence of C=C and C=O groups, respectively.

2. Effect of temperature on the inhibitor adsorption
The adsorption isotherm models the mechanism of inhibitor over the mild steel surface; the results are shown in Fig. 6. Adsorption isotherm modeling is essential to determine the mechanism of interaction involved in the adsorption of inhibitor molecules onto the metal surface [35]. Several models, such as Langmuir isotherm, can be used to describe the adsorption mechanism involved. The calculated model is given by plotting a graph between concentration (C inh ) and concentration versus surface coverage (C inh /θ) as depicted in Fig. 6. The surface coverage (θ) of API 5L as a function of the inhibitor concentration was calculated using (3) [36]: 1 .
inh inh ads The above equation shows K ads as the adsorption-desorption constant (ppm -1 ). Fig. 6 shows that the linear correlation coefficient (R 2 ) values approach near unity (0.9993, 0.9922, and 0.9703 for 30, 40, and 50 °C, respectively), indicating that the inhibitor adsorption follows the Langmuir isotherm. Furthermore, the relationship between K ads and ΔG ads is given in (4) where the R is a gas constant of 8.31 kJ/mol and T is the absolute temperature (K). On the one hand, the calculated thermodynamic value of ΔG ads shows the feasibility of the adsorption process and is intended to unveil the nature of inhibition. On the other hand, the relationship between ΔG ads , ΔH ads , and ΔS ads is provided in (5): In the above equation, ΔH ads is the enthalpy change of adsorption, and ΔS ads is the entropy of adsorption. Both parameters are measured in kJ/mol.  It is evident that the value of ΔG ads is more remarkable than -40 kJ/mol, which defines the physisorption in na-ture ( Table 5). The value of ΔH ads at 323 K shows that the adsorption process releases energy (exothermic). The higher value of ΔS ads shows the feasibility and spontaneity of the reaction from liquid to adsorbed monolayer inhibitor at -119.51 kJ/mol.

3. Surface modification studies
The Atomic Force Microscope (AFM) was utilized to examine untreated (blank solution) and treated metals immersed in the 500 ppm inhibitor at 30 °C (Fig. 7). It can be seen that the surface of API 5L was irregular and had heavy damage in the absence of the SCLE inhibitor, implying that the API 5L surface had been strongly corroded (Fig. 7, a, b). On the contrary, the surface in the presence of 500 ppm SCLE inhibitor at 30 °C was smoother ( Fig. 7, b, c). This is confirmed by the value of the skewness parameter at 0.06101 nm, which is smaller before immersing mild steel in 1M HCl (0.4080 nm).

Discussion of Syzygium Cumini as a green corrosion inhibitor
The anti-corrosion study of the Syzygium Cumini inhibitor was characterized using potentiodynamic polarization and electrochemical impedance spectroscopy. The Tafel polarization curves show that both cathodic and anodic curves displaced toward the negative and positive corrosion potential. The displacement indicates that SCLE acts as a mixed-type inhibitor that hinders both anodic and cathodic reactive sites due to the electroactive components of the inhibitor being adsorbed across the API 5L surface [37,38]. In addition, shifting potential corrosion is less than -85 mV from the blank to the anodic or cathodic directions; inhibitors can be classified as mixed-type inhibitors [39,40]. The displacement of potential corrosion was all less than -85 mV. It shows that SCLE is a mixed-type inhibitor for all the studied temperatures.
Based on Fig. 1 and Table 1, in the presence of the inhibitor, both anodic and cathodic curves were altered to the lower current density values (i corr ). The decrease in i corr due to the adsorbed phytochemical components of SCLE on the API 5L surface hindered both anodic and cathodic active sites. This indicates that a blocking mechanism exists by the phenolic compounds in the cathodic region by reducing the rate of hydrogen ions to accept electrons [41]. Simultaneously, a donor-acceptor mechanism occurs in the anodic region, in which the SCLE compounds move actively to donate free electrons into the vacant orbit of the metal atom. The decrease in i corr leads to the reduction of the corrosion rate. The phenomenon can be ascribed to the increasing rate of solubility of the metal and partial desorption of inhibitor molecules on the metal surface. Furthermore, the protective film formed is unstable, and the inhibition ability is weakened [42].
The EIS results show the appearance of a single depressed semicircle to indicate that the charge transfer mechanism controlled the corrosion processes [43]. As depicted in the Nyquist plot, the larger the diameter, the higher the charge resistance, making the metal less likely to be corroded [44]. Increasing the concentration of SCLE raises the absorption into the API 5L surface, which leads to the improvement of surface coverage and protection of the API 5L surface. Fig. 2 shows that due to the effectiveness of the protective film, the addition of an inhibitor only increases the impedance value, which is demonstrated by the similar shape between the absence and presence of SCLE addition [45].
On the contrary, the high-temperature effect decreases the semicircle curve diameter in the high-frequency area. This behavior implies that high temperatures accelerated corrosion. The addition of SCLE shows that the diameter of the semicircle retains the same length as the blank condition, demonstrating that SCLE has remarkable corrosion inhibition at high temperatures. The Nyquist plots are interpreted using a fitting line to determine the Randles equivalent circuit model. According to the impedance results, the higher charge transfer resistance is associated with the adsorption of inhibitors by blocking numerous active sites on the corroded materials [46].
The inverse influence of double-layer capacitance appears when the concentration of the inhibitor is increased.
There are a few considerations to the following fact: − the replacement of ions or water molecules with SCLE inhibitor organic molecules; − an increase in the thickness of the electrical double layer at the electrode-electrolyte interface; − a minor in the electroactive area, as reported in [47]. As a result, the C dl value is gradually lowered since a more incredible solidification process occurred when the inhibitor was added.
The effect of temperatures provides the key to revealing inhibitor thermal stability at various temperatures. The lower value of R ct is obtained when the temperature increases to show the metal's higher dissolution rate and the inhibitor's desorption at elevated temperatures [48]. The result of immersion time (Fig. 4) agrees well with the variation of temperature at a given range. Table 1 agrees with Table 2 in terms of the value of inhibition efficiency. Although it is a lower value at a higher concentration, the inhibitor remains stable to protect the metal from corrosion. However, to elevate any inhibition efficiency at a higher temperature, the addition of a natural phenolic and amine-based inhibitor is required to establish more hydrogen bonds and promote a chemical bond between metals and the inhibitor.
The immersion time defines the period frame where the adsorbed molecules are solidified on the surface of metals. In this work, the intention of modeling the effect of immersion time is to determine the performance of a single inhibitor dose. Based on Fig. 4, the longer residence time induces the formation of a thicker film due to more inhibitor molecules adsorbed on the metal surface. It is also noteworthy that increasing immersion time enlarges the diameter of the Nyquist curve's semicircle. The phenomenon corresponds to the adsorption of inhibitor molecules at the metal/solution interface, forming a time-stable protective layer [49]. Fig. 5 shows the result of the active functional groups involved in the adsorption, which is considerably related to the declining corrosion rate of the studied materials. Considering the functional group identified in the preceding section, the appearance of -OH and -NH induces the hydrogen bonding between the inhibitor molecules and the negative charge of the metallic surface [50]. The associate C=C and C=O enrich the adsorption process by extending the electron conjugation in p-orbital and lone pair of electrons in oxygen atoms. The same result was reported by [51]. In comparison, the rest of identified atoms and functional groups contribute to increasing the feasibility of the reaction, which is carefully inspected in the thermodynamic values of inhibition.
According to Table 5, the lower value of ΔG ads corresponds to the formation of chemical bonding of dative covalent bonds. The fact confirmed in Fig. 6 is that chemisorption is stronger adsorption compared to physisorption, in which its interaction are double orders in magnitude [52]. The value of ΔH ads is associated with the lower inhibition efficiency of the inhibitor as more energy is required to complete the accomplishment of the reaction [53]. At the same time, regular ΔS ads shows that the solidification at a higher temperature runs slower than that at a lower temperature. As a result, more adsorbed particles were retained on the surface of metals and smoothened the surface of metals and showed constant ΔH ads at all measured temperatures. This influences the protection of metals by obtaining a more thermodynamic stable adsorption process.
Atomic force microscopy measurement was carried out to evaluate the surface roughness of API 5L in 1 M HCl in the blank and non-blank solution. Fig. 7 shows a dramatic change in corroded metal surface roughness and topography. In particular, in the blank solution, the value of an average skewness parameter was higher than that of the inhibited surface. The skewness parameter defines the ratio between the peaks and valleys where the smaller the value, the smoother the surface (Fig. 7, b-d). It can be reported that before adding the inhibitor, the skewness value is 0.4080 nm, which contrasts with the treated inhibitor at 0.06101 nm. This indicates that the presence of SCLE in 1 M HCl provides a denser and uniform steel surface, resulting in adequate protection of the API 5L surface. The obtained result is comparable to the previously published work [54].
Therefore, it is valuable to propose an anti-corrosion mechanism using several previous characterization results. According to the Langmuir model, the adsorption mechanism of the SCLE inhibitor on the API 5L surface involves physisorption because the value of ΔG ads is closer to -20 kJ/mol and that of ΔH ads is less than 40 kJ/mol. The FTIR data reported in the previous section confirm that the functional compounds of SCLE molecules had been found on the API 5L surface. These active compounds are represented by C, OH, and N groups. In addition, the presence of alcohol, phenol, and aromatic compounds confirmed the presence of polyphenol and flavonoid components in the SCLE extract. These organic molecules are adsorbed onto the metal surface, providing a protective film and increasing corrosion inhibition. Also, the SCLE inhibitor is expected to fit the requirement to protect the API 5L metal from further metallic deterioration. This is proved by all the experimental results, which inevitably lengthens equipment life and reduces economic losses.
Despite the above, the research retains a few limitations related to applicability in determining the essential elements in corrosion products before adding inhibitors to the test solution. In this context, the X-ray diffraction (XRD) test offers the best trial related to identifying the type of corrosion product, such as the hematite of Fe 2 O 3 . A successful identification helps the researcher to propose a precise inhibition mechanism for the Syzygium Cumini leaf extract. In addition, this work remains inadequate to determine the actual elemental composition of the adsorbed metals. Using a scanning electron microscope aided by energy-dispersive X-ray spectroscopy (SEM-EDX) could facilitate overcoming the challenges. The primary benefit of the characterization is closely related to the adsorption of several functional groups identified in FTIR results onto the surface of inhibited metals.
The above difficulties disadvantage the study, especially when it attempts to determine the adsorption direction between the adsorbate and the substrate. In this case, the orientation toward the vertical direction gives a lower surface coverage area than that of the horizontal axis, as previously elaborated in [55]. In future work, a further examination of the orientation of the inhibitor paves the way to assert the obtained results in Tables 1, 2. With this in mind, the absence of the orientation study gives an uncertainty of whether the lower inhibition efficiency is related to the effect of temperature or the sharp disorientation of the inhibitor when adsorbed over the surface of metals. The modeling studies, such as the implementation of Density functional theory (DFT) and Monte Carlo simulation, realign the nature of the research. The method mentioned above intensively harnesses the atomic properties and their corresponding interaction to calculate the atomic parameter such as the energy of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). In contrast, Monte Carlo simulation helps evaluate the inhibitor molecules' lowest surface energy inclusively and their orientation on the surface of the substrate.

Conclusions
1. The anti-corrosion inhibition of the leaf extract was evaluated using electrochemical impedance spectroscopy and FTIR to show that the inhibitor is suitable to protect the API 5L metal from corrosion. The corrosion rate of the substrate decreased from 39.84 to 4.167 mmpy and the inhibition efficiency of SCLE increased to nearly 91 %. The EIS results demonstrate that increasing the immersion time increases the surface coverage and the inhibitor's efficiency.
2. The influence of temperature was observed to unveil the thermal stability of the inhibitor. The calculated thermodynamic result shows the adsorption process following the Langmuir model in which the nearness of the R 2 value is 1, despite the lower inhibitory capacity at an elevated temperature of 323 K.
3. The surface morphology study using AFM confirms the surface modification of an inhibitor film on the API 5L steel surface. This results in a decrease in corrosion rate from the potentiodynamic polarization, 4.167 mmpy, and η of 90 %. In addition, a more concentrated solution gives a lower skewness value of 0.061 to ensure the inhibition process, helping a smoother and more protective substrate.

Conflict of interest
The authors declare that they have no conflict of interest in relation to this research, whether financial, personal, authorship or otherwise, that could affect the research and its results presented in this paper.

Financing
The study was performed with the financial support of the Ministry of Research, Technology and Higher Education of Indonesia.

Data availability
Data will be made available on reasonable request.