Essential Oil of Origanum vulgare as a Green Corrosion Inhibitor for Carbon Steel in Acidic Medium

In this study, Oregano (Origanum vulgare) leaf essential oil was studied as an environmental-friendly anticorrosion agent for carbon steel in aggressive hydrochloric acid. The corrosion inhibition of O. vulgare was characterized by surface morphology, electrochemical, weight loss, theoretical and computational methods. It was found that the highest inhibition performance of O. vulgare was 85.64% at 2 g/l in 1 M HCl. The results of Langmuir isotherm and adsorption thermodynamics investigation demonstrated that the O. vulgare inhibitor adsorbed on the metal surface by the formation of rigid covalent bonds. The adsorption and inhibition centers of the selected inhibitor were studied by the computational methods, resulting in that the hydroxyl functional groups and benzoyl rings are mainly responsible for the high inhibition efficiency.

industries [1][2][3], because these materials have superior hardness, high tensile strength and low cost [4,5]. Carbon steel is normally severely corroded during the surface treatment, storage of gas oil, heating and cooling operations, and transportation of crude oil. The reason for this is that Fe is a more reactive metal; it can easily react with the water and oxygen molecules to form the corrosion products on the metal surface. In the acid-cleaning processes of metal surfaces, the hydrochloric acid solution is normally employed to remove the oxide deposits on the surface of steel [6,7]; as a result, a large number of corrosion products on the metal surface were depleted [8][9][10]. For example, the salts are accumulated inside steel-based pipes during crude oil transportation. Then, the acidic solutions are performed to clean the metal surface [2,11]. However, the corrosive nature of these acids leads the metal to dissolve during cleaning procedures, resulting in the deterioration of the metal substrate [12,13]. The addition of a corrosion inhibitor to the corrosive medium is a cost-effective way for the metal protection from acidic attack. Corrosion inhibitors are water-soluble compounds, and they are added to the pickling solution at low concentrations [2]. At that point, the metal surface is effectively insulated from the corrosion attacks of acidic solution [14][15][16]. Traditional inhibitors have harmful environmental effects. Due to their harmful impact on individuals and the environment, the use of these inhibitors has been limited [17,18]. This has augmented ecological awareness, and the use of green corrosion inhibitors has gained greater consideration recently [19][20][21][22][23][24].
Origanum vulgare (Oregano) is a native herb of the middle countries and belongs to the family Lamiaceae. The essential oil of O. vulgare contained more phenol compounds, such as hydroxycinnamic acid, caffeic acid, rosmarinic acid and other acid compounds [34], indicating that these biologically active compounds are good adsorbents on the metal surface.
The main aim of this investigation is to explore the inhibition efficiency of the essential oil of O. vulgare for steel in 1 M HCl medium as an environmental-friendly agent for mild steel by the surface morphology, electrochemical, weight loss, theoretical and computational methods.

Preparation of Inhibitor and Corrosive Environment
In this investigation, carbon steel was used as the working electrode. Its chemical composition is listed in Table 1. The working electrodes, whose contact surface with the corrosive medium is 1 cm 2 , were prepared by polishing them with 80to-2000-grit emery paper, then washing and drying them with distilled water and acetone. A 1 M solution of hydrochloric acid was used to prepare the corrosive solution. The corrosive hydrochloric acid solutions were made by diluting 37% HCl with deionized water.

Preparation of Inhibitor
The essential oil Oregano was used as the selected inhibitor. Oregano samples were collected from the region of Lekhmisset (Morocco). The dried aerial components were hydrodistilled for 4 h by a Clevenger-type device. The essential oil was extracted, dried with anhydrous sodium sulfate and kept at 4°C until it was needed. The essential oil of Oregano extracted by Ghanmi et al. [35], in which the major compounds are presented in Fig. 1 and Table 2, was used in a concentration range from 0.5 to 2 g/l.

Weight Loss Study
The dimensions of the utilized steel samples are 1.5 cm × 1.5 cm × 0.05 cm. The weight loss of metal coupons was measured after corrosion and inhibition according to ASTM G1-67 standard after 6 h of immersion time. The protection performance (E W , %) and corrosion rate (W corr ) are estimated by Eqs. (1) and (2), respectively, in which W corr is the corrosion rate in the corrosion solution, W corr/inh is the corrosion rate in inhibited solution, m is weight loss in mg, t is immersion time in hours and S is surface area in cm 2 ,

Electrochemical Study
Using a PGZ100 galvanostat potentiostat equipped with Volta Master 4 software and a three-electrode cell containing a working electrode (plain steel), a saturated calomel reference electrode (ECS) and a chemically inert counter electrode (platinum), the electrochemical tests were conducted. The system was connected to a computer. To get the OCP constant, the working electrodes were submerged in the solution for 0.5 h. The electrochemical potential reached a steady state during 0.5 h. The PDP analysis was done between − 900 and − 100 mV. The values of protection degrees were calculated using Eq. (3), where i 0 corr and i i corr show the corrosion current densities for corroded and inhibited metal samples, respectively.
The next part of electrochemical measurement is EIS, which was done between 100 kHz and 100 mHz with the 10 mV applied AC signal. It is noted that a thermostat bath was used to keep the solution temperature at the targeted value. The protection degrees of selected inhibitors were estimated using Eq. 4, where R i ct and R o corr are the charge transfer resistances in the inhibitor-free and inhibitor-contained systems.

Surface Study
The changes in the surface morphology of metal after corroding and inhibition (2 g/l inhibitors) processes were explored by scanning electron microscopy (SEM) (JOEL JSM-5500) at 20 keV (magnification is 500 ×) and 298 K. The surface destructions and improvements on the surface were identified by the SEM analysis.

DFT Study
The density functional theory (DFT) is frequently used in corrosion inhibition studies. The DFT method accurately describes the performance of organic compounds as corrosion inhibitors. In this study, the DFT calculations were performed using the Biovia Materials studio program with the Dmol3 module (Biovia, USA) [36,37]. In the geometry optimizations of selected inhibitors, the double numerical basis set with polarization (DNP) [38] and the SCAN (Strongly Constrained and Appropriately Normed) functional [39,40] in the frame of Generalized Gradient Approximation [41] were used. The DFT calculations were performed in the solvent (water) via a Conductor-like Screening Model (COSMO) [42,43].

MC and MD Simulations
The chemical interactions between the metal surface and selected corrosion inhibitors were theoretically investigated by the monte carlo (MC) and molecular dynamic (MD) simulations. The simulation analyses were performed in Material Studio (Biovia, USA) using COMPASS III (Version 1.0) force-field (condensed phase), Forcite module and NVT canonical ensemble at 298 K (simulation time is 1 ns) [6,44,45]. Before undertaking simulations, the whole system was geometrically optimized [46][47][48]. The temperature of the simulation box was controlled by the Berendsen thermostat [49][50][51]. The radial distribution function (RDF) was also studied in this analysis [52]. The adsorption sites, energies and chemical adsorption mechanisms were also predicted in this analysis. These simulations were done on Fe (110)

Weight Loss Study
The first way to evaluate the corrosion inhibition of a metal in an electrolyte solution is through weight loss method. The advantage of this approach is that it does not require a lot of equipment. The results of the weight loss tests are indicated in Table 3. It is noted that the inhibition efficiency and corrosion rate depend on the change in the concentration, confirming that the maximal protection degree was 85.64% while the minimal corrosion rate was 0.115 mg/cm 2 *h at 2 g/l. This demonstrates that the chosen essential oil is an  The aromatic rings are abundant in delocalized π-electron systems. This accounts for the excellent inhibition performance. With the support of π-electron systems, the inhibitor adsorbed onto the metal surface by forming strong covalent bonds.

EIS
EIS is regarded as one of the most prevalent and effective techniques; its purpose is to identify various electrochemical reaction phenomena and protective mechanisms [53]. Figure 2 represents the electrochemical impedance diagrams for blank and inhibitor in 1 M HCl. It is noted from these results that the presence of a single capacitive half-loop indicates that the corrosion and inhibition of metal surface are mainly controlled by the process of charge transfer [54], and the increase in the concentration of inhibitor is attributed to the rise the diameter of the capacitive loop. This effect  is responsible for enhancing the protective power of the inhibitor [55,56]. Table 4 summarizes the electrochemical parameters obtained by modeling the impedance spectra using the following equivalent circuit (Fig. 3). According to Table 4, when the inhibition concentration rises, then the double-layer capacitance C dl was decreased up to a value of 48.4 μF/cm 2 , while the charge transfer resistance was increased significantly reaching a maximum of 263.3 .cm 2 at 2 g/l. According to the literature [14], these observations can be interpreted and confirmed. These results also suggested a higher inhibition ability of the selected inhibitor. Oregano extract contained more hydroxyl functional groups, which are attached to benzoyl rings. These functional groups are effectively adsorbed to the metal surface by the formation of covalent bonds. Consequently, a protective film was formed on the steel surface. According to Qian et al. and Mourya et al. [57,58], the corrosion inhibitor electrostatically and chemically adsorbs on the metal surface, displacing the water molecules adsorbing on the surface. As a result, the values in double-layer capacitance (C dl ) markedly went down. The next reason for this decrease is that the defender layer was formed on the metal surface by the adsorption of corrosion inhibitors. The C dl is characterized related to Eq. (5), in which S is the metal surface dimension, d is the dielectric constant, ε is the environment constant and ε 0 is vacuum constant [59].
The protection values given in Table 4 show that the essential oil of oregano admits a good inhibitory power with a maximum inhibitory efficiency of 86.8% at 2 g/l. The values in charge transfer resistance (R ct ) and solution resistance (R s ) are high in the corrosion solution, because the metal surface was corroded. In comparison, the values of R s and R ct decrease with the presence of corrosion inhibitors. This is due to the formation of a protective film.

PDP
The electrochemical kinetic character of the metal/electrolyte interface was identified by PDP methods. Figure 4 indicates the Tafel curves obtained in the presence and absence of the essential oil of Oregano in a 1 M HCl medium. It is also demonstrated that the presence of oregano essential oil has a small effect on the anodic curves, whereas the addition of corrosion inhibitors reduces the corrosion density relative to the control. This suggests that the development of a protective coating reduces the rate of anodic and cathodic corrosion. Corrosion inhibitors have an effect on anodic corrosion (iron oxidation) and cathodic hydrogen reduction [60]. The values of EIS functions were found by extrapolation of the Tafel plots; the obtained data are indicated in Table 5. It is clear from the obtained results that (1) the selected corrosion inhibitor is a mixed type, which means that the corrosion and inhibition processes were depleted by the cathodic and anodic interactions; (2) the potential differences among the corrosion and inhibition processes were lower 80 mV, indicating that the electron transfers on the metal surface were maximally blocked with the presence of corrosion inhibitor [61]; (3) the addition of inhibitor is attributed to a decrease in the current density compared to the blank, indicating a slowdown in the rate of corrosion due to the adsorption of these molecules on the metal surface [62]. The obtained EIS data show that the selected compound protects 87% of the metal surface from corrosion attacks at 2 g/l.

Temperature Effect
The temperature is also the next important effect on corrosion and inhibition. The rise of the temperature modifies the inhibition and adsorption characteristics. The corrosion reactions are rose, and the adsorption processes are reduced with the rise of temperature. The literature indicates that the inhibitory actions of essential oils are temperature dependent [63,64]. In the present investigation, the temperature effect was studied by the PDP method. The results of the PDP curves at various temperatures are found in Fig. 5. According to Fig. 5, it is noted that the rise in temperature is responsible for a rise in the current density and the corrosion rate in both cases. It is also shown that the anodic and cathodic plots were decreased in the inhibited system, confirming that the selected inhibitor can protect the metal surface at high temperatures [43]. Table 6 indicates the electrochemical parameters obtained from the potentiodynamic polarization curves. It is observed that when the temperature rose, then the current density increased in both cases. The temperature rise cannot dramatically change the inhibition performances. The inhibition protection was 87.5% at 298 K while this value was 81.6% at 328 K, showing that there is little decrease in the inhibition efficiency. It is also indicated that the cathodic and anodic slopes have not been varied significantly; these observations reveal that the temperature of the medium does not affect the mechanism of protection and it acts only on the speed of the reactions involved. It decreased the speed of adsorption of essential oil of Oregano on the metal surface [65].

Changes in Activation Parameters
According to the literature [66,67], it was confirmed that the changes in activation were explored using the thermodynamic parameters such as activation entropy, enthalpy and energy. These thermodynamic parameters can quantify the effectiveness of corrosion inhibition and identify the adsorption type. Additionally, the surface phenomenon, the interaction between the inhibitor and surface, heat treatments of inhibition and electrostatic interactions on the surface were described by the thermodynamic analysis. The dependence of the value of corrosion density (i corr ) to the variation in temperature can be considered related to Arrhenius Eq. (6) [68], in which the T is temperature, E a is activation energy and A is constant. The thermodynamic parameters are estimated using Eq. (7), in which the S a is activation entropy, H a is activation enthalpy, h is plank constant, N is Avogadro number and R is universal gas constant.
According to Ivanov [69], the adsorption performance of corrosion inhibitors also depends on the values of activation energy. This is due to the corrosion inhibitor adsorbed on the metal surface by the covalent and electrostatic interactions. The rise in temperature is depleted the force of electrostatic interactions [70]. Table 7 indicates that the values of activation energy were enhanced with the presence of the organic compound, confirming that the energetic blocks were formed for the corrosion processes by the formation of a protective film on the metal surface [70]. The positive value of H a indicates that the dissolution process of steel is endothermic [71]. The increase in entropy in the inhibited system confirmed that the Fe reacted with organic compounds to form the active complex, which adsorbed on the metal surface effectively [66].

Surface Morphology Analysis
The surface morphology analysis indicates the surface phenomenon and changes on the metal surface after the corrosion and inhibition processes. In this research work, the SEM observations are made on steel samples after 6 h of immersion (Fig. 6). It is revealed from Fig. 6a that the surface of the metal was seriously scratched in the acidic solution. It also noticed that the surface is importantly destroyed with the corrosion ions on the entire surface in the absence of the inhibitor system. The presence of the essential oil of Oregano in 1 M HCl solution (Fig. 6b) made the surface more smooth, confirming that the metal surface was well insulated from the acidic medium. Figure 7 shows the results of the conformer search (6000 conformers found) and the corresponding lowest energy structures for α-terpinene, (E)-β-ocimene, thymol and carvacrol molecules [72][73][74][75]. As can be observed that the found lowest energy structures of α-terpinene, (E)-β-ocimene, thymol and carvacrol are efficient corrosion inhibitors. Figure 8 shows the σ-profile of the α-terpinene, (E)β-ocimene, thymol and carvacrol molecules. The selected inhibitors have good H-bonding acceptor and donor sites, which are responsible for the physical and chemical adsorption mechanisms. The accepting and donating abilities of the α-terpinene, (E)-β-ocimene, thymol and carvacrol molecules depend on the interaction between their functional groups and water molecules. Figure 9 shows the optimized structures of studied molecules and the estimation of the MAC values for O atoms. As indicated, the α-terpinene, (E)-β-ocimene, thymol and carvacrol molecules were good polar organic compounds. Their high polarity is rose in the aquatic solution. The The Mulliken Atomic Charges (MAC) provide a consistent and convincing indicator of the inhibition adsorption heteroatoms. It is revealed that the oxygen atoms mainly interacted with the Fe atoms [72,[76][77][78]. Figure 10 reveals optimized structure, HOMO, LUMO and ESP pictures of αterpinene, (E)-β-ocimene, thymol and carvacrol molecules. The values of basic theoretical parameters were found and are indicated in Table 8. Molecular activity is key to describing the chemical properties and structural positions for chemical reactions [3,5,7,47]. The HUMO regions are more electron-rich. The electrons of HOMO regions are shared with electrophilic regions of the substrate. In comparison, the electrons are accepted in LUMO regions from the nucleophilic species [5]. The α-terpinene, (E)-β-ocimene, thymol and carvacrol molecules are chemically adsorbed on the metal surface by the HOMO regions while the LUMO regions are attributed to promoting the adsorption.

DFT Results
Calculated theoretical parameters of α-terpinene, (E)-βocimene, thymol and carvacrol molecules are tabulated in Table 8 related to the previously reported equations [3,79]. The difference between the HOMO and LUMO energy was around 4-5 eV for the studied chemicals, suggesting that they are good adsorbents on the metal surface. The values of electron affinity and ionization potential for these inhibitors were around 1 and 6 eV, respectively, confirming that the electron transfer between the vacant d-orbitals of iron and HOMO regions is very effective [3,79]. It is also revealed that the α-terpinene, (E)-β-ocimene, thymol and carvacrol molecules are low hardness and high softness compounds, showing good corrosion efficiency. The values of energy of back-donation and fraction of transferred electrons also confirmed that the electrons of corrosion inhibitors are easily

MC Simulation
The interaction of the α-terpinene, (E)-β-ocimene, thymol and carvacrol molecules with the surface of Fe(110) was investigated by the MC simulations, and the obtained results are indicated in Fig. 11. The adsorption energy (E ads ) may be deduced from Eq. (8), in which the E inhibitor is inhibition energy; Fe(110) is Fe surface energy; E (110)//inhibitor is the total energy of the simulated system; the obtained values of E ads show the adsorption forces of optimized structure [82][83][84].    The adsorption configurations and positions of αterpinene, (E)-β-ocimene, thymol and carvacrol inhibitors on the studied surface were found. Figure 12 indicates the values of adsorption for selected optimized structures show the following order of inhibition efficiency: Carvacrol > (E)-βocimene > α-terpinene = Thymol (Fig. 12). The high values of binding energies show the stiffness of the interaction of inhibitors on the studied surface. It is found that carvacrol is the most effective inhibitor due to its benzoyl ring and functional groups. The delocalized electron systems of corrosion inhibitors are attributed to enhance the inhibition efficiency. The flat adsorption positions of α-terpinene, (E)-β-ocimene, thymol and carvacrol inhibitors are most favorable on the studied surface.

MD Simulation
The adsorption processes of corrosion inhibitors (αterpinene, (E)-β-ocimene, thymol and carvacrol) on Fe(110) surface in dynamic condition were investigated by MD simulations. On the other hand, the adsorption positions of inhibitors on the metal surface were also studied in dynamic conditions. The most energetic stable positions of inhibitors are found by studying the temperature variations in MD  simulation analyses. As can be seen in Fig. 13, the temperature change is minimal, suggesting that the MD of the selected system was stable and effective [79,85].
In the next part of the simulation analysis, the adsorption dynamics of α-terpinene, (E)-β-ocimene, thymol and carvacrol corrosion inhibitors on Fe (110) surface were investigated by the MD method and the obtained results are indicated in Fig. 14. In the corrosion inhibition analysis, the MD simulations were mostly used to describe the interaction between the optimized structures and metal surfaces [21,23,43,44]. As observed, the α-terpinene, (E)-β-ocimene, thymol and carvacrol corrosion inhibitors adsorbed on Fe (110) surface by the flat adsorption positions. The water molecules and chloride atoms were insulated from the metal surface.
In the radial distribution (RDF) analysis, the bond distance between the adsorption centers and Fe atoms was estimated [86][87][88]. The obtained RDF plots are cited in Fig. 15. As can be seen from the obtained results, the distance between the oxygen heteroatom ad Fe atom was under 3.5 Å, confirming that the inhibitors adsorbed on the metal surface by the dominant chemisorption mechanism [3,24]. The negative value of interaction energy shows that the α-terpinene, (E)β-ocimene, thymol and carvacrol molecules are forcefully binding to the metal surface.

Adsorption Mechanism
The obtained results of electrochemical, surface and quantum chemical calculation methods suggested that the selected corrosion inhibitors adsorbed on the metal surface by the chemisorption and physisorption mechanisms. In addition to this, the given theoretical investigations demonstrate a strong agreement with electrochemical experiments, which demonstrate that these inhibitors have a great performance against metal corrosion [20,89].
As indicated in Fig. 16, the lone pair electrons of the -OH group, the π-electrons of the benzene ring and the double bonds may share their electrons with the empty d-orbitals of the Fe atoms, resulting in the chemisorption [78]. Next, the accumulated electrons of the d-orbitals of metal atoms lead to inter-electron repulsions. As a result, some electrons are transferred to the antibonding orbitals of benzoyl rings. These processes promote the formation of protective film.
The comparison results with the previously published works are tabulated in Table 9 It is indicated that the suggested Oregano extract is more effective at low concentrations. It was extracted with easy operation and cost-effective methods. The corrosion efficiency of Oregano extract is more stable at high temperatures.

Conclusion
• In this research work, the following conclusions are found: • Oregano extract is a more effective inhibitor for steel corrosion in aggressive acidic solutions at low concentrations. It was extracted with easy operation and cost-effective methods. • Its maximum protection ability was 85.64% at 2 g/l at medium temperatures. • The PDP method revealed that the introduction of this extract to a corrosive solution leads to a noticeable decrease in the i corr and both cathodic and anodic slopes of carbon steel.
• The polarizing activities on the cathodic and anodic regions were maximally blocked by the presence of this extract. • Calculated thermodynamic parameters prove that this inhibitor acts by physical-sorption and chemical sorption mechanisms at the metal/electrolyte interface. • MD calculations indicated that inhibitors flat-adsorbed onto the metal surface and their adsorption energies are fairly high.  Citrus peel WL 90 [90] Spirogyra algae EIS 67 [91] Glycine max EIS 86 [91] Cuscuta reflexa EIS 80 [91] Cedrus atlantica EIS 80 [92] Watermelon rind EIS 83 [93] Watermelon seed EIS 85 [93] Watermelon peel EIS 79 [93] Phoenix dactylifera EIS 88 [94] Ircinia strobilina crude WL 82 [95] Origanum vulgare EIS 87 Present work Acknowledgements OD thankfully acknowledges the Centre for Materials Science, College of Science, Engineering and Technology, University of South Africa, for providing financial assistance under the postdoctoral fellowship scheme. AB expresses gratitude to the Kosovo Ministry of Education, Science, and Technology (Nr.2-5069) for their assistance in providing computing resources.
Author Contributions All the authors contributed equally to the manuscript.
Funding Open access funding provided by University of South Africa.

Declarations
Conflicts of interest Authors confirm that they have no conflict of interest.

Fig. 16 A pictorial illustration of Carvacrol corrosion inhibitor on the steel surface
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