Trachyspermum Leaves Extract as a Green Inhibitor for Corrosion Inhibition of Mild Steel in 0.1m HCl: Experimental and Computational Study

The present study aims at investigating the impacts of the extract of Trachyspermum leaves on the mild steel corrosion in 0.1M hydrochloric acid (HCl), while highlighting its inhibitory mechanisms. The effects of mild steel corrosion in solutions of HCl were examined using the gravimetric and galvanostatic polarization techniques, along with EIS analyses. According to the results, the maximum inhibitory effectiveness corresponding to the minimum corrosion rate could be observed at the highest desirable level of inhibitor concentration equal to 100 ppm, while the corrosion rate decreases with an increase in the extract concentration. The absorption examinations indicated the best description of the metal surface interaction by Langmuir isotherm, while obtaining the best exposure time for the Trachyspermum leaves extract adsorption into the surface of the metal in different concentration. Based on the results of polarization, the inhibitors can play the role of a mixed inhibitor, also confirmed by the computational data. solution is used 15 . Many acid inhibitors belong to organic molecules, which contain oxygen, nitrogen or sulfur atoms in a conjugated system, playing an efficient role as corrosion inhibitors. Different inhibitors that contain N have been studied by scholars concerning the corrosion inhibitory features for metals in acid media 16 . Organic inhibitors that contain N act strongly for metals in acid solutions 3, 17-19 . Different studies have investigated the effects of plant extracts as green inhibitors for corrosion because they do not pollute the environment and are eco-friendly, inexpensive and easy to access from nature. Research shows that products taken from nature 20 and coming from plants have different organic substances 3,12 – 14 such as alkaloids 21 , tannins 22 , pigments 23 , organic 24,25 and amino acids 3, 26 , making them popular for their inhibitory effects 3, 17, 27, 28 . This study aims at providing the Trachyspermum extract and investigating its corrosion resistance as a corrosion inhibitor for mild steel using a solution of 0.1M HCl according to gravimetric assessments, electrochemical impedance spectroscopy, and potentiodynamic polarization assessments. It also studies the influence of the inhibitors’ structural parameters on the effectiveness of the inhibition as well as the adsorption mechanisms on the surface of metal while correlating the experimental data with quantum chemical parameters.


Introduction
One of the significant problems faced by different industries is the mild steel corrosion 1 in acids, particularly hydrochloric acid. Moderation such adverse phenomena through application of organic materials is essential to inhibit corrosion. The collective work of different scholars has focused on developing efficient inhibitors to secure metallic materials against corrosion [2][3][4] . Research has shown that the direct as well as indirect costs of corrosion processes make up around 3-4% of the gross domestic production of developed nations each year 3,5 . Several major industrial processes, including steel pickling, oil wells acidification, ion exchanger regeneration, processing of leather, producing organic as well as inorganic compounds, and industrial cleansing use Hydrochloric acid (HCl) solution 5 .
Application of acid inhibitors in different industrial procedures is typically aimed at controlling metal corrosion [6][7][8][9] , making their application considerable convenient in protecting against corrosion 3,[11][12][13][14] . Hence, undesirable metal dissolution and acid consumption can be prevented, especially when an acid solution is used 15 . Many acid inhibitors belong to organic molecules, which contain oxygen, nitrogen or sulfur atoms in a conjugated system, playing an efficient role as corrosion inhibitors. Different inhibitors that contain N have been studied by scholars concerning the corrosion inhibitory features for metals in acid media 16 . Organic inhibitors that contain N act strongly for metals in acid solutions 3,[17][18][19] . Different studies have investigated the effects of plant extracts as green inhibitors for corrosion because they do not pollute the environment and are eco-friendly, inexpensive and easy to access from nature. Research shows that products taken from nature 20 and coming from plants have different organic substances 3,12-14 such as alkaloids 21 , tannins 22 , pigments 23 , organic 24,25 and amino acids 3,26 , making them popular for their inhibitory effects 3,17,27,28 . This study aims at providing the Trachyspermum extract and investigating its corrosion resistance as a corrosion inhibitor for mild steel using a solution of 0.1M HCl according to gravimetric assessments, electrochemical impedance spectroscopy, and potentiodynamic polarization assessments. It also studies the influence of the inhibitors' structural parameters on the effectiveness of the inhibition as well as the adsorption mechanisms on the surface of metal while correlating the experimental data with quantum chemical parameters.

 Preparation of the working electrode
A nominal (wt%) composition of Fe =97.84%, Mn =1.4%, P=0.045%, C =0.17%, N=0.009 % and Si=0.5%. was considered for the mild steel rod (a diameter of 1 cm) in the present work. Soldering the same steel samples to coated Cu-wires for electrical connections with a 1 cm 2 laid open zone aimed at conducting electrochemical investigations. Mechanical abrasion of the working electrode surface was performed using various grades of papers, 600, 800, 1000 and 1500 prior to measurement. Then, distilled water was used along with acetone to rinse the samples of mild steel, after which drying was done using warm air flow.

 Preparation of inhibitor and electrolytes
Distilled water was used to clean Trachyspermum leaves from fiery residues of mud. Leaf drying over a 48-74-hour period in the thermostat with a temperature of 60 °C, grinding, and obtaining the leaf powder were the next steps, followed by refluxing and shacking 100 gm of the Trachyspermum powder with a mixture of ethanol and water at a temperature of 75 °C over a 24hour period. After filtering the refluxed product, evaporation of the obtained solution was performed to 100 ml of dark brown reside followed by drying in a vacuum drying oven using a temperature of 60 °C during a 48-hour period. Procurement of the light brown deposit (about 2.5g material) and its saving in a vacuum desiccator were the next steps. Dilution of the hydrochloric acid to 0.1M HCl aimed at preparing the corrosion medium. Dilution of the extract with 0.1M HCl was carried out to obtain various concentrations of Trachyspermum leaves extract (30,45,70, 100 mg/L). The reagent grade, 36% HCl (Sigma Aldrich), as well as distilled water, were used to prepare the corrosion solution (0.1M HCl). Different concentrations (30,45,70, 100 mg/L) were considered to prepare the inhibitor solution, leading to the highest Trachyspermum extract solvency in 0.1M HCl up to 500 mg/L.

 Plant material
leaves of Trachyspermum were purchased from traditional herbal market, Tehran, Iran on June, 2020. A voucher specimen has been deposited at the Herbarium of the Department of Pharmacognosy, Faculty of Pharmacy, Tehran University of Medical Sciences, Tehran, Iran (PMP-657).

 Gravimetric measurements
Calculations of the mild steel samples were set up according to ASTM G 31-72 to perform gravimetric assessments 29 over a period of 24 hours at 298 K. After preparation of 0.1M HCl acid blank solution which contained 30, 45, 70, 100 mg/L of the extract of Trachyspermum leaves, soaking of the pre-weighted metal samples in the prepared solution was carried out. Exclusion of the metal samples, deliberate rinsing with acetone, drying with nitrogen flow, and weighing to electronic balance were all performed after drenching. The experimental temperature remained stable. Three replications have been presented in the present work.

 Electrochemical measurements
Electrochemical assessments were performed using Emstate Electrochemical Workstation. The cell system has three electrodes, including the mild steel, a platinum, and saturated calomel electrodes (SCE) in the form of working, counter, and reference electrodes, correspondingly. A working area of 1 cm 2 was considered for the samples to perform electrochemical assessments. A stable open circuit potential (OCP) was initially obtained by soaking the working electrode into the test solution. The frequencies of 100 kHz to 0.01 Hz were considered to scan the electrochemical impedance spectroscopy (EIS) considering a signal amplitude perturbation of 5 mV at OCP. Considering OCP at 1 mV/s scan rate, a scan of ± 250 mV against SCE was used to record the potentiodynamic polarization curves. Three replications of the electrochemical assessments resulted in reliable values.

 Quantum chemical studies
Quantum chemical estimations were performed using the density functional theory (DFT) at a B3LYP function considering 6-311G+ (d, p) basis set for molecules with Gaussian 03 project programming to examine the effects of inhibitor molecular structure as well as electronic features on the effectiveness of inhibition. The optimized structures in the gas phase were considered to obtain the main parameters such as the lowest unoccupied molecular orbital (ELUMO) energy, the highest occupied molecular orbital (EHOMO), energy gap (ΔE) of the LUMO and HOMO, total energy (T.E.), electronegativity (χ), molecules softness (σ) as well as hardness (η), the energy distinction of the molecule electron exchange and dipole moment (μ), chemical potential (π), the number of transferring electrons (ΔN), etc. the use of the gas phase to perform theoretical estimations is supposed to be a suitable strategy as there are no significant differences with the results of the aqueous phase, while there is a significant decrease in the estimation time 30 .

 Surface morphology studies (SEM analysis)
The scanning electron microscope was used to observe the samples' surface morphology following the immersion of the pre-heated samples in 0.1M HCl solution in the absence and presence of 500 mg/L Trachyspermum leaves extract over a 24-hour period at 298 K. Doubled distilled water and acetone were used to rinse the surface of the samples, followed by drying to obtain the necessary information on the surface morphology of the samples.

 Potentiodynamic polarization measurements
Polarization tests were recorded and some electrochemical parameters were attained from the polarization curves containing corrosion current density (jcorr), corrosion potential (Ecorr), anodic and cathodic Tafel slopes (βa, βc), and inhibition efficiency (IE, labeled also as η). The IE was calculated using the following equation: in which jcorr and jcorr inh represent the current densities of corrosion in blank solution and 0.1M HCl consisting of inhibitor, correspondingly. Figure 1 indicates the steel polarization plots, taken five minutes following immersion in blank solution and 0.1M HCl consisting of inhibitor. Table  When the shift in the Ecorr is <85 mV due to the inhibitor presence, the inhibitor will belong to the mixed type 31,32 , while it will be cathodic or anodic based on the shift direction in other conditions. As the Ecorr shift was <85 mV in this research, the molecule belonged to the mixed type. Nevertheless, as shown in Figure 1, Trachyspermum functions as a mixed type when it is immersed for five minutes. Meantime, Trachyspermum affected cathodic and anodic reactions significantly. According to Table 1, less values of jcorr of Trachyspermum are observed as inhibition concentration, indicating that Trachyspermum has highly active inhibition characteristics.

 Electrochemical impedance spectroscopy
The Nyquist plots associated with the mild steel in 0.1M HCl solutions are shown in Figure 2 in the absence and presence of different inhibitor concentrations following immersion for five minutes. Fitting of the EIS results with the equivalent circuit was performed according to Figure  3, in which Rs, Rct, and CPE indicate solution resistance, charge transfer resistance, and the constant phase element, correspondingly. Figure 2 indicates an increase in the semicircle diameter based on the inhibitor concentration. Table 2 indicates Impedance data collected through the test, in which the equation below was used to calculate the IE (η): In which Rct inh and Rct reflect the charge transfer resistance in the inhibited as well as uninhibited system, correspondingly.    Table 2, the Rct values face an increasing trend based on the inhibitor concentration after immersing for five minutes. However, there is a general reduction in the CPE values, reflecting an increasing trend in the corrosion inhibition. The decrease in CPE is associated with the increase of the thickness of the protective layer or the reduction of the local dielectric constant (D) 33 . The findings show similarities between the results obtained by EIS investigations and polarization assessments. The results showed higher values for Trachyspermum corrosion inhibitory effects with the greatest value at 100 mg/L.

 Surface morphology
The SEM images of currently polished steel specimen and those soaked in 0.1M HCl are displayed in Figure 4 in conditions that inhibitor (100 mg/L Trachyspermum) has been absent and present following two hours. Obviously, the damage and pits occur when the inhibitor is absent, but there is a significant reduction in the damage and pits resulting from corrosion when 100mg/L inhibitor is present. A protective layer is created on the steel by the inhibitor, which preserve the steel against the corrosive attacks that lead to fewer black pits. Besides, as shown in Figure 4, when Trachyspermum is present, there are no black pits or holes, reflecting that Trachyspermum has better corrosion inhibitory effects. The findings are completely in line with the data collected through electrochemical assessments.

 Quantum calculation
Many studies on the corrosion inhibitors use quantum chemical estimations of the electronic parameters along with their correlations with the effectiveness of corrosion inhibition calculated through experimental methods. The DFT method was used to calculate the molecular electronic parameters (Tables 3 & 4), including energy gap (ΔEg), chemical potential (μ), electrophilicity (ω) as well as global hardness (η) of Trachyspermum (Fig 5) and also the structure of the main materials in Trachyspermum all of which have been considered in research on corrosion inhibition. A critical discussion of the application of this methodology has been provided in another study; it is not possible to provide a straightforward explanation of the corrosion inhibition efficiency just by the use of quantum chemical parameters, reflecting the importance of the nature of the inhibitors' effects on the surface. The supporting information represents the estimated electronic parameters along with the relevant discussions on the inhibitors under study. As shown by the specification of the electronic features of isolated molecules, Trachyspermum has the ability to play the most efficient role, supported by the experimental data of this study.

 Weight loss
The corrosion loss was compared when the inhibitor was absent (Wu) and present (Wi) to calculate the inhibition degree of surface coverage (θ) at different concentrations of Trachyspermum with the use of the equation below: The increased efficiency observed for the inhibition based on the concentration can be explained considering the adsorption scope of the inhibitor molecules on the metal surface. It is supposed that the formation of the film by the inhibitor molecule adsorption on the surface of the metal is the only criterion to lower the surface area of the cathodic and anodic reactions. The inhibitor molecules cover a fraction θ of the metal surface at a given instant, while there is a reaction between the uncovered fraction (1-θ) and the acid when the inhibitor is absent. It is possible to deduce inhibitor's adsorption features according to the nature of the inhibitor interaction with the corroding surface. The adsorption features can be also explained using the values of the surface coverage (Table 5). In general, it is possible to explain the adsorption behavior of inhibitors using adsorption isotherms, indicating critical information on the interactions between metals and inhibitors 34 . As shown by simplifying assumptions, inhibition effectiveness can be considered proportionate to surface coverage 35 . Examination of Frumkin, Temkin, and Langmuir adsorption isotherms as well as the El-Awady kinetic-thermodynamic model 36 aims at fitting the inhibitors' adsorption. Figure 6 shows the curve with a good fitness to Langmuir adsorption isotherm obtained with the use of the equation below 37 : In which, Kads, C, θ, indicate the equilibrium constant used in the adsorption process, the inhibitor's molar concentration, and the surface coverage, respectively.
the equation below was used to calculate the adsorption free energy (ΔG o ads) 38 : In which, T and R represent the absolute temperature and universal gas constant (8.314 J mol −1 K −1 ), respectively, while water's molar concentration is considered to be 55.5. When ΔG°ads has a value ranging from -20 to −40 kJmol −1 , there is evidence of both physical and chemical adsorption types 39 . Table 6 indicates the values of ΔG°ads.

Conclusion
Evidence showed that the extract obtained from Trachyspermum had inhibitory effects on mild steel using HCl, while there was an increase in the inhibitory efficacy as the extract concentration increased. The highest value of inhibitory effectiveness was 98% which was obtained at a concentration of 100 ppm. Based on the Potentiodynamic polarization curves, the extract of Trachyspermum leaves could be considered as a mixed inhibitory agent. The adsorption isotherms follow the Langmuir adsorption isotherms.