Mentha Piperita Extract as a Natural Product for the Corrosion Inhibition of Low Carbon Steel in a Polluted NaCl Environment: Chemical, Electrochemical and Biological Studies

The impact of aqueous Mentha Piperita Extract (MPE) as a corrosion inhibitor for low carbon steel (LCS) in NaCl/Na 2 S solution was investigated using chemical and electrochemical techniques to evaluate the protection performance of MPE. “As the concentration of the extract increased, the protection efficiency (PE) increased, reaching 91.2% at 300 ppm, whereas the increase in temperature favored to slow decrease”. The extract was physically adsorbed on the metal surface according to the Temkin isotherm. Polarization data revealed that this extract acts as a mixed inhibitor. The mechanism of corrosion protection of MPE was discussed from electrochemical techniques. MPE was adsorbed on LCS obeying Temkin isotherm. Surface morphology techniques such as scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to demonstrate the creation of a protective coating on the LCS in the presence of MPE. The extract inhibits the bacterial activity of Escherichia coli.


Introduction
Because of its low cost, low carbon steel (LCS) appears to be widely employed for structural purposes. However, its susceptibility to rusting in humid air and rapid dissolution rate in acidic solutions are significant drawbacks for its application on a broader scale without protection. "Corrosion is a naturally occurring phenomenon that deteriorates a material or its properties because of a reaction with its environment. Corrosion can cause dangerous and expensive damage to everything from pipelines, bridges and public buildings to vehicles, water, and wastewater systems, and even home appliances. It is one of the most serious problems in the oil and gas industry. In general, hydrochloric acid is employed in industries for cleaning, descaling, and pickling steel structures such as reactors, agitators, pumps, drains, and so on, which are prone to significant metal dissolution. The use of inhibitors is the most feasible corrosion prevention approach for slowing down the aggressiveness of this acid. The use of organic extracts is one of the most widely practical methods for protection of metals and alloys against corrosion, since adding extracts does not cause disruption of the industrial process". In previous years, numerous extracts have been extracted or selected from existing plants, and it has been found that the finest extracts are those having a center for π electron donation (usually heightened by the presence of heteroatoms in aromatic compounds), while others may be obtained from extracts of obviously occurring compounds [1][2][3] . Extracts derived from plants are significant because they are environmentally friendly 4,5 , nontoxic and do not contain heavy metals. "It has been established that sulfur and/or nitrogen containing heterocyclic compounds with various substituents are effective corrosion extracts in different solutions over a wide pH range. The PE of an organic compound mainly depends on its ability to adsorb on the metal surface". The protective nature of the adsorbed compact barrier film is influenced by the nature of the metal surface and electronic structure of inhibiting molecules. The adsorption of nitrogenous compounds is ascribed to the effects of the functional groups connected with aromatic rings parallel to the metal surface 6 . Indeed, various investigations on corrosion inhibition utilizing plant extract [7][8][9] , pure chemicals 10,11 and essential oils [12][13][14][15][16][17][18][19][20][21] have been performed in the last decade. They have all been identified as excellent inhibitors for various metals and alloys in various acidic settings. The current study aims to evaluate the protection efficiency (PE) of the investigated MPEM in directing the corrosion of LCS in polluted NaCl and to analyze the protective film formed on the LCS surface using scanning electron Methods. The weight loss approach is well known as the most extensively used methodology for assessing inhibition effectiveness. The procedure for WL determination was similar to that reported earlier 24 . Seven samples of LCS (2 × 2 × 0.1 cm as a dimension) were reweighed using a digital balance (Model Denver Instrument) and dipped in 100 ml of corroding medium (in open beakers) with and without different concentrations (50-300 ppm) of the extract at temperatures from 25-40°C for a total period of 180 min dipped period. The samples were withdrawn from the solution at intervals of 30 min immersion, rinsed with water, scrubbed with a bristle brush under running water, dried and reweighed". Triplicate experiments were set up for each concentration of extract for reliability. "Cathodic and anodic polarization curves were recorded in the potential region OCP ± 250 mV (SCE) as a function of concentration at a sweep rate of 0.2 mV/s. A cell with three electrodes was used. LCS as the working electrode, reference electrode was saturated calomel electrode (SCE) and counter electrode was Pt foil". From PP curves, corrosion data such as E corr , corrosion current i corr , anodic Tafel slopes, β a and cathodic slopes β c were computed. EIS spectra were recorded in the frequency ranges 105 Hz and 0.1 Hz with amplitude of 10 mV peak to peak at the open-circuit potential. The values of charge transfer resistance (R ct ) and double layer capacitance (C dl ) were calculated. The EFM was carried out at two different frequencies: 2 and 5 Hz. Because the base frequency was 0.1 Hz, the waveform was repeated every 1 s, which was required for good resolution of the current response.
The largest peaks were utilized to calculate the corrosion current density (i corr ), Tafel lines (c and a), and fitting together factors CF-2 and CF-3 25 . All electrochemical tests were carried out using a Gamry PCI300/4 Potentiostat/Galvanostat/Zraanalyzer. For data fitting and calculation, a personal computer running DC 105 software for PP, EIS300 program for EIS, EFM140 software for EFM, and Echem Analyst 5.21 was utilized.
Surface examination study. The coin surfaces were examined using a JSM-6390AL model before and after dipping in corrosive conditions for one day. Meanwhile, the components in the material were analyzed using an energy dispersive spectrometer (EDX). The nature of the film generated on the surface of the metal coins was investigated using these techniques. The roughness of the surfaces was checked utilizing atomic force microscopy (AFM) at the Faculty of Engineering, Mansoura University, Egypt.
Results data and discussion WL measurements. Figure 1 contains the WL-time plots of LCS in Polluted NaCl solution with existing and nonexistent altered concentrations of MPE. The corrosion rate (mg cm -2 min -1 ), degree of surface coverage (Ɵ) and PE% at 25°C are tabulated in Table 1

% PE=
× 100 = θ x100 (1) where W o and W i are the mass loss in the absence and presence of extract, respectively. As revealed from the data in Table 1, the extract has quite a high %PE that reaches 91.2% for 300 ppm of the extract. By increasing the extract concentration, the percent PE increased. The extract can be adsorbed on the LCS surface due to unpaired electrons present on their oxygen atoms and/or electrostatic attraction, according to the chemical structures of the extract. The extract's adsorption on the metal provides a charge and mass transfer barrier, which reduces the metal's contact with the corrosive environment. As a result, the metal's corrosion rate is lowered.   Table 2 and Fig. 2 A-C, which decreases as temperature rises. Arrhenius processes are commonly used to describe corrosion reactions, and the rate (k corr ) can be stated using the equation in Eq. (2): where k corr is the corrosion rate, E * a is the apparent activation energy, R is the universal gas constant, T is the absolute temperature, and A is the frequency factor. Fig. 3 shows an Arrhenius plot (log k corr against the reciprocal of temperature (1/T)) followed by straight lines with high correlation coefficients. For the blank and extract, the activation energy E a * ranged from 8.5 to 42.7 (Table 3). Physical adsorption or weak chemical interaction between the extract molecules and the LCS surface is indicated by a rise in E a * when the extract is present 27 where h is Planck's constant and N is Avogadro's number. A plot of log k corr /T as a function of 1/T for LCS is shown in Fig. 4. Straight lines with a slope of ∆H * /R and an intercept of ln R/Nh + ∆S * /R were drawn, from which the values of ∆H * and ∆S * for the blank and extract were computed". "The activation enthalpy, ∆H * , was 6.0 and 40.8 kJ mol -1 for the blank and the extract, respectively, while the activation entropy, ∆S * , was 177.9 and 271.0 J mol -1 K -1 for the extract (Table 3). In the absence and presence of extract, the ∆S * is substantial and negative. This implies that the activated complex in the rate determining step represents an association rather than a dissociation step, meaning that a decrease in disordering takes place on going from reactants to the activated complex 28,29 . The positive sign of ΔH * indicates that the corrosion process is endothermic.   Adsorption isotherms. To study the adsorption behavior of MPE on the LCS surface in the given medium, the adsorption isotherm must be defined to explain the mechanism of the extract reaction. The best fit isotherm is Timken adsorption isotherm. On its active sites, the extract is adsorbed on the metal surface. Ɵ values for altered extract doses at various temperatures were tested by fitting to various isotherms. Therefore, the relation between the dose of the extract (C), (Ɵ) and the adsorption equilibrium constant (K ads ) was obtained from the Timken adsorption isotherm: aƟ = ln K ads C (4) where (a) is the heterogeneity factor, C is the concentration of extract and K ads is the constant of adsorption equilibrium, "which is related to the standard free energy of adsorption (ΔG o ads ) by the equation": "K ads = (1/55.5) exp (ΔG o ads /RT)" (5) where 55.5 is the concentration of water in mol/L at the metal solution interface. The plot of (Ɵ) as a function of logarithm of the extract dose is shown in Fig. 5. From the plot, straight lines were obtained for the extract, indicating that the experimental data fit well into the Temkin adsorption isotherm. The Temkin isotherm characterizes the chemisorption of uncharged molecules on heterogeneous surfaces. The regression coefficients (R 2 ) are about 0.99. The departure of the slopes from unity is due to molecular interaction between the adsorbed extract species, which is not considered during the deviation: ΔG°a ds is consistent across all sites and is unaffected by the degree of surface coverage. 4th Table the adsorbate does not interact with one another, i.e., the adsorbate on G°a ds has no effective interaction". Table 4 shows that as the temperature rises, the K ads value drops, indicating that the extract adsorption on the metal surface is reduced. In general, K ads refers to the strength of the bond between the adsorbent and the adsorbate. The ∆G o ads values were derived from this plot and were negative, indicating that the extract molecules were stable and spontaneously adsorbed on the LCS surface. The heat of adsorption (∆H o ads ) can be determined using Vant Hoff's formula: "Log K ads = (−ΔH o ads /2.303 RT) + constant" (6) To calculate the heat of adsorption (ΔH°a ds ), log K ads was plotted against 1000/T (Fig. 6). A straight line was obtained. The absolute values of ΔH°a ds obtained in this study were lower than 100 kJ mol −1 , which is indicative of physisorption and supports the above mechanism of adsorption. The negative value of ΔH o ads (−32.520 kJ mol −1 ) in the presence of the extract reflects the exothermic nature of the LCS dissolution process. The activation enthalpies vary in the same manner as the activation energies, supporting the proposed protection mechanism. According to the basic Eq.: The adsorption entropy, ∆S o ads , was determined. The large negative entropy values in the presence of this investigated extract indicates that the activated complex in the rate-determining step represents an association rather than a dissociation step, implying that disordering decreases as one moves from reactants to the activated complex, and the activated molecules are in a higher order state than at the start 30,31 .  Previously, electrochemical approaches could be used to assess corrosion rates and describe corrosion processes. The inclusion of the extract during the polarization process is a criterion for classifying the kind of extract action, such as anodic, cathodic, or mixed. At 25°C, the Tafel polarization curve of LCS in polluted NaCl in existing and absence altered doses of MPE is shown in Fig. 7. The anodic and cathodic current densities of LCS in the presence of MPE were substantially lower than those of the blank. This decline becomes more pronounced as the MPE dose rises. Furthermore, when MPE is added to a 1 M HCl solution, a slight shift in the corrosion potential toward greater positive potentials is seen. The cathodic component shows the emergence of parallel Tafel lines, showing that activation controls hydrogen development. As a result, hydrogen discharge is predominantly accomplished at the LCS surface via a charge transfer mechanism [32]. As a result, the inhibition process might be induced by a simple blocking surface effect, such as a decrease in reaction area on the corroding surface0 33,34 . At each concentration, the values of PE and Ɵ were determined using Eq. 8 35,36 .
where i corr and i corr(inh) are the corrosion current densities of uninhibited and inhibited solutions, respectively. Table 5 shows that when the extract concentration increases, the i corr values drop. The increase in extract concentration causes a considerable shift in ß a & ß c , indicating a mixed type of inhibitor 37 . The protective efficacy of the extract improved as the concentration of the extract increased. MPE is a mixed-type inhibitor with cathodic action as its main feature. Following the addition of the extract, the polarization curves altered toward a lower negative potential and lower current density values, as shown in Fig. 7, indicating that MPE's protection mode is simple surface blockage by adsorption. The extract species are adsorbed on the metal surface by the donor atoms of oxygen and the electron clouds of the benzene rings, increasing the (θ) as the extract concentration increased. The bulky group of the extract also prevents ions from diffusing to or from the electrode surface 38 . As a result, the anodic dissolution current density of LCS is reduced.  EIS measurements. Impedance measurements are frequently used to gain a better understanding of the process at LCS contact. EIS was carried out in a blank and inhibited solution containing varying concentrations of MPE under potentiostatic conditions at 25°C. Impedance tests reveal how the extract can be produced into a thin layer film on the metal surface. Figure 8 shows the corrosion behavior of LCS with and without MPE in 3.5 percent NaCl and 16 ppm Na 2 S. As indicated by Tsuru et al. [39], the charge-transfer resistance (R ct ) values are determined from the difference in impedance at lower and higher frequencies. The following equation yields the double-layer capacitance (C dl ) and the frequency at which the imaginary component of the impedance is maximal (-Zmax): t max R f 2 1   dl C (9) where f max is the maximum frequency at which the imaginary component of the impedance (Z im ) is at its maximum at R t is the diameter of the loop". The PE ct obtained from the charge transfer resistance is calculated by: R t and R t/inh are the charge-transfer resistance values without and with extract, respectively. The semicircle indicates that the corrosion process was primarily controlled by charge transfer [40]. "The general form of the curves is extremely similar for all samples, showing that the addition of the extract had essentially little effect on the corrosion mechanism 41 ". The diameter of the Nyquist plots grew as the extract concentration rose. These findings suggested that increasing extract concentrations improves PE. Table 6 shows that raising the extract concentration improve the charge transfer resistance (R ct ) while decreasing the double layer capacitance (C dl ). The steady replacement of water molecules by extract molecules adsorbing on the metal surface to produce an adherent layer on the metal surface could explain the increase in R ct values and, as a result, protection efficiency. Because of roughness and potential oxide surface imperfections, the n value appears to be connected to the no uniform distribution of current. CPE indicates a perfect capacitor when n = 1. Because genuine capacitive behavior is rare, CPE is commonly employed for data fitting instead of a perfect capacitor. The current investigation demonstrates that n < 1 in both uninhibited and inhibited media, demonstrating surface heterogeneity despite MPE adsorption on the LCS surface's most active adsorption sites. According to the literature, the modest drop in n is a sign of surface inhomogeneity caused by inhibitor adsorption 42 . In our situation, the reverse tendency is seen, indicating an increase in homogeneity when MPEO is adsorbed. The results show that the corrosion efficiency values obtained using the impedance approach and polarization measurements are in good agreement. The corrosion rate is determined by the chemical composition of the electrolyte rather than the technology used 43 . The EIS data were reproduced using equivalent electric circuits, as illustrated in Fig. 9, with R S denoting solution or electrolyte resistance, C dl denoting double layer capacitance, and R ct denoting charge transfer resistance. The chi square value (χ2) 44 is used to assess the quality of fitting to the analogous circuit. Table 6 shows the appropriate values (0.0281-0.0951), which demonstrate the high-quality fitting with the suggested circuit.  EFM measurements. With the EFM technique, a potential perturbation by two sine waves of different frequencies is applied to a corroding system. As a corrosion process is nonlinear in nature, responses are generated at more frequencies than the frequencies of the applied signal. The AC current response due to this perturbation consists of current components (peaks) of different frequencies. Analysis of these current responses can result in the corrosion rate and Tafel parameters 45 . Fig. 10 shows intermodulation spectra derived from EFM measurements as examples of LCS in 3.5 percent NaCl and 16 ppm Na 2 S with and without 300 ppm MPE at 25°C. (i corr , ß c , ß a , CF-2, CF-3, and percent IE) are all corrosion kinetic characteristics of MPE with varied doses in polluted NaCl at 25°C in Table 7.As the extract concentration rises, the i corr falls, and the protection efficiency rises. The causation factors in Table 7 are quite close to the theoretical values, which should ensure the validity of the Tafel slope and corrosion current densities, according to EFM theory. The %IE obtained from the WL, PP, EIS, and EFM approaches were found to be in consistent with each other.  Figure 10. "Intermodulation spectrum for LCS in 3.5% NaCl+16 ppm Na 2 S solution in the absence and presence of different concentrations of MPE at 25°C.
Surface characterization. EDX spectra and the corresponding SEM micrographs of the LCS surface only after dipping in the corrosive media with and without the extract for approximately 5 days are shown in Fig. 11(a-c). The image of LCS surface is presented in Fig. 11a. An image of the LCS surface after dipping in corrosive media for approximately 5 days is presented in Fig. 11b, showing distortion due to the porous oxide film layer. The result of adding the extract to the metal surface is shown in Fig 11c, where there is no surface damage, and the surface is smooth. The metal sample is linked to the distinctive peaks. The metal surface has peaks of Fe, Mn, Cl, Na, S, and oxygen atoms after immersion in the corrosive fluids for approximately 5 days, as shown in Fig.  11b, which is connected to the oxide film generated on the metal surface. Fig. 11c shows the SEM peaks of the effect of the addition of the extract on the metal surface. In Fig. 11c, the peaks of Fe decrease, and the peaks of oxygen increase, which indicates great coverage of the extract on the metal surface, leading to a decrease in the peaks of the Fe atoms, and the extract passivates the metal surface from the corrosive media. Therefore, MPE is a good extract for LCS at a 300-ppm concentration. agriculture of Escherichia coli is carried out. There was no influence on the activity of Escherichia coli, as shown in Table 8 and Fig. 12. The MPE, which has an oxygen donor atom, is hypothesized to connect to the proteins and lipids of bacterial tissues, assisting in the respiration process. As a result, this extract has no toxicity on bacterial activity and may be safely used in sanitation facilities without causing any problems in the treatment process.  Mechanism of corrosion protection. Some MPE molecules can be protonated in the corrosive medium (acquire + ve charge), and the LCS has a positively charged surface, so there is repulsion between it and the protonated molecules. The Clions are adsorbed on the LCS surface and turn its surface negative, after which the protonated molecules are adsorbed on the -ve LCS surface ( Figure 13). Diterpenes, steroids, tannins, flavonoids, carbohydrates, alkaloids, phenols, coumarin, and saponins are the primary chemical elements of the phytochemical MPE extract 22 . Most of these phytochemicals are organic molecules with polar functional groups of oxygen (O) and nitrogen (N). A structure such as this promotes the creation of a complex with dissolved iron ions. The production of insoluble complexes is indicated by the result. These complexes are adsorbed onto the LCS by weak pressures, forming a protective layer against the corrosive environment and therefore slowing the dissolving process.

Conclusions
The following data can be drawn based on the obtained data: In sulfide-polluted saltwater, MPE works as a corrosion extract for LCS, with an efficiency of 87.43 percent at a 300-ppm concentration. MPE adsorption on metal surfaces follows the Timken adsorption isotherm, indicating that it is an effective eco-friendly and low-cost extract. As the temperature rises, the effectiveness of the protection system decreases. Strong and spontaneous adsorption of MPE on the metal surface is indicated by the negative free energy (-ΔG o ads ) of adsorption. The Tafel constant values (ß a and ß c ) indicate that the extract is a mixed type with a cathodic component. Adsorbed species formed insoluble complex compounds when they came into contact with dissolved iron ions. The protection efficiencies reported using various electrochemical methods is consistent. Because this extract has no influence on Escherichia coli biological activity, it can be used safely in sanitation plants.