Mitigation effect of natural lettuce oil on the corrosion of mild steel in sulfuric acid solution: chemical, electrochemical, computational aspects

ABSTRACT The inhibition vigor of natural lettuce oil against corrosion of mild steel (MSt) 1.0 M H2SO4 solutions was investigated using chemical, electrochemical, and computational studies. The anti-corrosion efficacy (%AE) increases with the increase in the amount of doses of lettuce oil and reaches 95.83% from the galvanostatic polarization. The anti-corrosion process was demonstrated by the adsorption of lettuce oil onto the surface of MSt. The adsorption is spontaneous due to the negative signs of free energy of adsorption. The adsorption is subjected to Langmuir isotherm. Lettuce oil inhibits the pitting corrosion of MSt in chloride -containing solutions by transferring the pitting potential to the noble directions. Density functional theory (DFT) and Monte Carlo (MC) simulations were performed on the four components of natural lettuce oil. The adsorption of the inhibitors on the Fe(110) in the solutions is in parallel form. The results obtained from DFT and MC simulations predicted that α-Lactucerol and Lactupicrin give the MC simulations were performed on the four components of natural lettuce oil. The adsorption of the inhibitors on the Fe(110) in the solutions is in parallel form. The results obtained from DFT and MC simulations predicted that α-Lactucerol and Lactupicrin give the highest %AE. GRAPHICAL ABSTRACT


Introduction
Mild steel (MSt) is one type of steel applied in many industrial applications such as car body components, structural forms, plates, etc. due to multiple properties such as strength, ductility, toughness, ductility, and weldability.In general, 1.0 M H 2 SO 4 solution is usually involved in industrial exercises such as acid pickling, industrial acid cleaning, acid descaling, and oil well acidizing processes and severely attacks carbon steel, we are choosing a high concentration because this concentration is recommended by the industrial sector.Scientists are faced to solve this problem in many ways; one of the most important and effective is the application of corrosion inhibitors [1,2].
Most of the corrosion inhibitors used to protect steels from aggressive acidic solutions are organic compounds, polymer compounds, surfactant molecules, and pharmaceutical drugs.These compounds contain in their chemical structure donor or repelling groups or hetero atoms in their chemical structure [3][4][5][6][7][8][9][10][11][12][13][14][15].These compounds inhibit the corrosion of steel in acidic solutions mainly by its adsorption on their surface.The efficacy of the adsorption is based on some factors for example the functional group, the steric effect, electron density of heteroatoms, the existence of than one active center for adsorption, and the capability to form complex [16].
These organic compounds give high efficiency in inhibiting the corrosion of steel in acidic media, but unfortunately, these compounds cause harm to human health and the environment, very expensive and harmful to the national economy of any country.For this reason, scientists use plant extracts and natural oils to solve this problem, as a result of its many advantages including their low cost, safety for human health, environmental friendliness, and high efficiency in inhibition because its chemical composition contains many effective groups which facilitate the adsorption process on the steel surface and increases its corrosion resistance [17][18][19][20][21][22][23][24][25].
The main goal of this research is to inhibit the corrosion of MSt in 1.0 M H 2 SO 4 solutions by lettuce oil as a safe, ecofriendly inhibitor.Chemical, electrochemical, and computational studies are applied to determine the oil inhibition efficacy.In addition, some activation and adsorption Thermodynamic parameters were determined and explained.Computational methods (DFT and MC simulation) are used to indicate the more effective compound in the chemical structure of lettuce oil.

Chemical and electrochemical measurements
In this work, we used mild steel (MSt) containing composition (weight percent% C = 0.17, Si = 0.15, Mn = 0.40, P = 0.06, S = 0.04, Cr = 0.02 and the rest is Fe.For Mass loss (ML) methods, a MSt coupon with the dimension of 1.10 × 2.83 × 0.12 cm was used.For electrochemical methods such as galvanostatic polarization (GP), potentiodynamic anodic polarization (PDAP), and electrochemical impedance spectroscopy (EIS)impedance, a cylindrical rod immersed in Araldite with an uncovered surface area of 0.39 cm 2 is used.The electrode surface was polished with different grades of emery papers ranging between 200and 1500, washed with acetone, and rinsed with bidistilled water and dried at room temperature.A.R. grade H 2 SO 4 acid was used as the corrosive medium.ML measurements were performed as previously used [26].In this study, the desired concentration of lettuce oil was prepared as follows: lettuce oil stock solution (1000 ppm) was prepared by dissolving it in a 30% volume of carbon tetra chloride in water and the required concentration of lettuce oil was prepared by dilution using bi-distilled water and when added to a 1M H 2 SO 4 solution no precipitate formed.The amount of carbon tetra chloride in all aqueous solutions in the presence and the absence of the investigated letteuce oil were kept constant to remove the effect of carbon tetra chloride on the anticorrosion efficacy.
PGSTAT30 potentiostat/galvanostat was utilized for the GP and PDAP methods.The cell used includes 3 electrodes which are calomel, platinum, and MSt electrode.The MSt electrode is placed in the solution to be examined until steady state potential is reached (after about 35 min) and after this time, the polarization is initiated.For GP the scan rate is 2 mVsec −1 while in PDAP is 0.2 mV sec −1 .The potential range for the GP experiment is −1200 mV to +200 mV.EIS method was established in a frequency range of 100 kHz to 0.1 Hz with an amplitude of 4.0 mV from peak-to-peak exploitation of AC signals in open circuit potential.All the measurements were performed at a constant temperature of 301 K in a temperature-controlled system.

Chemical constituents of lettuce oil
Lettuce oil contains four major compounds such as: (1) αlactucerol (taraxasterol); (2) β-lactucerol (lactucon, lactucerin); (3) lactucin; and (4) lactucopicrin [27,28].The chemical structure of these compounds is represented in Table 1 2.3.Computational details Quantum calculations were carried out on four compounds, namely α-Lactucerol, β-Lactucerol, Lactucin, and Lactupicrin inhibitors using the density functional theory (DFT).The optimizations of the inhibitors were carried in the aqueous phase using Conductor-like Polarizable Continuum Model (CPCM) without any constraints.The quantum parameters were obtained using B3LYP level of theory and 6-31 g (d,p) basis set.B3LYP/6-31d (d,p) is a popular computational methodology for studying the electronic and structural properties of organic molecules, including inhibitors.The B3LYP functional is a hybrid exchange-correlation functional that combines the Becke three-parameter exchange functional with the Lee-Yang-Parr correlation functional.All the computations were done using the Gaussian 09 code [29].Some parameters such as the highest occupied/the lowest unoccupied molecular orbitals (HOMO/LUMO), and energy gap (ΔE) were determined.The chemical softness (s), chemical hardness (h), and the fraction of electron transferred (ΔN) have been calculated as follows: where w (the work function of Fe) = 4.82 eV [30] and h Fe = 0.

Monte Carlo (MC) simulation
MC simulation is an effective method for realizing the reaction of the inhibitors with Fe (110) surface in the presence of 100 molecules of H 2 O, 10 ions of H 3 O + , and 5 ions of SO 2−  4 .Simulations are done by the adsorption locator module executed in Materials Studio 7.0 [31] using a supercell (13 × 13Å) and a vacuum 25 Å.The MC was carried out utilizing COMPASS force field.
The adsorption energy can be calculated by the following equation: where E Fe/inh , E Fe , and E inh are the overall energy of the constituent compounds of lettuce oil on Fe(110) in the presence of 100 molecules of H 2 O, 10 ions of H 3 O + , and 5 ions of SO 2− 4 , the energy of Fe(110) in the presence of 100 molecules of H 2 O, 10 ions of H 3 O + , and 5 ions of SO 2− 4 , and of isolated inhibitor respectively.

GP method
Figure 1 presents the GP curves of the MSt electrode in a free 1.0 M H 2 SO 4 solution including specific concentricity of lettuce oil ranging from 100 to 300 ppm at 299 ± 0.1 K.These curves are characterized by the occurrence of a transition region at the beginning of the GP curves where the potential increases or decreases slowly with augmentation of current and beyond this region the potential decreases(cathodic polarization) or increases(anodic polarization) [32].Some corrosion functions such as cathodic and anodic Tafel slopes (β c and β a ), corrosion potential (E corr. ) and corrosion current density (I corr ), and anti-corrosion efficacy (%AE) were determined.
The %AE was calculated using Equation ( 5) where I free and I add are the corrosion current densities in the absence and presence of an inhibitor, respectively.These results were collected in Table 2.We found that as the doses of lettuce oil increased, The β c and β a values  of the examined lettuce oil were shifted to a negative and positive potential, respectively.E corr values are approximately constant.The shift in the presence of lettuce oil is about 10 mV.These outcomes demonstrate that lettuce oil has been categorized as an inhibitor of mixed type.It is noted that the values of β c are higher than the value of β a .This emphasizes t that the lettuce oil is a mixed inhibitor mainly cathodic.I corr values are reduced by increasing lettuce oil doses and increasing %AE values.It reached %95.83 at 300 ppm doses of lettuce oil.This confirms the vigor inhibitory effect of lettuce oils.

PDAP studies
PDAP method was applied to measure the ability of the lettuce oil to inhibit the pitting attack of MSt in the presence of chloride ions.Figure 2 presents the PDAP curves of MSt in 1M H 2 SO 4 + + 0.5M NaCl solution containing certain doses of lettuce oil at a scanning rate of 0.2 m V Sec −1 .The slow scan rate allows pitting initiation to occur at a less potential.At the beginning of the PDAP curves the cathodic current increases, and remains constant at zero current with the augment of potential and when a given potential is reached, the anodic current increases immediately and a pitting attack begins This potential is known as pitting potential (E pitt ) [33,34].It is evident, from this figure that, there is no anodic peak which demonstrates the fastness of the layer constructed on the MSt.E pitt is moved to the positive trend by increasing the doses of lettuce oil.
This behavior is described by the following equation [33,34]: where α and ß are constant depending on the number of doses of lettuce oil and the type of electrode used.The correlations between E pitt and the logarithm of lettuce oil concentrations are presented in Figure 3.A Linear relationship was obtained illustrating that with increasing lettuce oil doses, the pitting potential is transferred to the positive trend.This emphasizes that the lettuce oil inhibits the pitting corrosion of MSt [35,36].

EIS studies
The electrochemical processes related to MSt corrosion in 1M H 2 SO 4 solution we inspected through EIS experiments using Lettuce oil as an inhibitor.A Nyquist diagram is shown in Figure 4(a) for MSt in 1 M H 2 SO 4 without the tested additive and with it at selected concentrations.A single depressed semicircle appears on the EIS spectrum [37] and the diameter of the semicircle increases with additive concentration.It can never be altered by adding more additives because the shape of the resulting spectrum always indicates the mechanism controlled by charge transfer resistance.Figure 5 presents the equivalent circuit setup of this model which includes corrosion solution resistance R s , charge transfer resistance R ct , and double layer capacitance C dl , which forms the constant phase element.As shown in Table 3, the R ct values increase as the amount of additive increases, possibly due to a protective layer forming on the MSt surface.Increasing the concentration of lettuce oil promotes the adsorption of its active ingredients on the surface of MSt causing a protective layer to form on the surface, thereby decreasing the charge transfer between the MSt surface and the H 2 SO 4 medium and hence increasing the% AE [38].Furthermore, the C dl values decrease due to the replacement of a water molecule by the adsorption of lettuce oil which forms an adsorbent layer on the surface of MSt which leads to a decrease in the local dielectric constant of the MSt/solution interface and hence increases the efficiency of inhibition.
A possible explanation for this is the adsorption of extract components onto the CS/electrolyte interface, protecting MSt from the corrosive environment.According to Helmholtz model given by the following equation [33]: where f max is the maximum frequency.The values of anti-corrosion efficacy listed in Table 3 utilizing the subsequent equation:where (R ct )°and (R ct ) are the values of the charge transfer resistance in the 1M H 2 SO 4 solution and when added certain concentricity of the investigated oil, respectively.Clearly from Table 3 the %AE rise with rising concentration of the lettuce oil.Outcomes confirm that the investigated lettuce oil inhibits MSt in 1M H 2 SO 4 in a promising manner.A further investigation shows the Bode and phase angle plots in Figure 4(b) where the impedance modulus increases with increased lettuce oil amounts at low frequencies.As a consequence, the adsorption of this compound on MSt surfaces was confirmed and, in turn, the inhibition process was improved [39].Additionally, the phase angle plots demonstrate the occurrence of a single time constant at the interface between MSt surface and solution.

Influence of lettuce oil doses and temperatures
The ML method is applied to measure the anticorrosion efficacy (%AE) of MSt in 1.0 M H 2 SO 4 solution alone and when including some doses of lettuce oil ranging from 100 to 300 ppm after inundation time reached to 8 h at various temperatures ranging from 299 to 329 K.The rate of corrosion (K corr. ) (mg.cm −2 .h−1 ) was determined from the subsequent equation [40]: where ΔML = ML i -ML u ML u and ML i are the mass loss in 1.0 M H 2 SO 4 solution alone and in the existence of lettuce oil, respectively, A is the surface area of MSt coupons and t is the inundation time in hours.
The anticorrosion efficacy (%AE) and surface coverage (Ɵ) from ML method was computed from the subsequent equation: where K corr and K corr.i are the corrosion rates in the 1.0 M H 2 SO 4 and in the occurrence of lettuce oil.The determined corrosion functions from ML method such as K corr , %AE ML , and Ɵ at different temperatures are collected in Table 4. Obviously from these functions, with the augment of doses of lettuce oil, K corr.values decrease, the %AE Ml and Ɵ values are increased demonstrating the inhibiting impact of lettuce oil.As the temperature elevated from 299 K to 329 K, the K corr values increase and therefore, %AE Ml and Ɵ values are reduced which elucidates the adsorbent layer formed on MSt from lettuce oil is desorbed.This confirms the adsorption of lettuce oil on the MSt surface is physical [41].

Activation thermodynamic functions
Arrhenius and alternate transition state equations is applied to the determine the activation energy (E • a ), enthalpy and entropy of activation (ΔH°, ΔS°) for the dissolution of MSt in 1.0 M H 2 SO 4 solution in the absence and presence of some doses of lettuce oil utilizing the  next equations [42,43]: [log where R is the gas constant, A is the constant, h is the Plank's constant and N is the Avogadro's number.The E a °values are determined from the slope of the relationship between [log K corr and 1/T] for the dissolution of MSt in 1.0 M H 2 SO 4 and when including some doses of lettuce oil as presented in Figure 6.Straight lines were acquired with slope equal (−E a °/R) The computed values of E a °are registered in Table 5.
From these data, the E a °values increases with increasing doses of lettuce oil.This confirms that the corrosion of MSt is under activation control.E a °values in the occurrence of lettuce oil are more than those obtained in free 1.0 M H 2 SO 4 solution.This confirms the adsorption of the main constituents of lettuce oil on the surface of MSt by forming an adherent for mass and charge transfer and increasing the thickness of the double layer.E a °values above 20 kJ/mol to ensure physical adsorption of lettuce oil on the surface of MSt.
The plots between [log K corr /T and 1/T] for the dissolution of MSt in 1.0 M H 2 SO 4 solution and when including some doses of lettuce oil as presented in Figure 7.A linear relation was acquired with slope equal to (-ΔH°/2.303R) and intercept equal to (log R/ Nh +ΔS* /2.303R).The values of ΔH°and ΔS°are registered in Table 5.
The initial impression from these results was that the positive signs of ΔH °indicated that the nature of the corrosion of MSt in the 1.0 M H 2 SO 4 solution was endothermic processes, which means that the dissolution nature of the mild steel is low.The value of ΔH°was 7.98 kJ mol −1 in the free 1.0 M H 2 SO 4 solution.ΔH°rises with rising the doses of lettuce oil elucidating that this oil inhibits the corrosion of MSt.The negative signs of ΔS°demonstrated that the addition of lettuce oil formed a stable protective on MSt increasing the inhibition efficacy.With increasing doses of lettuce oil, the ΔS °values become more negative.This confirmed that the increase in stability of the film formed and the activated complex in the rate-limiting step appears as an association rather than dissociation, implying that a decrease in disordering occurs at the transition from reactants to the activated complex [44].

Adsorption consideration and mechanism of inhibition
The inhibitory effect of lettuce oil is principally due to the adsorption of its main components on the surface Table 5. Activation parameters for MSt in 1.0 M H 2 SO 4 alone and also when some doses of lettuce oil are included.of MSt.The adsorption operation can be deem as an exchange process in which a lettuce oil in the aqueous phase (L.oil) aq .Substitutes an 'z' amount of water molecules adsorbed on the MSt surface to give a lettuce oil adsorbed on the MSt surface.(L.oil) surf .

Conc. of oil (ppm)
L.Oil (aq) + zH 2 O (surf.)L.oil (surf.)+ zH 2 O (aq) (13) The adsorption potent depends on the chemical structure of the four main components of the lettuce oil, this may be due to the presence of the advantages of the steric effect and also the electronic density represented in the aromatic rings, including the conjugated double bonds and oxygen atoms which stimulate the strong adsorption processes on the metal surface, acid concentration, temperature, and other factors.Figure 8 represents the suggested adsorption pathway of the oil constituents on the metal surface.The determined values of θ from ML method were entered into some adsorption isotherms to find the preferable isotherm suitable for the experimental data.We found that the most appropriate relationship matching our results is the Langmuir adsorption isotherm according to the next equation: where C is the concentration of lettuce oil, K ads is the equilibrium constant of the adsorption, which is related to the free energy of adsorption (ΔG ads ) by the next equation [45]: The value 55.5 is the concentration of water in mol/ l. Figure 9 symbolizes the relationship between (C /θ versus C) for the corrosion of MSt in 1.0 M H 2 SO 4 containing some doses of lettuce oil at certain temperatures ranging from 299 K to 329 K.A straight line with a slope equals nearly one.This demonstrates that the adsorption of lettuce oil on the MSt surface is obeyed to Langmuir isotherm indicating no interaction between the adsorbed species.
The values of K ads were determined from the intercept of Langmuir plots.The Values of K ads are equal to (6.66, 6.27, 5.83, and 5.54) X10 −3 .In general, the large value of K ads indicates that the inhibitor is strongly and easily adsorbed onto the MSt surface, which improves the anti-corrosion efficacy.The values of ΔG ads were determined from Equation ( 10) and equal to -91.05, −89.82.−85.34 and −83.49kJ.mol −1 at 299,309,319 and 329 K, respectively.The negative marks of ΔG ads elucidates the spontaneous adsorption of lettuce oil on the MSt surface.
The enthalpy of adsorption (ΔH ads ) can be computed from the following equation (Van't Hoff equation) [46]: Figure 10 presents the plots (log K ads .vs 1000/T) for the adsorption of lettuce oil on the MSt surface.The value of     showed that the corrosion vanished on the MSt surface upon the addition of 300 ppm lettuce oil and that the surface was mainly covered with the components of lettuce oil.This may be due to the strong adsorption of this oil on the surface of MSt.The micrograph in Figure 11(a) depicts a smooth surface free of any corrosion damage.The micrographs of Figure 11(b) show the appearance of MSt after immersion in 1.0 M H2SO4 for a period of 5 h, at 25°C.A corroded surface appears with corrosion products contaminated on it as a result of the corrosive nature of sulfuric acid.The micrographs in Figure 11(c) showed that the corrosion vanished on the MSt surface upon the addition of 300 ppm lettuce oil and that the surface was mainly covered with the components of lettuce oil.This may be due to the strong adsorption of this oil on the surface of MSt.

DFT study and MC simulation
The four inhibitors, such as α-Lactucerol, β-Lactucerol, Lactucin, and Lactupicrin which were simulated in the aqueous phase, are the components of the lettuce oil.The optimized geometrical of each compound were presented in Figure 12.The experimental result of lettuce oil gives high inhibition efficiency, therefore to know which compound gives high inhibition efficiency, we calculated the quantum parameters to investigate the properties of each compound.The computed values for each compound inhibitor are listed in Table 6.α-Lactucerol and β-Lactucerol structures have a similarity.Also, Lactucin and Lactupicrin structures have a similarity, therefore we compared the quantum parameters of α-Lactucerol with β-Lactucerol and Lactucin with Lactupicrin inhibitors.
As we observed, a molecule that has a larger HOMO energy can make it easier for electrons to move from inhibitor molecules to the metal's d-free empty orbital and the high-energy surfaces of MSt (as an electron-donating component).As a result, these compounds can act as effective inhibitors on the surface of metals and are easily adsorbed on the steel surface.The results shown in Table 6, higher HOMO energy of the inhibitors are in the following order α-Lactucerol > β-Lactucerol, and Lactupicrin > Lactucin that gives indication α-Lactucerol has higher inhibition efficiency than β-Lactucerol, and the inhibition efficiency of Lactupicrin is higher than Lactucin inhibitor.The other quantum descriptor is the energy gap (ΔE) the low value of ΔE of the inhibitor indicates high molecular reactivity and can easily adsorb on the mild steel surface [47,48].As appear in Table 6, the order of their energy gap is as follows α-Lactucerol > β-Lactucerol and Lactupicrin > Lactucin which confirms that α-Lactucerol and Lactupicrin have higher inhibition efficiency than β-Lactucerol and Lactucin inhibitors respectively.The visual HOMO and LUMO of the four inhibitors were shown in Figure 13, the HOMO and LUMO for the α-Lactucerol and β-Lactucerol inhibitors were mainly distributed on the 3,1-diMethyl-4-methylenecyclohexane.In the case of Lactucin inhibitors, the HOMO and LUMO are mainly located on the azulene ring.For Lactupicrin, the HOMO is located on the azulene ring while LUMO is localized on the 4-hydroxyphenylacetic acid.
The images of the electrostatic potential mapping (ESP) of the four inhibitor molecules are presented in Figure 13.ESP of any chemical molecule is related to the polarity, dipole moment, and charge distribution over the molecule.It helps to locate −ve and +ve potential centers on the molecule so that regions of nucleophilic and electrophilic attack can be easily predicted.In Figure 13, the regions of the red-colored area located on oxygen atoms on four inhibitors show the potential of the −ve charge and blue colored area on the whole inhibitor molecules shows the potential of the +ve charge.
High values of chemical softness and low values of chemical hardness were desired values to support the belief that these molecules are highly reactive species with an adsorptive propensity when they are on metal surfaces.The fraction of transferred electrons (ΔN) for the inhibitors varying from 0.12e-0.28edemonstrates their strong capacity to donate electrons to the metal surface.
The interaction between lettuce oil and MSt can be further studied based on MC approaches.As shown in Table 7, the adsorption energy of the four inhibitors is all negative.The adsorption energy of the α-Lactucerol, β-Lactucerol, Lactucin, and Lactupicrin inhibitors are −130.65,−130.39,−119.81 kcal/mol, and −139.34 respectively.α-Lactucerol and Lactupicrin inhibitors show large adsorption on the mild steel' that is predicted to give high inhibition efficiency.Figure 14 shows that the four components of lettuce oil are parallel adsorbed on the Fe (110) surface.

Conclusions
Lettuce oil acts as an effective corrosion inhibitor for MSt in 1.0 M H 2 SO 4 .The anti-corrosion efficacy increases as the concentration of lettuce oil increases and the temperature decreases.Inhibition power of lettuce oil due to its spontaneous adsorption on the surface of MSt.The adsorption isotherms obeyed Langmuir.Lettuce oil acts as a mixed inhibitor.Lettuce oil prevents the corrosion of MSt in the presence of ions by shifting the pitting potential to a more noble direction.The DFT study predicted that α-Lactucerol and Lactupicrin would give the highest anti-corrosion efficacy, since the adsorption of α-Lactucerol, β-Lactucerol, Lactucin and Lactupicrin on the Fe(110) surface parallel to the surface.

Figure 3 .
Figure 3.The relation between E pitt and log concentrations of lettuce oil.

Figure 5 .
Figure 5. Electrical equivalent circuit applied to fit the outcomes of EIS.

Figure 6 .
Figure 6.Plots between log K corr and 1/T for MSt in 1.0 M H 2 SO 4 alone and also when some doses of lettuce oil are included.

Figure 7 .
Figure 7. Relationship between log K corr /T and 1/T for MSt in 1.0 M H 2 SO 4 alone and when some doses lettuce oil are included.

Figure 8 .
Figure 8.A schematic representation of the adsorption of the lettuce oil constituents on the MSt.Surface.

3. 6 .
Figure 11(a-c) presented the SEM micrographs of (a)freshly polished MSt surface, (b) after immersion for

Figure 10 .
Figure 10.Van't Hoff plots (log K ads .vs 1000/T) for the adsorption of lettuce oil on the MSt surface.

Figure 11 .
Figure 11.SEM images of MSt surface (A) freshly polished (B) after immersion for an interval of 5 h in 1.0 M H 2 SO 4 solution, (C), 1.0 M H 2 SO 4 and 300 ppm lettuce oil.

5 h in 1
.0 M H 2 SO 4 solution without any inhibitors, and (c) in 1.0 M H 2 SO 4 solution containing 300 ppm lettuce oil, respectively.The micrograph in Figure 11(a) depicts a smooth surface free of any corrosion damage.The micrographs of Figure 11(b) show the appearance of MSt after immersion in 1.0 M H2SO4 for a period of 5 h, at 25°C.A corroded surface appears with corrosion products contaminated on it as a result of the corrosive nature of sulfuric acid.The micrographs in Figure 11(c)

Table 1 .
The chemical structure of lettuce oil.

Table 2 .
Corrosion parameter acquired from GP curves of MSt electrode in 1M H 2 SO 4 solution containing different concentrations of lettuce oil.

Table 3 .
EIS data of MSt in free 1M H 2 SO 4 and in 1M H 2 SO 4 contains various concentrations of lettuce oil at 300 K.

Table 4 .
Corrosion parameters computed from ML method.