Cistus monspeliensis extract as antioxidant and corrosion inhibitor of ordinary steel in 1 M hydrochloric acid medium

The aim of this study is the valorization of the Cistus monspeliensis plant, native to North of Morocco, as antioxidant and corrosion inhibitor. Firstly, the plant is extracted by maceration in a mixture of water/acetone solvents. Phytochemical tests are carried out on the extract obtained. The antioxidant power of Cistus monspeliensis extract is evaluated by two methods: the test of reduction of the free radical DPPH (1,1-diphenyl2-picryl hydrazyl) and that of Ferric reducing antioxidant power (FRAP). The electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization are used to study the anticorrosion effect of Cistus monspeliensis extract. The results showed that the extract, 27.6% yield, contains phenolic compounds in the form of flavonoids, hydrolysable and condensed tannins, saponins, reducing sugars and glycosides. This extract has an antioxidant capacity similar to that of ascorbic acid with an inhibition concentration of 0.077 mg/mL 0.102 mg/mL for DPPH and FRAP test, respectively. Tafel plots show that the extract is an excellent cathodic inhibitor. The maximum inhibition efficiency of 92 % was obtained with 0.25 g/L of the inhibitor at 298 k. The impedance plot is characterized by a single capacitive loop attributed to the charge transfer process. The results also showed that the inhibitor acts on the surface of the metal principally by adsorption, leading to the formation of a protective film limiting the corrosion of ordinary steel.


Introduction
Acid pickling of surfaces is one of the industrial processes used for the removal of scales. One of the severe problems associated with the use of acids is the corrosion of metallic substrates. For that, synthetic inhibitors have been used to overcome this problem 1 . However, most of these inhibitors are toxic 2,3 . Therefore, ecological inhibitors, based on plant extracts, of a non-toxic nature are suitable candidates for replacing synthetic inhibitors because of their abundant sources and low cost [4][5][6][7] . Indeed, the ability of these inhibitors is generally attributed to the presence of secondary metabolites (polyphenols, alkaloids, saponins, etc.) having antioxidant power, readily available and renewable 8,9 . Currently, natural inhibitors are a topic of research in development based on the number of publications coming out each year [10][11][12][13][14] . The tests carried out on the study of anticorrosive properties of plant extracts gave promising results [15][16][17] . Hence, Table 1 represents certain previous studies on the corrosion inhibition of steel in 1 M HCl medium by green inhibitors, using polarization tests and electrochemical impedance spectroscopy.
The natural plant products are also used as natural antioxidants to replace the synthetic antioxidants, which have toxic and dangerous effects 25 . It has been suggested that some plants, which have antioxidant activity, could decrease the risk of disease linked to oxidative stress in humans, thanks to the improvement of its antioxidant defence system, inhibition of the production of reactive oxygenated species and also to the redox properties of antioxidants [26][27][28] . There is a large variety of methods to determine this activity (DPPH, FRAP, CUPRAC, ORAC, ABTS, betacarotene, etc.). Most are spectrophotometric methods based on the coloring or discoloration of a reagent in the reaction medium. The best results are often obtained by the combination of several methods [29][30][31][32] .   Table 1. List of plant extracts used as corrosion inhibitors on corrosion of steel in acidic medium.

Name of inhibitor Metal Electrolyte Efficiency
Chinese gooseberry fruit shell extract 18 Mild steel 1M HCl solution 92% Rosa canina fruit extract 19 Mild steel 1M HCl solution 86% Ammi visnaga L. Lam Seeds extract 17 41 . This Moroccan medicinal plant is known locally as "shtapa" is traditionally used for the treatment of diabetes disease and to fight against insects 33,42,43 .
To our knowledge, Cistus monspeliensis has never been studied for corrosion inhibition of ordinary steel in hydrochloric acid. The present study investigates the antioxidant activity of the Cistus monspeliensis extract using free radical DPPH (1,1-diphenyl-2picryl hydrazyl) reducing capacity and the ferric reducing antioxidant power (FRAP), and examines its inhibition action on corrosion, of ordinary steel in 1M HCl solution, by electrochemical methods such as potentiodynamic polarization and electrochemical impedance spectroscopy.

Preparation of Cistus monspeliensis extract
Firstly, the aerial part of dried Cistus monspeliensis was ground and extracted using the Soxhlet apparatus in hexane for 2 hours. The residue of the extraction is then extracted by maceration in an acetone/water mixture (70/30, v / v) for 2 hours. The mixture was then filtered, concentrated on evaporating the solvents and lyophilized.

Phytochemical tests
The phytochemical tests are used to search for the different families of chemical compounds present in the plant by well-known qualitative reactions. The detection of these chemical compounds is based on precipitation reactions, a specific color change, or examination under ultraviolet light. The tannins are detected using a ferric chloride solution (FeCl3, 2%). The presence of flavonoids is obtained by the reaction of the plant extract with concentrated hydrochloric acid and magnesium turnings. The presence of alkaloids is indicated by the appearance of an orange precipitate in the presence of the Dragendorff's reagent. Saponins are characterized by the presence of foam. The detection of the reducing compounds consists of treating the extract with Fehling liquor and then heating it in a water bath; a positive test is indicated by the formation of a red brick precipitate. Besides, to test the presence of glycosides, the reaction consists in adding to the solution of the extract acetic acid, concentrated sulfuric acid and drops of FeCl3 at 2% 44-50 .

Study of the antioxidant activity
The antioxidant activity of Cistus monspeliensis extract was determined by two methods: the DPPH • free radical reduction test (1,1-diphenyl-2-picryl hydrazyl) and Ferric Reducing Antioxidant Power (FRAP).

DPPH free radical reduction test
In this test, the reducing power of Cistus monspeliensis extract was measured according to the protocol described by Olugbami et al. 51  This formula allowed to trace the straight line (y = ax + b) representing the variation of the percentage inhibition of each sample. From this regression equation, it is possible to calculate the concentration that reduces 50% of DPPH in each sample and ascorbic acid. This concentration, termed as IC50, is usually calculated by the following equation: Where a: slope of the line, b: intercept of the line

Ferric Reducing Antioxidant Power FRAP
The ferric reducing antioxidant power (FRAP) of Cistus monspeliensis extract was evaluated following the method described by Gonçalves et al. 52 . In test tubes each containing 1 mL of the sample solution, 2.5 mL of phosphate buffer (0.2 M, pH 6.6) and 2.5 mL of potassium hexacyanoferrate [(K3Fe(CN)6), 1%] were added. The mixture was incubated at 50°C for 20 minutes. 2.5 mL of 10% trichloroacetic acid was then added, and the mixture was centrifuged at 3000 rpm for 10 minutes. Finally, 2.5 mL of supernatant, 2.5 mL of distilled water were mixed with 0.5 mL of ferric chloride [(FeCl3), 0.1%] are added. A blank is prepared under the same operational conditions. The reading of the optical density is performed at 700 nm, three measurements are determined for each solution. The antioxidant power of the extracts is evaluated by comparing curves obtained with the straight line of the ascorbic acid used as standard. The results are obtained by calculating IC50, which expresses the concentration equivalent to the absorbance 0.5 53 .

Specimen preparation
The material used, as the working electrode for polarization and EIS measurements, in this study is ordinary steel, it is chemical, and mass composition are illustrated in Table 2. Coupons were mechanically cut into 1×1 cm, polished with emery paper (from 120 to 2000 grade), rinsed by distilled water and dried at room temperature before each test.

Solution preparation
The corrosive solution used in this study was an acid solution (1 M HCl), obtained by dilution of concentrated hydrochloric acid (37%) with distilled water. The concentration range of Cistus monspeliensis extract studied was varied from 0.05 to 1 g/L.

Electrochemical tests
Electrochemical tests including potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) were performed using a threeelectrode cell: Platinum foil, saturated calomel electrode (SCE) and ordinary steel strips as a counter, reference and working electrodes, respectively. All experiments were recorded using PGZ 100 Potentiostat/ Galvanostat controlled by a computer associated with "Volta Master 4" software that was used for evaluating the experimental data. Electrochemical experiments were analyzed using electrochemical software ORIGIN 6. Electrochemical parameters were then extracted using to EC-Lab V10.02 software.

Polarization measurements
Polarization measurements were carried out using a three-electrode cell: platinum as a counter electrode, ordinary steel as a working electrode, and a saturated calomel electrode (SCE) as the reference electrode. A mixture of 50 mL HCl (as blank) and different concentrations of the extract were used as test solutions for this study. Before any tests, the electrochemical system was stabilized for 30 minutes to attain steady-state corrosion potential (Ecorr). The anodic and cathodic polarization curves were recorded by a constant sweep rate of 1 mV/s. Corrosion current density (icorr) and corrosion potential (Ecorr) values were obtained by software, and inhibition efficiency (ηpp) at different inhibitor concentrations was calculated using the following equation 54 : x 100 Where i 0 corr and icorr are the corrosion current densities of ordinary steel in the absence and presence of inhibitor, respectively.

Electrochemical impedance spectroscopy (EIS)
Electrochemical impedance spectroscopy experiments were carried out under the same conditions as the plot of polarization curves, with a frequency range from 100 kHz to 100 MHz using (peak to peak) alternating amplitude signals of 10 mV. The EIS diagrams were plotted in the Nyquist representation. From these plots, the charge transfer resistance (Rct) and the double-layer capacitance (Cdl) values were investigated, and inhibition efficiency (ηimp) was calculated at each inhibitor concentration using the following equation 55 : ct)/Rct] x 100 Where Rct and R 0 ct are the charge transfer resistance values of ordinary steel in the presence and absence of inhibitor, respectively.

The yield of extraction and phytochemical tests
After delipidation by hexane, Cistus monspeliensis extraction by maceration in an acetone/water mixture gave a 27.6% yield of polar extract. In addition, the phytochemical characterization tests carried out on the extract obtained provided the results illustrated in Table 3. These results show that this extract is rich on phenolic compounds in the form of hydrolysable tannins, condensed tannins and flavonoids; it also contains sugars and glycosides. In previous work, Karim et al. 56 reported that the aqueous extract of Moroccan Cistus monspeliensis is obtained with a yield of 18.83%. Another study showed that the yield of the ethanolic extract of the same plant from northwest Morocco is 25.18% 57 .

Result of antioxidant activity
The straights in Figure 1 represent the variation, as a function of the concentration of the extract, of the absorbance obtained in the FRAP test (A) and the percentage of inhibition obtained in the DPPH reduction test (B). In the same figure, these lines are compared with those obtained with ascorbic acid, the reference antioxidant. Table 4 shows the IC0.5 and IC50 values obtained respectively in the FRAP and DPPH test for the extract and ascorbic acid. The extract showed a very high ability to reduce ferric ion with an IC0.5 value of 0.102 ± 0.006 mg/mL, which is close to that of ascorbic acid at 0.093 ± 0.003 mg/mL for the test FRAP. This reducing power can be attributed to the presence of phenolic compounds; previous studies showed a good correlation between the reducing power of iron and the amount of polyphenols [58][59][60] .
Cistus monspeliensis extract also has significant antioxidant activity (IC50 = 0.077 ± 0.009 mg / mL) for the DPPH reduction test. This exciting result, which exceeds the antioxidant power of ascorbic acid (IC50 = 0.082 ± 0.012 mg/mL), is probably due to its richness in various phenolic compounds identified by phytochemical tests. These molecules reduce the DPPH radical because of their ability to release a hydrogen atom 61-63 .

Potentiodynamic polarization measurements
Potentiodynamic anodic and cathodic polarization curves for ordinary steel in 1 M HCl solution in the absence and presence of different concentrations of Cistus monspeliensis extract at 298 K are shown in Figure 2. The kinetics parameters of the corrosion including corrosion potential Ecorr, corrosion current density (icorr), cathodic and anodic Tafel slopes (βc and βa) and inhibition efficiency (ηpp) are represented in Table 5. It appears from these results that the addition of the extract decreases corrosion current density, indicating that the extracts retarded corrosion rate of ordinary steel samples in HCl solutions. Also, it can be seen that the inhibition efficiency ηpp(%) of the extract increases with the inhibitor concentrations, to an optimum concentration (0.25 g/L), above that concentration, the value of this parameter decreased. Compared with the blank solution, when the change of the corrosion potential is above 85 mV, the inhibitor can be classified as cathodic or anodic, depending on the direction of displacement 64 . Therefore, the corrosion potential (Ecorr) of the ordinary steel has been cathodically displaced from 35 to 135 mV relatively to blank, indicating that the Cistus monspeliensis extract exhibits cathodic inhibition effects.

Electrochemical Impedance Spectroscopy
Electrochemical impedance diagrams relating to the ordinary steel / 1 M HCl interface in the absence and presence of Cistus monspeliensis extract at different concentrations are represented in Figure 3. The Nyquist plots show that the diameter increased for all concentrations. The analysis of the shape of these spectra exhibits that the curves are represented by a single capacitive semi-circle, showing that the corrosion was mainly controlled by charge transfer process 65 .
The simple equivalent circuit, shown in Figure 4, is used to investigate technical details of impedance spectra. It allows calculating various parameters, such as solution resistance Rs, charge transfer resistance Rct, double layer capacitance Cdl and magnitude of constant phase element (CPE). CPE is used to give a more accurate fit. Its impedance function (ZCPE) could be explained as follows 66 : Where Y0 is the CPE constant, n is a CPE exponent which signifies interface surface properties of the working electrode, j 2 = -1 is the imaginary number, w is the angular frequency in rad.s -1 (ω = 2πf), and f is the frequency in Hz 67 .
The double-layer capacitance (Cdl) is calculated from the equation: Where ω max = 2πfmax is the angular frequency at the maximum value of the imaginary part of the impedance spectrum, Q is the constant phase element (CPE).
The parameters associated with the impedance spectrums are reassembled in Table 6. As it can be seen from Table 5, the inhibitory efficiency increases to reach a maximum value 91.6%, polarization resistance Rct increases from 23.36 to 279.60 Ω cm 2 , Q and double-layer capacitance (Cdl) decreases from 190 to 65µF.cm -2 with the increase of Cistus monspeliensis extract concentration until 0.25 g/L. The increase in Rct values implied that an inhibitoradsorption film was formed on the steel substrate, thus retarding the charge transfer 68 . The thickness of the electrical double layer is related to Cdl by the following equation 69 : Where ε0 is the dielectric constant, εr is the relative dielectric constant, and A is the effective surface area. Therefore, a decrease in the capacity (Cdl) may be attributed to a decrease in the local dielectric constant of film, or an increase in the thickness of the electrical double layer, or both simultaneously occurred 70 .
Hence, the results gave good agreement between the inhibition efficiency of corrosion as obtained from the impedance study and polarization measurement.

Effect of temperature 3.5.1. Potentiodynamic polarization
Temperature is one of the parameters that can modify the behavior of a material in a corrosive environment. For that, we examined the effect of the temperature variation (298 K, 308 K, 318 K and 328 K), on the one hand on the interactions of the metal with the corrosive solution in the absence and presence of the inhibitor, and its influence on the inhibitory efficacies on the other hand. This variation was studied by potentiodynamic polarization (Figure 5) in the presence of 0.25 g/L of Cistus monspeliensis extract. The values of the corrosion potential (Ecorr), the corrosion current density (icorr), the slopes of the anode βa and cathode βc and the inhibition efficiency (ηpp) are presented in Table 7. It appears that the corrosion current density increases while the inhibition efficiency decreases with the increase of the extracted temperature from 92.0% to 84.8% in 1 M HCl. Which probably indicates desorption of inhibitor molecules 71,72 . It can be seen also that the extract investigated have inhibitory properties at all temperatures studied, and the values of inhibition efficiency remain slightly constant with the increase in temperature.

Thermodynamic study
The thermodynamic study quantifies the inhibition efficiency against corrosion, facilitates the calculation of thermodynamic parameters, in exploring the activation energy and interpret the type of adsorption adopted by the inhibitor. Exploring the activation energy, the dependence of the corrosion current obtained was studied as a function of temperature, using Tafel extrapolation method ( Figure 6). The corrosion reaction can be considered as an Arrhenius type process.

= −
Where Ea is the apparent activation corrosion energy, T is the absolute temperature, A is the pre-exponential constant of Arrhenius and R is the universal gas constant. Table 7 shows the calculated values of apparent activation corrosion energy in the absence and presence of Cistus monspeliensis extract. It appears that Ea in the presence of inhibitor is higher than that corresponding to 1 M HCl solution. The increase in apparent activation energy can be interpreted as physical adsorption, which means that the inhibitor is adsorbed on the substrate by interaction electrostatic (weak bonds) 54,73 . A previous study explained that the increase in activation energy

A B
can be ascribed to a significant decrease in the adsorption of the inhibitor on the ordinary steel surface with an increase in temperature, with consequentially increased in corrosion of ordinary steel about the fact that the metal is exposed to the acid medium 71,74 .
An alternative formula of the Arrhenius equation makes it possible to determine the enthalpy and the entropy of activation according to the following equation: Where h is Planck's constant, N is Avogadro's number, ΔSa is the entropy of activation, and ΔHa is the enthalpy of activation.  Table 8. Examination of these data revealed that the thermodynamic parameters (ΔHa and ΔSa) for dissolution reaction of ordinary steel in 1 M HCl in the presence of extract are higher than that obtained in the absence of inhibitor. The positive sign of enthalpies reflects the endothermic reaction of the steel dissolution process 75 . It is pointed out in the literature that a positive sign of the enthalpies reflects the endothermic nature of the steel dissolution process. The presence of inhibitors tested reveals that the corrosion process becomes more and more endothermic when compared to blank 76 .
The negative sign for the entropy of activation in both inhibited and uninhibited solutions indicates that the activation complex in rate determination step shows the association instead of the dissociation step, indicating that a decreased disorder occurs from reactant to the activated complex 77 .

Conclusion
Based on this study, the following conclusions were derived: -Phytochemical tests revealed the presence of several families of chemical compounds such as polyphenols (flavonoids, condensed tannins and hydrolysable tannins), saponins, glycosides, and reducing sugars.
-The antioxidant power of Cistus monspeliensis extract showed that this extract has an antioxidant capacity similar to that of ascorbic acid (the reference antioxidant). This activity seems to be related to the presence of phenolic compounds.
-The Cistus monspeliensis extract is efficient corrosion inhibitor for carbon steel in 1M HCl medium.
-Tafel plots showed that the extract is an excellent cathodic inhibitor.
-The effectiveness of the inhibition increases with increasing concentration of the extract to reach a maximum value of 92% at 0.25 g/L of Cistus monspeliensis extract.
-The extract remains active at the temperatures studied. The thermodynamic parameters obtained reveal physical adsorption between the extract and the surface of the metal.

A B
-A correlation was observed between the measurements obtained from the potentiodynamic polarization curves and the electrochemical impedance spectroscopy techniques.
-The Cistus monspeliensis extract can find applications as well as in the medicinal and pharmaceutical fields than industrial.