The antioxidant activity by the DPPH test was performed following the method described by BEKTAS with a few modifications (Moussaoui et al., 2021). Briefly, different concertinos of Wf-EO were mixed with 750 µl of DPPH (4.10⁻² mg/ml). After incubation at ambient temperature for 30 min, the reading of absorbance was conducted at 517 nm. The results were expressed as percentage inhibition according to the following formula: PI ( % ) = ( A 0 − A / A 0 ) ∗ 100 , (1) where PI is percentage of inhibition; A₀ is absorbance of the DPPH solution (negative control); and A is absorbance of the DPPH solution mixed with sample.

The FRAP test was performed according to Moattar’s method. In summary, 500 µl of a buffer phosphate solution with 0.2 M, pH = 6.6, and 500 µl of potassium ferricyanide [K₃Fe(CN)₆] (10 mg/ml) were mixed with 100 µl of different concentrations of Wf-EO. After incubation in a water bath (50°C) for 20 min, 500 µl of both aqueous TCA solution (10%), and distilled water, and 100 µl FeCl₃ (0.1%) were supplemented to the reaction medium. Next, the absorbance of the sample (Wf-EO) was measured at 700 nm, and the results were expressed as half-maximal effective concentration (EC-50) (EL Moussaoui et al., 2019c).

Twenty-five microliters of the Wf-EO were mixed with reagent solution including sulfuric acid (0.6 M), sodium phosphate (28 mM), and ammonium molybdate (4 mM). Afterward, the reaction medium was incubated at 95°C for 90 min. The absorbance of the solution was measured at 695 nm using a spectrophotometer). The results were expressed in mg EAA/g Wf-EO (EL Moussaoui et al., 2019c).

β-carotene (1 ml) was mixed with chloroform (0.2 mg/ml), and then the whole was then mixed with 10 µl linoleic acid and Tween 80 (100 mg). Next, the chloroform solvent was evaporated under reduced pressure at 45°C and 25 ml of hydrogen peroxide (H₂O₂) was added to the reaction medium. The resulting mixture (2.5 ml) was mixed with 0.1 ml of the diluted Wf-EO and then incubated at 50°C for 120 min with BHT reagent. The antioxidant percentage was calculated by the following formula: AA % = ( AE / ABHT ) × 100 , (2) where AA% is the antioxidant activity percentage, AE is the absorbance of the sample, and ABHT is the absorbance of the sample without BHT.

Mild steel (MS) with following chemical composition: 0.3700% C, 0.1600% Cu, 0.2300% Si, 0.6800% Mn, 0.0770% Cr, 0.0160% S, 0.0090% Co, 0.0110% Ti, 0.0590% Ni, and the remainder iron (Fe) was used as a substrate for anticorrosion activity testing. Before all measurements, the sample was abraded using emery papers, rinsed with water before being degreased in acetone, and finally washed again using distilled water.

The electrolyte was obtained through dilution of hydrochloric acid (37%) (d = 1.18). In this sense, Wf-EO was previously dissolved in 10 ml of ethanol bi-distilled before being added to 90 ml of 1.0 M HCl. The concentrations used in this work ranged from 1 g L⁻¹ to 0.1 g L⁻¹.

Electrochemical measurements were performed using the BioLogic SP-150 device controlled with software (Ec-Lab® V11.200). All experiments were conducted in the conventional three-electrode cell with MS described earlier as a serving electrode, platinum and Ag/AgCl 3M KCl were used as reference and counter electrodes, respectively. For the potentiodynamic polarization (PP) measurements, the potential was swept from −250 to +250 mV at a scan rate of 1 mV s ⁻¹ relative to the potential were swept. Electrochemical impedance spectroscopy (EIS) measurements were carried out after a steady-state was achieved in a frequency ranging from 100.00 kHz to 100.00 mHz with ten points per decade and AC voltage amplitude of 10 mV peak to peak. Before each measurement, the electrode was kept at E _corr for 30 min at a temperature of 298 K and the test was repeated twice.

PP measurements are a fundamental technique used to collect information related to an electrochemical system. The perturbations of MS in 1.0 M HCl medium, in the absence and presence of Wf-EO, were carried out after 30 min of stabilization at the corrosion-free potential called open circuit potential of the electrochemical system at 298 K. As reported in Figure 4. The electrochemical parameters related to this system such as corrosion current density (icorr), corrosion potential (Ecorr), and Tafel cathodic coefficient (β _c) were listed in Table 2. The icorr were determined from the extrapolation of cathodic Tafel lines, which exhibit a real Tafelian behavior (McCafferty, 2005). The inhibition efficiency (ɳ _Tafel ) was calculated via using η T a f e l   = ( 1 − i c o r r / i n h i c o r r ) × 100 , (3) where i _corr and i _corr/inh represent corrosion current density values without and with Wf-EO, respectively.

As can be shown from Figure 4, both cathodic and anodic reactions were inhibited through the addition of Wf-EO in acidic solution. The cathodic parallel Tafel lines (Figure 4) and the slight changes in the cathodic Tafel slopes β _c (Table 2) indicated that the Tafel’s law was verified in this domain (Rice-Evans et al., 1996). In this sense, the mechanism of H⁺ proton discharge was activation controlled by the addition of Wf-EO in acidic solution without modification of the process mechanism. Nevertheless, in the anodic domain with potentials higher than −270 mVAg/AgCl, 3M KCl, Wf-EO did not modify the current versus potential characteristics when compared to the uninhibited medium. This phenomenon could be attributed to the desorption of the Wf-EO compounds from the electrode surface at a potential and can therefore be defined as desorption potential E _d , commonly reported in the literature (Beniken et al., 2018; Aourabi et al., 2021).

The corrosion potential E _corr is shifted toward more anodic values as shown in Table 2. In this sense, if the displacement in E _corr(inh) is larger than 85 mV from E _corr in free inhibitor, the inhibitor can be seen as a cathodic or anodic type (Bensouda et al., 2019), and if the displacement in E _corr(inh) is less than 85.00 mV, the inhibitor can be considered as a mixed type (Soltaninejad and Shahidi, 2018; Guruprasad et al., 2019). In this study, the largest value of potential displacement achieved is of the magnitude of 74 mV indicating that Wf-EO acts as a mixed type inhibitor with the domination of the anodic character.

Table 2 shows that the values of current densities of i _corr decrease considerably with rising Wf-EO concentrations from 611.61 µA cm ⁻² in 1.0 M HCl alone to 107.72 µA cm ⁻² at a concentration of 1.00 g L⁻¹ of Wf-EO. This diminution in current density provides the inhibition efficiency to reach a maximum value of 82.39%. It can also be noted that no enhancement in the inhibiting efficiency is observed despite increasing the concentration of Wf-EO, that is a plateau of ɳ _Tafe % was registered from 1 g L⁻¹ of Wf-EO concentration.

For understanding the mechanism that took place at the electrode/solution interface, EIS constitutes a very powerful technique for investigating corrosion inhibition processes. The Nyquist and Bode diagrams of MS immersed into the corrosive solution 1.0 M HCl, without and with concentrations of Wf-EO are displayed in Figure 5.

The Nyquist diagrams show at first glance only one single depressed capacitive loop, which showed that corrosion of MS can be monitored by the charge transfer process. This phenomenon of capacitance dispersion at the electrode interface strongly depends on the state of the electrode surface, that is, its roughness, degree of polycrystallinity, and also an anion adsorbtion. The deviation from ideal capacitive behavior can be empirically represented by a constant phase element (CPE), a complex impedance having a special property with a phase angle independent of frequency, as reported by Fricke (Fricke, 1932). The impedance of CPE component can be expressed by using Z CPE = Q − 1 ( j ω ) − n , (4) where Q is a related parameter to the electrode capacitance (F sⁿ⁻¹ cm⁻²), and n is the constant phase exponent often used as a gauge of the roughness or heterogeneity of the surface (0 < n < 1) associated with the deviation of the straight capacitive line from 90° by an angle α, whether α can calculated by Eq. 5 and j ² =−1 is a y number of imaginary whilst ω is the angular frequency. α = 90 ° ( 1 − n ) . (5)

Therefore, Bode-plots are recommended for the display of EIS data in terms of frequency. Bode-plots permit the detection of regions that are dominated by resistive elements such as resistance of solution R _s and polarization resistance R _p in which a slope of zero was remarked; meanwhile, regions dominated by pure capacitive element for which a slope of (−1) were noticed in the ideal case. However, the width of Gaussian (φ-log f) obtained in the presence of different Wf-EO concentrations is much larger than that obtained in 1.0 M HCl alone. This result stipulates most likely the appearance of a second time constant, although not well separated from that of charge transfer in the presence of the different Wf-EO concentrations. This second phenomenon of relaxation or time constant is generally attributed to the presence of an additional obvious process on the metal surface. This simple qualitative analysis confirms the formation of a barrier layer.

Among the numerous equivalent circuits that have been used to describe the present electrochemical interface to extract the necessary parameters for understanding the studied system, only the following circuit with two-time constants R _s + CPE _dl /[R _ct + CPE _a /R _a ] has been retained as presented in Figure 6. R _s is the solution resistance, R _ct is the charge transfer resistance, and the time constant (CPE _dl ) at high frequency referring to the charge transfer process. R _a is the resistance of the adsorbed inhibitor whilst CPE _a represents the relaxation time constant of the adsorbed layer to the MS surface. Therefore, an excellent parametric readjustment of the experimental impedance spectra in the Nyquist and Bode planes was obtained. Moreover, the experimental and simulated spectra were well correlated with a χ′²/|Z| coefficient, and therefore, they support the validation of this model.

The impedance parameter recorded from the fitting of diagrams and η E I S % are presented in Table 3. The inhibition efficiency issued from the polarization resistance (PR) in the presence of the different Wf-EO concentrations is calculated according to η E I S % = ( 1 −   R p R p / i n h ) × 100 , (6) where R _p is the polarization resistance calculated through the following equation (R _P = R _ct + R _a ).

The data displayed in Table 3 clearly shows that in the concentration range, the PR enhances with Wf-EO concentration; hence, the best inhibition efficiency reached was 87.77%. The parameter of CPE _dl component Q _dl decreased abruptly with the addition of Wf-EO when compared to the uninhibited solution through presumably the formation of a protective adsorbed layer. The improvement of this layer was associated with an increase in n _d when compared to the blank solution that was associated with a slight decrease in the surface heterogeneity (Aourabi et al., 2020). It was noted that the charge transfer resistance R _ct increased with increasing concentration of Wf-EO and the total resistance R _p was rather dominated by R _a .

According to the model of Bonnel et al. (1983), the resistance associated with the high-frequency portion of the spectrum is hypothesized to contain cathodic and anodic contributions of the charge transfer reaction. This confirms the mixed character obtained from PP measurement. The resistance related to the low-frequency portion is regarded to contain the explicit contribution from mass transport across the porous corrosion products. This latter result was reflected by the lowest value of n _a which can be attributed as a measure of the energy distribution since the adsorbed molecules were electronic charge carriers (Popova and Raicheva, 1996).

During the process of inhibitor adsorption on the surface of metal - assisted adsorption, the process is controlled by the residual charge on the chemical structure and the inhibitor metal. The increasing addition of a corrosion inhibitor results in a progressive decrease in the rate of corrosion and/or enhancement of corrosion inhibition. The relationship between the concentration of inhibitor and corrosion rate was investigated by several models of isotherms. In this sense, it was reported that the common adsorption isotherms used to adjust the experimental data are Freundlich, Temkin, Langmuir, and the El-Awady kinetic–thermodynamic model. The conventional and linearized forms of isotherms are figured in Table 4.

In the present research work, θ values corresponding to different concentrations of Wf-EO are easily determined from EIS (θ _EIS ) method, by the ratio η%/100. Figure 7 displays different models of adsorption isotherm such as Langmuir, Freundlich, Temkin, and El-Awady and also the related parameters of adsorption deduced from these isotherms.

These results show that all the coefficients are close to unity with the better fit provided by the use of the adsorption isotherm Langmuir model with R ² = 0.9999; meanwhile, the slope of the corresponding formula slightly deviates from unity. Accordingly, the model of Langmuir seems to be more appropriate to describe the adsorption process as reported in earlier works (Durnie et al., 2001; Obot et al., 2009). On the contrary, the reciprocal of ‘y’ obtained from the El-Awady model is nearly equal to 4 and can therefore suggest that four water molecules were replaced by one from Wf-EO in the inhibition process. Based on this result, it can be concluded that the first is contradictory with the hypothesis on which the Langmuir isotherm is based, which stipulates that one molecule of inhibitor replaces one molecule of water. Moreover, the adsorption process of Wf-EO molecules also follows the Temkin isotherm, which indicated the presence of molecular interaction in the adsorbed layer. However, this hypothesis is practically consistent with the Langmuir isotherm model and it is also confirmed by the negative sign of intermolecular interaction in the adsorption layer ‘a’ of the Temkin model (Oguzie, 2007). It is thus fitting that the adsorption process investigated in the present work could not be reasonably modeled by the Freundlich isotherm model even though R ² values obtained from the corresponding plots were good.

In summary, the possibility of ideal Langmuir-type adsorption is unthinkable due to fact that the surface heterogeneity is evidenced by EIS measurements where the CPE exponents are far from unity. Furthermore, a molecule of water would probably be replaced by three or even four molecules of oil, identical or not, as testified by the El Away isotherm with a possibility of strong interaction between them by reference to the isotherm of Temkin.