1. Introduction

2. Experimental

3. Results and discussion

3.3. Electrochemical studies

3.3.1. Potentiodynamic polarization between - 0.4 and + 0.4 V/SCE

Figure 4 reveals the potentiodynamic polarization curves of copper after a 90-minute immersion in 3 wt% NaCl solution without (blank) and with SCG oil extracts at 25°C.

We can confirm from this figure that the cathodic branch of the copper in the blank chloride solution can be divided into three regions: region I corresponds to the weak polarization region near OCP; next, region II corresponds to nearly flat current density, associated with the reduction of dissolved oxygen (Equation (2)); and region III, almost below -380 mV, is the result of the hydrogen evolution reaction mentioned in Equation (3) [24,41]

$${\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-}\to 4{\mathrm{O}\mathrm{H}}^{-}$$

(2)

$$2{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2+2{\mathrm{O}\mathrm{H}}^{-}$$

(3)

$${\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-}\to 4{\mathrm{O}\mathrm{H}}^{-}$$

(2)

$$2{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2+2{\mathrm{O}\mathrm{H}}^{-}$$

(3)

Both metal electro-dissolutions and metal soluble diffusion in the blank solution control the anodic reaction. As it was stated in our previous research, the dissolution mechanism can be represented by the following reactions [42]:

$$\mathrm{C}\mathrm{u}\to {\mathrm{C}\mathrm{u}}^{+}+{\mathrm{e}}^{-}$$

(4)

$${\mathrm{C}\mathrm{u}}^{+}+{\mathrm{C}\mathrm{l}}^{-}\to {\mathrm{C}\mathrm{u}\mathrm{C}\mathrm{l}}_{\left(\mathrm{f}\mathrm{i}\mathrm{l}\mathrm{m}\right)}$$

(5)

$$\mathrm{C}\mathrm{u}\mathrm{C}\mathrm{l}+{\mathrm{C}\mathrm{l}}^{-}\to {\mathrm{C}\mathrm{u}\mathrm{C}\mathrm{l}}_{2\left(\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{e}\right)}^{-}$$

(6)

Figure 4 also shows that the presence of SCG extract shifted the cathodic branch of the polarization curves towards more negative potentials and toward a lower current density. Thus, confirming that this oil extract has a stronger impact on oxygen cathodic reduction than on copper oxidation reaction. The E_corr values measured for the blank, 0.2g/L; 0.4g/L and 0.6g/L solutions are respectively equal to -176, -272, -287 and -274 mV/SCE. Consequently, the E_corr shift reaches 111 mV/SCE for 0.4 g/L, a value which is sufficiently high to classify the SCG extract as a cathodic inhibitor [5,24].

3.3.2. Voltammetry around OCP (ΔE=± 60 mV)

To further explain the SCG oil extract corrosion resistance performance, potentiodynamic polarization curves were obtained after a 90-minute immersion in 3 wt% NaCl solution with the addition of oil extract at different concentrations on a limited potential range (ΔE= ±60 mV). These curves were computer fitted using the EC-Lab program V10.32 (Bio-Logic) as described in section 2.3 to first determine the corrosion current density (J_corr), the Tafel coefficients (β_a and β_c) and the corrosion rate (CR) and to later calculate the polarization resistance (R_p) and the inhibition efficiency (η %) according to Equations (7,8) and (9), respectively.

$${\mathrm{R}}_{\mathrm{p}}=\frac{\mathrm{B}}{{\mathrm{J}}_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}}$$

(7)

Here B is a constant that is calculated by using Stern-Geary Equation [43],

$$\mathrm{B}=\frac{{\mathsf{\beta }}_{\mathrm{c}}{\mathsf{\beta }}_{\mathrm{a}}}{2.303({\mathsf{\beta }}_{\mathrm{c}}+{\mathsf{\beta }}_{\mathrm{a}})}$$

(8)

$$\mathsf{\eta }\left(\mathrm{\%}\right)=\frac{{\mathrm{C}\mathrm{R}}^0-\mathrm{C}\mathrm{R}}{{\mathrm{C}\mathrm{R}}^0}\times 100$$

(9)

where CR⁰ and CR are the corrosion rate in the absence and presence of inhibitor, respectively.

In Table 2, all of these electrochemical parameters were reported.

Figure 5 illustrates an example of comparison between the experimental polarization curve and Tafel curve simulated with EC-Lab software for copper with addition of SCG oil for 0.4g/L in saline solution.

In the experimental conditions that we used, the cathodic branch reveals the reactions of oxygen reduction and the anodic branch corresponding to the copper dissolution [44]. It can be noted that in the presence of SCG oil extract, the anodic Tafel slopes (β_a) have roughly constant with an average value of 50 ±7 mV/dec. This finding indicates that the presence of SCG oil has no major effect on the copper dissolution. The cathodic Tafel coefficient (β_c) was determined at 120 mV/dec in the blank solution. These values decrease after adding inhibitor to reach 60.7 ± 14 mV/dec for 0.4 g/L SCG oil concentration, which may be linked to the fact that the inhibitor mainly impacts the cathodic reaction. As it was shown in Table 2, the values of J_corr drop with the inhibitor concentration’ rise, indicating a remarkable decrease in the CR which reaches a minimum value of 0.0026 mm/year and a significant increase in the polarization resistance, especially with 0.6 g/L (Rp = 59.89 ± 2.8 kΩ.cm²). In addition, the inhibition efficiency (η) reached 95.78 % in the 0.6 g/L concentration of SCG oil extract. This confirms that SCG oil shows a good inhibition effect on copper in chloride media. Thus, the high concentration could boost protection ability against copper corrosion.

3.3.3. Electrochemical impedances spectroscopy (EIS)

Figure 5 reveals the Nyquist diagrams obtained after a 90-minute immersion in 3 wt% NaCl solution without and with various concentration of inhibitor at 25°C. The Nyquist diagram obtained without inhibitor (blank) shows a capacitive semicircle at high frequencies with a straight line inclined characteristic of a Warburg-type diffusion process at low frequencies. This semicircle shows the combination of charge transfer resistance as well as double layer capacitance. It should be noted that this capacitive loop is not completely perfect; this can be attributed to the roughness and inhomogeneity of the electrode surface [24,45,46]. The straight line could be attributed to the anodic diffusion of soluble copper species of CuCl₂^- from the electrode to the chloride solution, and to the cathodic oxygen’s diffusion [25].

Figure 5. Nyquist plots for copper exposed to 3 wt% NaCl solutions devoid of (blank) and containing different concentrations of SCG oil at RT (~25 °C).

Figure 5. Nyquist plots for copper exposed to 3 wt% NaCl solutions devoid of (blank) and containing different concentrations of SCG oil at RT (~25 °C).

The appearance of copper’s Nyquist curves in a 3 wt% NaCl solution in the presence of different concentrations of the SCG oil extract are fairly distinct from that of the copper in the absence of the SCG oil. This means that the change in the corrosion mechanism occurs following the inhibitor’s addition. The size of the capacitive loops seems to grow with the increase in extract concentration. In fact, the Nyquist loop’s diameter for a concentration of 0.6 g/L of SCG oil extract was significantly larger than that for the other electrode. This might be because the extract’s protective layer adheres to copper surface and compacts when the concentration increases [47]. Hence, it forms a stable layer on the metal surface functioning as an efficient barrier that prevents corrosion.

The logarithm of impedance amplitude (log |Z|) as a function of the logarithm of frequency (log freq) is represented in Figure 6a. At low frequencies, the amplitude of the bare copper's impedance |Z| is lower (around 10³ Ω cm²), indicating that the material easily corrodes because the CR and the value of |Z| are inversely related [48]. For the same frequency range, the |Z| value of copper was significantly higher after the inhibitor’s addition, particularly at 0.6 g/L (around 10⁴ Ω cm²). Finally, the log |Z| becomes practically constant when the frequency is low. We can also observe an increase in the resistive response of the copper electrode [49].

Figure 6. Bode plots (a) and phase angle (b) versus frequency for SCG oil for copper exposed to 3 wt % NaCl solutions at RT (~25 °C).

Figure 6. Bode plots (a) and phase angle (b) versus frequency for SCG oil for copper exposed to 3 wt % NaCl solutions at RT (~25 °C).

Figure 6b displays the phase angle plots of copper without and with various concentration of SCG oil extract after a 90-minute immersion in 3 wt% NaCl solutions. The argument of the complex impedance Z (phase shift between the current and the potential), Arg (Z) is plotted as the logarithm of frequency’s function. The phase angle plots obtained for the copper in blank saline solution reveals two-time constants: the first in the high frequency region, related to the relaxation process of the double layer capacitance and the second in the low frequency region, corresponding to the Warburg diffusion (corrosion process) [41].

Two-time constants can be seen on the curves after adding the working oil. One was in the higher frequency caused by the presence of the inhibitor while the other was shown at medium frequencies related to the double layer capacity.

In harmony with the earlier EIS findings’ description, the impedance data’s analysis was carried out with two equivalent circuits presented in Figure 7.

Figure 7. Equivalent circuits used to fit the EIS experimental data.

Figure 7. Equivalent circuits used to fit the EIS experimental data.

We describe the behavior of copper or copper alloys in chloride-containing solutions by means of these models in the literature, either without or with the adsorption of inhibitors [50,51]. Figure 7a proves suitable for the experimental impedance data of copper in the blank solution. In this model, R_s is the solution resistance, R_ct is the charge transfer resistance, Q_dl corresponds to the capacitance of the double layer as well as W is the Warburg impedance linked to the diffusion processes in the low frequency region [24]. In contrast, it was fair to use the equivalent circuit R_s(Q_f(R_f(Q_dlR_ct))) (Figure 7b) to analyze the copper’s EIS data in solutions containing SCG oil. This equivalent circuit consists of passive film resistance (R_f) and passive film capacitance (Q_f).

The "dispersion effect" at the solid/liquid interface was clearly present because the impedance loops' centers - located below the real axis (see Figure 5) - are not perfect semicircles. The solid electrode surface’s inhomogeneity and roughness are behind this phenomenon [52]. It is, therefore, required to replace the pure capacitor with a constant phase element CPE (Q) when fitting the EIS data in order to obtain a fit that is more accurate. We can calculate the impedance of CPE as follows to Equation (10) [53]:

$$Z_{\mathrm{C}\mathrm{P}\mathrm{E}}=\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$Q_0{\left(j\omega \right)}^{\mathrm{n}}$}\right.$$

(10)

where Q₀ is the magnitude of CPE, j is the imaginary root, ω is the angular frequency, and n value is attributed to the electrode’s inhomogeneous nature due to the surface roughness, porous layer formation, inhibitor adsorption, etc[54].

The electrochemical parameters obtained after EIS data’s computer fitting through electric circuits shown in Figure 7 are listed in Table 3.

Inhibition efficiency (η) can be calculated by the polarisation resistance as it was shown in Equation (11) [53]:

$$\eta =\frac{R_{\mathrm{p}}-R_{\mathrm{p}}^0}{R_{\mathrm{p}}}\times 100$$

(11)

R⁰_p and R_p are the polarization resistance of copper in 3 wt % NaCl solutions devoid of and containing SCG oil extract, respectively.

The value of R_p can be applied as an anti-corrosion ability’s indicator calculated according to the following of Equation (12) [55]:

$$R_{\mathrm{p}}=R_{\mathrm{c}\mathrm{t}}+R_{\mathrm{f}}$$

(12)

The first remark derived from the data in Table 3 is that the parameters obtained without inhibitor (blank) correlate with previously published research [24,48,55]. Furthermore, compared to the parameters we got after adding the working oil, we can see that the lowest value of R_ct (1466 Ω cm²) was obtained for the blank sample, showing the inhibition effect of SCG oil on copper electrode. Table 3 also shows that R_ct values increased from 1466 to 17980 Ω cm² with the rise in SCG oil concentrations. Increased R_ct values with SCG oil concentration are based on the rise in inhibitor surface coverage, resulting in an increase in inhibitor efficiency [56]. In addition, the R_f value rose from 3698 Ω cm² in 0.2 g/L SCG oil concentration to 8932 Ω cm² in 0.6 g/L oil concentration. Thus, the R_p value concerning 0.6 g/L of SCG oil equals 26822 Ω cm² which is 18-fold higher than that of the blank solution. As it is noticed in Table 3, the inhibition efficiency (η) of SCG increases from 85.93% to 94.55%, when the oil’s concentration rises from 0.2 g/L to 0.6 g/L%. These values are in accordance with η values derived from the potentiodynamic polarization curves recorded around OCP (Table 2). Hence, it has been demonstrated that the inhibitor forms a stable layer on the copper surface that acts as an efficient barrier that prevents corrosion.

3.3.4. Adsorption isotherms modeling

It is generally known that organic molecules inhibited corrosion by adsorption at the metal/solution interface. The adsorption isotherms can be used to provide fundamental details regarding the interaction between inhibitors and a metal surface. The adsorption of an organic adsorbate at the metal-solution interface can occur because of the substitution in the aqueous solution (Org_(sol)) and water molecules previously adsorbed on the metallic surface (H₂O_(ads)) according to the following Equation (13) [57]:

$${\mathrm{O}\mathrm{r}\mathrm{g}}_{\left(\mathrm{s}\mathrm{o}\mathrm{l}\right)}+\mathrm{x}{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{a}\mathrm{d}\mathrm{s}\right)}\to {\mathrm{O}\mathrm{r}\mathrm{g}}_{\left(\mathrm{a}\mathrm{d}\mathrm{s}\right)}+\mathrm{x}{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{s}\mathrm{o}\mathrm{l}\right)}$$

(13)

where Org_(sol) and Org_(ads) are the organic compounds present in the aqueous solution and the organic molecules adsorbed onto the metal surface, x is the ratio indicating the number of water molecules that are substituted by one organic adsorbate molecule [58].

In order to evaluate the adsorption process of SCG oil extract on the copper surface, various isotherms were used namely Langmuir, Frumkin, Temkin, Freundlich and Flory Huggins isotherms according to the following Equations (14–18) [25]:

$$\mathrm{Langmuir} \frac{C}{\theta }=\frac{1}{K_{\mathrm{a}\mathrm{d}\mathrm{s}}}+C$$

(14)

$$\mathrm{Frumkin} \mathrm{l}\mathrm{n}\left[\frac{\theta }{\left(1-\theta \right)C}\right]=2\mathrm{a}\theta +\ln K_{\mathrm{a}\mathrm{d}\mathrm{s}}$$

(15)

$$\mathrm{Temkin} \ln C=\mathrm{a}\theta -\ln K_{\mathrm{a}\mathrm{d}\mathrm{s}}$$

(16)

$$\mathrm{Freundlich} \mathrm{l}\mathrm{o}\mathrm{g}\theta =\mathrm{n}\mathrm{l}\mathrm{o}\mathrm{g}C+\log K_{\mathrm{a}\mathrm{d}\mathrm{s}}$$

(17)

$$\mathrm{Flory}\text{–}\mathrm{Huggins} \mathrm{l}\mathrm{o}\mathrm{g}\frac{\theta }{C}=\log \left(\mathrm{x}{\mathrm{K}}_{\mathrm{a}\mathrm{d}\mathrm{s}}\right)+\mathrm{x}\mathrm{l}\mathrm{o}\mathrm{g}(1-\theta )$$

(18)

where θ is the surface coverage determined through the relation θ = η/100, and from the impedance data, C is the concentration of SCG oil, K_ads is the adsorption-desorption equilibrium constant, a is the parameter of later interaction between the adsorbate species which is a strong indicator of the non-homogeneity of the surface, n is the Freundlich adsorption isotherm parameter [25,59].

To select the isotherm that best fits the experimental data, various correlation coefficients (R²) was determined for several classical adsorption isotherms. R² determined from the Langmuir adsorption isotherm gives the best fit (R²= 0.999). This behavior suggests that the species present in the SCG oil extract were adsorbed onto the copper surface according to a Langmuir adsorption isotherm. It involves the formation of a protective monolayer on the metal surface with a fixed number of actives sites [60].

A straight line is acquired by plotting of C/θ versus C as shown in Figure 8. The intercept of the straight line obtained in the Langmuir adsorption isotherm was used to calculate K_ads for the SCG oil extract. The high value of K_ads (32.89 g^-1.L) exhibits the high adsorption ability of the working oil on the metal surface [61].

Figure 8. Langmuir adsorption isotherm plot and corresponding modeling parameters for SCG oil extract on the copper surface in 3 wt % NaCl solution at RT.

Figure 8. Langmuir adsorption isotherm plot and corresponding modeling parameters for SCG oil extract on the copper surface in 3 wt % NaCl solution at RT.

The adsorption’s standard free-energy (ΔG⁰_ads) was calculated from this isotherm through the following relation (19) [62]:

ΔG⁰_ads = RT ln(1000 K_ads)

(19)

where R is the universal gas constant, T is the thermodynamic temperature and 1000 is the water’s concentration (g.L^-1). The negative values of ΔG⁰_ads (-25.782 kJmol^-1) refer to the spontaneity of the process and the strong interaction between the adsorption layer and the copper surface [63].

Generally, the level of the adsorption’s standard free energy - around - 20 kJ mol^-1 or less negative - can reveal that there is an electrostatic interaction between the inhibitor and the charged metal surface (physical adsorption); those around - 40 kJ mol^-1 or less indicates a coordination between the lone pair of O atoms or $\pi$-electrons cloud and the metallic surface (chemical adsorption) [58,63]. Therefore, the calculated value of ΔG⁰_ads is between - 40 kJ mol^-1 and - 20 kJ mol^-1, indicating that the inhibitor adsorption on the copper surface occurred by a mixed process and involves both physisorption and chemisorption [25,64].

4. Conclusions

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