## 3 Results and Discussion
### Electrochemical Impedance Spectroscopy  
In order to understand the action mode of several synthetic and natural corrosion inhibitors on mild steel in acid medium, EIS was exploited in many published studies [41; 42; 43]. Regarding the applied potential used in this method, a small sinusoidal excitation is imposed and the electrochemical metal-solution interface results in impedance. Using the different impedance data, Nyquist plots for mild steel in 1 M HCl at 308 K containing different concentrations of AO and HERA (Figure 6) were established. Table 4 includes the impedance parameters derived from the Nyquist plots. Figure 5: Langmuir adsorption isotherm of mild steel in 1 M HCl in the presence and the absence of AO and HERA. The difference in impedance at lower and higher frequencies allows the calculation of charge transfer resistances [44]. At the imaginary component's maximum Nyquist plot, double layer capacitances (\(C_{dl}\)) were calculated by use of Equation (10):  
\[C_{dl}=\frac{1}{2\pi\times f_{max}\times R_{t}} \tag{11}\]  
where \(R_{t}\) is the charge transfer resistance (\(\Omega\cdot\)cm\({}^{2}\)), \(f_{max}\) is the maximum frequency (Hz), and \(C_{dl}\) is double layer capacitance (\(\mu\)F\(\cdot\)cm\({}^{-2}\)). The obtained plots were depressed and semi-circular in shape with the center below the impedance's real component. Deviation of perfect circular shape is often related to the frequency dispersion of interfacial impedance which arises because of surface roughness and interfacial phenomena [45]. The plots' sizes and shapes were increased by increasing the inhibitors' concentrations, indicating that the corrosion of mild steel is mainly controlled by a charge transfer process [46]. We notice that an increase in inhibitors' concentrations provokes an increased charge transfer resistances value; whereas, the double-layer values decrease. It has been reported that the adsorption process on the metal surface is characterized by a decrease in double layer values [44]. [PERSON] et al. proved that the salt is added to the oil in order to ameliorate its preservation [1]. This study proves that in addition to its preservative role, salt ameliorates corrosion inhibition efficiency: HERA, the tested \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \begin{tabular}{c} **Concentration** \\ **(g/L)** \\ \end{tabular} & \begin{tabular}{c} **\(R_{s}\)** \\ **(\(\Omega\cdot\)cm\({}^{2}\))** \\ \end{tabular} & \begin{tabular}{c} **\(R_{t}\)** \\ **(\(\Omega\cdot\)cm\({}^{2}\))** \\ \end{tabular} & \
\begin{tabular}{c} **\(C_{dl}\)** \\ **(\(\mu\)F/cm\({}^{2}\))** \\ \end{tabular} &
\begin{tabular}{c} **\(E\) (\%)** \\ **(\%)** \\ \end{tabular} \\ \hline \multirow{4}{*}{**A**O} & **Bank** & **-** & **2.63** & **40.31** & **29.59** & **133.41** & **-** \\ \cline{2-7}  & **0.06** & **2.91** & **92.59** & **24.21** & **75.91** & **56.47** \\  & **0.12** & **9.84** & **114.7** & **18.28** & **72.81** & **64.86** \\  & **0.25** & **3.68** & **152.5** & **15.44** & **70.99** & **73.57** \\  & **0.5** & **7.83** & **182.7** & **12.01** & **70.33** & **77.94** \\  & **1** & **6.19** & **187.2** & **12.09** & **67.68** & **78.47** \\  & **2** & **3.28** & **212.3** & **12.31** & **60.92** & **81.01** \\ \hline \multirow{4}{*}{**HERA**} & **0.06** & **2.75** & **79.22** & **14.35** & **140.07** & **49.12** \\  & **0.12** & **2.86** & **112.1** & **15.42** & **92.08** & **64.04** \\ \cline{1-1}  & **0.25** & **2.80** & **122.9** & **19.55** & **77.72** & **67.20** \\ \cline{1-1}  & **0.5** & **2.85** & **132.3** & **15.48** & **74.15** & **69.53** \\ \cline{1-1}  & **1** & **2.97** & **138.6** & **15.28** & **66.24** & **70.92** \\ \cline{1-1}  & **2** & **3.04** & **182.7** & **15.55** & **56.05** & **77.94** \\ \hline \hline \end{tabular}
\end{table} 
Table 4: EIS parameters of mild steel in 1 M HCl in presence and absence of AO and HERA. Figure 6: Nyquist plots for mild steel in 1 M HCl in presence and absence of AO and HERA.