## 3 Results and Discussions
### Electrochemical Analysis
This second stage corresponds to the prolonged time of adsorption, where the available binding sites decrease and the ionic species in the solution no longer have sufficient space to adsorb directly on the surface (observed as a plateau in Figure 2a). Further adsorption results in the formation of a reversible layer, as the ionic species in the solution tend to stay close to the layer formed in the first stage and no longer attach directly to the surface but rather near the formed layer. This corresponds to a weak binding, and as a result, the ionic species can be easily desorbed. The adsorption of species onto metallic surfaces was found to well describe the experimental data for this work. Complete saturation appears at ~60 min. Using EIS, the fractional surface coverage (\(\theta\)) can be calculated (according to Equation (2)) because the saturation capacitance represents the capacitance of the fully covered electrode surface. \[\theta=(C_{dl\text{o}}-C_{\text{dl}})/C_{dl\text{o}} \tag{2}\]  
Figure 2: Variation of: (**a**) \(C_{dl}\) parameter, (**b**) surface cover \(\theta\) during the adsorption process of EEA4 NB over time at different concentrations, and (**c**) Langmuir isotherm. The IE is associated with the degree of inhibitor coverage at the metal surface. The adsorption isotherm needs to be determined to understand how the inhibitors interact with the metal surface. The adsorption isotherms calculated at different concentrations were found to best fit the Langmuir isotherm (Equations (3) (Langmuir Isotherm) and (4)) [1,20]:  
\[\frac{C}{\theta}=\frac{1}{K_{ads}}+C \tag{3}\]  
\[R_{Langmuir}=\frac{1}{1+K_{ads}C} \tag{4}\]  
where \(\theta\) is the surface coverage, \(C\) is the EEA4 NB concentration, \(K_{ads}\) is the adsorption equilibrium constant, and \(R_{Langmuir}\) is the separation factor. Experiments related to the adsorption kinetics of EEA4 NB on carbon steel were well-described by the Langmuir model, as indicated by the high linear regression coefficient (\(R^{2}\)). The results indicate that adsorption is a time-dependent process and serves as the rate-limiting mechanism. The experimental data fit well with the \(\theta\) values of the Langmuir isotherm, with an \(R^{2}\) value closest to unity (\(R^{2}\) = 0.9997). Figure 2c displays the linear plots for the considered isotherm obtained using Equation (3). The \(K_{ads}\) determined for the Langmuir isotherm is 3460.514 mol\({}^{-1}\) and represents the strength between the adsorbate and adsorbent R\({}_{\text{Langmuir}}\) is less than one, indicating the existence of interactions between the adsorbed EEA4 NB molecules and metal surface. The free energy of adsorption (\(\Delta G^{\circ}{}_{ads}\)) is \(-\)30.141 kJ\(\cdot\)mol\({}^{-1}\) and was calculated using Equation (5) [1,20]:  
\[\Delta G^{\circ}{}_{ads}=-RTLn(55.5K_{ads}) \tag{5}\]  
where \(R\) is the universal gas constant (8.314 J mol\({}^{-1}\) K\({}^{-1}\)), \(T\) is the absolute temperature in Kelvin, \(\Delta G^{\circ}{}_{ads}\) is the standard free energy of adsorption, and 55.5 is the concentration of water in the solution in mol dm\({}^{-3}\). The negative \(\Delta G^{\circ}{}_{ads}\) value, falling within the range of \([-\)20, \(-\)40] kJ/mol, and the high value of \(K_{ads}\) indicate that the adsorption process occurs spontaneously, leading to the formation of a stable layer on the metal surface. This process takes place through a mixed process of physical and chemical adsorption [35]. Potentiodynamic polarization curves were obtained for carbon steel after a 60-min immersion in 3% NaCl solutions, both with and without EEA4 NB, as shown in Figure 3.  
Figure 3: Potentiodynamic polarization curves for carbon steel in a 3% NaCl solution, both with and without EEA4 NB, at a scan rate of 1 mV/s.