## 3 Results and Discussion
### EIS Analysis of Inhibition Performance  
EIS is a reliable electrochemical technique, widely used to quantitatively evaluate the interfacial resistance between metals and corrosive media. Figure 9 presents the impedance response of FCD-500 in a 100 ppm Cl\({}^{-}\) solution at 80 \({}^{\circ}\)C, with different inhibitor concentrations. As shown in the Nyquist plot, the blank condition exhibited a linear impedance response, indicating that no protective film was formed on the FCD-500 surface and that corrosion proceeded directly at the metal-solution interface. In contrast, as the inhibitor concentration increased, the impedance spectra exhibited a more evident capacitive semicircle, along with an overall increase in diameter. This trend reflects an increase in the charge transfer resistance with higher inhibitor concentrations [31], suggesting enhanced corrosion protection and the formation of a more stable passive film on the FCD-500 surface [32]. In the Bode plot shown in Figure 9b, the blank condition exhibited a consistently low phase angle over a wide range of frequencies, with a broad and ill-defined peak. This implies a weak capacitive response and an insufficient passive layer at the interface [27]. In contrast, under inhibitor-added conditions, a distinct single peak was observed in the mid-frequency region, accompanied by a sharp increase in the phase angle. Moreover, as the inhibitor concentration increased, the maximum phase angle gradually increased, reaching its highest value at 17,000 ppm. The higher phase angle with an increasing inhibitor concentration suggests the formation of a more stable and compact passive film on the metal surface, which effectively suppresses charge transfer at the metal-solution interface and confirms the improved corrosion inhibition performance. Figure 8: Corrosion current density of FCD-500 with varying inhibitor dosage (0–17,000 ppm) and Cl\({}^{-}\) concentration (0–500 ppm) at 80 \({}^{\circ}\)C.  
Figure 10 shows the equivalent circuit model using the fitted EIS data, and the detailed parameters are provided in Table 4. Here, \(R_{s}\) represents the uncompensated solution resistance, and \(R_{ct}\) is the charge transfer resistance. \(Q\) denotes the constant phase element (CPE), which is used in place of the double-layer capacitance (\(C_{all}\)) to account for the non-ideal capacitive behavior of the working electrode. Moreover, \(\chi^{2}\) was used to evaluate the goodness of fit, \(Y_{o}\) represents the magnitude of the pseudo-capacitance, and \(n\) indicates the deviation from the ideal capacitive behavior. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Inhibitor** & \(R_{s}\)**, \(\Omega\)** & \(\begin{array}{c}Y_{o}\times\\ \mathbf{10}^{-4}\)**, \(\mathbf{S}\cdot\mathbf{s}^{\mathbf{n}}\end{array}\) & \(n\) & \(R_{ct}\)**, \(\Omega\)** & \(IE\)**, \%** & \(\begin{array}{c}\mathbf{x}^{2}\times\\ \mathbf{10}^{-3}\end{array}\) \\ \hline Blank (0) & 528.1 & 25.81 & 0.86 & 991.6 & 0 & 1.08 \\ \hline
7000 & 182.8 & 6.44 & 0.59 & 3492 & 71.6 & 0.3 \\ \hline
9000 & 161.0 & 0.65 & 0.88 & 30,280 & 96.7 & 1.04 \\ \hline
15,000 & 99.75 & 0.62 & 0.9 & 35,990 & 97.2 & 0.33 \\ \hline
17,000 & 95.87 & 0.71 & 0.88 & 36,100 & 97.3 & 0.61 \\ \hline \end{tabular}
\end{table} 
Table 4: EIS parameters and inhibition efficiency for the FCD-500 in 100 ppm Cl\({}^{-}\). Figure 10: Equivalent circuits used to fit the EIS data of FCD-500 in regard to the experiment conditions. Figure 9: EIS results for FCD-500 in a 100 ppm Cl\({}^{-}\) solution with different inhibitor concentrations at 80 \({}^{\circ}\)C: (**a**) Nyquist plot, and (**b**) Bode plot. The \(R_{s}\) was measured to be 528.1 \(\Omega\) under the blank condition and decreased progressively with an increasing inhibitor dosage, reaching 95.87 \(\Omega\) at 17,000 ppm [33]. This trend is attributed to the increase in solution conductivity, as the nitrate-based inhibitor acts as an electrolyte in the solution [34; 35].