## 3 Results and Discussion
### Adsorption Isotherm Analysis  
The inhibition effect of TSCA on carbon steel is determined by the adsorption process from electrochemical impedance spectroscopy and potentiodynamic polarization tests. To study the adsorption of TSCA, the adsorption isotherm in a thermodynamic way is applied considering a good inhibition effect. The surface coverage (\(\theta\)) is the inhibition efficiency from the weight loss, EIS and potentiodynamic polarization experiment. Langmuir, Temkin, and Frumkin adsorption isotherms are used to study the relationship between the surface coverage and the concretration of TSCA (\(C\)), as shown in Figures 5-7. The adsorption of TSCA follows the Langmuir adsorption isotherm, as the linear regression coefficient of the Langmuir adsorption isotherm is closer to 1 than that of the Temkin or Frumkin adsorption isotherm from the weight loss and EIS results. In addition, the linear regression coefficients of the Langmuir adsorption isotherm and Frumkin adsorption isotherm are almost the same. So, the adsorption of TSCA follows the Langmuir adsorption isotherm from the results. The Langmuir adsorption isotherm can be expressed by the following equation:  
\[\frac{C}{\theta}=C+\frac{1}{K_{\text{ads}}} \tag{6}\]  
where \(K_{\text{ads}}\) represents the equilibrium constant for the adsorption/desorption process. The value of \(K_{\text{ads}}\) (2.40 \(\times\) 10\({}^{5}\) L/mol) can be calculated from the intercept of the fitted line. It is related to the free adsorption energy (\(\Delta G_{ads}^{0}\)), the absolute temperature (\(T\)), and the gas constant (\(R\)), as shown in the following equation:  
\[\Delta G_{\text{ads}}^{0}=-RT\ln(55.5K_{\text{ads}}) \tag{7}\]  
A large negative value of -40.64 kJ/mol for \(\Delta G_{ads}^{0}\) was obtained from the equation, indicating a strong interaction between TSCA and carbon steel on the solid/solution interface [48; 49]. Since the obtained \(\Delta G_{ads}^{0}\) is less than -40 kJ/mol, the adsorption of TSCA can be ascribed to chemical action. However, the small difference of -0.64 kJ/mol between the obtained true value and reference value reveals physical action between TSCA and carbon steel [50]. As a result, chemical adsorption contributes much to the adsorption of TSCA on the carbon steel surface, while physical adsorption contributes relatively little. The lone pair electrons of N, O and S and \(\pi\) electronics of double bonds in TSCA molecules form stable covalent coordination bonds with the empty d orbitals of iron atoms, which is beneficial to the surface coverage and inhibition efficiency of carbon steel.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \(C_{\text{TSCA}}\) (mg L\({}^{-1}\)) & \(E_{\text{corr}}\) (mV) & \(i_{\text{corr}}\) (uA\(\cdot\)cm\({}^{-2}\)) & \(-\beta_{\text{c}}\) (mV\(\cdot\)dec\({}^{-1}\)) & \(\beta_{\text{a}}\) (mV\(\cdot\)dec\({}^{-1}\)) & \(\eta_{\text{T}}\) (\%) \\ \hline
0 & \(-\)478 & 846 & 132 & 72 & - \\
1 & \(-\)480 & 284 & 123 & 75 & 66.4 \\
10 & \(-\)483 & 121 & 126 & 73 & 85.7 \\
20 & \(-\)481 & 65 & 129 & 71 & 92.3 \\
40 & \(-\)483 & 23 & 128 & 68 & 97.3 \\ \hline \end{tabular}
\end{table}
Table 3: Polarization parameters and corresponding inhibition efficiency for carbon steel immersed in 1 M HCl solutions with different contents of TSCA.  
Figure 5: Different adsorption isotherm plots for the adsorption of TSCA on the surface of carbon steel in 1 M HCl at 298 K from weight loss results: (**a**) [PERSON]’s isotherm, (**b**) [PERSON]’s isotherm, (**c**) [PERSON]’s isotherm.  
Figure 6: Different adsorption isotherm plots for the adsorption of TSCA on the surface of carbon steel in 1 M HCl at 298 K from EIS results: (**a**) [PERSON]’s isotherm, (**b**) [PERSON]’s isotherm, (**c**) [PERSON]’s isotherm.