## 3 Results and Discussion
### Equivalent Electrochemical Circuits Analysis  
Different equivalent electrochemical circuits are used to fit the Nyquist plots for the analysis of the EIS results, which are distinguished based on the real conditions on adsorption mechanism of different corrosion inhibitors. The change transfer resistance of passive film on different groups is fitted by following equivalent electrochemical circuits, as shown in Figure 6. All of the parameters fitted by the following equivalent electrochemical circuits were accurate to 97%. R\({}_{8}\) represents the resistance of solution, C\({}_{\text{dl}}\) represents the ideal double-layer capacitance, the coefficient \(n\) is close to 1, and Q reflects the constant phase element (0 \(<n<\) 1, being equal to the value of C\({}_{\text{dl}}\) in real conditions). The function of Q\({}_{CPE}\) can be calculated by Equation (1) [38]. Significantly, \(n\) reflects the phase shift regarding to the surface homogeneous degree. Therefore, homogeneous and smooth surface can be described as C\({}_{\text{dl}}\), and inhomogeneous and rough surface can be described as Q. R represents the resistances of passive film formed by different corrosion inhibitors. R1 and R2 represent the resistance of organic components or organic components, and Rct represents the charge transfer resistance. \[Q_{CPE}=Y_{0}^{-1}(j\omega)^{-n} \tag{1}\]  
Figure 5: EIS curves of steel bars immersed in SCP solutions with different corrosion inhibitors at 60 days. It was evident in Figure 6b that the simple equivalent electrochemical circuit could reflect the adsorption characteristics of calcium nitrite. It mainly contained the charge transfer resistance (Rct) in parallel with C\({}_{\text{dl}}\). This proves that the additional contribution provided by calcium nitrite contributes to the formation of passive film [39]. As shown in Figure 6e, the equivalent electrochemical circuit of inorganic corrosion inhibitor is similar to that of calcium nitrite [40]. R1 is in parallel with Q in equivalent electrochemical circuits. The composition and characteristics of passive film are accurately represented by many equivalent circuit elements, such as Q and C. Along with the converse phenomenon of Q to C, more inhomogeneous and rougher passive film are formed [38]. Compared with calcium nitrite and inorganic corrosion inhibitors, passive film forms in more complicated structures with organic or multi-component corrosion inhibitors. The R1 and R2 display on the fitting circuit are in parallel with capacitor C\({}_{\text{dl}}\) and element Q, respectively. It corroborates that the double-layer structure of passive film could form on the steel bars with the soaking process. Table 3 illustrates the EIS fitting parameters in terms of different Nyquist plots (Figures 3-5) and equivalent electrochemical circuits (Figure 6). The value of Rct is analyzed in detail, so that the protective efficiency of corrosion inhibitors on steel bars are reflected [38]. A great Rct value confirms a thick passive film grows on steel bar lateral. Moreover, the thickness and stabilization of passive film directly determines the steel bar protecting effect. With the attack of chloride, steel bars begin to corrode. The resistance of reference group changes from 93.85 k\(\Omega\cdot\)cm\({}^{2}\) at 5 days to 5.63 k\(\Omega\cdot\)cm\({}^{2}\) at 60 days, indicating the destruction of the passive film. The addition of all kinds of corrosion inhibitors could improve the Rct reference group. Thus, all the corrosion inhibitors are effective in corrosion resistance. For calcium nitrite, the Rct value was 69.54 k\(\Omega\cdot\)cm\({}^{2}\) at 5 days, which is quite close to the reference group. Absolutely, during 60 days' immersion, the resistance of the steel bar raises to 465.17 k\(\Omega\cdot\)cm\({}^{2}\). In other words, the anti-corrosion efficiency of calcium nitrite appears in  
Figure 6: Equivalent electrochemical circuits used for fitting EIS results.