## 3 Results and Discussion
### Electrochemical Measurements  
#### 3.4.1 The Electrochemical Impedance Spectroscopy  
The electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves were applied to investigate the corrosion resistance of EP/GP, EP/PDA, EP/GPP41, EP/GPP21, EP/GPP11, EP/GPP12 and EP/GPP14 coatings. The reference electrode was an Ag/AgCl electrode, which was calibrated before use to ensure that the same reference potential for each experiment. The working electrode was placed in 3.5 wt% NaCl solution for 3600 s to test the stability of the open circuit potential [45]. When the potential fluctuation did not exceed 0.5 mV in the last 10 min, electrochemical impedance spectroscopy and potentiodynamic polarization curves were measured using an electrochemical workstation [46; 47]. The experiment was carried out twice to ensure the reproducibility of different samples, and then ZSimpWin software was used to fit and analyze the EIS data. Figure 7 shows the EIS results of different coatings prepared by EP, EP/PDA, EP/GP and EP/GPP nanocomposites. In Nyquist plots, the imaginary part of the impedance is related to the real part by semicircles. The height and width of these semicircles are related to the capacitive and resistive behavior of the coating, respectively. The diameter of the semicircle represents the total resistance of the coating, while a resistance value is too small, meaning that severe localized corrosion may have occurred [48; 49; 50]. Figure 7a shows that the addition of GO-PANI-PDA increased the diameter of the semicircular ring, leading to the high corrosion resistance of the EP/GPP coating. It can be seen that the corrosion resistance of coatings is significantly affected by the weight ratio between GO-PANI and PDA. When the ratio of PDA to GO-PANI increases from 1:4 to 1:2 (GPP41 to GPP21), the anti-corrosion performance of the coating was greatly improved, while it turned to decline with the further increase the ratio of PDA. Interestingly, we found that the EP/GPP 21 coating has the best anti-corrosion performance among all coatings. The impedance of the coatings undergoes a similar trend as shown in Figure 7b, indicating that the corrosion  
Figure 6: FT-IR spectra of GO-PANI, PDA and GO-PANI-PDA nanocomposites. inhibition effect of EP/GPP increased first and then decreased with the increasing weight ratio of PDA to GO-PANI. Figure 8 displayed the equivalent circuit model of the EP/GPP 21 coatings in 3.5 wt% NaCl solution, where R\({}_{\text{e}}\) is the solution resistance/electrolyte resistance, R\({}_{\text{po}}\) is the coating resistance, R\({}_{\text{ct}}\) is the charge transfer resistance, C\({}_{\zeta}\) is the coating capacitance, C\({}_{\text{dl}}\) is the electric double-layer capacitance/film interface constant phase element [32], msd represents the measured value, and cal represents the fitted value. It is well known that a large value of R\({}_{\text{ct}}\) of coating corresponded to good corrosion resistance. The value of C\({}_{\zeta}\) can reflect the permeability resistance of the coating and the value of C\({}_{\text{dl}}\) corresponds to the defects in coatings, and the wet adhesion between the coating layer and the metal. For the EP/GPP 21 coating, the values of R\({}_{\text{ct}}\) and R\({}_{\text{po}}\) reached 5.279 \(\times\) 10\({}^{16}\)\(\Omega\cdot\)cm\({}^{2}\) and 3.183 \(\times\) 10\({}^{6}\)\(\Omega\cdot\)cm\({}^{2}\), respectively, indicating the coating has good corrosion resistance, whereas the values of C\({}_{\zeta}\) and C\({}_{\text{dl}}\) are as small as 7.257 \(\times\) 10\({}^{-10}\)\(\Omega\cdot\)cm\({}^{2}\) and 6.071 \(\times\) 10\({}^{-9}\)\(\Omega\cdot\)cm\({}^{2}\), respectively, indicating that a dense layer of PDA pre-formed on the surface of the substrate. It can be reasonably concluded that GPP nanocomposites can fill the defects of epoxy resin in addition to the physical barrier effect of the coating with the adhesion properties of PDA.