## 3 Results and Discussion
### Electrochemical Measurements
#### 3.4.2 Potentiodynamic Polarization Curves  
The polarization experiments of EP/GPP nanocomposite coatings in 3.5 wt% NaCl solution were carried out. The corresponding Tafel curves and specific data are shown in Figure 9 and Table 1. Compared with the anodic and cathodic polarization curves of EP/GP coatings, the curves of EP/GPP coatings are shifted positively, which means that EP/GPP coating exhibits high corrosion potential during anodic and cathodic polarization. The cathodic and anodic branches are extrapolated to their intersections to obtain the corrosion current (Icorr), the corrosion potential (Ecorr) and the corrosion rate (CR) [51]. The corrosion potential of EP/GPP21 coating (\(-\)0.51 V) is shifted to the positive direction than that of EP/GP coating (\(-\)0.64 V). Its corrosion current density (3.83 \(\times\) 10\({}^{-8}\) A/cm\({}^{2}\)) is also nearly an order of magnitude lower than that of EP/GP (7.05 \(\times\) 10\({}^{-7}\) A/cm\({}^{2}\)), and the coating exhibits the smallest current density during cathodic polarization, corresponding to the corrosion rate of 4.50 \(\times\) 10\({}^{-4}\) mm/year is also an order of magnitude lower than that of EP/GP of 8.27 \(\times\) 10\({}^{-3}\) mm/year. These results indicated that the passivation zone was formed and EP/GPP21 coating has the best corrosion resistance. The similar results from Tafel analysis also suggested the same conclusion as that obtained by the EIS analysis.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multirow{2}{*}{**Eccor (V)**} & \multirow{2}{*}{**Iccor (A/cm\({}^{2}\))**} & **Corrosion Rate** \\  & & & **(mm/year)** \\ \hline EP/PDA & \(-\)0.873 & 1.78 \(\times\) 10\({}^{-5}\) & 2.08 \(\times\) 10\({}^{-1}\) \\ \hline EP/GP & \(-\)0.637 & 7.05 \(\times\) 10\({}^{-7}\) & 8.27 \(\times\) 10\({}^{-3}\) \\ \hline EP/GPP41 & \(-\)0.737 & 9.90 \(\times\) 10\({}^{-8}\) & 1.16 \(\times\) 10\({}^{-3}\) \\ \hline EP/GPP21 & \(-\)0.509 & 3.83 \(\times\) 10\({}^{-8}\) & 4.50 \(\times\) 10\({}^{-4}\) \\ \hline EP/GPP11 & \(-\)0.550 & 3.92 \(\times\) 10\({}^{-7}\) & 4.60 \(\times\) 10\({}^{-3}\) \\ \hline EP/GPP12 & \(-\)1.029 & 2.80 \(\times\) 10\({}^{-6}\) & 3.29 \(\times\) 10\({}^{-2}\) \\ \hline EP/GPP14 & \(-\)0.810 & 1.12 \(\times\) 10\({}^{-5}\) & 1.31 \(\times\) 10\({}^{-1}\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: The summarized Tafel plot data for the different coatings.  
Figure 8: Analysis of the EIS data using an equivalent circuit model.  
The low values of Icorr and Ecorr exhibited by EP/GPP can be attributed to the surface-adsorbed GO-PANI-PDA molecules, and the addition of uniformly dispersed GO-PANI-PDA nanosheets can compensate for the defects in the epoxy resin matrix and improve the dispersibility and interfacial adhesion of GO-PANI nanoparticles to epoxy resin. It also further prevents the steel from being corroded by corrosive medium through the \"labyrinth effect\" of corrosive ion penetration, which significantly reduces the CR of GO-PANI-PDA and improves the corrosion resistance of the coating. As a result, the EP/GPP21 has a highest corrosion inhibition efficiency. It is worth noting that the thickness of the electrode coating is very critical when making the electrode coating, and finding a more accurate coating thickness control method will be of great help to the experiment.