## 3 Results and Discussion
### Evaluation of the Self-Healing Ability of the Coatings
#### 3.7.2 Evaluation of the Self-Healing Ability by EIS Tests in NaCl and HCl Solutions  
The EIS measurements were carried out to assess the self-healing performance of the coatings. Figure 8 shows the Nyquist and bode diagrams for the scratched samples, which were immersed in 3.5 wt.% NaCl solution for 8 days. The corresponding fitted curves according to the proposed electrical equivalent circuit are also presented. Similarly, the same results for the coatings that were immersed in 0.5 M HCl solution are presented in Figure 9. Comparing the results for the samples immersed in the NaCl solution reveals that Sample C, the coating containing 2 wt.% of the nanocapsules, has the best corrosion resistance, while acidic media sample B showed the best self-healing performance. According to the EIS results, adding the capsules increased the corrosion resistance of scratched coatings for both the immersion solutions. However, increasing the nanocapsule content more than 2 and 1 wt.%, reduced the Nyquist semi-circle radii for the coatings immersed in NaCl and HCl solutions, respectively, indicating the reduction in the corrosion resistance of the samples. Figure 10 shows the equivalent electrical circuit utilized to fit the EIS data. In this circuit, R\({}_{\text{s}}\), R\({}_{\text{coat}}\) and R\({}_{\text{corr}}\) are the solution resistance, the coating resistance, and charge transfer resistance, respectively. Table 3 represents the impedance data extracted from the equivalent electrical circuit of the samples  
Figure 8: (**a**) Nyquist and (**b**) bode diagrams of the scratched samples immersed in the NaCl solution for 8 days. Figure 9: (**a**) Nyquist and (**b**) bode diagrams of the scratched samples immersed in the HCl solution for 8 days. immersed in the 3.5 wt.% NaCl solution. In addition, Table 4 summarizes those acquired for the samples immersed in the 0.5 M HCl solution. In Nyquist plots, the high-frequency time constant is attributed to the capacitive behavior of the epoxy coating containing nanocapsules. Whereas the low-frequency one can be related to the charge transfer at the interface of the metallic substrate and self-healing coating. According to Figure 8a, in the Nyquist data of sample C, there are two capacitive loops that are attributed to the adequately released healing agent for the self-healing process. In fact, there is a significant difference between the semi-circle radii of sample C and the other samples, indicating the effectiveness of the corrosion protection of the self-healing coating. When nanocapsules in the epoxy coating increased from 2 to 4 wt.%, the corrosion resistance clearly decreased due to smaller dimensions of the capacitive loop. This is due to the agglomeration of the nanocapsules in the epoxy coating leading to localized corrosion and penetration of the corrosive NaCl solution into the surface of the self-healing coated substrates. Generally, the agglomerated area provides a localized form of corrosion through some defects, such as holes, cavities, and cracks [45]. It can be seen that the radius of the capacitive loop increased when the self-healing nanocapsule was incorporated in the epoxy coating, proving the self-healing agent's role in the protection of the metallic surface. Comparing the corrosion resistance, the coatings with various amounts of nanocapsules show that the corrosion resistance of the samples in the NaCl solution was significantly higher than those acquired for the samples immersed in the HCl solution. For example, Rcorr for the sample C in NaCl solution was 697,000 \(\Omega\) cm\({}^{2}\), which reduced to 10,100 \(\Omega\) cm\({}^{2}\) when it was immersed in the HCl solution. In addition, Figures 8b and 9b demonstrate the bode plots of the coatings immersed in NaCl, and HCl solution, respectively. It should be noted in bode plots that the impedance modulus \(|Z|\) at lower frequencies shows the total corrosion resistance of the utilized system and consequently the efficiency of the self-healing performance.