## 4 Results and Discussion
### Characterization of the Hydroxyapatite Particles
The blank coating revealed impedance values around \(10^{5}\) Ohm\(\cdot\)cm\({}^{2}\) that slowly decayed to half of this value after 5 days of immersion, while the modified coating showed impedance values initially around \(10^{6}\) Ohm\(\cdot\)cm\({}^{2}\) that evidenced some fluctuations and a small drop after 5 days of immersion. Overall, the modified coating revealed impedance values that were approximately one order of magnitude above the blank coating, stating its superior corrosion protection performance. This was confirmed by the fitting results presented below in Figure 6c. These results demonstrate that even in the presence of a defect in the coating, the addition of HAP modified with tannic acid can slow down the corrosion activity and, consequently, the extent of the damage. Figure 5: EIS Bode plots for the (**a**) reference coating and (**b**) coating modified with HAP particles loaded with tannic acid during immersion in 0.05 M NaCl with an artificial defect. To better quantify the differences between the two non-damaged coatings, the spectra were simulated using the equivalent circuit presented in Figure 7. This equivalent circuit reflects two-time constants. The high-frequency time constant was introduced to account for the high-frequency barrier properties, while the CPEdI and Rct were used to simulate the low-frequency response, i.e., the interfacial activity at the steel coating interface. Constant phase elements (CPE) were used instead of capacitors. The fitting results are depicted in Figure 6.  
Figure 6: Evolution of the fitting parameters extracted from the numerical simulation of the experimental EIS data: (**a**) pore resistance; (**b**) admittance of the CPE for the coating; (**c**) charge transfer resistance; (**d**) admittance of the low-frequency CPE. Figure 7: The equivalent circuit used to fit the experimental EIS data. Figure 6a,b evidenced higher values of pore resistance and lower admittance values for the blank coating compared to the modified coatings. The faradaic resistance values determined by fitting evidenced significant differences. In the modified coating (Figure 6c), the faradaic resistance increased after the first day of immersion attaining values around 1 MOhm.cm\({}^{2}\) and being very stable over the 5 days of testing. Concomitantly, the admittance of the CPE (Figure 6d) also remained stable and more than one order of magnitude below that of the blank coatings (Figure 6b). Contrarily, the faradaic resistance in the blank coating showed a monotonous drop, decaying from 100 kOhm.cm\({}^{2}\) to 5 kOhm.cm\({}^{2}\) (Figure 6c). The trends observed in Figure 5 demonstrated that the modified coatings provide very effective barrier properties, with a higher and stable pore resistance as well as marked corrosion inhibition. It is observed that the faradaic resistance is more than one order of magnitude higher in the modified coating, evidencing that the corrosion process is effectively inhibited. The positive effects related to modification of the coating with HAP loaded with tannic acid can be summarized as follows: the particles are compatible with polyolefin coatings and do not disrupt the original barrier properties. When the electrolyte and chloride ions reach the bare steel corrosion starts, local pH changes occur. Thus, the pH increases at the cathodic sites, while at the anodic sites the pH acidifies as a consequence of iron cations hydrolysis. HAP is sensitive to pH and may enter a slow dissolution process, as previously reported elsewhere [25]. During this process, the inhibitor can be released from the particles leading to the formation of protective species over the exposed steel. Additionally, phosphate ions released from HAP dissolution may form stable and protective compounds with iron hydroxides blocking anodic activity.