## 4 Results and Discussion
### Characterization of the Hydroxyapatite Particles
These particles are expected to dissolve below pH 4, releasing the corrosion inhibitor; however, the process shows some reversibility. As the pH decreasing reversibility is more difficult and in the pH range below 3.5, no solid products are expected, as demonstrated in a previous work [25]. Figure 3 shows that the release of tannic acid occurred in all pH ranges, including in the weak alkaline region. A significant increase in the inhibitor release was observed when the pH decreased below 3 and a complete release (and dissolution) of the particles was observed. Although hydroxyapatite particles are not supposed to dissolve above pH 8, the release of tannic acid was also observed. This effect may be due to the release of the superficial inhibitor adsorbed on the surface or in the pores of the hydroxyapatite particles. This release of inhibitor may introduce some early leaching decreasing the availability of the inhibitor at the later stages of corrosion. Figure 2: (a) FTIR for tannic acid, blank HAP, and tannic acid-loaded HAP; (b) TGA for blank HAP, tannic acid, and tannic acid-loaded HAP. Corrosion Performance of Polyolefin Coated Steel Modified with Tannic Acid-HAP Particles Electrochemical Impedance Measurements  
EIS was used to investigate the barrier properties of reference coatings and coatings modified with HAP particles loaded with tannic acid. Changes in the high-frequency range can be correlated to the coating porosity and presence of defects or small pinholes. The coated steel samples were immersed for 30 days in 0.05 M NaCl. The thickness of the produced modified coatings and reference coatings was 18 \(\pm\) 0.5 \(\upmu\)m and 20 \(\pm\) 0.5 \(\upmu\)m, respectively. Figure 4 shows sets of representative EIS Bode plots, obtained on the reference (blank polyolefin coating without additives) and modified coatings after immersion in 0.05 M NaCl for 30 days. Figure 4: 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. Figure 3: The release of inhibitors at different pH. Standard deviation was less than 0.9% for each depicted data point. Both coatings showed a very stable high-frequency capacitive loop from the early stage of immersion. The phase angle was more negative than \(-80^{\circ}\) and approached \(-90^{\circ}\) above 100 Hz for the modified coating. The same trend was observed for the blank coating but only above 1 kHz. The wider frequency range displaying phase angles close to \(-90^{\circ}\) confirms the better barrier properties of the modified coating and accounts for a lower porosity compared to the blank coating. The overall impedance of the blank coating showed very stable values, around \(10^{7}\) Ohm\(\cdot\)cm\({}^{2}\), since the start of the immersion test until its end. The modified coatings behaved differently. The initial impedance values were above \(10^{8}\) Ohm\(\cdot\)cm\({}^{2}\) and decayed slowly, remaining above \(10^{7}\) Ohm\(\cdot\)cm\({}^{2}\) at the end of 30 days of immersion. Overall, this coating also showed very stable barrier properties, and the EIS results evidenced that the presence of HAP particles loaded with tannic acid did not have any detrimental effects. To better detail the corrosion inhibition efficacy of the HAP loaded with tannic acid, a round-shaped defect, with a diameter of around 200 \(\upmu\)m and reaching the bare metal, was created in the coated steel samples. The samples were studied for 5 days in 0.05 M NaCl. As expected, both coatings revealed a strong drop in the impedance values compared to the intact coatings--Figure 5. The high-frequency barrier was disrupted, and at medium frequencies, the time constant present in all the spectra can be assigned to the faradaic processes that occur at the steel interface. In both cases, at low frequencies, an incipient time constant points out the presence of mass transfer-controlled corrosion activity. The impedance values for both coatings were slightly different.