## 4 Results and Discussion  
### Characterization of the Hydroxyapatite Particles  
Figure 1a,b show the TEM images of hydroxyapatite particles without and with tannic acid, respectively. The blank hydroxyapatite particles consist of packed nanorods with sizes varying from 50-150 nm that tend to form clusters with sizes up to several micrometers. Tannic acid-loaded HAP particles (Figure 1b) are thinner and longer compared to the blank HAP particles (Figure 1a). In fact, due to the shape and size of the thin rods, a larger quantity of corrosion inhibitors can be filled inside the rod-like particles. When the particles are loaded with tannic acid, this one seems to uniformly cover the particles. The amount of tannic acid loading will be discussed below in light of the TGA analysis. XRD was performed to study the structural properties of bare HAP particles and tannic acid-loaded HAP particles. The XRD results (Figure 1c) revealed the main crystalline peaks of blank hydroxyapatite particles which were in good agreement with pure hydroxyapatite particles according to the JCPDS card number 09-0432. The aligned peaks were (002), (102), (210), (211), (112), (202), (310), (213), and (004), and the main peaks were (002), (211), and (112) which can be assigned to the characteristic peaks for the HAP particles [29]. The same peaks were present in tannic acid-loaded HAP particles. Since tannic acid is non-crystalline, it can be assumed that the introduction of tannic acid in HAP particles is responsible for the changes in crystal planes and therefore, broader peaks were observed in the case of tannic acid-loaded HAP. The presence of tannic acid was confirmed by ATR-FTIR (Figure 2a). Well-defined P-O stretching bands were located at 1056 cm\({}^{-1}\), 1085 cm\({}^{-1}\), and 1096 cm\({}^{-1}\). O-P-O bending bands were also found near 660 cm\({}^{-1}\) and 520 cm\({}^{-1}\)[30]. Near 3570 cm\({}^{-1}\) and 632 cm\({}^{-1}\), it is possible to identify the stretching and bending bands of O-H [31]. Figure 1: TEM images of (**a**) blank HAP particles, (**b**) HAP particles loaded with tannic acid, and (**c**) XRD analysis for blank and tannic acid-loaded HAP. In the case of tannic acid-loaded HAP particles, O-H bands were observed near 3325 cm\({}^{-1}\). The bands near 2710 cm\({}^{-1}\) and 1698 cm\({}^{-1}\) correspond to the asymmetric stretch of C-H and C=O stretch, respectively. Aromatic C-O symmetrical stretch bands were detected near 1606 cm\({}^{-1}\). The bands spotted in the range 1443-1532 cm\({}^{-1}\) can be assigned to aromatic C=C. The asymmetrical stretch of aromatic C-O is located at 1307 and 1177 cm\({}^{-1}\). C-O-C stretch bands were found near 1016 cm\({}^{-1}\). The band near 754 cm\({}^{-1}\) corresponds to C-H. The fingerprints of tannic acid agreed with the expected bands for the pure tannic acid [32]. TG measurements were carried out to quantify the amount of tannic acid in the HAP particles and to study the thermal stability of the particles (Figure 2b). HAP particles showed thermal stability until 1000 \({}^{\circ}\)C. In the first step, mass losses assigned to loss of water, CO, CO\({}_{2}\), and breakdown of high molecular weight phenols into small chain fragments were expected. HAP particles may enter phase transformation into various phosphate groups after 600 \({}^{\circ}\)C, as previously discussed [28]. Tannic acid decomposition starts at 150 \({}^{\circ}\)C and it is expected to decompose until 600 \({}^{\circ}\)C [33]. Tannic acid-loaded HAP particles are evidenced in three regions: the first one at a lower temperature is due to water loss; the second region corresponds to the degradation of tannic acid; and the third one is due to the decomposition of HAP particles at higher temperatures. The TGA results allowed to estimate the tannic acid content in the HAP particles that was approximately 11%. This reveals a very interesting corrosion inhibition loading. Release from the Particles  
The dissolution of the hydroxyapatite particles is a complex process.