## 3 Results
### Wettability, Polarization Curves, and SBF Corrosion Tests
The magnitudes of the natural corrosion current Icorr and natural corrosion potential Ecorr act as driving forces dictating the type and rates of anodic and cathodic electrochemical processes that could take place at the metal-corroding solution interface. It is generally agreed that the larger the corrosion current density, the higher the corrosion rate, that is, the lower the corrosion resistance of the metal [53; 54]. The results of the 15-day SBF corrosion test on PHB-coated and uncoated Mg AZ31 alloy carried out according to the ASTM-G31-72 are reported in Figure 3. In particular, the pH change of the SBF corroding medium, the mass of the Mg\({}^{2+}\) corrosion ions released in the SBF, and the gravimetric samples' weight losses were recorded each day before solution refreshing and are reported in Figure 3a-c, respectively. The curves interpolating the experimental data reported in Figure 3 were obtained by mobile mean curve fitting with a period of two. The PHB-coated Mg AZ31 samples clearly exhibited in all three diagrams a significantly different corrosion behavior compared to that of the uncoated system. At the end of each exposure day, the initial pH value of the corroding medium rose from 7.4 to values steadily ranging from 7.6 to 7.8 over the entire 15-day test for the PHB-coated metal samples (red curve in Figure 3a), while that of the uncoated alloy (blue curve in Figure 3a) presented a more scattered distribution and a higher basic character with pH values risen to 8.0-9.5. For all the uncoated samples tested, an abrupt increase of pH after day 9-10 of the corrosion test was observed. This event could be attributed to the previously described porous Mg(OH)2 layer collapsing due to the corrosion reactions and Mg\({}^{2+}\) ion ablation from the metal-hydroxide interface [52; 53]. Magnesium, when exposed to aqueous environments, develops a superficial porous hydroxide film that only moderately protects the metal from external aggressive liquid environments, thus only partially hindering and slowing down any potential corrosion process [56; 57]. However, Mg(OH)2, although only slightly soluble in water, undergoes severe modifications in presence of chloride ions, which could occur in the case of use in contact with physiological solutions. Mg(OH)2, reacting with Cl\({}^{-}\), forms the highly soluble MgCl2 and molecular hydrogen [57]. This continuous dissolution process is accompanied by a progressive increase of the corroding medium basicity (as we have observed in our corrosion tests reported in Figure 3a). In these conditions, pitting of magnesium and embrittlement of the Mg(OH)2 layer could occur at advanced stages of the corrosion process. The abrupt pH increase at days 9-10 can thus be associated to the collapse of the magnesium hydroxide layer, which caused the direct exposure of the metal surface to the corroding medium. This occurrence, consistently with our experimental data, i.e., observed daily released Mg\({}^{2+}\) ions (reported in Figure 3b as mg per sample cm\({}^{2}\)) and daily gravimetrically measured sample weight losses (reported in Figure 3c as mg per sample cm\({}^{2}\)), led to an increase of Mg\({}^{2+}\) ion concentration in the corroding medium and to a massive sample weight loss. The mass of Mg\({}^{2+}\) released from the uncoated magnesium alloy, in fact, was observed to be constantly higher than that from the PHB-coated ones. This higher tendency to release Mg\({}^{2+}\) ions is consistent with the previously described measurements of anodic and cathodic polarization curves and values of the natural corrosion current reported in Figure 2c; the natural corrosion current I\({}_{\rm corr}\) of AZ31 coated with PHB was, in fact, two orders of magnitude lower than that of the uncoated alloy (1.68 \(\times\) 10\({}^{-3}\) mA cm\({}^{-2}\) vs. 2.26 \(\times\) 10\({}^{-1}\) mA cm\({}^{-2}\)).