## 3 Results  
### Morphological and Electrochemical Characterization of the Samples' Surfaces  
Scanning electron micrographs of the surface morphologies of the uncoated and PHB-coated AZ31 samples are shown in Figure 1a-d. The surface of the thermally treated AZ31 magnesium alloy exhibits a relatively smooth noncontinuous surface with snaking lines that were probably due to the formation of a nonhomogeneous magnesium hydroxide layer during the hydrothermal treatment. Conversely, PHB-coated samples are characterized by much more homogeneous surfaces (Figure 1b). Details of the fractured sample section are reported in Figure 1c,d, where it is possible to see the PHB surface-coating film. From these images, it appears that the polymer film was uniformly distributed across the surface and that it well adhered to the hydrothermally treated metal surface. From the image of the section of the cold fractured coated samples (Figure 1c and, in detail, Figure 1d), both adhesive and cohesive failures of the polymeric coating are visible. Direct measurements of the adhesion strength during our time testing were carried out; however, no evidence of PHB corrosion was microscopically noticed after the 2 weeks of the tests. The molecular characteristics and chemical compositions of AZ31 and PHB-coated AZ31 sample surfaces were spectroscopically investigated. FTIR spectra are compared in Figure 2a,b. The left FTIR spectrum, Figure 2a, which represents the uncoated alloy, shows the presence of an absorption band at 3343 cm\({}^{-1}\), which represents the existence of an (OH) bond possibly associated to a superficial layer of the magnesium hydroxides. The surface absorption spectrum of the polymer coated samples, reported in Figure 1d, consistently with the standard spectrum of PHB, presents a 3437 cm\({}^{-1}\) peak associated to the stretching vibration of hydroxyl OH; the 2978 cm\({}^{-1}\) peak of the stretching of saturated CH; the 1725 cm\({}^{-1}\) of the stretching of C = O; the 1456 cm\({}^{-1}\) and 1381 cm\({}^{-1}\) of CH\({}_{3}\)- and -CH\({}_{2}\)- deformation vibration; the 1291 cm\({}^{-1}\)\(\sim\)1058 cm\({}^{-1}\) PHB ester band; and, finally, the 826 cm\({}^{-1}\) absorption peak of CH\({}_{2}\) groups. Figure 1: Characterization of the PHB coating. (**a**) Surface morphology of AZ31 and (**b**) AZ31 coated with PHB under the scanning electron microscope. (**c**) PHB-coated sample fractured section and (**d**) detail of the PHB coating. These results confirm that the AZ31 alloy sample developed a magnesium hydroxide surface layer [43] after hot water treatment that was successively homogeneously covered by the thin film of PHB polymer. Cathodic and anodic polarization plots are described in Figure 2c. The natural corrosion current (Icorr) and natural corrosion potential (Ecorr) determined by Tafel extrapolation (dotted lines) for the PHB-coated and uncoated Mg alloys are also reported in Figure 2c. The Icorr value for the PHB-coated sample is two orders of magnitude lower than that of the uncoated AZ31 alloy. Since Icorr is directly proportional to the corrosion rate [38], a drastic improvement in corrosion resistance is achieved by PHB coating that can be attributed to the hindering of the oxidation and reduction reactions occurring at the metal interface. Magnesium corrosion in aqueous media first implicates reaction of H2O with Mg to produce Mg(OH)2 and H2. The overall reaction comprises the anodic reaction Mg \(\rightarrow\) Mg2+ + 2e- and the cathodic reaction 2H2O + 2e- \(\rightarrow\) H2 + 2 OH-, which accounts for the increase of the solution pH [53; 54; 55]. After a hydrothermal passivation treatment (as was the case for our samples), the Mg AZ31 alloy surface was covered by a Mg(OH)2 layer. During the polarization tests, metal cation transport controls the anodic Mg dissolution kinetic at the metal interface beneath the Mg(OH)2 layer. The unstable in aqueous environment and porous layer cannot hinder electrolyte penetration and diffusive transport.