## 3 Results
### Wettability, Polarization Curves, and SBF Corrosion Tests
The electrochemical observations confirm the hypothesized corrosion mechanism of the SBF immersion corrosion tests, where a higher diffusion resistance and lower cation and anion mobility are induced by the PHB coating in AZ31 Mg alloy. It can be supposed, then, that the degradation rate of the AZ31 coated with PHB was significantly reduced by the presence of the polymeric coating film, which acted as a  
Figure 3: (a) pH of the SBF corroding medium, (b) mass of Mg\({}^{2+}\) corrosion ions released in the SBF, and (c) gravimetric sample weight losses. Test were run according to ASTM-G31-72; SBF corroding medium was daily refreshed. membrane steadily controlling the diffusion flux of the ionic species (especially Cl\({}^{-}\) anions) from the SBF solution to the metal surface and the counterdiffusion of Mg\({}^{2+}\) metal ions toward the external solution. Conversely, the mere presence of the less stable (especially in presence of chloride ions) porous magnesium hydroxide layer cannot equally guarantee constant and effective corrosion control. The hypothesized diffusive nature of the control exerted by the PHB membrane protecting the metal can be confirmed by the experimental data in Figure 4, where the cumulative amount of Mg\({}^{2+}\) ions released in the daily refreshed SBF corroding solutions (data reported in Figure 3b) is plotted as a function of the test time for PHB-coated and uncoated Mg AZ31 alloys. Figure 4: Cumulative Mg ion release (**a**) and weight loss (**b**) kinetics during corrosion tests. Details of the initial kinetic and time lag in the upper left part of the figure. A steady state constant flux of Mg corrosion ions for the uncoated system (blue line in Figure 4) can be observed from the early stages of the test, while a _time lag_ before reaching stationary flux is observed for the PHB-coated system. A detail of the initial part of the Mg ions release curve is reported in the upper-left corner of Figure 4. The upward shape of the Mg ion release curve before reaching a steady, constant slope is a characteristic observed in permeability tests due to the initial diffusion and sorption of the solution components in the still unpenetrated polymer. In presence of the PHB membrane, a diffusive control is exerted on electrolyte mobility before steady state conditions of ionic species flux can be reached. This diffusive resistance (time lag) in our tests can be evaluated from the intercept on the time scale (tlag) of the linear part of the curve, namely, 1.9 days. Corrosion rates can be directly evaluated for PHB-coated and uncoated alloys from the slopes of the curves at their stationary conditions. The corrosion rate for the PHB-coated Mg alloy was almost four times slower than that of the uncoated system at 1.25 and 5.5 mg/cm2 day, respectively (which, according to ASTM-G31-72, correspond to respective metal corrosion rates of 7.2 and 31.6 \(\mu\)m/day). Permeability test and time lag measurements in permeation tests have been proven [58; 59] to be effective methods to characterize permeability and diffusion coefficients of dissolved species. In the case of the corrosion test, the driving force for diffusion was the difference in concentration of Mg2+ ions between the metal interface, where they are steadily generated by corrosion reactions, and the corroding solution, which is maintained at a low Mg2+ concentration guaranteed by the daily solution refreshments. Due to the simple nature of the permeation experiment, it can be schematized in our case that diffusive resistance can be directly obtained from experimental transient data preceding steady state permeation (time lag).