# In Vitro Corrosion Behavior of Zn3 Mg0.7Y Biodegradable Alloy in Simulated Body Fluid (SBF)
## 3 Results
### Electro-Corrosion Behavior of Zn, ZnMg and ZnMgY Materials in SBF Electrolyte  
The corrosion potential, E\({}_{\rm corr}\) = E (I = 0), is a measure of the corrosion tendency of a metal or alloy immersed in a given electrolytic medium (thermodynamic probability of corrosion). In fact, this is the potential value (measured in relation to the reference electrode--in this case, the saturated calomel electrode) where the anodic and cathodic reactions rates meet. Very high negative corrosion potential values indicate a very high tendency for zinc and zinc-based alloys to corrode. In the case of pure zinc, the increase in the negative value of the corrosion potential indicated a slight increase in the tendency of corrosion, probably caused by the increase in the surface of the sample due to corrosion (the surface was no longer flat but rough). In the case of the Zn3 Mg0.7Y alloy, the evolution of the corrosion potential indicated a slight tendency of passivation (very small), while for the Zn-Mg alloy, the variation was more complex. The corrosion current and, directly related to it, the corrosion rate had different evolutions for the three alloys depending on the immersion time in SBF. Thus, in the case of pure zinc, the reaction rate increased appreciably with the storage time, which was also caused by the increase in the surface roughness corresponding to generalized corrosion (Figure 7). When calculating the corrosion rate, the initial plan surface was taken. The parameters of the instantaneous corrosion process evaluated by the Tafel method are presented in Table 4.  
Figure 6: SEM scratch stain: (**a**) pure Zn; (**b**) Zn3 Mg; (**c**) Zn3 Mg0.7Y. In the case of the Zn-Mg alloy, the corrosion rate was still high in the initial moments, this being probably due to the much higher reactivity of magnesium than that of zinc. After 8 days of immersion, the reaction rate increased, which is unlikely due to the roughness, but after 18 days of immersion, it decreased appreciably, reaching a value very close to the corrosion rate of pure zinc. This may be due to the prolonged immersion in the solution of this initial alloy in the solubilization of magnesium until total depletion on the surface and the subsequent corrosion of zinc. The deposition on the surface of the sample of some solid reaction products can be added to this, as in the case of zinc (Figure 7). In Figure 8, for the sample maintained for 18 days in SBF, the cavities from which the magnesium dissolved can be observed. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & \multicolumn{3}{c}{**Zn**} & \multicolumn{3}{c}{**Zn-3 Mg**} & \multicolumn{3}{c}{**Zn3 Mg0.7Y**} \\ \cline{2-10}  & **0 Days** & **8 Days** & **18 Days** & **0 Days** & **8 Days** & **  
In the case of the Zn3 Mg0.7Y alloy test, the corrosion rate increased in the initial moments and after 18 days decreased appreciably. This behavior is due to the formation of a crust from insoluble solid reaction products, as can be seen in Figure 9.  
Tafel slopes provide information on the reaction mechanism. In this case, the low value of the anodic slope indicated that the anodic reaction (Zn \(\rightarrow\) Zn2 ++ 2e\({}^{-}\)) is the active reaction. As suggested by the higher cathodic slope, the corrosion process is under concentration polarization control. Activation control is determined by the rate of electron transfer from the anode to the cathode. In the presence of dissolved oxygen at the cathode, the following reaction takes place: \(\
icefrac{{1}}{{2}}\) O\({}_{2}\) + H\({}_{2}\)O + 2e\({}^{-}\)\(\rightarrow\) 2 OH\({}^{-}\), followed by the reaction Zn\({}^{2+}\) + 2 OH\({}^{-}\) = Zn (OH)\({}_{2}\)[36]. All the linear polarization curves recorded for the three alloys after various storage periods in SBF had the general appearance shown in Figure 10a.