# 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 curve has two distinct segments: a nonlinear segment at very low currents, denoted as the mixed potential domain, located around the corrosion potential, and a linear portion, starting from a threshold potential, E\({}_{\rm gc}\), corresponding to generalized corrosion. For potentials greater than E\({}_{\rm gc}\), the corrosion current increased in direct proportion to the overpotential applied to the metal and can be expressed by the following equation:  
\[\rm I(mA)=a.E(mV)+b\]  
Unfortunately, for these systems, the polarization curves and, thus, the Evans diagram were very different from the classical curves, in which the two branches (anodic and cathodic) were symmetrical. Due to this, the values of the Tafel slopes highly depended on the way the data were processed, see Figure 10b, especially the size of the potential range around the chosen corrosion potential. The linear potentiometry parameters obtained are presented in Table 5. The corrosion current (i\({}_{\rm corr}\)) values increase along with the increase from 2.28 uA, for Zn to 15.57 uA for Zn3 Mg.7Y. The corrosion rate of Zn3 Mg0.7Y is 83.38 mpy, higher than Zn and Zn3 Mg. This is because of the non-homogeneous structure caused by the formation of new compounds with Y. The values of the constants a and b for the alloys studied as a function of the immersion time in the solution are presented in Table 6. In the last column of Table 6, the linear correlation coefficients for the respective straight sections are presented. The corrosion potential presented was evaluated by the Tafel method, from the linear polarization curves recorded at a sweep speed potential of 1 mV/s. Their values were comparable to those in Table 4, obtained by the same method from the curves recorded at a scan rate potential of 0.5 mV/s, being only slightly higher, but the increases were not significant.Unfortunately, for these systems, the polarization curves and, thus, the Evans diagram were very different from the classical curves, in  
Figure 9: The surface evolution of the alloy sample after maintaining in SBF (500\(\times\)). (**a**) insoluble solid reaction products; (**b**) solid reaction products. which the two branches (anodic and cathodic) were symmetrical. Due to this, the values of the Tafel slopes highly depended on the way the data were processed, see Figure 10b, especially the size of the potential range around the chosen corrosion potential. The linear potentiometry parameters obtained are presented in Table 5. The corrosion current (i\({}_{\text{corr}}\)) values increase along with the increase from 2.28 \(\mu\)A, for Zn to 15.57 \(\mu\)A for Zn3 Mg.7Y. The corrosion rate of Zn3 Mg0.7Y is 83.38 mpy, higher than Zn and Zn3 Mg. This is because of the non-homogeneous structure caused by the formation of new compounds with Y. An order of variation of the corrosion potential could not be established either in the case of the same alloy or between the alloys, the differences being located in the limits of the experimental errors. The main metal in the alloy, in very large quantities, was zinc, and an average value of the average corrosion potential was determined: (E\({}_{\text{corr}}\))\({}_{\text{average}}=-1114\) mV for dE/dt = 1 mV/s and (E\({}_{\text{corr}}\))\({}_{\text{average}}=-1086\) mV for dE/dT = 0.5 mV/s. Figure 10: (**a**) The linear polarization curve for Zn with freshly ground surface in SBF, (**b**) Tafel slopes. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **E\({}_{\text{corr}}\)** & **i\({}_{\text{corr}}\)** & **V\({}_{\text{corr}}\)** \\  & **(V)** & **(\(\mu\)A)** & **(mpy)** \\ \hline Zn & 1.05 & 2.28 & 8.58 \\ \hline Zn3 Mg & 1.07 & 13.32 & 50.05 \\ \hline Zn3 Mg0.7Y & 1.13 & 15.57 & 83.38 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Linear potentiometry parameters.