# 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 slope of the lines describing the influence of the overcurrent applied to the metal on the corrosion current, and implicitly on the corrosion rate, in the case of zinc increased with the increase in the intercept with the potential axis (b), which translated into a slight increase in the general corrosion process. In the case of the Zn3 Mg alloy, both the slope and the original cut decreased with the immersion time in the solution, thus marking a decrease in the corrosion rate over time. This is explained by the fact that in the initial period, magnesium dissolves first, which is the most electronegative metal. In the case of the Zn3 Mg0.7Y, there was no orderly variation for either the slope or the intercept at the origin. The cyclic voltammograms for the three alloys for various immersion intervals in SBF all had the same force as that shown in Figure 11.  
For all curves, the linear current-voltage dependence started from a voltage value of \(-1000\) mV. It was noticed that the return branch (cathodic curve) overlapped almost perfectly on the direct branch (anodic curve). This means that the generalized corrosion maintained by a large voltage value did not appreciably alter the active surface, nor did it produce passivation phenomena by the deposition of reaction products. The electrochemical impedance spectroscopy data were processed with the SZSimp-Win software, which uses the least nonlinear squares method to obtain the most appropriate values of the equivalent circuit elements tested. The parameters that best describe (fit) an equivalent circuit were obtained by minimizing the function \(\chi^{2}\), defined as the sum of the squares of the residuals (the differences between the calculated values and the experimental values):  
\[\chi^{2}=\sum_{i=0}^{n}\left[W_{i}^{\prime}\left(Z_{i}^{\prime}\left(\omega^{ i},\overrightarrow{p}\right)-a_{i}\right)^{2}+W_{i}^{\prime\prime}\left(Z_{i}^{ \prime\prime}\left(\omega_{i},\overrightarrow{p}\right)-b_{i}\right)^{2}\right] \tag{1}\]  
where \(n\) is the number of points and \(W_{i}\) is the weighting coefficients. A value of \(\chi^{2}\) equal to \(10^{-4}\) translates into a relative error of the measured current of 0.01, i.e., 1%. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Alloy** & **Time** & \begin{tabular}{c} **E\({}_{\text{cor}}\)** \\ **(mV)** \\ \end{tabular} & \begin{tabular}{c} **E\({}_{\text{gc}}\)** \\ **(mV)** \\ \end{tabular} & \begin{tabular}{c} **I(mA) = a.E(mV) + b** \\ **a (mA/mv)** \\ \end{tabular} & \begin{tabular}{c} **r\({}_{\text{L}}\)** \\ **b (mA)** \\ \end{tabular} \\ \hline Zn & \begin{tabular}{c} 0 days \\ 8 days \\ 18 days \\ 18 days \\ 18 days \\ \end{tabular} & \
\begin{tabular}{c} \(-1121\) \\ \(-1108\) \\ \(-1136\) \\ \(-1100\) \\ \(-1108\) \\ \(-1101\) \\ \(-1092\) \\ \(-1092\) \\ \(-1106\) \\ \(-116\) \\ \(-116\) \\ \(-116\) \\ \(-117\) \\ \end{tabular} &
\begin{tabular}{c} \(-950\) \\ \(-1000\) \\ \(-1000\) \\ \(-1000\) \\ \(-1000\) \\ \(-1000\) \\ \(-1000\) \\ \(-1000\) \\ \(-1000\) \\ \(-1000\) \\ \(-1000\) \\ \(-1000\) \\ \(-1000\) \\ \(-1000\) \\ \(-1000\) \\ \(-1000\) \\ \(-1000\) \\ \(-1000\) \\ \(-1000\) \\ \(-1000\) \\ \(-1000\) \\ \(-1000\) \\ \(-1000\) \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Dependence of the corrosion current on the overcurrent applied to the alloy, at potentials higher than E\({}_{\text{gc}}\). Figure 11: Cyclic voltammogram for zinc with freshly ground surface in SBF; dE/dt = 10 mV/s. For a certain equivalent circuit to be suitable for describing the physical condition of the alloy surface, the minimum value of \(\chi^{2}\) is not sufficient, but the errors associated with each circuit element must be below 5%. Taking into account these conditions, for the optimal fit of the experimental data, the circuits shown in Figure 12 were established. The R (QR) circuit is suitable for describing a system in which a single reaction takes place on the surface of the alloy and corrosion is controlled entirely by the transfer of charges through the double-electric layer.