# 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
In the case of the Zn-Mg alloy, the R (QR) circuit satisfactorily described the experimental data only for the freshly ground sample and for the sample maintained for 8 days in SBF, but here too the polarization resistance decreased and the corrosion rate increased. Moreover, the frequency exponent in the expression of the constant phase element showed unexpectedly low values, probably marking an appreciable deterioration of the alloy surface and maybe some local deposits. In the case of Zn3 Mg0.7Y, only the equivalent circuit could not be used for the test held for 8 days in solution. Analyzing the evolution of the constant \(Q\) as a function of immersion time, this varied randomly, with very large oscillations, both between different alloys and for the same alloy at different immersion times, this circuit element being more sensitive to experimental errors, exemplified by the percentage errors, between 3 and 5%, obtained when fitting the curve. For the sample maintained for 18 days in SBF, the frequency exponent was very close to a value of 0.5, indicating the possibility of a diffusion phenomenon. As in the case of the other two alloys, some values close to 0.5 were encountered; we tried to use the second equivalent circuit, as shown in Figure 12, which contained a diffusion impedance. The values of the circuit elements, evaluated on the basis of the same experimental data, are presented in Table 8.  
Comparing these values with the values of the parameters for the R (QR) circuit, it was found that they were very close, which is additional proof that these circuits also describe the same state. The need to introduce a diffusion impedance, even in the absence of a film adsorbed or adhering to the surface of the metal, may seem at least risky. This can be explained by considering the existence of a local diffusion in a nanometer-sized film in the reaction zone on the metal surface [37]. Zinc is a very active metal in ionic media containing chlorine, with its surface suffering a generalized, uniform corrosion. Corrosion occurred in a single reaction, and the reaction products were soluble. There was a uniform concentration of ions and electrons in a nanometer-sized layer on the surface. The mobility of electrons is much higher than that of ions [38]. As the electrons became free in the system, the electric field relaxed rapidly, which led to a local charge neutrality, and the transport of the charge carriers was limited by diffusion through this overloaded nano-layer with charges. The \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Alloy**} & \multirow{2}{*}{**Immersion Time**} & \multirow{2}{*}{\(\mathbf{10^{3}\)\(\boldsymbol{\chi}^{2}\)}} & \multicolumn{2}{c}{\(\mathbf{R_{s}}\)} & \multicolumn{2}{c}{\(\mathbf{10^{3}\)\(\mathbf{Q}\)}} & \multirow{2}{*}{\(\boldsymbol{n}\)} & \multicolumn{2}{c}{\(\mathbf{R_{ct}}\)} & \multicolumn{1}{c}{\(\mathbf{W}\)} \\  & & & & & & & \(\mathbf{Ohm.cm^{2}}\) & \(\mathbf{S.s^{th}/cm^{2}}\) \\ \hline \multirow{3}{*}{Zn} & 0 days & 1.52 & 28.22 & 3.323 & 0.718 & 867.9 & 0.008637 \\  & 8 days & 1.18 & 47.58 & 3.004 & 0.643 & 447.1 & 0.06193 \\  & 18 days & 1.15 & 35.92 & 1.876 & 0.531 & 205.9 & 0.04964 \\  & 0 days & 0.90 & 29.51 & 0.0710 & 0.655 & 291.4 & 0.05335 \\ Zn3 Mg & 8 days & 0.89 & 47.19 & 0.3250 & 0.636 & 503.8 & 0.065730 \\  & 18 days & 0.68 & 25.77 & 0.1659 & 0.420 & 455.2 & 0.008236 \\ Zn3 Mg0.7Y & 0 days & 0.62 & 26.18 & 0.03832 & 0.620 & 524.1 & 0.03609 \\  & 8 days & 1.35 & 32.46 & 0.2468 & 8.837 & 28.75 & 0.008623 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Values of circuit elements R (Q (RW)). values of the parameter \(\chi^{2}\) indicated that the last two circuits (one is enough) described the experimental data much better, followed by those for the R circuit (QR).