# 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 this circuit, R\({}_{\rm{s}}\) is the resistance of the solution between the electrode surface and the reference electrode, R\({}_{\rm{ct}}\) is the opposite resistance to the charge transfer and CPE is a constant phase element introduced instead of the capacity of the double-electric layer (C\({}_{\rm{dl}}\)) for a better adjustment. This showed good experimental data. The introduction of this element was necessary due to the fact that the surface of the working electrode is not homogeneous and the electrical capacity is frequency dependent. If for a capacitor the impedance is equal to \(Z_{C}=1/(j\omega\rm{C})\), in the case of the constant phase element, the impedance is evaluated in accordance with [6; 7; 8; 9]:  
\[Z_{CPE}=\frac{1}{Q(j\omega)^{n}} \tag{2}\]  
where \(Q\) is a constant proportional to the active area (area exposed to corrosion), \(<\)Q\(>\) = \(\Omega^{-1}\rm{s}^{n}/\rm{cm}^{2}\)\(\equiv\) S.\(\rm{s}^{n}/\rm{cm}^{2}\), \(\omega\) is the angular frequency (\(\omega=2\pi\)f; f = frequency of the applied alternating current), \(j\) is the imaginary number and \(j=(-1)^{\ 1/2}\). A consequence of this simple relationship is that the phase angle of the CPE is independent of frequency and has a value of (90\({}^{\circ}\))\({}^{n}\), which is also the reason that it is called a constant phase element. The values of the circuit elements for this equivalent circuit are shown in Table 7. The Nquist and Bode diagrams for Zn, ZnMg and ZnMMgY obtained are presented in Figure 13a-c. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Alloy**} & \multirow{2}{*}{**Immersion Time**} & \multirow{2}{*}{\(\mathbf{10^{3}.\chi^{2}}\)} & \(\mathbf{R_{s}}\) & \(\mathbf{10^{4}.Q}\) & \multirow{2}{*}{\(\mathbf{R_{ct}}\)} \\  & & & **Ohm.cm\({}^{2}\)** & & **S.s\({}^{n}\)/cm\({}^{2}\)** & \\ \hline \multirow{4}{*}{Zn} & 0 days & 2.40 & 27.89 & 0.371 & 0.705 & 920.4 \\  & 8 days & 1.52 & 47.93 & 2.660 & 0.663 & 269.6 \\  & 18 days & 0.56 & 36.47 & 1.539 & 0.554 & 218.2 \\  & 0 days & 5.09 & 27.89 & 0.1245 & 0.597 & 358.4 \\ \cline{1-1}  & 8 days & 0.80 & 45.36 & 30.170 & 0.379 & 171.9 \\ \cline{1-1}  & 18 days & - & - & - & - & - \\ \cline{1-1}  & 0 days & 0.76 & 25.93 & 0.4056 & 0.614 & 535.1 \\ \cline{1-1} Zn3 Mg0.7Y & 8 days & - & - & - & - & - \\ \cline{1-1}  & 18 days & 7.28 & 27.71 & 5.987 & 0.515 & 220.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Circuit R(QR) element values. Figure 12: Equivalent circuits used for processing EIS data on the three alloys depending on the storage time. This circuit satisfactorily describes the experimental data for zinc, and for the freshly ground surface and for samples immersed in SBF 8 or 18 days. The percentage errors for the circuit elements were approximately 1% and even lower, except for \(Q\), where they were of the order of 3-4%, but located in the reliable range. It should be noted that in the case of zinc, the load transfer resistance (R\({}_{\text{ct}}\)) decreased appreciably with the immersion time in the solution, which increased the reaction rate, resulting in very good agreement with the experimental data. Figure 13: Nquist and Bode diagrams for Zn, ZnMg and ZnMMgY at: (a) 0 days; (b) 8 days; (c) 18 days. that found for the instantaneous corrosion rate evaluated in the linear polarization curve (Tafel method). The exponent n, which gives an indication of the deviation from the ideality of the capacity of the double-electric layer, which also decreases with the immersion time, a decrease attributed to the increase in the degree of surface roughness due to corrosion was observed. This behavior also indicates that, although it is possible that insoluble reaction products, such as ZnO or (Zn\({}_{5}\)(OH)\({}_{8}\)Cl\({}_{2}\cdot\)2H\({}_{2}\)O) (simonkolleit) [25], may form during storage in solution, they are probably porous and do not act as a barrier to the reaction.