# Microstructural, Electrochemical and In Vitro Analysis of Mg-0.5 Ca-xGd Biodegradable Alloys
## 3 Results and Discussions
### Electrochemical Corrosion Resistance Analysis
The open circuit potential (OCP) usually varies with the immersion time because of the changes in the nature of the sample's surface during its exposure to a corrosive medium. The lower OCP value in the case of Mg-Ca-3 Gd confirms the formation of a passivation layer on the sample's surface. This shows that the intermetallic compounds retain the cathodic reactions more than anodic, and positive ions have time to diffuse. The Mg-0.5 Ca alloy had a more negative potential (\(-\)3511.2 mV) than the Mg-Ca-xGd alloy (\(-\)1447.7 mV), indicating that the addition of Gd shifted to a more noble direction, thus having a positive effect on the corrosion resistance of the alloys. It can be observed that the cathodic polarization in the Mg-0.5 Ca-1 Gd alloy was significantly higher than that of the Mg-0.5 Ca-2 Gd, indicating that the corrosion potential (\(E_{0}\)) was more negative in the alloy with reduced amounts of Gd. It can also be seen that the anodic polarization curve of the Mg-Ca-1 Gd alloy was greater than that of the Mg-Ca alloy. The lower \(J_{corr}\), along with the longer anodized polarization curve and more negative \(E_{0}\) results, demonstrates that the corrosion resistance was improved for the Mg-Ca-Gd alloy (Figure 5). The corrosion current density of the Mg-0.5 Ca alloy was 170 \(\upmu\)A/cm2, which is significantly higher than that on Mg-Ca-3 Gd (5.92 \(\upmu\)A/cm2). As a result, the formation of a specific phase, namely the \(\alpha\)-Mg-based Mg5 Gd solid solution had a higher influence on the corrosion behavior of the alloys. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Sample** & \(\mathbf{E_{0}}\) & \(\mathbf{b_{a}}\) & \(\mathbf{b_{c}}\) & \(\mathbf{R_{p}}\) & \(\mathbf{I_{cor}}\) & \(\mathbf{V_{cor}}\) & **OCP** \\  & **mV** & **mV** & **mV** & **ohm\(\cdot\)cm2** & **μA/cm2** & **mm/Y** & **V** \\ \hline Mg-0.5 Ca & \(-\)3511.2 & 217.8 & \(-\)188.6 & 0.114 & 170.11 & 4.2 & \(-\)1.72 \\ Mg–Ca–0.5 Gd & \(-\)1740.8 & 283.8 & \(-\)240.4 & 0.993 & 42.42 & 1.0 & \(-\)1.56 \\ Mg–Ca–1 Gd & \(-\)2002.8 & 307.4 & \(-\)265.0 & 1.54 & 32.2 & 0.8 & \(-\)1.62 \\ Mg–Ca–1.5 Gd & \(-\)1576.0 & 264.1 & \(-\)250.2 & 2.55 & 16.53 & 0.41 & \(-\)1.68 \\ Mg–Ca–2 Gd & \(-\)1491.9 & 196.9 & \(-\)179.8 & 2.76 & 12.71 & 0.28 & \(-\)1.61 \\ Mg–Ca–3 Gd & \(-\)1447.7 & 256.1 & \(-\)236.3 & 2.25 & 5.92 & 0.16 & \(-\)1.58 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Parameters obtained from the electro-corrosion resistance tests of the experimental alloys Mg–0.5 Ca–xGd (x = 0.5; 1; 1.5; 2 and 3 wt.%). During cyclo-voltammetric sweeps (Figure 5) changes developed in the passive films and in the solution composition at the metal-electrolyte interface (e.g., hydroxide formation and pH increase), as well as development of the general corrosion form. All specimens had a generally corroded surface, particularly due to linear and cyclic polarization tests. In all cases, in addition to the detachment of the material from the surface of the alloys, the growth of the different compounds took place through contact between the surface of the experimental materials and the electrolyte. Corrosion compounds appeared on the surface of magnesium-based alloys, exhibiting many micro-cracks and various morphological details. The polarization cyclic curves present the generalized corrosion of the surface of all specimens. Cyclic voltammetry consists of scanning of the electrode potential in the positive direction up to a determined value for current or potential, after which, the scanning is immediately reversed towards more negative values until it reaches the initial potential; in some cases, this scanning is repeated to determine the modifications produced on the current-potential curve obtained by scanning. However, in our case, the cyclic voltammograms no longer show a hysteresis loop; the cathode branch of the voltammogram practically overlaps the anodic branch, except for a narrow potential range located in the immediate vicinity of the potential from which dissolution starts.