# Microstructural, Electrochemical and In Vitro Analysis of Mg-0.5 Ca-xGd Biodegradable Alloys
## 2 Experimental Details
### Electrochemical Analysis  
Based on the protocol applied in [43], the experimental magnesium-based alloys' behavior was tested in simulated body fluid (SBF) electrolyte with the ion composition as shown in Table 3. An electro-corrosion resistance VoltaLab-21 potentiostat (Radiometer, Denmark) was used to analyze linear and cyclic curve determinations in SBF. The results were acquired and processed using the Volta Master 4 program package. For experiments, an experimental cell was used to expose the sample (work electrode) to an electrolyte solution at the same time with the Pt auxiliary electrode and a calomel-saturated one. The samples were isolated with Teflon, so only a specific area was exposed to the electrolyte, namely 0.78 cm\({}^{2}\). The solution was permanently aerated with a magnetic stirrer in order to remove the gas bubbles formed on the metal surface due to hydrogen elimination. Electrochemical polarization test methods established the criteria for susceptibility for several forms of corrosion and anodic or cathodic protection. Cyclic voltammetry, potentiodynamic and potentiostatic techniques were necessary for evaluation of the passive region, aptitude to passivation, stability and quality of passivation (corrosion rate). Direct current (DC) electrochemistry, particularly the potentio-dynamic polarization test, was used to record much relevant information based on the behavior of the electrodes. Using direct current polarization mechanisms, we could obtain detailed information about the working electrodes' (MgCaGd samples) corrosion rate and the type of surface corrosion: generalized or pitting, passivation layer formation and stability, and anodic (oxidation) or cathodic (reduction) reactions. The linear polarization resistance method is probably one of the most widespread applications of electrochemical measurements in the laboratory and is widely applied in the case of uniform (generalized) corrosion. The following coordinates were chosen by the authors for the function representation: current density [mA/cm\({}^{2}\)] and potential [V], because this variation allows emphasis on the corrosion potential (\(E_{0}\)) and corrosion current (\(J_{corr}\)). The experiments were realized at room temperature (24 \({}^{\circ}\)C) and the potential records were registered (linear plots were registered at a scan rate of 1 mV/s and the cyclic plots at a scanning rate of 10 mV/s). The tests were repeated 4 times to achieve proper repeatability of the results. The corrosion current density, \(J_{corr}\), can be determined using 2 equations by extrapolation of the Tafel lines: a complete polarization curve consisting of a cathodic part and an anodic part. The cathodic portion of the polarization curve contains information concerning the kinetics of the reduction reactions occurring for a particular system. The corrosion potential is deduced from the intersection of the anodic and cathodic Tafel slopes. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Specimens** & \multicolumn{2}{c}{**Chemical Composition**} & **Mg [Grams]** & **Mg–15 Ca** & **Mg–30 Gd** \\  & & **Composition** & **Mg [Grams]** & **(Grams)** & **(Grams)** \\ \hline MgCa0.5 Gd & Mg-0.5\%Ca-0.5\%Gd & 21.82 & 0.77 & 0.41 \\ MgCa1 Gd & Mg-0.5\%Ca-1\%Gd & 21.42 & 0.77 & 0.82 \\ MgCa1.5 Gd & Mg-0.5\%Ca-1.5\%Gd & 21.00 & 0.77 & 1.23 \\ MgCa2 Gd & Mg-0.5\%Ca-2\%Gd & 20.59 & 0.77 & 1.64 \\ MgCa3 Gd & Mg-0.5\%Ca-3\%Gd & 19.77 & 0.77 & 2.46 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Initial materials masses used formetal charges in order to obtain the experimental samples. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Electrolyte Ions (mmol/dm\({}^{3}\))** & **Na\({}^{+}\)** & **K\({}^{+}\)** & **Mg\({}^{2+}\)** & **Ca\({}^{2+}\)** & **CI\({}^{-}\)** & **HCO\({}_{3}\)** & **HPO\({}^{2-}\)** & **SO\({}^{2-}\)** \\ \hline Simulated body fluid & 142 & 5 & 1.5 & 2.5 & 147.8 & 4.2 & 1 & 0.5 \\ Human blood plasma & 142 & 5 & 1.5 & 2.5 & 103 & 27 & 1 & 0.5 \\ \hline \hline \end{tabular}
\end{table}