# Microstructural, Electrochemical and In Vitro Analysis of Mg-0.5 Ca-xGd Biodegradable Alloys
## 3 Results and Discussions
### Electrochemical Corrosion Resistance Analysis  
Normally, in the process of electrochemical corrosion, the cathodic polarization curve is assumed to represent the cathodic H\({}_{2}\) evolution through solution reduction, while the anodic polarization curve represents the dissolution of the magnesium alloy. The alloys in contact with the electrolyte solution present both anodic and cathodic reactions, with higher activity for oxidation process (in all cases \(b_{a}>b_{c}\)), with an increase in the difference at the beginning of Gd alloying (first three samples) and a drop in the differences between anodic and cathodic reactions for samples with 2 or 3 wt.% Gd. The highest oxidation rate was observed in the Mg-0.5 Ca-1.5 Gd samples and the lowest with the reduction reactions for the Mg-0.5 Ca-0.5 Gd sample. However, the rate of corrosion depended on the intensity of both reactions [51]. The experimental conditions of the electro-corrosion experiment realized in SBF electrolyte are given in Table 5. Because magnesium and magnesium-based alloys discharge large amounts of hydrogen into the exposed solution part of the sample, the gaseous gaps appear to be in the form of continuous bubbles [52]. Removal of the H\({}_{2}\) voids from the metallic surface was realized with a magnetic stirrer at a controlled speed; in our case, we used a much-reduced rate of SBF shuffling in order to avoid the acceleration of the corrosion rate. After every determination, based on the fact that the electrolyte solution changed its color, a new quantity of SBF was used. At the contact with the SBF electrolyte solution, the metallic specimen presented an oxidation process to metal ions based on the following reactions:  
\[\mathrm{Mg}\rightarrow\mathrm{Mg}^{2+}+2\mathrm{e}^{-} \tag{4}\]  
\[\mathrm{Ca}\rightarrow\mathrm{Ca}^{2+}+2\mathrm{e}^{-} \tag{5}\]  
The electrons from the anodic reaction were consumed by a corresponding cathodic Reaction (4) and the reduction of oxygen dissolved in H\({}_{2}\)O.  
\[2\mathrm{H}_{2}\mathrm{O}+2\mathrm{e}^{-}\rightarrow\mathrm{H}_{2}+2\mathrm{ OH}^{-} \tag{6}\]  
Figure 4: XRD pattern of Mg–0.5 Ca–xGd samples. The compounds formed on the surface after immersion was based on the alloys' main elements, especially magnesium and calcium oxides. At the beginning of the contact between the experimental alloys and the electrolytes, a protective oxide layer appeared on the surface and, for a certain period of time, provided protection against corrosion; the layer was generally formed from Mg(OH)2. The formation and growth of the Mg(OH)2 layer was followed by a hydrogen discharge, as directly observed by visual inspection. As a conclusion, the formed Mg(OH)2 layer cannot provide effective protection to the Mg substrate, at least not for a longer period of time. The pH of the solution varied with the chemical reactions that occurred on the surface as a basic general characteristic and sometimes, especially with the formation of HCl, decreased due to the influence of acid formation. The corrosion current (\(J_{corr}\)) expresses the reduction value of the experimental materials in contact with an electrolyte solution. The corrosion density presented different values for the experimental alloys from 5.92 \(\upmu\)A/cm2 to 170.11 \(\upmu\)A/cm2 (Table 5). The influence of the gadolinium element added to the magnesium-calcium alloy on the electro-corrosion resistance properties of the system is highlighted in Table 5. A reduction in the corrosion rate of up to 20 times with the addition of 3 wt.% of Gd was observed compared with the Mg-0.5 Ca-based experimental alloy, Mg_0.5 Ca. In quantitative terms, the percentage of Gd significantly influenced the behavior of the alloy at values higher than 2 wt.% of Gd. It is known that both intermetallic compounds have a significant influence on the electro-corrosion resistance of Mg-based materials [48; 49]. The open circuit potential (OCP) shows close values for all samples, decreasing with the elevated percentage of the alloying element.