## 6 Real-Time In-Vivo and In-Vitro Corrosion Monitoring
### Electrochemical-Based Monitoring System
The Faradaic and non-Faradaic impedance changes at the metal-solution interfaces were monitored by EIS measurements. At low frequencies, the Faradaic impedances occur from the redox reactions of corrosion while at high frequencies, the non-Faradaic impedances are monitored through surface charging and obstruction by degraded products or passivation. After the immersion tests were completed, the results were analyzed using atomic absorption spectrometry (AAS) to determine the concentration of Mg present in the solution. The limitations of this method was potential interference by other ions or simultaneous usage of potentiometric sensors and EIS instrumentation [28]. In addition, the electrode life affects the EIS technique for in-vivo recording. As time passes, its quality and accuracy deviates [4]. Figure 5: (**a**) An amperometric sensor circuit; (**b**) An experimental setup to monitor corrosion of biodegradable metal alloy [4]. In another study [30], electrochemical methods complemented with real-time imaging technique were adopted to observe the magnesium corrosion in order to determine the local processes as well as the evolution of hydrogen gas. The magnesium specimen used for testing and plan view imaging was exposed to six different electrolytes. A layout of three electrode cell was selected where the working electrode was the magnesium specimen while the reference electrode and counter electrode are calomel electrode and platinum wire, respectively. They performed a potentiodynamic polarization test in which measurement was obtained after three minutes of immersion in the test solution. It was proposed that disruption of the oxide/hydroxide film was caused by the generation of streams of bubbles when an anodic current is available along with environmental conditions that induce depassivation. As a result of the direct exposure of electrolyte and Mg specimen, as well as the larger overpotential due to release of hydrogen bubbles, the corrosion front amplified the current where oxidation rate of magnesium is proportional to the total'remote' anodic current (linked to the evolution of medium to large bubbles) and local anodic current (linked to the streams of hydrogen). Figure 7a,b represents the six different anodic and cathodic polarization curves. Based on the diagrams, their passive behavior is analyzed. Both curve A and B showed passive behavior in the anodic region indicating presence of a protective film on their surfaces. Despite chloride ions being present (from curve B's electrolyte solution), there was no significant effect during the whole anodic polarization on the passive film. With a lower concentration of electrolyte (curve C and D), curve C exhibits a passive behavior while curve D represents a very small passive region followed by a quick rise in current density. As for the remaining last two curves (curve E and F), the current densities observed for curve E at anodic and cathodic polarization is lower than that of curve F's. This is due to the chloride ions disrupting the protective film. Real-time imaging work was also conducted during potentiodynamic polarizations for certain electrolytes (e.g., 0.01 M NaOH) to comprehend the mechanism of hydrogen evolution on magnesium [30]. In general, measurements of potentiodynamic polarization can help determine the instantaneous corrosion rate (i\({}_{\text{corr}}\)) at the corrosion potential, E\({}_{\text{corr}}\)[10]. During the corrosion process, the surface released hydroxyl ion and metallic ions as cell growth has different stages, depending on the distance from the sample. Due to the progress in technology, two methods have been developed which makes the investigation of corrosion current density and ion distribution at microscopic spatial resolution possible throughout the corrosion process. These two methods are scanning vibrating electrode technique (SVET) and scanning ion-selective electrode technique (SIET). A study conducted by [PERSON] et al. [31] used SVET and SIET with two representative  
Figure 6: CCS system instrument and cell layout.