# Mesoporous Carbon as Galvanic-Corrosion Activator Accelerates Fe Degradation
## 3 Results and Discussion
### Degradation Behavior  
The typical polarization curves of Fe and Fe/MC composites were depicted in Figure 5a. The derived E\({}_{\text{corr}}\) and I\({}_{\text{corr}}\) using Tafel extrapolation were shown in Figure 5b. It was obvious that the E\({}_{\text{corr}}\) was greatly decreased with MC increasing. Specifically, the E\({}_{\text{corr}}\) was \(-0.60\pm 0.01\) V for Fe, and \(-0.67\pm 0.03\) V for Fe/1.0 MC. Meanwhile, the I\({}_{\text{corr}}\) increased from \(6.45\pm 0.36\) uA/cm\({}^{2}\) for Fe to \(23.6\pm 0.9\) uA/cm\({}^{2}\) for Fe/1.5 MC. In the Nyquist plots (Figure 5c), the change in the impedance imaginary part (z\") was expressed as a function of the impedance real part (z\"), which exhibited series semicircles. It was well known that the diameter of the high-frequency capacitive loop represented the polarization  
Figure 4: (**a**) Representative stress–strain curves obtained from tensile tests; (**b**) The calculated tensile strength and elongation from the stress–strain curves. n = 3. Fracture surface morphologies of as-built (**4**) Fe, (**d**) Fe/0.5 MC, (**e**) Fe/1.0 MC and (**f**) Fe/1.5 MC. resistance [34; 35]. Obviously, the diameters of semicircles gradually decreased with MC increasing, indicating that their corrosion resistance turned weak. The corrosion rate was also calculated from electrochemical parameters, as shown in Figure 5d. The Fe/1.0 MC composite showed a significantly accelerated corrosion rate of 0.19 \(\pm\) 0.02 mm/year as compared to Fe. The SEM images of the corroded surface after polarization test were presented in Figure 6. Clearly, Fe exhibited an intact surface with few corrosion products (Figure 6a). For the Fe/0.5 MC composite, a few corrosion products appeared. As a comparison, Fe/1.0 MC and Fe/1.5 MC showed a large amount of corrosion product, which should be Fe oxides, as confirmed by EDS analysis (Figure 6f). This indicated that enhanced corrosion occurred for Fe/1.0 MC and Fe/1.5 MC. During potentiodynamic polarization, the external driving force was applied to dissolve the Fe matrix. In this condition, the galvanic interactions between the Fe matrix and MC could be ignored. In fact, the introduction of MC created defects in the Fe matrix, as presented in Figure 2, which might be the major reason for the enhancement of electrochemical corrosion. [36; 37]. Figure 6: Surface of (**a**) Fe, (**b**) Fe/0.5 MC, (**c**) Fe/1.0 MC and (**d**) Fe/1.5 MC after polarization tests; (**e**) Locally enlarged image of Fe/1.0 MC and (**f**) energy dispersive spectrometer (EDS) analysis of the marked area in Figure 6e. Figure 7: Corrosion surface of (**a**) Fe, (**b**) Fe/0.5 MC, (**c**) Fe/1.0 MC and (**d**) Fe/1.5 MC composites after immersion for 21 d; (**e**) and (**f**) were the corresponding EDS analyses of points A and B, respectively. The Fe concentration in SBF at different soaking time was measured by ICP-AES, as presented in Figure 8a. It could be seen that Fe ion concentration increased, with the immersion time extending. For pure Fe, the released Fe ion concentration increased slowly from 3.9 \(\pm\) 1.1 to 7.1 \(\pm\) 2.3 mg/L during four different soaking cycles. After incorporating MC, the released Fe ion concentration increased significantly, indicating an increased degradation rate. In particular, the Fe ion concentration rose rapidly from 7.9 \(\pm\) 2.2 mg/L at day 7 to 14.8 \(\pm\) 3.6 mg/L at day 28 for the Fe/1.0 MC composite. The degradation rate of Fe/MC composites basing on immersion tests were shown in Figure 8b. Specifically, Fe exhibited a relatively slow degradation rate of 0.09 \(\pm\) 0.01 mm/year. In contrast, Fe/1.0 MC showed a significantly accelerated degradation rate of 0.24 \(\pm\) 0.03 mm/year. A previous research reported that Zn-based bone implants exhibited a corrosion rate of 0.18 mm/year, while Mg-based bone implants exhibited a corrosion rate of 1.7 mm/year. In fact, the bone healing period is usually 3-6 months, which requires a degradation rate of 0.2-0.5 for bone implants.