Fig. 11(a) plots the evaluation of OCP values corresponding to alloys tested in SBF at different time. Except plunges at day 15, the OCP values increase with the immersion time, indicating a stable corrosion layer with higher potential forms on the surface. The plunge at day 15 is attributed to the distinct pitting corrosion and the transformation of corrosion layer on the surface (Fig. 7(f)), which exposures the active substrate again to the SBF.PDP testing on the materials is conducted to compare the corrosion resistances of immersed alloys with time (Fig. 11(b)). Both anodic and cathodic Tafel slopes change with immersion time, indicating that changes of corrosion layer affect dissolution of the Zn matrix and oxygen consumption. Corresponding parameters including corrosion potential (Ecorr), icorr, ˇaand ˇcslopes are listed in Table 5. Passivation zones of 100−150 mV appear in anodic branches of the PDP curves within first 5 days, due to the formation of Li-containing passive layer. The passive area increases within the first 3 days, then decreases afterwards. Meanwhile, the film break down potential decreases with time, indicating the decreasing of kinetic barrier effects of Li-containing film with time. Ecorrval-ues of immersed alloys increase slightly with time, consistent with the OCP results. The icorr values at day one to day 5 are much lower than that of other time points, then icorr values increase gradually with immersion time except a sudden increase at day 15, which is associated with the growth of un-protective dissolution products upon freshly exposed alloy surface sites. Therefore, the polarization characteristics of immersed alloys above demonstrate that there is a time dependent surface layer formation that provides a kinetic barrier to the anodic dissolution, especially pronounced at initial immersion period.Fig. 11(c) and (d) shows the EIS spectra of WR alloy immersed in SBF for different days. As shown in the Nyquist curves (Fig. 11(c)), a diffusion impedance character is observed in the low frequency region for immersed alloys within 5 days, evidenced by the rising slope of the curve in the low frequency region and the rising impedance modulus (|Z|) at the lowest frequency (Fig. 11(d)). The diameters of capacitive loop increase within first 5 days and then decrease till day 30. Two-time constants exist in the Bode curves of the specimens (Fig. 11(e)), so that a two-time constants model with the Warburg (W) or Gerischer (G) diffusion impedance is used to fit the EIS data of all the specimens. The fitted equivalent circuit is shown in Fig. 11(e). Rsvalue represents the solution resistance. R1 and CPE1 represent charge transfer resistance and the electric double layer at the metal/electrolyte interface, respectively. The two quantities describe the first capacity loop at high frequency. R1and CPE1represent the film resistance and capacity, respectively, which describe the second capacity loop at low frequency. CPE is a constant phase element and is employed here to compensate the non-homogeneity in the system.Figure 11(e) shows the equivalent circuit of the installation. Rsvalue represents the resistance of the solution. Rdland represents the charge transfer resistance and electric double layer at the metal/electrolyte interface, respectively. These two quantities describe the first high-frequency capacity loop. R1 and cpe1 represent thin-film resistors and capacitors, respectively, and they describe the second capacitor loop at low frequencies. CPE is a constant phase element used to compensate for non-uniformities in the system.
