Electrochemical activity

Cyclic voltammetry (CV) is one of the most effective ways in determining the electrochemical properties of samples. Figure 4.13 presents the CV curves of pure Ti, two TiZr-based MGs and the Cu-rich Ti-based MG in the Hank’s solution. Results showed that the EC activity of the two MGs of Ti42Zr40Si15Ta3 and Ti40Zr40Si15Cu5 exhibited stable electrochemical property since there was no significant redox peaks in the CV scans. The measured current responses for these two MGs were in the same scale in compare with that of the reference pure Ti. On the contrary, there was an obvious electrochemical response for the Cu-rich MG of Ti45Cu35Zr20 at the positive applied potential, indicating the occurrence of oxidation reaction. The Cu-rich ribbon was not able to sustain the rapid corrosion response and was decomposed after 3 CV scan cycles. Results confirmed that the high Cu content would lower corrosion resistance. This could be caused by the strong polarization of Cu in

the MG, especially for immersing the MG in an environment with high concentration of chloride ions [165,169,170]. Figure 4.13 also presents the magnified current response for the three CV curves with low current response. It is clear that the current levels for the Ti42Zr40Si15Ta3 and Ti40Zr40Si15Cu5 MGs are even lower than that of the reference pure Ti, indicating the excellent electrochemical stability of the two produced MGs.

Alternatively, the implantable biomaterials may suffer from the electrochemical corrosion due to the potential difference across the cell membrane (membrane potential). The values of membrane potential are typically ranging from 40 mV to 80 mV, depending on the concentration of the sodium and potassium ions of a cell. In order to mimic the real biological environment, the specimens were immersed into the Hank’s solution with an applied potential of 80 mV. Figure 4.14 displays enlarged amperometric i-t curves showing the close-up current density responses of the pure Ti and the two TiZr-based metallic glasses. The measured current response for the MG of Ti45Cu35Zr20 was too high and not in the same scale such that it was excluded from Figure 4.14. The measured current values of the two TiZr-based metallic glasses were monitored for 30 min, illustrating the low electrochemical response in a biological environment for the Cu-free and low-Cu containing TiZr-based specimens. In short, the potential state tests also revealed superior electrochemical stability of Ti42Zr40Ta3Si15 andTi40Zr40Si15Cu5metallic glasses, consistent with the above CV tests.

Although the CV measurement is a straight-forward way for evaluating the electrochemical properties of the developed MGs, the rapid electrochemical tests were not able to analyze the corrosion mechanisms and the polarization dynamics. Therefore, a more accurate electrochemical method with lower scanning rates at the positive applied potential

understanding the corrosion behavior of these MG materials. There are three important electrochemical parameters can be obtained for analyzing TiZr-based and Ti-based metallic glasses via the potentiodynamic polarization measurement. The measured current response at a slow scan rate of 0.33 mV/s is shown in Figure 4.15. The corrosion potential (Ecorr) and the corrosion current density (Icorr) are the parameters for evaluating the driving force and activity of corrosion reaction, respectively. Table 4.5 presents the average Ecorr for measuring the MGs and the pure Ti reference.

The Ecorr readings of Ti42Zr40Si15Ta3 andTi40Zr40Si15Cu5 were slightly lower than those of Ti45Cu30Zr20 and pure Ti, but the difference is only less than 0.2 V (about -0.4 V versus -0.26 V). The slightly lower Ecorr indicated that the MGs with low Cu-contents were slightly easier to be polarized or oxidized than the Cu-rich MG of Ti45Cu30Zr20 and pure Ti. Although Ecorr is an important parameter for examining the material corrosion property, this value is not an entire indicative of the material corrosion resistance [171]. It is clear in Table 4.5 that the Cu-free and low Cu-content TiZr-based MGs, Ti42Zr40Si15Ta3 and Ti40Zr40Si15Cu5, exhibit lower Icorr than those of the Ti45Cu30Zr20 and pure Ti.

The more critical issue that influences the electrochemical property is the pitting corrosion pitting. Pitting corrosion could happen particularly under the high concentration of chloride ions which are likely to react with the Cu atoms in the MGs. The sudden current increases in the Tafel curves also indicated that there were significant electrochemical reactions for the MGs in the SBF Hank’s solution. The pitting potential (Epit) was a significant index to determine the occurrence of the pitting corrosion via observing the sudden signal rise in Figure 4.15. The Ti45Cu35Zr20 exhibited a negative Epit at -0.078 V, indicating the spontaneous pitting corrosion of this material in SBF. In contrast, the

Ti42Zr40Si15Ta3 and Ti40Zr40Si15Cu5 MGs have positive Epit at +1.2 and +0.7 V, respectively (Table 4.5), indicating that the non-spontaneous corrosion reaction of these two MGs in SBF.

The pitting potential for these two MGs are much higher than the membrane potential of 80 mV which confirms the satisfactory stability and corrosion resistance for these two MGs in SBF.

Moreover, the low passivation current density, Ipass, of pure Ti (Table 4.5) also indicated that there was a dense passivation layer formed on the Ti surface, preventing the inner Ti from further oxidation. The lower Ipass readings for the Ti42Zr40Si15Ta3and Ti40Zr40Si15Cu5

MGs (compared to pure Ti)also revealed that these two MGs possessed nice corrosion resistance in SBF since the protective passivation layer was formed on the MG surface [37,38].