*3.2. Electrochemical Measurements*

Figure 7 shows the electrochemical behaviors of the prepared Mg-RE alloy ingot sample and the sheet with TRC sample in PBS solution at 310 K. In addition, the fitting results are summarized in Table 3. As stated above, after the two-roll casting process, the Mg-RE alloy sheet presented a more positive potential (−1.08 *VAg*/*AgCl*), with comparable potentials to the Mg-RE alloy ingot (−1.37 *VAg*/*AgCl*). Based on electrochemical theory, in the process of electrode reaction, the ions in the solution were mainly in charge of conveying the transformation to the surface of the electrode. In the cathode area, the Mg-RE alloy dissolves into metal cation. Because the metal cation ion concentration is too high, the charge exchange process cannot be carried out as soon as possible. The cloud of ions blocks the electrodes' ability to charge, which is called polarization resistance (*Rp*). In general, the larger the Rp of the metal materials, the larger the ion cloud on the electrode surface, thus preventing charge exchange. The corrosion potential (*Ecorr*) of the samples is mainly determined by the relative size between anode and cathode reaction rates, which reflects the reaction trend [31]. Furthermore, the corrosion current density (*Icorr*) shows a decreasing trend: Mg-RE sheet with TRC sample (1.51 × 10−<sup>4</sup> μA) < Mg-RE ingot sample (1.74 × 10−<sup>3</sup> μA). Lower *Icorr* indicates better corrosion resistance. Therefore, it is demonstrated that the Mg-RE sheet sample with TRC possesses a higher corrosion potential, a smaller current density, and a better corrosion resistance. Due to the influence of part of the amorphous phase being formed in the Mg-RE alloy after rapid cooling solidification by two-roll casting, the corrosion resistance was enhanced.

The electrochemical impedance data were determined for the corrosion potential in PBS and presented in Nyquist plots (Figure 7b, and Bode plots (Figure 7c). The equivalent circuit for electrochemical impedance is shown in Figure 7b. *Rs*, *Rt*, and *Rf* represent the solution resistance between the reference electrode and the working alloy sample, the resistance of charge transfer and the resistance of the corrosion product layer on the surface of the sample, respectively. Additionally, CPE1 and CPE2 illustrate the capacitance of the corrosion product layers and charge separation at the positions where hydrogen evolution increases. In the Nyquist plots shown in Figure 7b, the magnitude of the radius curvature has different values, showing a decreasing trend: Mg-RE ingot sample < Mg-RE sheet with TRC sample, which is also illustrated by the decreasing impedance modulus trend of the Mg-RE alloys in the curves in Figure 7c. It is well known that the size of the Nyquist curve is an important parameter that reflects corrosion resistance. That is to say, better corrosion resistance and

behavior of the metal and alloy matrix is related to higher |Z| modulus at lower frequency, which is inversely proportional to the corrosion rate of the alloy. The Bode phase plot is shown in Figure 6d, and it can be found that the phase angles corresponding to high frequency are in a decreasing order as follows: Mg-RE sheet with TRC sample > Mg-RE ingot sample, which could be attributed to the protective properties of the surface film layers.

**Figure 7.** Electrochemical behaviors of the Mg-RE alloy ingot sample and the sheet with TRC sample in PBS solution: (**a**) polarization curves; (**b**) equivalent circuit and Nyquist plots of the real part Z vs. the imaginary part Z"; (**c**) Bode plots of |Z| vs. frequency; and (**d**) Bode plots of phase angle vs. frequency.


