*3.3. Hydrogen Evolution Tests*

Figure 8 presents the hydrogen evolution test results for the cast ZK60 alloy to show the change in its corrosion rate in detail, in which *v*<sup>H</sup> is the differentiation of the *V*H-t curve in Figure 8a and *P*<sup>H</sup> is calculated by Equation (4). The *<sup>V</sup>*<sup>H</sup> at 24 h (Figure 8a) was about 0.7 <sup>±</sup> 0.03 mL cm<sup>−</sup>2, which is higher than that of an extruded ZK60 (~0.5 mL cm<sup>−</sup>2) [54]. The *P*<sup>H</sup> value (Figure 8b) increased with time in the initial corrosion period (0–1.5 h) and then decreased over 1.5–48 h; at last, it increased again over 48–72 h. These results imply that there exist different corrosion stages in the periods of 0–2 h, 2–48 h, and 48–72 h, which may be related to the change in the alloy surface condition. According to Song [24], the total volume of hydrogen collected should equal the total amount of metal lost, and both the weight loss and hydrogen evolution tests were reliable methods. Even though the hydrogen evolution rate *P*<sup>H</sup> we measured is slightly lower than the weight loss corrosion rate *P*w, which may be caused by the difference in the test methods, they generally show similar change tendencies.

**Figure 8.** Changes in (**a**) *V*<sup>H</sup> and (**b**) *P*<sup>H</sup> of the cast ZK60 alloy as a function of time measured by the hydrogen evolution test in 0.1M NaCl.

#### *3.4. Polarisation Curve Measurements*

Figure 9 presents the polarization curves of the cast ZK60 alloy after immersion in 0.1 M NaCl for different times and the changes in its corrosion rates (icorr and Pi) with time. All the polarization curves showed the typical features of activation-controlled processes [55,56]. A so-called breakdown potential (Ebreak) occurred in the anodic polarization curves (except t = 2 h), owing to the breakdown of the oxide film on the alloy [57], as shown in Figure 9b. Ecorr (Figure 9a) and Ebreak (Figure 9) moved positively in the period of 0–24 h and then became negative again, while Pi and icorr (Figure 9b) displayed a similar change with corrosion time to that of PH as shown in Figure 8b, which is consistent with previous reports [38,58]. The icorr and Pi values in Figure 9b are much smaller than the Pw (Figure 7) and PH (Figure 8b) values. Similar results from other Mg alloys have been discussed in detail in [44]. However, they displayed a similar change tendency. All these results also suggest that there may be different corrosion stages in the corrosion periods of 0–2 h, 2–24 h, and 24–72 h.

**Figure 9.** (**a**) Polarization curves of the cast ZK60 alloy immersed in 0.1 M NaCl for different times; (**b**) Changes in the icorr, Pi, and Ebreak with time (25 ◦C).

#### *3.5. EIS Measurements*

Figure 10 presents the EIS of the cast ZK60 alloy immersed in 0.1 M NaCl for different times and the change in *R*<sup>p</sup> (polarization resistance) as a function of time. All the Nyquist plots display two capacitive loops in the high-frequency region and an inductive loop in the low-frequency region, which is similar to the EIS features of other Mg alloys [59]. The capacitive loops are related to the processes in the surface film and the electrical double layer [60], and the inductive loop should be attributed to the initiation of the localized corrosion on the surface of the cast ZK60 alloy according to [61–63]. Based on these EIS features, Figure 11 presents an equivalent circuit to fit the EIS results in Figure 10a [58,59,64,65]. *R*<sup>s</sup> is the solution resistance. CPEf and CPEdl represent the constant phase

elements (CPE) for the surface film and the electrical double layer, respectively. *R*<sup>f</sup> and *R*ct represent the surface film resistance and charge-transfer resistance, respectively. *R*<sup>L</sup> and *L* represent equivalent resistance and inductance to describe the low-frequency inductance. It should be noted that *R*ct is the parallel of the charge-transfer resistance of the anodic process and the cathodic process (*R*ct,a and *R*ct,c) at *E*corr [63]. The fitting curves are also displayed in Figure 10a, and the fitting parameters are listed in Table 2.

**Figure 10.** (**a**) Nyquist plots measured at *E*corr for the cast ZK60 alloy immersed in 0.1 M NaCl for different times; (**b**) *R*p–t curves at 25 ◦C.

**Figure 11.** Equivalent circuit used to model the electrochemical impedance spectroscopy (EIS) response of the cast ZK60 alloy in 0.1 M NaCl.


**Table 2.** Fitting parameters of the EIS results in Figure 9a.

According to [59,66,67] and the results in Table 2, the fitting curves can be extrapolated to the zero-frequency limit to obtain the polarization resistance (*R*p) at different times. Therefore, the *R*p–t curve was obtained as shown in Figure 10b. *R*p decreased with time over 0–2 h, indicating corrosion acceleration, and then, *R*p clearly increased between 2 and 24 h before decreasing slightly over 24–72 h. In this case, the change in *R*<sup>p</sup> with *t* is consistent with that of *P*<sup>i</sup> (Figure 9b). The *R*<sup>t</sup> and *R*<sup>f</sup> values in Table 2 show the same change tendencies as those for *R*p. The change in the CPEdl, CPEf, *R*L, and L with *t* should be closely related to the surface layer of the cast ZK60 alloy [59]. The specific changes in these EIS results are discussed later.
