*2.5. Hydrogen Evolution Tests*

Following the work of Shi [46], plug-in specimens were employed to perform the hydrogen evolution test in 0.1 M NaCl at room temperature (~25 ◦C). Figure 1 presents the schematics of the test system. The cast ZK60 specimens (10 <sup>×</sup> 10 <sup>×</sup> 10 mm3) were treated as the above weight loss samples. The evolved hydrogen was collected into a burette, and its volume (*V*H, mL cm−2) was recorded at different times. The hydrogen evolution rate, vH (mL·cm−<sup>2</sup> day<sup>−</sup>1), can also be converted to a general corrosion rate, *P*<sup>H</sup> (mm y<sup>−</sup>1), using [5,44].

$$P\_{\rm H} = 2.2 \text{ } \upsilon\_{\rm H} \tag{4}$$

**Figure 1.** Schematic diagram of the test system for the hydrogen evolution measurement.

### *2.6. Corrosion Morphology Characterization*

The etched metallographic specimens were placed in 0.1 M NaCl, where the solution thickness on the sample surface was about 1 mm, to observe the corrosion development in the initial period (0–1 h) in situ using a 3D optical microscope (KEYENCE VHX-1000). The corrosion morphologies of the cast ZK60 samples corroded in 0.1 M NaCl for different times were measured by SEM and with the 3D optical microscope. The cross-section corrosion morphologies and the elemental distributions were analyzed by EPMA (EPMA-8050G).

#### **3. Results and Discussion**
