*3.1. Microstructure of the Cast ZK60 Alloy*

Figure 2 shows the optical and the back-scattered electron (BSE) SEM micrographs of the cast ZK60 alloy. The microstructure of the cast ZK60 alloy was composed of an α-Mg phase and large second-phase particles, which were mainly deposited along the grain boundaries (Figure 2a,b). According to [47,48], the main components of these particles are Mg and Zn and may be MgZn2 and MgZn. The XRD pattern of the cast ZK60 alloy in Figure 3 only displays the presence of MgZn2. No discernable diffraction peaks from MgZn were detected in this work, which suggests no MgZn phase in the studied alloy; also, there is no Zr detected, maybe due to its low content in the test sample. However, the XRD patterns cannot give accurate structural information for the second phase. Therefore, the crystal structures of the abovementioned second phases were characterized using TEM in detail as follows.

To further verify the second phase in the as-cast ZK60 alloy, Figure 4 presents the TEM micrograph of the as-cast ZK60 alloy. Block-shaped and globular second-phase particles can be observed in Figure 4. The blocky phase has a size of about 500 nm, and the globular phase is about 100 nm. Figure 4 also presents the corresponding selected area electron diffraction (SAED) patterns of the second phase. The SAED patterns proved that the two second-phase particles were MgZn2 [49]. No MgZn phase was found in the as-cast ZK60 alloy.

**Figure 2.** (**a**) Optical and (**b**) BSE-SEM micrographs of the cast ZK60 alloy.

**Figure 3.** XRD pattern of the cast ZK60 alloy.

The BSE-SEM micrograph in Figure 2b indicates the nonuniform distribution of the alloying elements in the cast ZK60 because the brighter areas contain more elements of higher atomic weight than the darker regions [50]. Figure 5 presents the area distribution of the alloying elements in the cast ZK60, proving that the center area of the grains with light color was richer in Zr and Zn (Zr-rich region) than the neighboring darker zones (grain boundary region). Here, the "grain boundary region" of the as-cast ZK60 is denoted as the areas between the Zr-rich region, i.e., the dark areas in Figure 2b. This uneven distribution of the alloying elements in the cast ZK60 alloy may cause the inhomogeneous electrochemical activity resulting in the micro-galvanic corrosion [51].

Figure 6 presents the SKPFM maps of the cast ZK60 sample, which clearly show that the second phase had the highest potential. Furthermore, the Volta potential profiles (Figure 6c) along the line A and line B indicated that the central region of the grains (i.e., Zr-rich region) exhibited higher potential than those of the grain boundary regions, owing to the higher Zr and Zn contents in the center of grains and higher Mg content in the grain boundary regions (Figure 5). Thus, the second-phase particles and the central region of the grains should be more stable than the Mg matrix in grain boundary regions and more likely to become cathodes in the micro-galvanic cells. A second-phase particle occurred in the grain area (Figure 6a), which is consistent with Figure 2a, so the micro-galvanic corrosion may also have been initiated in the grains.

**Figure 4.** TEM micrograph of the cast ZK60 alloy and the corresponding selected area electron diffraction (SAED) patterns of the second-phase particles.

**Figure 5.** BSE-SEM image of the cast ZK60 alloy and the corresponding area distributions of Mg, Zn, and Zr.

**Figure 6.** (**a**) Scanning Kelvin probe force microscopy (SKPFM) topography map; (**b**) Potential map of the same area; (**c**) Volta potential profiles along lines A and B in the SKPFM image.
