**1. Introduction**

Magnesium (Mg) alloys have been widely applied as lightweight engineering materials due to their unique properties [1–8]. As commercial Mg-Zn-based alloys, ZK60 (Mg–Zn–Zr) alloys [9] have attracted great interest from researchers due to their high strength [10–12]. The microstructure [13,14], mechanical properties [15–17], and biological applications [18,19] of ZK60 alloys have been studied widely in recent decades. It has been verified that microstructure evolution is essential for the mechanical properties of ZK60 alloys, grain refinement, and stable precipitates, having vital effects on improving the mechanical properties [20–23]. Nevertheless, the weak corrosion resistance of ZK60 alloys limits their further applications.

The microstructure of Mg alloys, especially their second phases, has an evident impact on their corrosion behavior [3,5]. The second phases of Mg alloys may have a dual role in their corrosion, i.e., a galvanic acceleration effect or a corrosion blocking effect, depending on their quantities and distribution [24–26]. In Mg-Al alloys, when the amount of aluminum (Al) is low (e.g., Mg-5Al), the β-phase (Mg12Al17) is relatively discontinuous in the Mg matrix and mainly acts as a cathode

phase to accelerate the dissolution of the matrix. As the content of the Al element increases (e.g., Mg-10Al), the β-phase precipitates are tiny and continuously distributed along the grain boundaries, producing a barrier to prevent corrosion [27]. The similar effect of the second phase was reported in other Mg alloys [28,29]. However, the effect of the second phase in ZK60 alloys on their corrosion behaviors is barely reported. Some researchers tried to enhance the corrosion resistance of ZK60 alloys by modifying their microstructure through heat treatment [30,31], deformation processing [32], and alloying [33–36]. Even so, the relationship between the microstructure and corrosion behavior of ZK60 alloys needs more studies for it to be investigated.

Some studies reported the corrosion behavior of ZK60 alloys on different occasions. Cheng et al. [37] pointed out that the Zr element in ZK60 alloys refined the grain and purified the alloy composition, which could improve its corrosion resistance in 1 M NaCl. Zeng et al. [38] investigated the effects of the microstructure and concentration of NaCl (3.5 and 5.0 wt.%) on the corrosion behavior of an extruded ZK60 alloy. They found that an increase in the grain size of the ZK60 alloy accelerated its corrosion rate. The alloy microstructure played a crucial role in the pitting and intergranular corrosion. Xu et al. [39] reported that the corrosion rate of a cast ZK60 alloy decreased with the immersion time in solutions containing 3.5 wt.% NaCl, NaBr, and NaI, while it displayed passivation in 3.5 wt.% NaF solution. Apart from the above reports, some studies focused on the biodegradation behavior of ZK60 alloys in Hank's solution, Ringer's solution, simulated body fluid, and artificial urine for biomedical applications [40–43]. The biodegradable property of the ZK60 alloys is the interesting issue in these investigations. In general, the above research usually concentrated on the uniform corrosion of ZK60 alloys, and little attention was paid to the development of their local corrosion. The influence of microstructure on the local corrosion of ZK60 alloys is still not clearly understood, especially the effect of the second phase.

In this study, a commercial cast ZK60 alloy was selected as the test material. Its microstructure was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and scanning Kelvin probe force microscopy (SKPFM) analysis. The general and local corrosion of the alloy in 0.1 M NaCl was investigated using weight loss tests, hydrogen evolution tests, and electrochemical measurements, as well as corrosion morphology monitoring with an optical microscope and SEM. The effect of the microstructure of the cast ZK60 alloy, especially the second phase and the distribution of alloying elements, on the corrosion initiation and developmental features of the alloy were investigated, and the mechanisms involved were studied. This work will help to verify the relationship between the microstructure and the corrosion behavior of ZK60 alloys. Moreover, it may also provide a theoretical basis for improving the corrosion resistance of ZK60 alloys by adjusting the microstructure in future research.

### **2. Materials and Methods**

#### *2.1. Test Material and Solution*

A commercial as-cast ZK60 alloy was used in this study. Table 1 presents its chemical composition, analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES, SPECTRO, Kleve, Germany). All the solutions used in this work were prepared with analytical-grade reagents and distilled water. The test solution was 0.1 M NaCl under an open-air condition, which was controlled at 25 ± 1 ◦C with a water bath.


**Table 1.** Chemical composition of the cast ZK60 alloy (wt.%).
