4.1.1. Corrosion Reactions on the Cast ZK60 Alloy

A thin oxide film can be formed on the surface of the cast ZK60 alloy in a moist atmosphere at room temperature according to Reactions (5) and (6), which may have a bilayer structure with an inner layer of MgO and an outer layer of Mg(OH)2 [72,73].

$$2\text{Mg} + \text{O}\_2 = 2\text{MgO}\tag{5}$$

$$\text{MgO} + \text{H}\_2\text{O} = \text{Mg(OH)}\_2\tag{6}$$

Because MgO and Mg(OH)2 are both relatively soluble in water according to Reactions (7) and (8) [3,74], where Ksp is the solubility product constant, MgO will be gradually dissolved and converted to Mg(OH)2 when they are immersed in NaCl solution.

$$\text{MgO} + \text{H}\_2\text{O} = \text{Mg}^{2+} + 2\text{OH}^-\\K\_{\text{sp}} = 10^{-6} \tag{7}$$

$$\text{Mg(OH)}\_{2} = \text{Mg}^{2+} + 2\text{OH}^{-} \quad K\_{\text{sp}} = 10^{-11} \tag{8}$$

In this case, the MgO/Mg(OH)2 oxide film formed on the cast ZK60 alloy was partly protective in a neutral NaCl solution. The anodic and cathodic partial reactions of the corrosion process can be written as Reactions (9) and (10), respectively, and the corrosion product is formed as Reaction (11) [75].

$$\text{Mg} = \text{Mg}^{2+} + 2\text{e}^- \tag{9}$$

$$2\text{H}\_2\text{O} + 2\text{e}^- = \text{H}\_2(\text{g}) + 2\text{OH}^- \tag{10}$$

$$\text{Mg}^{2+} + 2\text{OH}^- = \text{Mg}(\text{OH})\_2 \tag{11}$$

4.1.2. Initiation of Corrosion on the Cast ZK60 Alloy

Based on the microstructure of the cast ZK60 alloy, Figure 19 presents a schematic model to illustrate the initiation and development of the corrosion on the cast ZK60 alloy. As discussed above, a partly protective MgO/Mg(OH)2 oxide film will be formed on the cast ZK60 alloy in neutral 0.1 M NaCl (Figure 19a). Because the large second-phase particles and the Zr-rich region in the grains were more stable than the Mg matrix in grain boundary regions, as shown in Figure 6, the oxide film in these regions should be more stable than that in their adjacent grain boundary regions (i.e., darker areas in Figure 2b), which was denoted as a stable and active oxide film in Figure 19a [76,77].

**Figure 19.** Schematic model for the initiation and development of the corrosion on the cast ZK60 alloy in NaCl solution. (**a**) original state; (**b**) corrosion initiation; (**c**) filaments growth; (**d**) filaments propagation; (**e**) pit formation and (**f**) pit development.

Compared to the Mg matrix in the grain boundary regions, those with stable oxide film act as micro-galvanic cathodes. Under the attack of Cl−, the film around the second phase could be easily broken due to the higher numbers of imperfections [78]. Therefore, the micro-galvanic corrosion was initiated in the areas around the large discontinuous second-phase particles in the grain boundaries, owing to the strong galvanic effect between these second-phase particles and the Mg matrix in the grain boundaries (Figure 13b). It should be noted that there were some second-phase particles deposited in the grains (Figures 2a and 6), and therefore, the corrosion may also be initiated in the grains. However, we did not observe this phenomenon, which may be due to the stable oxide film in the grains (Zr-rich region) and the light galvanic effect between them.

After the corrosion initiation (Figure 19b), the oxide film near the second-phase particles in the grain boundaries cracked first. However, there were no black Mg(OH)2 products observed in this beginning period (Figure 12a), because they were dissolved in the NaCl solution (Equation (5)). After the concentration of Mg(OH)2 reached saturation in the NaCl solution, black Mg(OH)2 precipitation occurred (Figure 12b).

#### 4.1.3. Growth of Corrosion Filaments on the Cast ZK60 Alloy

Because the Zr-rich area within the grains was more stable than the Mg matrix in grain boundary regions, the corrosion preferentially propagated in the grain boundary regions in the initial corrosion period (Figure 19c), displaying the characteristics of filiform-like corrosion, as shown in Figures 12 and 13. According to [69,70], the front edges of the corrosion filaments acted as intense anodes via Equation (6), while the dark tracks behind the anodes acted as locally activated cathodes, where cathodic Equation (7) occurred. In NaCl solution, Cl− may be enriched around the anode areas to attack and break the oxide film [28]. By contrast, the activated cathodes behind the anodes provide the driving force for the development of the corrosion filaments. Then, the precipitates of Mg(OH)2 cover the surface of the active cathodes behind the anodes to make them gradually turn into inert cathodes.

The second phase in the grain boundaries and the Zr-rich region within the grains have an essential influence on the origination and propagation of the corrosion filaments on the cast ZK60. These filiform-like corrosion characteristics of the cast ZK60 were quite different from those of Mg-3Zn and Mg-8Li alloys reported previously [65,71], in which the corrosion filaments seem not to be affected by the second phases and can originate from the center of the grains and spread to various directions. These differences should be ascribed to their different microstructures, especially the quantity and size of the second phase and the distribution of alloying elements in these alloys.

As the corrosion time was increased, the active "dark areas" along the grain boundaries were gradually covered with a black Mg(OH)2 layer (Figure 19c) and potentially became cathodic areas [79]. This change would impede the anodic corrosion process in these areas and make their potential positive, reducing and even eliminating the potential difference between these areas and the Zr-rich regions. Therefore, the corrosion filaments could gradually develop to the central area of grains (Figure 19d) to form some broad corrosion areas, as shown in Figures 13 and 14. Additionlly, the black Mg(OH)2 layer was porous and imperfect and could not prevent the entry of the NaCl solution (Figure 18). Thus, the anodic process still existed beneath the Mg(OH)2 layer, developing an apparent corrosion depth under the corrosion filaments, as shown in Figures 13 and 14. When t > 24 h, most of the alloy surface was gradually covered with the Mg(OH)2 layer (Figure 15), and finally, it would be covered completely (Figure 19e,f).

#### 4.1.4. Development of Corrosion Pits on the Cast ZK60 Alloy

With the development of the corrosion filaments, the corrosion pits will be formed when the Mg matrix surrounding the large second-phase particles is corroded completely (Figure 19e). Some small corrosion pits occurred in the corrosion filaments when t = 2 h (Figure 14c,d), which may be due to the loss of the second-phase particles when the corrosion product was removed. The visible corrosion pits occurred after 12 h of immersion (Figure 15), and their number and depth increased with time over 24–72 h, in which Pdepth was about one magnitude order higher than those corrosion rates (Pw, PH, and Pi), as shown in Figures 15–17. The high Pdepth of the cast ZK60 alloy indicates a severe localized corrosion state and warrants further investigation.

The development of the corrosion pits was related to the occluded environment under the corrosion filaments, covered by the second-phase particles and Mg(OH)2 precipitate. The solution in the occluded corrosion pit would be alkalized to increase the pH to around 10.5 through Reaction (5). At the same time, Cl− would migrate in due to the charge neutralization and become enriched at the bottom of the pit (Figure 18). These effects would accelerate the anodic process in the corrosion pits and make them develop deeply [80].

According to the above discussion, the microstructure of the cast ZK60 alloy had an essential influence on the corrosion initiation and propagation in NaCl solution. Thus, it is necessary to improve its corrosion resistance by microstructure modification, which will be studied later.
