*3.6. Corrosion Morphology of the Cast ZK60 Alloy*

To observe the real-time corrosion development in the initial period (0–1 h), Figure 12 presents the in situ corrosion images of the etched sample after immersion in 0.1 M NaCl for different times. In the initial period (20 min, Figure 12a), there was no apparent corrosion, with a limited number of H2 bubbles observed on the surface, implying a slight corrosion state. This slight corrosion should be related to the protective film on the alloy surface impeding the corrosion in the initial period [68]. Over 40–60 min (Figure 12b,c), some dark threads appeared on the surface, which lengthened with time and displayed filiform-like corrosion characteristics. In this period, H2 bubbles constantly evolved at or near the leading edges of the dark threads, leaving cloudy trails behind them. The magnified picture in Figure 12d suggests that the corrosion filaments seem to traverse within the grain boundary regions and do not extend toward the center of the grains in the initial stage of expansion, which is further verified later. These corrosion characteristics are consistent with those of other Mg alloys in similar corrosion media and should be related to their microstructure features [50,65,69].

**Figure 12.** Optical in situ corrosion images of the cast ZK60 etched sample immersed in 0.1 M NaCl for different times: (**a**) 20 min; (**b**) 40 min; (**c**,**d**) 60 min.

Figure 13 presents the BSE-SEM micrographs of the etched samples after immersion in 0.1 M NaCl for 1 and 2 h. The black corrosion filaments (Figure 13a) covered by Mg(OH)2 [70] mainly occurred on the "darker zones" in grain boundary regions (see Figures 2b and 5) and encompassed the second-phase particles. Some second-phase particles in the boundary regions (Figure 13b) were surrounded by a small number of black corrosion products, implying that the corrosion filaments may have started from the surrounding areas of these second-phase particles. Most of the central regions of the grains, i.e., the Zr-rich areas with a light color (Figures 2b and 5), were still uncorroded. When t = 2 h (Figure 13c), only a small number of second-phase particles occurred in the corroded areas, implying that most of them were removed due to the dissolution of their surrounding Mg matrix. Some broad corrosion areas in Figure 13 show that the corrosion could develop to the central area of grains with increasing corrosion time. Moreover, the corrosion filaments in Figure 13c,d show

an apparent corrosion depth, implying that the corrosion also developed under the black corrosion product layer.

**Figure 13.** BSE-SEM micrographs of the cast ZK60 etched samples immersed in 0.1 M NaCl for different times: (**a**,**b**) 1 h (with corrosion products); (**c**,**d**) 2 h (without corrosion products).

As shown in Figures 12 and 13, the etched samples were used to observe the origination and developmental features of the corrosion filaments on the cast ZK60. To avoid the influence of the etching treatment on the corrosion process, Figure 14 presents the secondary electron (SE) SEM images of the raw alloy sample after immersion in 0.1 M NaCl for 0.5 and 2 h. The corrosion morphologies in Figure 14a,b also exhibit the characteristics of filiform corrosion, similar to those in Figure 12b. The polishing scratches on the surface of the test samples do not appear to influence the initiation and extension of the corrosion filaments, suggesting that they are controlled by the microstructure of the cast ZK60 alloy. When t = 2 h, the corrosion images in Figure 14c,d display features similar to those in Figure 13c and some small corrosion pits occur in the corrosion areas, which may have been the result of the loss of the second phase-particles. The raw cast ZK60 sample also displayed the filiform corrosion features in the early corrosion period (0–2 h).

Figure 15 presents the secondary electron (SE) SEM images of the cast ZK60 alloy after immersion in 0.1 M NaCl for different times to observe the corrosion development over a long period (12–72 h). The corrosion filaments gradually extended to the whole surface of the test samples after 24 h, and the number and depth of the corrosion pits increased with time. These results further prove that with increasing corrosion time, the corrosion of the cast ZK60 alloy can develop toward the central area of grains and toward the depth of the alloy matrix, which is discussed below.

**Figure 14.** SE-SEM micrographs of the cast ZK60 sample immersed in 0.1 M NaCl for different times: (**a**,**b**) 30 min; (**c**,**d**) 2 h (without corrosion products).

**Figure 15.** SE-SEM micrographs of the cast ZK60 alloy immersed in 0.1 M NaCl for different times: (**a**) 12 h; (**b**) 24 h; (**c**) 48 h; (**d**) 72 h (with the corrosion products removed).
