*3.7. Characterization of the Corrosion Pits on the Cast ZK60 Alloy*

To characterize the development of corrosion pits, the deepest pit on the cast ZK60 sample was selected after immersion in 0.1 M NaCl for different times, and their 3D morphologies and depth profiles were measured, as shown in Figure 16. The density and depth of the corrosion pits increased over 12–72 h, and the size of the pit mouth was generally larger than the pit depth, which is similar to the corrosion morphologies in Figure 15. The corresponding pit depth at different corrosion times was employed to calculate the average penetration rate (Pdepth, mm y−1), which is also presented in Figure 17. The pit depth increased with time, but the fastest Pdepth occurred in the period of 12–24 h (~24 mm y<sup>−</sup>1) before gradually decreasing to ~16.6 mm y−<sup>1</sup> in the period of 24–72 h.

**Figure 16.** 3D corrosion images and depth profiles 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 (without corrosion products).

**Figure 17.** Depth of the deepest corrosion pit and the corresponding *P*depth of the cast ZK60 alloy immersed in 0.1 M NaCl for different times.

Figure 18 is the cross-sectional EPMA pattern of a corrosion pit on the cast ZK60 sample immersed in 0.1 M NaCl for 72 h. The corrosion pit was covered by the corrosion product, and the second-phase particles were encompassed in it. Only Mg, O, and Cl elements were found in the corrosion product layer, which should result from Mg(OH)2 and Cl<sup>−</sup> in the test solution. Moreover, a large number of Cl− through the whole corrosion product layer suggests that Cl− could penetrate the product layer easily and may become enriched in the bottom of the corrosion pit to propagate the pit corrosion [71]. The second-phase particles in the corrosion pit imply that they may be related to the formation of corrosion pits, which is discussed later.

**Figure 18.** Electron probe micro-analyzer (EPMA) maps of the elemental distribution in the cross-section of a corrosion pit on the cast ZK60 alloy immersed in 0.1 M NaCl for 72 h.

#### **4. Discussion**

### *4.1. Initiation and Development of Corrosion on the Cast ZK60 Alloy*
