*4.2. Change in the Corrosion Rate in Di*ff*erent Corrosion Periods*

The PH, Pi, and Rp values of the cast ZK60 alloy generally showed different change tendencies in different immersion periods (Figures 8, 9b and 10b). In the early corrosion period (0–2 h), the corrosion rates (PH and Pi) increased with time (Figures 8b and 9b), while Rt, Rf, and Rp decreased (Table 2 and Figure 10b). In this period, the filiform corrosion was initiated and developed quickly on the cast ZK60 alloy without visible corrosion pits (Figures 12–14), corresponding to the states in Figure 19a–d. In this case, with the development of the corrosion filaments, the broken oxide film increased steadily to accelerate the anodic and cathodic processes, as shown in Figure 9, and made the corrosion rates increase. The increase in the broken oxide film may be the reason why no Ebreak occurred in the anodic polarization curve when t = 2 h (Figure 9). Furthermore, the increase in the broken oxide film enlarged the reactive areas on the cast ZK60 surface to increase CPEdl and CPEf, meanwhile decreasing RL and L in this period (0–2 h, Table 2).

The second corrosion period was determined to be within 2–24 h, according to the above discussion about the corrosion development on the cast ZK60 alloy. In this period, the corrosion filaments gradually occupied most of the active oxide film, and a small number of corrosion pits occurred and developed quickly (Figures 15 and 16), which corresponds to the state in Figure 19e. Because most of the active oxide film was corroded and covered with a black Mg(OH)2 precipitate, the anodic process, Reaction (9), was inhibited significantly to make Ecorr more positive, and Ebreak occurs again with a more positive value (Figure 9). In this case, the corrosion rate (PH and Pi) decreased significantly (Figures 8b and 9b), while Rt, Rf, and Rp increased (Table 2 and Figure 10b). The difference is that Pi and 1/Rp reached the lowest value at t = 24 h, but PH decreased persistently until t = 48 h, while Pw as shown in Figure 7 displayed the highest value at t = 24 h, which may be ascribed to the differences among these test methods. In this period, the decrease in the broken oxide film may also result in a clear increase in RL and L and decrease in CPEdl (Table 2). Meanwhile, CPEf firstly increased and then decreased, which may have been due to the increase in the area and thickness of the corrosion product layer. Even though very few corrosion pits occurred in this period (Figures 15 and 16), their Pdepth increased significantly and reached a large value of about 24 mm y−<sup>1</sup> (Figure 17), which was much larger than the general corrosion rates of Pw, PH, and Pi in this period (Figures 7, 8 and 9b). Therefore, the pitting corrosion became more critical than the filiform corrosion in this period.

Over 24–72 h, most of the surface of the cast ZK60 was gradually covered with Mg(OH)2 layer, but the number and depth of the corrosion pits increased (Figures 15–17), corresponding to the state in Figure 19f. Because of the increase in corrosion pits, the anodic process was accelerated again. In this case, the Ebreak and Ecorr of the cast ZK60 alloy became negative, and Pi increased again (Figure 9), while Rt, Rf, and Rp decreased (Table 2 and Figure 10b). However, Pw and Pdepth decreased with time in this period (Figures 7 and 17), indicating that the increase in the corrosion product layer still impeded the growth rate of the pitting depth and the general weight loss, possibly due to the alkalizing effect of the cathodic reaction and the low solubility of Mg(OH)2. In this period, the increase in CPEf may have been due to the dissolution of the corrosion product layer reducing its thickness; meanwhile, the decrease in RL and L may have been related to the acceleration of the pitting corrosion process, similar to in the processes on passive metals [61]. In general, pitting corrosion became more severe in this period and dominated the corrosion behavior of the cast ZK60 alloy. In this case, Pdepth should be adequate to describe the corrosion rate of the cast ZK60 alloy in this period.
