**4. Discussion**

#### *4.1. Corrosion Rate of UNS C95810*

Defects can affect the diffusion of Cl− and the corrosion driving force, thus accelerating the corrosion of NAB. The presence of defects causes the passivation film formed on NAB to become uneven. Generally, the electric field strength in the passivation film (ε) obeys the following formula: ε = *Vl* , where *V* is the potential difference between the two sides of the passivation film and *l* is the thickness of the passivation film [19]. Because the passivation film at the defects is thinner than that at other locations [19], the field strength at the defects is higher than that at other locations. This causes Clto diffuse more easily to the defects and to enrich there (Figure 11 and Table 6), resulting in faster destruction of the passivation film.

**Table 6.** The composition of S-Defect immersed in artificial seawater for 3 h.


Once the passivation film is broken, the substrate is directly in contact with the corrosive medium, which accelerates the corrosion rate. Therefore, the presence of defects affects the diffusion of Cl− and the corrosion process.

**Figure 11.** SEM image of S-Defect immersed in artificial seawater for 3 h.

#### *4.2. Corrosion Product Film Structure of S-Defect*

The results of the XRD test show that the X-ray diffraction pattern of both the S-Cast and S-Defect samples are basically the same, as shown in Figures 8 and 9, but the intensity of some phases in the corrosion products is different. Therefore, they have similar corrosion product film structures but different corrosion product film thicknesses (Figures 2, 8 and 9). It had been reported that a corrosion product film with Cu2O in the outer layer and Al2O3 in the inner layer was formed on a NAB [4] and a ternary Cu-Al-Ni alloy [23,24]. Cu2O and Al2O3 were formed by the following reactions [12,13,22,24,25]:

$$\rm Al(s) + 4Cl^- \rightarrow AlCl\_4^- + 3e^- \tag{1}$$

$$\rm AlCl\_4^- + 2H\_2O \to Al\_2O\_3(s) + 4Cl^- + 3H^+ \tag{2}$$

$$\text{Cu}(\text{s}) \rightarrow \text{Cu}^{+} + \text{e}^{-} \tag{3}$$

$$\text{Cu}^+ + \text{Cl}^- \rightarrow \text{CuCl}(s) \tag{4}$$

$$\text{CuCl}(s) + \text{Cl}^- \rightarrow \text{CuCl}\_2^- \tag{5}$$

$$2\text{CuCl}\_2^- + \text{H}\_2\text{O} \rightarrow \text{Cu}\_2\text{O}(\text{s}) + 4\text{Cl}^- + 2\text{H}^+ \tag{6}$$

Cu2O is a p-type semiconductor with cation vacancies. It can accept foreign ions, such as Ni and Fe ions, to incorporate into inner cation vacancies, further improving the protection from the film [14,26,27]. Based on previous research [4,14,21,23,24,26,27], Cu2O should be the main corrosion product in the inner layer of the film for both S-Cast and S-Defect. Fe and Ni is enriched in the inner layer by incorporating into the lattice of Cu2O. The microstructures of the S-Cast and S-Defect are complex, resulting in different Cu dissolution rates in different phases. Because Al2O3 cannot form simultaneously on different phases, the discontinuous Al2O3 layer on the surface is not impermeable to the passage of cuprous cations [20]. As a result, the inner layer of the corrosion product film consists of Al2O3 and Cu2O with the incorporation of Fe and Ni [20]. In the XRD tests, Cu(OH,Cl)2 and Cu2(OH)3Cl are detected in the corrosion products of S-Cast and S-Defect. They could form by the following reactions [13,25,26,28]:

$$\text{Cu}^+ + \frac{1}{2}\text{O}\_2 + \text{e}^- \rightarrow \text{CuO}(\text{s})\tag{7}$$

$$\text{CuO(s)} + \text{H}\_2\text{O} \rightarrow \text{Cu(OH)}\_2\text{(s)}\tag{8}$$

$$\text{Cu}\_2\text{O}(\text{s}) + \text{Cl}^- + 2\text{H}\_2\text{O} \rightarrow \text{Cu}\_2(\text{OH})\_3\text{Cl}(\text{s}) + \text{H}^+ + 2\text{e}^- \tag{9}$$

According to Song's research [21] on a NAB corrosion product film, Cu2(OH)3Cl was located in the outer layer of the NAB corrosion product film. In the structure of the NAB corrosion product film proposed by Du [22], Cu(OH)2 or CuO was located between Cu2O and Cu2(OH)3Cl. Therefore, combined with the above analysis and the results of the XRD test, it can be concluded that the structure of the S-Defect corrosion product film is as follows (Figure 12): the inner layer is Al2O3 and Cu2O with the incorporation of Fe and Ni, the middle layer is Cu(OH,Cl)2 or CuO, and the outer layer is Cu2(OH)3Cl.

**Figure 12.** The corrosion product film structure of S-Defect immersed in artificial seawater for a long time.

#### *4.3. Microstructure and Corrosion Behavior of UNS C95810*

The stability of the phase and its corrosion products can affect the corrosion behavior, and the stability of the phase can be expressed with a work function. There is a corresponding relationship between work function and Volta potential: *VCPD* = -<sup>Φ</sup>*tip* − <sup>Φ</sup>*sample*/*e,* where *VCPD* is the Volta potential, <sup>Φ</sup>*tip* and <sup>Φ</sup>*sample* are the work functions of the tip and the sample, respectively, and *e* is the value of the electronic charge [29]. The value of <sup>Φ</sup>*tip* is constant. Thus, higher *VCPD* results in lower <sup>Φ</sup>*sample*. The work function is defined as the minimum energy required for an electron to escape from the surface of a solid. A lower work function of a material represents more likely occurrences of corrosion [30]. Therefore, a substance with a higher Volta potential is more susceptible to corrosion. From the measurements of the Volta potential of an as-cast NAB by Song, it can be seen that the Volta potential of the κIV phase was higher than that of the α phase but lower than that of the κII phase [21]. Hence, the stability of the phases from low to high should be κII, κIV and α. However, in near-neutral artificial seawater, Al2O3 is formed on the surface of the Al-rich κ phase and its stability is much higher than that of Cu2O, so the κ phase as the cathode phase will not continue to corrode but accelerates the corrosion rate of the surrounding microstructures [31]. Since the κIV phase and κII phase are located in the α phase and the boundary of the α phase, corrosion preferentially occurs in the αphase boundary and some locations in the α phase. The Volta potential between the κII phase and α phase is 60–80 mV, while that between the κIV phase and α phase is 20–40 mV [21]. The areas with a higher Volta potential have a stronger driving force for corrosion. Therefore, the corrosion in the α phase boundary is more prone to occur than that in the α phase.

The existence of defects accelerates the corrosion process to a certain extent, mainly due to the following reasons. First, the defect microstructures mainly include oxides of copper and aluminum, along with pure copper. This is a more complex phase composition than the as-cast microstructures. Such a phase composition makes it easier for the inside of defects and between the defects and the surrounding microstructures to form a galvanic corrosion, thereby accelerating the corrosion of the defects and the microstructures near the defects. Second, defects not only make the microstructures

of as-cast NAB more complex but also cause the inhomogeneity of the corrosion product film, resulting in greater stress in the growth process of the film [21]. The increase in growth stress can cause the film to become loose or even crack, thereby causing the corrosive medium to more easily enter the film and corrode the substrate. Finally, the corrosion potential of UNS C95810 decreases due to defects. The lower corrosion potential provides a more powerful driving force for corrosion, thereby accelerating the corrosion of UNS C95810.

#### *4.4. Corrosion Mechanism of UNS C95810*

According to the experimental results obtained in this paper and related literature [21], the stability of the phases in NAB from low to high is κII, κIV, κIII and. Since the κ phase is an Al-rich phase and Al is a more active metal than Cu, the κ phase will preferentially corrode and form Al2O3 on the surface when NAB is in contact with seawater. Al2O3 is a more stable oxide than that of Cu2O and CuO, so the κ phase covered by Al2O3 will no longer corrode as an anode phase but will act as a cathode phase in galvanic corrosion to accelerate the corrosion of the surrounding microstructures. The above results in the formation of Cu2O, Cu(OH,Cl)2 and Cu2(OH)3Cl on the surface of the surrounding microstructures. The Volta potential between the κII, κIII, κIV phases and the α phase is 60–80 mV, 10–30 mV and 20–40 mV, respectively [21]. Therefore, the corrosion near the κII phase is the fastest and the most serious, followed by that near the κIV phase, and finally that near the κIII phase. In summary, the specific corrosion behavior of S-Cast can be summarized as Figure 13.

**Figure 13.** Process mechanism of corrosion behavior of S-Cast immersed in artificial seawater.

For S-Defect, in addition to the corrosion characteristics of S-Cast, it has some additional characteristics due to the presence of defects. The presence of defects not only promotes the formation of galvanic corrosion between the internal microstructures of the defects but also promotes that between the defects and the surrounding microstructures, thereby greatly increasing the tendency toward galvanic corrosion. Therefore, the corrosion behavior of S-Defect can be obtained by combining the corrosion process of S-Cast with an additional galvanic corrosion process introduced due to the existence of defects. Its corrosion mechanism can be summarized as shown in Figure 14.

**Figure 14.** Process mechanism of corrosion behavior of S-Defect immersed in artificial seawater.
