**3. Result**

#### *3.1. Weight Loss*

The test results of S-Cast and S-Defect weight loss in artificial seawater are shown in Figures 1–3. The results show that the weight loss and film weight of S-Cast and S-Defect generally show a gradual increase trend with corrosion time but their weight loss rates gradually decrease with corrosion time. Moreover, the weight loss, film weight and weight loss rate of S-Defect are larger than that of S-Cast. Therefore, the defects accelerate the corrosion of UNS C95810. Defects induce a discontinuous corrosion product film on NAB in seawater, which results in a decrease in the protective effect of the film on the substrate and an increase in the corrosion rate and the thickness of the film.

**Figure 1.** Weight loss of S-Cast and S-Defect immersed for different time periods in artificial seawater.

**Figure 2.** Film weight of S-Cast and S-Defect immersed for different time periods in artificial seawater.

**Figure 3.** Weight loss rate of S-Cast and S-Defect immersed for different time periods in artificial seawater.

#### *3.2. Microstructure and Corrosion Morphology*

The normal microstructure of UNS C95810 is mainly composed of a α phase matrix and a dispersed κ phase (Figure 4).

**Figure 4.** The microstructure of S-Cast before corrosion.

According to the composition and shape of the κ phase, it can be divided into κII, κIII and κIV. The phase κII is an intermetallic compound based on Fe3Al, which is flower-like or spherical with a size of 5 to 10 μm and distributed at the boundary of the α phase [9]. The κIII phase is a lamellar intermetallic compound based on NiAl [9]. The κIV phase is an intermetallic compound based on Fe3Al and is distributed in the α phase with a size smaller than 2 μm [9]. The size of the α phase is generally more than 100 μm [20]. To determine the chemical composition of the defects, EDS tests of selected area and selected points (Figure 5) are carried out. The results show that there is only copper in selected point while there are many oxygen and few chlorine elements in alloys in selected areas, as shown in Table 3.



It can be inferred that the microstructures in defects mainly include pure copper, an oxide of copper and aluminum and the κ phase.

S-Cast and S-Defect differ in their corrosion processes. S-Cast is preferentially corroded at the boundary of the αphase and some locations within the α phase, and the corrosion near the boundary of the α phase is more serious (Figure 6a,b).

As the corrosion progresses, the α phase also begins to undergo significant corrosion (Figure 6c,d). When the corrosion reaches 30 days, obvious corrosion products have formed on the surface of the alloy (Figure 6e). S-Defect is preferentially corroded at the defects, and the corrosion near the defects is more serious (Figure 7a,b).

**Figure 5.** Surface and point scan of S-Defect before corrosion.

After the corrosion occurs at the defects and their vicinity, the corrosion at the αphase boundary and some locations within the αphase gradually becomes apparent (Figure 7a–d). When the corrosion reaches 30 days, a large area of severe corrosion is occurred at the defects compared to other locations (Figure 7e). In addition to affecting the location of preferential corrosion, defects increase the complexity of UNS C95810 microstructures, leading to an increase in tendency toward galvanic corrosion, thus accelerating the corrosion of UNS C95810 (Figure 6 vs. Figure 7). To determine whether there is a difference between the corrosion product composition of the defects and that of the surrounding as-cast microstructures, EDS analysis is performed on the position shown in Figure 7e. The results (Table 4) show that there is almost no composition difference between the corrosion product of the defects and that of the surrounding as-cast microstructures.

**Figure 6.** *Cont*.

**Figure 6.** The corrosion morphology of for S-Cast immersed in artificial seawater at different times: (**a**) 3 d; (**b**) local magnification at 3 d; (**c**) 10 d; (**d**) local magnification at 10 d; (**e**) 30 d.

**Figure 7.** *Cont*.

**Figure 7.** The corrosion morphology of for S-Defect immersed in artificial seawater at different times: (**a**) 3 d; (**b**) local magnification at 3 d; (**c**) 10 d; (**d**) local magnification at 10d; (**e**) 30 d.


**Table 4.** The composition of the studied position by SEM with EDS.

#### *3.3. Corrosion Product Film*

The XRD results of S-Defect and S-Cast for different immersion periods are shown in Figures 8 and 9. Cu2O, Cu(OH,Cl)2 and Cu2(OH)3Cl are detected in the XRD results of S-Defect and S-Cast. Cu2O changes little with corrosion while Cu(OH,Cl)2 and Cu2(OH)3Cl vary greatly with corrosion (Figure 8). Cu(OH,Cl)2 is more abundant in the corrosion products after 10 days than that of Cu2(OH)3Cl (Figure 8). Therefore, it is considered that during the corrosion process, Cu dissolves to Cu<sup>+</sup>, and then it reacts with Cl- to form CuCl2− [13,20].

**Figure 8.** The XRD patterns of S-Defect immersed for different time periods in artificial seawater.

Over time, it converts to Cu2O [20,21]. As the amount of Cu2O increases, it will react with Cl− to form more stable phases, such as Cu2(OH)3 Cl [20–22]. This is similar to the experimental results of Song [21] and Du [22]. Song [21] proposed that Cu2(OH)3Cl was located on the outermost layer of a NAB corrosion product film. Du [22] proposed that Cu(OH)2 and CuCl2 were located between Cu2O and Cu2(OH)3Cl. It is believed that since Al is more active than Cu, it first dissolves to form Al2O3, and then Cu dissolves to form CuO and Cu2O. Cu(OH)2, CuCl2 and Cu2(OH)3Cl are formed on the basis of products formed by the dissolution of Cu. Therefore, they should be located on the outer layers of the Al2O3, CuO and Cu2O. The XRD results in this study indicate that the Al2O3, Cu2O, CuO, Cu(OH)2, CuCl2 and Cu2(OH)3Cl phases were detected on the corrosion product film with increasing corrosive time. In summary, the corrosion product film of UNS C95810 has a three-layer structure.

**Figure 9.** *Cont*.

**Figure 9.** The XRD patterns of (**a**) S-Cast immersed for different time periods in artificial seawater and (**b**,**<sup>c</sup>**) S-Cast partial magnification.
