*3.3. Corrosion Resistance*

Figure 6 shows the potentiodynamic polarization curve of samples processed with different conditions. The results of electrochemical experiments are shown in Table 5. Table 5 shows that the pitting potential of the S3 sample solution treated at 1100 ◦C for one hour was relatively higher (1196 mV) compared to the others. The ratio of α to γ of the S3 sample was 59.5:40.5. This suggests that good phase arrangemen<sup>t</sup> gave the material better pitting resistance. Table 5 also shows that the pitting potential of the S2 sample with σ phase precipitation is the lowest (1055 mV). Zhang et al. [19] studied the transformation mechanisms of the σ phase in UNS S32707aged at nose temperature, the σ phase preferentially formed along the α/α and α/γ phase boundaries and then penetrated into α phase, resulting from the eutectoid reaction α → γ2 + σ. Meanwhile, a few σ phases nucleated at the γ/γ phase boundaries. The precipitation of the σ phase in S2 may have led to poor Cr at <sup>α</sup>/<sup>α</sup>, <sup>α</sup>/γ, γ/γ, <sup>α</sup>/<sup>σ</sup>, and γ/σ phase boundaries, which resulted in decreased pitting resistance. Figure 7 shows the EDS spectrum of the α and γ phases in the S3 sample. The element distribution of the α and γ phases is shown in Table 6. In the α phase, Cr, Mo, and other α-forming elements were enriched. In the γ phase, the γ-forming element of Ni was enriched. The results of references [20,21] show that the solubility of nitrogen in ferrite is generally less than 0.05%. The ratio of α to γ of the S3 sample was 59.5:40.5. It can be estimated that the nitrogen content of ferrite and austenite in the S3 sample is 0.05% and 0.52%, respectively. The PREN (PREN = mass% Cr + 3.3 mass% Mo + 16 mass% N) of α and γ were 50.7 and 50.6, respectively. The pitting potential of the S4 and S5 samples is very close and difficult to distinguish. Figure 6 shows that the S4 sample presents a notable re-passivation process after its breakdown with the increment of the potential from 0.6 to 1.0 VSCE, while S5 shows a relatively higher dissolve rate with the increased potential from 0.6 to 1.1 VSCE. This indicates that the passive films of the S4 sample are much more stable than that of S5. Zheng et al. [22] found that more grain boundaries due to grain refinement could improve the chromium diffusion and promoted to the forming of compact passive

film for the duplex stainless steel. In this study, the grain size of the S4 sample is smaller than that of S5, which is beneficial to the corrosion resistance of S4 with the refined grain size.

**Figure 6.** The potentiodynamic polarization curve of samples processed with different conditions.

**Table 5.** The results of electrochemical experiments.


**Figure 7.** The EDS spectra of the S3 sample.

**Table 6.** The element distribution of the α and γ phases (mass%).


Figure 8 shows the full spectrum of XPS on the surface of the S3 sample after solution annealing at different sputtering depths. As shown in Figure 8a, C, O, N, Fe, and Cr were notable, while Mo and Ni were weak before sputtering. Furthermore, Figure 8 shows that the surface of the passivation film is mainly composed of compounds formed by Fe, Cr, N, and O. As shown in Figure 8b,c, the intensity of C1s peak decreases greatly after sputtering, which indicate that the C peak in Figure 8a comes from the contamination of the vacuum chamber. The decrease of N1s peak indicates that N was mainly concentrated on the surface of the passive film. The strong peaks at the positions of Fe2p and Ni2p indicate that Fe and Ni elements mainly existed in the interior of the passive film. Lastly, Figure 8 also shows that the XPS spectra of the samples remain after different sputtering depths (1 nm and 2 nm), indicating that the chemical composition of the passivation film was relatively stable in the thickness of 1 nm to 2 nm.

**Figure 8.** XPS full spectrum of the S3 sample after solution annealing with different sputtering depths (**a**) 0 nm; (**b**) 1 nm; (**c**) 2 nm.

Figure 9 shows the result of the high-resolution XPS spectrum of O1s at different sputtering depths by PeakFit. As shown in Figure 9a, XPS spectra of O1s had three peaks before sputtering, corresponding to binding energies 529.5, 531.3, and 533.1 eV, respectively. According to the binding energy values of O1s reported by Wang et al. [23], the first peak is the characteristic peak of M-O compound, corresponding to <sup>O</sup>2<sup>−</sup>, the second peak is the characteristic peak of M-(OH) or M-(OH)2 compound, corresponding to OH<sup>−</sup>, and the third peak is the characteristic peak of H2O. The characteristic peak of 533.1 eV disappeared after sputtering, indicating that the water on the surface of the passivation film came from residual water on the electrode surface. From Figure 9b,c, it can be seen that the O1s high-resolution spectra of sputtered samples only had a characteristic peak of O2− after spectral analysis, which indicates that oxygen in the passive films mainly exists in the form of oxides.

**Figure 9.** Narrow XPS of O1s with different sputtering depths (**a**) 0 nm; (**b**) 1 nm; (**c**) 2 nm.

Figure 10 shows the high resolution XPS spectra of Mo3d5/2 and N1s with different sputtering depths. Mo mainly exists in the form of Mo4<sup>2</sup>− with a corresponding binding energy of 231.8 eV before sputtering. When sputtering depth is 1 and 2 nm, Mo mainly exists in MoO2 form with a corresponding binding energy of 231.2 eV. N mainly exists in the form of NH4+ with a corresponding binding energy of 399.8 eV before sputtering. When the sputtering depth is 2 nm, the peak of NH4+ disappears and N exists in the form of Cr2N with a corresponding binding energy of 397.5 eV.

**Figure 10.** The high resolution XPS spectra of Mo3d5/2 (**a**) and N1s (**b**) at different sputtering depths.
