*3.2. Metallographic Analysis*

**Table 3.** Sample labels.

**Full Name Femtosecond Laser Treatment in Air Femtosecond Laser Treatment in Argon Superhydrop hobic Ferrite Sample Martensite Sample**  Abbreviation FLAR FLAN SH F-sample M-sample Preparing a periodic micro-nanostructure is usually necessary to obtain an SH surface. As shown in Figure 3d, with the increased laser fluence and scanning spacing, micro-scale grooves with a width of 10 μm were observed on the laser-ablated surface (Figure 3d). The textured surface manifested superhydrophilicity with a CA of nearly 0°. After the annealing process, the SH surface showed a high CA of 154° and a sliding angle Figure 4 shows the metallographic images and the XRD patterns of the SLM-ed 17- 4PH SS samples before and after heat treatment. Before heat treatment, the metal sample was dominated by columnar body-centered cubic (BCC) ferrite (F) grains, composed of elongated subgrains with different growth directions. The sample without heat treatment was labeled F-sample. After heat treatment, the peak value of α 0 (110) was significantly increased. This was because the solution treatment refined and homogenized the microstructure. The large-grain ferrite was transformed into a fine acicular martensite (M) lath. However, more intergranular defects could be observed, and the subgrain was not apparent. The heat-treated sample was labeled M-sample. However, the high-heat treatment-induced transformation from ferrite to acicular martensite had little effect on the wetting behaviors of the SLM-ed SS samples treated with different parameters (Figure 3f).

#### *3.3. Electrochemical Analysis*

#### 3.3.1. Potentiodynamic Polarization Studies

Figure 5 shows the polarization characteristics of each sample with different processes. The corrosion potential (*E*corr) and current density (*i*corr) were calculated using the Tafel extrapolation method (Table 4). Typically, a high corrosion potential indicates excellent corrosion resistance. The heat treatment caused the charge in the corrosion potential of the original samples to not be apparent. After laser treatment in argon, the F-sample showed a high corrosion potential. The laser-treated M-sample showed the highest corrosion voltage of −0.3820 V. The heat treatment caused an improvement in the corrosion voltage, which can also be observed in the samples treated in air. However, the oxidized nanoparticles on the surface fell away easily, and the exposed matrix accelerated the corrosion. Therefore, the corrosion voltage was relatively low and unstable. The SH M-sample showed the lowest

self-corrosion potential. The reason for this is that the polarization current destroyed the low surface energy property caused by the annealing process; therefore, the material's wetting behavior changed from SH to superhydrophilic. In general, the increased hydrophilicity can lead to an attenuation in the material's corrosion resistance. apparent. The heat-treated sample was labeled M-sample. However, the high-heat treatment-induced transformation from ferrite to acicular martensite had little effect on the wetting behaviors of the SLM-ed SS samples treated with different parameters (Figure 3f).

of 3° (Figure 3d). The advancing and receding CAs were 151.03° and 146.18°, respectively,

Figure 4 shows the metallographic images and the XRD patterns of the SLM-ed 17- 4PH SS samples before and after heat treatment. Before heat treatment, the metal sample was dominated by columnar body-centered cubic (BCC) ferrite (F) grains, composed of elongated subgrains with different growth directions. The sample without heat treatment was labeled F-sample. After heat treatment, the peak value of α′ (110) was significantly increased. This was because the solution treatment refined and homogenized the microstructure. The large-grain ferrite was transformed into a fine acicular martensite (M) lath. However, more intergranular defects could be observed, and the subgrain was not

*Micromachines* **2022**, *13*, x FOR PEER REVIEW 6 of 16

resulting in a CA hysteresis of 4.85°.

*3.2. Metallographic Analysis* 

**Figure 4.** Microstructures of the SLM-ed 17-4PH SS (**a**) before and (**b**) after heat treatment. (**c**) XRD patterns of the SLM-ed 17-4PH SS before and after heat treatment. **Figure 4.** Microstructures of the SLM-ed 17-4PH SS (**a**) before and (**b**) after heat treatment. (**c**) XRD patterns of the SLM-ed 17-4PH SS before and after heat treatment. μA·cm2. In addition, the refined grain induced by the transformation from ferrite to acicular martensite is beneficial to the corrosion resistance [19,20].

groups. The FLAN M-sample showed the lowest corrosion current density of 0.1346

**Figure 5.** Potentiodynamic polarization curves of the (**a**) F-samples and (**b**) M-samples. (**c**) Corrosion voltages and (**d**) corrosion currents of SLM-ed samples with different post-treatment processes. **Figure 5.** Potentiodynamic polarization curves of the (**a**) F-samples and (**b**) M-samples. (**c**) Corrosion voltages and (**d**) corrosion currents of SLM-ed samples with different post-treatment processes.

**Table 4.** Quantitative information about the potentiodynamic polarization curves of 8 samples in

FLAR F-samples 0.7107 ± 0.6% −0.5389 ± 2.2% FLAR M-samples 0.5963 ± 6.1% −0.4786 ± 16.5% FLAN F-samples 0.2518 ± 9.7% −0.4017 ± 0.6% FLAN M-samples 0.1346 ± 10.7% −0.3820 ± 5.5% SH F-samples 0.7583 ± 2.9% −0.4575 ± 2.8% SH M-samples 0.7988 ± 2.0% −0.6940 ± 3.8%

**Sample Average Log** *i***corr (μA·cm−2) Average** *E***corr (V)** 

Figure 6 shows the measured and simulated impedance characteristics of the eight samples tested in Figure 5. Capacitive arcs appeared on the Nyquist plots for all the samples, revealing that the corrosion reactions occurred at the SS/electrolyte interfaces. Without the heat treatment, the original sample's arc radius was significantly larger than

3.3.2. Electrochemical Impedance Spectroscopic (EIS) Studies

0.5 mol/L NaCl solution.


**Table 4.** Quantitative information about the potentiodynamic polarization curves of 8 samples in 0.5 mol/L NaCl solution.

The corrosion current density (*i*corr) belongs to the dynamic category. The smaller the *i*corr, the slower the corrosion rate. As shown in Figure 5d, the corrosion current densities of the original and FLAN samples were significantly lower than those of the other two groups. The FLAN M-sample showed the lowest corrosion current density of 0.1346 <sup>µ</sup>A·cm<sup>2</sup> . In addition, the refined grain induced by the transformation from ferrite to acicular martensite is beneficial to the corrosion resistance [19,20].

#### 3.3.2. Electrochemical Impedance Spectroscopic (EIS) Studies

Figure 6 shows the measured and simulated impedance characteristics of the eight samples tested in Figure 5. Capacitive arcs appeared on the Nyquist plots for all the samples, revealing that the corrosion reactions occurred at the SS/electrolyte interfaces. Without the heat treatment, the original sample's arc radius was significantly larger than that of the other three samples. The FLAR sample showed the smallest arc radius. For the heat-treated samples, the laser texturing in argon resulted in the largest capacitive arc radius, and the SH M-sample showed the smallest arc radius. The capacitive arc radius is an essential parameter for evaluating the corrosion resistance of metal materials [21,22]. The larger the capacitive arc radius, the greater the impedance value of the corrosive ions passing through the material surface, and the better the material's corrosion resistance. This indicates that laser texturing in argon significantly improved the corrosion properties of the M-sample. The high impedances and phase angles indicate that the formed passivation films were more stable for the original and FLAN samples.

To obtain the detailed characteristics of the passivation films, the two equivalent circuit models shown in Figure 7 were chosen to fit the impedance data. *R<sup>s</sup>* represents solution resistance. The equivalent circuit (EC) shown in Figure 7a was named EC-1, and that shown in Figure 7b was named EC-2. The high agreement between the simulated curves and the experiment results fully verifies the circuit's validity (Figure 6). *R*<sup>f</sup> and *Q*<sup>f</sup> represent passivation film resistance and capacitance, respectively. *R*ct and *Q*dl represent charge transfer resistance and double-layer capacitance. The chi-square values (*χ* 2 ) were all less than 0.01. The correspondence between the tested samples and ECs, and the fitting results after 72 h are shown in Table 5.

The results indicate that, after 72 h of immersion, the passivation films of the original, FLAN, and SH F-samples remained intact. A similar phenomenon was also observed on the FLAN and SH M-samples. Among them, the FLAN M-sample possessed the largest *<sup>R</sup>*<sup>f</sup> of 0.598 MΩ·cm<sup>2</sup> , indicating its high resistance and corrosion-resistant passivation film. However, the high *Q*<sup>f</sup> reveals that there may be many defects in the FLAN M-sample.

The EIS test showed that, with a small *R*<sup>f</sup> and a large *Q*<sup>f</sup> , the fabricated SH surfaces did not show a good anti-corrosion performance. In addition, compared with other samples, the micro-sized groove structure enabled the SH surface to process a higher *Q*<sup>f</sup> value. The results also show that the passivation films of the FLAR F-sample and M-sample were damaged during the test. This is attributed to the oxide particles formed on the laser textured surface. In NaCl solution, these particles tend to fall off, resulting in the destruction of the passivation film.

that of the other three samples. The FLAR sample showed the smallest arc radius. For the heat-treated samples, the laser texturing in argon resulted in the largest capacitive arc radius, and the SH M-sample showed the smallest arc radius. The capacitive arc radius is an essential parameter for evaluating the corrosion resistance of metal materials [21,22]. The larger the capacitive arc radius, the greater the impedance value of the corrosive ions passing through the material surface, and the better the material's corrosion resistance. This indicates that laser texturing in argon significantly improved the corrosion properties of the M-sample. The high impedances and phase angles indicate that the formed

passivation films were more stable for the original and FLAN samples.

**Figure 6.** Measured and simulated (**a**) Bode phase angle, (**b**) Bode impedance, and (**c**) Nyquist curves of SLM-ed samples with different post-treatment processes. **Figure 6.** Measured and simulated (**a**) Bode phase angle, (**b**) Bode impedance, and (**c**) Nyquist curves of SLM-ed samples with different post-treatment processes. all less than 0.01. The correspondence between the tested samples and ECs, and the fitting results after 72 h are shown in Table 5.

Original F-samples (EC-1) 0.142 ± 16.9% 35.269 ± 5.3% 0 0 Original M-samples (EC-1) 0.173 ± 23.7% 59.290 ± 10.9% 0 0

FLAN F-samples (EC-1) 0.313 ± 9.3% 99.970 ± 21.8% 0 0 FLAN M-samples (EC-1) 0.598 ± 5.2% 206.392 ± 11.8% 0 0 SH F-samples (EC-1) 0.126 ± 28.6% 243.610 ± 5.1% 0 0 SH M-samples (EC-1) 0.00562 ± 6.6% 780.84 ± 5.5% 0 0

destruction of the passivation film.

3.3.3. XPS Characterization

**Figure 7.** Electrochemical equivalent circuits for fitting the measured impedance data of (a) the FLAR samples and (b) other samples. **Figure 7.** Electrochemical equivalent circuits for fitting the measured impedance data of (**a**) the FLAR samples and (**b**) other samples.

The results indicate that, after 72 h of immersion, the passivation films of the original, FLAN, and SH F-samples remained intact. A similar phenomenon was also observed on the FLAN and SH M-samples. Among them, the FLAN M-sample possessed the largest *R*f of 0.598 MΩ·cm2, indicating its high resistance and corrosion-resistant passivation film. However, the high *Q*f reveals that there may be many defects in the FLAN M-sample.

The EIS test showed that, with a small *R*f and a large *Q*f, the fabricated SH surfaces did not show a good anti-corrosion performance. In addition, compared with other samples, the micro-sized groove structure enabled the SH surface to process a higher *Q*<sup>f</sup> value. The results also show that the passivation films of the FLAR F-sample and Msample were damaged during the test. This is attributed to the oxide particles formed on the laser textured surface. In NaCl solution, these particles tend to fall off, resulting in the

Figure 8 shows the XPS spectra of the passivation films on different samples. The peak spectra, such as Fe2p, Cr2p, Ni2p, Cu2p, Nb3d, and C1s, were fitted to investigate the surface components. The Fe2p spectra of the eight samples show seven peaks around 706.5 eV, 706.9 eV, 708.1 eV, 709.1 eV, 711.5 eV, 718.9 eV, and 723.7 eV, which are related

FLAR M-samples (EC-2) 0.0000166 ± 16.9% 33.936 ± 19.9% 8.049 ± 4.2% 115.330 ± 10.6%

**Sample** *R***f (MΩ·cm2)** *Q***f (μF·cm−2)** *R***ct (MΩ·cm2)** *Q***dl (μF·cm−2)** 

**Table 5.** Fitting results of EIS for 8 samples in 0.5 mol/L NaCl for 72 h.


**Table 5.** Fitting results of EIS for 8 samples in 0.5 mol/L NaCl for 72 h.

#### 3.3.3. XPS Characterization

Figure 8 shows the XPS spectra of the passivation films on different samples. The peak spectra, such as Fe2p, Cr2p, Ni2p, Cu2p, Nb3d, and C1s, were fitted to investigate the surface components. The Fe2p spectra of the eight samples show seven peaks around 706.5 eV, 706.9 eV, 708.1 eV, 709.1 eV, 711.5 eV, 718.9 eV, and 723.7 eV, which are related to Fe, FeO/Fe2O3, and Fe3O4. The Cr2p spectrum has six peaks around 573.9 eV, 575.4 eV, 576.1 eV, 577.4 eV, 583.5 eV, and 586.4 eV, which are related to Cr, Cr2O3, and CrCl3/Cr(OH)3. The Ni2p spectrum contains three peaks around 851.8 eV, 855.0 eV, and 869.9 eV, corresponding to Ni, NiO, and Ni. Two peaks around 933.2 eV and 952.7 eV can be observed in the Cu2p spectra, corresponding to Cu/CuO and Cu2O/Cu/CuO. There are three peaks in the Nb2p spectrum, namely, 202.4 eV, 207.2 eV, and 209.5 eV, which are associated with NbO, NbO/Nb2O5, and Nb2O5. The C1s spectrum has four peaks around 284.8 eV, 285.6 eV, 286.5 eV, and 588.6 eV, which are related to C.

As we all know, FeO, Fe2O3, Cr2O3, and other metal oxides are the main components of SS passivation films [23,24]. It can be seen that the PLAN samples with relatively high polarization and impedance performances showed high peak values of Fe, Cr, Cu, and Nb oxides. This is because the femtosecond laser ablation can induce the precipitation of Cr, Cu, and Nb elements to form an oxide film on the surface, resulting in increased corrosion resistance. This is consistent with the EIS results. Moreover, since the grain is refined after the heat treatment, these metal oxides tend to grow at the grain boundaries with high Gibbs free energy. The increased boundaries in the unit area are conducive to forming a dense and stable passivation film [25]. The SH surface showed the lowest peak values for all the metal oxides, which is consistent with the above impedance results. Meanwhile, this also further confirms the relatively short corrosion reaction time of the SH surface. The low peak values for the C-O bond may have contributed to the lost superhydrophobicity of the SH samples [26].

#### 3.3.4. Electrochemical Corrosion Morphology

Figure 9 shows the surface morphologies after the impedance tests. The FLAN sample showed the fewest defects. The reason may be that the material surface possessed a dense passivation film, so the destruction speed of Cl− to the passivation film was lower than the passivation film's repair speed. However, the corrosion pits on the surface of the FLAR sample were very obvious. This can be attributed to the severely destroyed passivation film.

Interestingly, the SH surface that had relatively low polarization and impedance performances showed a good corrosion morphology. On the SH M-sample surface, the corrosion traces were very inconspicuous. Some micro-particles were distributed on the laser-fabricated micro-grooves on the SH F-sample. The EDS test showed that carbonaceous particles were derived from the surrounding air. The surface morphologies reveal that the SH surfaces had high corrosion resistance.

**Figure 8.** *Cont.*

**Figure 8.** High-resolution XPS spectra of (**a**) Fe 2p, (**b**) Cr 2p, (**c**) Ni 2p, (**d**) Cu 2p, (**e**) Nb 3d, and (**f**) C 1s for SLM-ed samples after being immersed in 0.5 mol/L NaCl solution for 72 h. **Figure 8.** High-resolution XPS spectra of (**a**) Fe 2p, (**b**) Cr 2p, (**c**) Ni 2p, (**d**) Cu 2p, (**e**) Nb 3d, and (**f**) C 1s for SLM-ed samples after being immersed in 0.5 mol/L NaCl solution for 72 h. particles were derived from the surrounding air. The surface morphologies reveal that the SH surfaces had high corrosion resistance.

formances showed a good corrosion morphology. On the SH M-sample surface, the corrosion traces were very inconspicuous. Some micro-particles were distributed on the laser-fabricated micro-grooves on the SH F-sample. The EDS test showed that carbonaceous **Figure 9.** Surface morphologies of (**a**) original F-samples, (**b**) FLAR F-samples, (**c**) FLAN F-samples, (**d**) SH F-samples, (**e**) original M-samples, (**f**) FLAR M-samples, (**g**) FLAN M-samples, and (**h**) SH M-samples after being immersed in 0.5 mol/L NaCl solution for 72 h. 3.3.5. Influence Mechanism of Laser Polishing Treatment on Corrosion Resistance **Figure 9.** Surface morphologies of (**a**) original F-samples, (**b**) FLAR F-samples, (**c**) FLAN F-samples, (**d**) SH F-samples, (**e**) original M-samples, (**f**) FLAR M-samples, (**g**) FLAN M-samples, and (**h**) SH M-samples after being immersed in 0.5 mol/L NaCl solution for 72 h.

Interestingly, the SH surface that had relatively low polarization and impedance per-

Cr → Cr3+ + 3e− (1) Fe → Fe2+ + 2e− (2) Fe → Fe3+ + 3e− (3) Cu → Cu+ + e− (4) Nb → Nb2+ + 2e− (5) Nb → Nb5+ + 5e− (6)

O2 + 2H2O + 4e<sup>−</sup> → 4OH− (7)

With the little difference in the corrosion morphologies of the original, FLAR, and FLAN samples, the corresponding corrosion mechanism is explained in Figure 10a. For the SH surface with the special structure, the corrosion mechanism is shown in Figure 10b. At room temperature, the chemical reactions of the 17-4PH SS in 0.5 mol/L NaCl solution

resented by Fe in the SLM-ed SS samples gradually dissolve into the solution after losing

electrons at the anode, and the oxidation reaction occurs near the anode:

The reduction reaction occurs at the cathode:

solution:

3.3.5. Influence Mechanism of Laser Polishing Treatment on Corrosion Resistance Fe(OH)2 + 2Fe + O2 → 3FeO + H2O (16)

Around the anode, the above metal ions react with the Cl−/OH− in the NaCl solution to form metal chloride or hydroxide, which causes the metal to continue to dissolve in the

Next, these metal compounds and residual oxygen in the water together form stable Cr2O3 and Cu2O oxides on the matrix surface, as shown in the XPS results (Figure 9). The formation of these oxides causes the passivation film to be gradually repaired as follows:

> With the little difference in the corrosion morphologies of the original, FLAR, and FLAN samples, the corresponding corrosion mechanism is explained in Figure 10a. For the SH surface with the special structure, the corrosion mechanism is shown in Figure 10b. At room temperature, the chemical reactions of the 17-4PH SS in 0.5 mol/L NaCl solution are as follows: 4Cu+ + 4e<sup>−</sup> + O2 → 2Cu2O (17) Nb(OH)2 + 2Nb + O2 → 3NbO + H2O (18) 2Nb(OH)5 + 4Nb + 5O2 → 3Nb2O5 + 5H2O (19)

Cr3+ + 3Cl−/OH<sup>−</sup> → CrCl3/Cr(OH)3 (8)

Fe3+ + 3Cl−/OH<sup>−</sup> → FeCl3/Fe(OH)3 (9)

Fe2+ + 2Cl−/OH<sup>−</sup> → FeCl2/Fe(OH)2 (10)

Nb2+ + 2Cl−/OH<sup>−</sup> → NbCl2/Nb(OH)2 (12)

Nb5+ + 5Cl−/OH<sup>−</sup> → NbCl5/Nb(OH)5 (13)

4Cr(OH)3 + 4Cr +3O2 → 4Cr2O3 + 6H2O (14)

4Fe(OH)3 + 4Fe + 3O2 → 4Fe2O3 + 6H2O (15)

Cu + Cl<sup>−</sup> → CuCl (11)

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**Figure 10.** Schematic illustrating the corrosion mechanism of (**a**) hydrophilic and (**b**) SH 17-4PH SS. **Figure 10.** Schematic illustrating the corrosion mechanism of (**a**) hydrophilic and (**b**) SH 17-4PH SS.

Due to the potential difference between various elements, many micro-cells appear on the material surfaces at the initial stage. These micro-cells can facilitate the migration of Cr3+, Fe2+, Fe3+, Cu+, Nb2+, Nb5+, and Cl− in the solution, decreasing the stability of the Firstly, since the oxidation reaction occurs at the cathode, the metallic elements represented by Fe in the SLM-ed SS samples gradually dissolve into the solution after losing electrons at the anode, and the oxidation reaction occurs near the anode:

$$\text{Cr} \rightarrow \text{Cr}^{3+} + 3\text{e}^- \tag{1}$$

$$\text{Fe} \rightarrow \text{Fe}^{2+} + 2\text{e}^- \tag{2}$$

$$\text{Fe} \rightarrow \text{Fe}^{3+} + \text{3e}^{-} \tag{3}$$

$$\text{Cu} \rightarrow \text{Cu}^{+} + \text{e}^{-} \tag{4}$$

$$\text{Nb} \rightarrow \text{Nb}^{2+} + 2\text{e}^- \tag{5}$$

$$\text{Nb} \rightarrow \text{Nb}^{5+} + \text{5e}^- \tag{6}$$

nanostructures may hinder the diffusion of oxides, which is beneficial to the stable The reduction reaction occurs at the cathode:

$$\rm O\_2 + 2H\_2O + 4e^- \rightarrow 4OH^- \tag{7}$$

Around the anode, the above metal ions react with the Cl−/OH− in the NaCl solution to form metal chloride or hydroxide, which causes the metal to continue to dissolve in the solution:

$$\rm{Cr^{3+}} + \rm{3Cl^{-}/OH^{-}} \rightarrow \rm{CrCl\_{3}/Cr(OH)\_{3}} \tag{8}$$

$$\rm{Fe^{3+} + 3Cl^{-} / OH^{-} \to FeCl\_{3}/Fe(OH)\_{3}} \tag{9}$$

$$\rm{Fe^{2+} + 2Cl^{-} / OH^{-} \to FeCl\_{2} / Fe(OH)\_{2}} \tag{10}$$

$$\text{Cu} + \text{Cl}^- \rightarrow \text{CuCl} \tag{11}$$

$$\mathrm{Nb}^{2+} + 2\mathrm{Cl}^-/\mathrm{OH}^- \rightarrow \mathrm{NbCl}\_2/\mathrm{Nb}(\mathrm{OH})\_2 \tag{12}$$

$$\text{Nb}^{5+} + \text{5Cl}^{-} / \text{OH}^{-} \rightarrow \text{NbCl}\_{5} / \text{Nb(OH)}\_{5} \tag{13}$$

Next, these metal compounds and residual oxygen in the water together form stable Cr2O<sup>3</sup> and Cu2O oxides on the matrix surface, as shown in the XPS results (Figure 9). The formation of these oxides causes the passivation film to be gradually repaired as follows:

$$4\text{Cr(OH)}\_{3} + 4\text{Cr} + 3\text{O}\_{2} \to 4\text{Cr}\_{2}\text{O}\_{3} + 6\text{H}\_{2}\text{O} \tag{14}$$

$$4\text{Fe(OH)}\_{3} + 4\text{Fe} + 3\text{O}\_{2} \to 4\text{Fe}\_{2}\text{O}\_{3} + 6\text{H}\_{2}\text{O}\tag{15}$$

$$\text{Fe(OH)}\_{2} + 2\text{Fe} + \text{O}\_{2} \rightarrow 3\text{FeO} + \text{H}\_{2}\text{O} \tag{16}$$

$$4\text{Cu}^+ + 4\text{e}^- + \text{O}\_2 \rightarrow 2\text{Cu}\_2\text{O} \tag{17}$$

$$\text{Nb(OH)}\_{2} + 2\text{Nb} + \text{O}\_{2} \rightarrow 3\text{NbO} + \text{H}\_{2}\text{O} \tag{18}$$

$$\text{2Nb(OH)}\_{5} + 4\text{Nb} + 5\text{O}\_{2} \rightarrow \text{3Nb}\_{2}\text{O}\_{5} + 5\text{H}\_{2}\text{O} \tag{19}$$

Due to the potential difference between various elements, many micro-cells appear on the material surfaces at the initial stage. These micro-cells can facilitate the migration of Cr3+, Fe2+, Fe3+, Cu<sup>+</sup> , Nb2+, Nb5+ , and Cl− in the solution, decreasing the stability of the passivation films. The dissolved metal cations are combined with the chloride ions and then rapidly oxidize to form metal oxides such as Cr2O3, FeO, Fe2O3, Cu2O, NbO, and Nb2O<sup>5</sup> (Figure 8). The metal oxides tend to grow at the grain boundaries with a high density, which is conducive to forming passivation films. The passivation film grown along the dense grain boundaries can block the corrosion of metals, resulting in the inhibition of pitting corrosion [25]. However, if there are defects in the passivation, the Cl− can penetrate the metal matrix, resulting in corrosion pits [27].

The Cr, Cu, and other alloying elements precipitated on the FLAN sample surface can lead to an increased thickness of the passivation film. Moreover, the surface nanostructures may hinder the diffusion of oxides, which is beneficial to the stable formation of passivation films. Therefore, the passivation films on the FLAN samples showed relatively high corrosion resistance in the impedance test (Table 5). On the FLAR samples, the loose oxide particles fell off easily during the corrosion process, resulting in substrate exposure. The passivation film was destroyed too fast to be repaired, so the pitting corrosion on the FLAR samples was very obvious (Figure 9f).

The relaviely weak anti-corrosion performance of the SH samples is attributed to the unique surface structure and wetting property. Due to the surface superhydrophobicity, there was an air film between the NaCl solution and the material surface at the initial stage of the test. Therefore, the NaCl solution could not penetrate the micro-nanostructure. However, as the site in contact with the solution was corroded, the liquid gradually penetrated the rough structure (Figure 10b). After 24 h, the superhydrophobicity was wholly lost, and the material surface exhibited superhydrophilicity. As a result, the actual corrosion reaction time of the SH sample was less than that of the other samples, which may be the main reason for the relatively low resistance of the surface passivation film.

#### **4. Conclusions**

In this study, the corrosion behaviors of SLM-ed 17-4PH SS treated with different femtosecond laser parameters were investigated.


(3) Since the wetting behavior was transformed from SH to superhydrophilic, the fabricated SH surfaces did not show a good anti-corrosion performance. However, the air film between the solution and the material surface delayed the surface corrosion, resulting in inconspicuous corrosion pits.

**Author Contributions:** Conceptualization, H.Y. and C.Y.; methodology, L.M.; validation, M.W., J.L. (Jiaming Li), and C.L.; formal analysis, L.M.; investigation, W.S.; writing—original draft preparation, L.M.; writing—review and editing, H.Y. and J.L. (Jiazhao Long); supervision, C.Y.; funding acquisition, H.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Natural Science Foundation of Top Talent of SZTU (2020103), the Project of Characteristic Innovation in Higher Education of Guangdong (2020KQNCX070), the National Natural Science Foundation of China (62005081), and the Guangdong Basic and Applied Basic Research Foundation (2021A1515011932).

**Acknowledgments:** The authors would like to thank the Laboratory of Advanced Additive Manufacturing, Sino-German College of Intelligent Manufacturing in SZTU, and the Analytical and Testing Centre of JNU for the XRD and XPS.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**

