Introduction

Nickel-based superalloys are widely used in the medical biomaterial, aerospace, and national defense fields because of their excellent mechanical properties and superior corrosion resistance (Ref 1,2,3,4,5,6). As a popular nickel-based superalloy, the Inconel 718 alloy is generally used to fabricate engine turbine disks, turbine rotor blades, and other mechanical fastening parts (Ref 7,8,9). Usually, these components possess complex structures and cannot be manufactured using normal manufacturing techniques (e.g., wrought, rolling, and casting) (Ref 10). Under these circumstances, additive manufacturing (AM) is adopted to fabricate these components with complex structures.

Although the AM technique has a series of unique features (e.g., shortens the processing steps, can build components with complex shapes, and saves labor costs) and has been successfully applied in practice; however, alloys made using this technique have demonstrated higher corrosion susceptibility than alloys made using traditional manufacturing techniques (Ref 11,12,13,14). When exposed to corrosive media, passive films usually form on the surfaces of these alloys, which are found to be defective (Ref 10, 15). Kong et al. (Ref 16) found that the superhigh subgrain boundary density and the microgalvanic effect contributed to the lower passive current density of as-received selective laser-melted 316L stainless steel (SLM 316L SS) without obvious pores. The corrosion performance of AM alloys generally relies on the structures and properties of the passive films that form; thus, it is important to consider the changes in the corrosion properties of AM alloys fabricated using new techniques, e.g., selective laser melting (SLM), wire arc additive manufacturing (WAAM) and laser engineered net shaping (LENS) (Ref 17). Wu et al. (Ref 18) compared the corrosion behavior of a Ti-6Al-4V alloy made using the WAAM technique to that of a wrought alloy; the corrosion resistance of the WAAM-made Ti-6Al-4V alloy in a 3.5 wt.% NaCl solution was lower than that of the wrought alloy due to the change in the passive film thickness. Moreover, the anisotropic microstructure would lead to anisotropic corrosion behavior for the AM parts (Ref 19, 20). Ni et al. (Ref 19) reported that the grain sizes of SLM 316L SS in the XOZ plane were smaller than those in the XOY plane and that the molten pool boundaries would be preferentially corroded. Kong et al. (Ref 20) found that when the aggressive environment was intensified, the existence of voids and molten pool boundaries accelerated the dissolution rate. Juillet et al. (Ref 21) investigated the isothermal oxidation behaviors of casted, forged, and additive manufactured IN-718 superalloys by SLM at 600 °C, 700 °C, and 800 °C and found that the morphologies of the Cr2O3 scales depended on the manufacturing process. The oxide scale grown on the AM IN-718 surface appeared more compact than that grown on the forged IN-718 surface. The electrochemical characteristics in the anodically polarized region were similar to those of the traditionally manufactured and AM Ti-6Al-4V alloys. At potentials higher that 2.0 V, the AM samples exhibited a sharp increase in current by over two orders of magnitude due to the microstructural defects and voids (Ref 22). Zhang et al. (Ref 12) found that the corrosion resistance of IN718 fabricated using WAAM was lower than that of wrought alloy, which was due to the presence of a high percentage of porous and less protective NiO in the passive film, which formed on the surface of WAAM IN718. Research on the corrosion behaviors of SLM alloys remains in its infancy (Ref 23, 24). Thus, further research is needed to better elucidate the effect of SLM on the corrosion behavior of materials.

Inconel 718 alloy is a common superalloy that is created for turbine blades in planes. For the planes that are used in marine environments, turbine blades undergo marine corrosion, causing them to fail. However, the corrosion behavior and passivation properties of Inconel 718 alloys are rarely reported in marine environments. This work aims to contribute to a better understanding of the passive film formed on an Inconel 718 alloy fabricated using SLM and to compare it with a commercially rolled Inconel 718 alloy.

Experimental Details

The materials used in the present work were an Inconel 718 alloy fabricated using selective laser melting (SLM 718) and a commercially rolled Inconel 718 alloy (R 718, which was used as a reference material). SLM 718 was manufactured at a power of 138 W, a scanning rate of 429 mm/s, and a material plate thickness of 0.04 mm. The chemical compositions (wt.%) of SLM 718 and R 718 are shown in Table 1. The samples were cut into cylinders with a diameter of 10 mm and sealed in a mixture of epoxy and polyamide resins with 0.785 cm2 of the surface exposed. Then, samples were gradually ground with SiC papers up to 1000 grit, cleaned in ethanol, and finally dried in cold air.

Table 1 Chemical compositions of SLM 718 and R 718 (wt.%)
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The electrochemical tests were performed in a 3.5 wt.% NaCl solution at 25 ± 1 °C. The horizontal plane of the SLM 718 alloy was selected as the research plane. The potentiodynamic polarization measurement was performed at a scan rate of 0.1667 mV/s from −250 mVSCE below the open-circuit potential (OCP) to 1100 mVSCE. Electrochemical impedance spectroscopy (EIS) measurements were conducted under natural corrosion conditions. The frequency ranged from 104 Hz to 10−2 Hz, and the AC amplitude was 10 mV. Potentiostatic polarization tests were carried out at 0.4 VSCE for 1 h. Mott-Schottky tests were performed at an alternating current (AC) amplitude of 10 mV within an applied potential range of −600 mVSCE to 1200 mVSCE at a successive step of 20 mV after the potentiostatic polarization test. The electrochemical measurement methods used in the present work were described in detail in a previous study (Ref 25).

The phase constituents of the alloys were analyzed using X-ray diffraction (XRD, D/Max 2400 Rigaku Corporation, Tokyo, Japan) with a Cu Kα radiation source at 10 kV and 35 mA along with a step size of 0.02° and a scan rate of 4°/min. OM (optical microscopy) images were obtained using a Keyence VHX-700 (Keyence Co. Ltd., Osaka, Japan). The samples subjected to X-ray photoelectron spectroscopy (XPS) analysis were potentiostatically polarized at 400 mVSCE for 1 h. XPS spectra were collected using an X-ray photoelectron spectrometer (ESCALAB 250Xi T) with an Al Kα (1486.6 eV) monochromatic radiation source. All the high-resolution spectra were fitted by Avantage software, and then the oxide data were obtained. After the immersion of freshly polished samples in a 3.5 wt.% NaCl solution for 24 h, the morphology of the corroded surface was studied using atomic force microscopy (AFM, ICSPI. Corp, Canada) in noncontact mode.

Results and Discussion

Microstructural Characterization

The XRD patterns of SLM 718 and R 718 are shown in Fig. 1. The diffraction peaks presented in Fig. 1 corresponded to the characteristics of γ-face centered cubic Ni-Fe-Cr, γ′-face centered cubic Ni3(Al,Ti)C (Ref 26) and γ′′-body-centered cubic Ni3Nb phases (Ref 27, 28). The (OM) images of SLM 718 and R 718 are shown in Fig. 2. Regarding SLM 718, various deposition traces were found, which clearly showed the laser scanning line by line in the horizontal direction and the molten pool with arc-shaped features formed layer by layer in the vertical direction, as shown in Fig. 2(a). R 718 has an equiaxed grain structure (Ref 29), with an average grain size of approximately 20 μm, as shown in Fig. 2(b). The chemical compositions of SLM 718 and R 718 were very similar, but their microstructures differed significantly.

Fig. 1
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XRD patterns of SLM 718 and R 718

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Fig. 2
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OM images of the SLM 718 alloy (a) and R 718 alloy (b)

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Electrochemical Response

Open-Circuit Potential

Figure 3 shows the open-circuit potential (OCP) curves of SLM 718 and R 718 in a 3.5 wt.% NaCl solution. Figure 3 shows that the OCP of both alloys continuously shifted in the positive direction with an increasing immersion time and achieved its steady-state potential when the immersion time was 3000 s. This finding suggested that the passive film formed spontaneously on the surface of the two tested alloys (Ref 30). The steady-state potentials of SLM 718 and R 718 were −0.19 VSCE and -0.13 VSCE, respectively.

Fig. 3
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OCP evolution of the SLM 718 alloy and R 718 alloy over time

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Potentiodynamic Polarization

Figure 4 presents the potentiodynamic polarization curves of SLM 718 and R 718 in a 3.5 wt.% NaCl solution. As seen from Fig. 4, both alloys exhibited typical active-passive behavior. The corrosion behaviors of SLM 718 and R 718 were similar but their Ecorr and ip were notably different. The corrosion potential (Ecorr), corrosion current density (icorr), and passive current density (ip) derived from Fig. 4 are listed in Table 2. SLM 718 exhibited a relatively high passive current density, suggesting a faster dissolution rate. The increase in ip and dissolution rate was related to the decrease in the Cr and Ni content in the passive film, which is illustrated later in this article (Ref 31). It is well known that the corrosion resistance of metals/alloys is closely related to the properties of their passive films (Ref 32). Therefore, it is obvious that the corrosion resistance of SLM 718 was inferior to that of R 718.

Fig. 4
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Potentiodynamic polarization curves of the SLM 718 alloy and R 718 alloy

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Table 2. Electrochemical parameters extracted from the potentiodynamic polarization curves
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Electrochemical Impedance Spectroscopy

The Nyquist and Bode plots of SLM 718 and R 718 in a 3.5 wt. % NaCl solution are shown in Fig. 5. The Nyquist plots showed an incomplete semicircle arc, which indicated highly capacitive behavior (Ref 30, 33). This capacitive behavior suggested that the corrosion mechanism of SLM 718 and R 718 was dominated by the passive film. The Bode plots showed that the phase angles tended toward −80° in the frequency range from approximately 10−1 to 102 Hz, indicating the superior protectiveness of the passive film (Ref 34). The impedance of the passive film on SLM 718 was lower than that on R 718, indicating that SLM 718 had inferior corrosion resistance (Ref 35). The EIS only had one time constant (Ref 30, 36, 37), as shown in Fig. 5(b). The capacitance corresponding to the double layer was not observed. Figure 6 shows the eq

uivalent electric circuit of the EIS results. Rs is the solution resistance, Rp is the resistance of the passive film, and Q is the passive film capacitance that is expressed using the CPE. CPE was used due to the uneven current distribution at the surface or in the event of increased surface roughness. Table 3 lists the fitting results of the EIS data in Fig. 5. The Rp of R 718 was much higher than that of SLM 718, suggesting that the passive film of SLM 718 had inferior protective capacity (Ref 13). Therefore, the corrosion resistance of SLM 718 was inferior to that of R 718. This finding was in good agreement with the results obtained from the potentiodynamic polarization tests.

Fig. 5
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Nyquist (a) and Bode (b) plots of the SLM 718 alloy and R 718 alloy.

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Fig. 6
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Equivalent circuit used for the quantitative evaluation of electrochemical impedance spectroscopy

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Table 3 EIS fitted data of the SLM 718 alloy and R 718 alloy
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Potentiostatic Measurements

Figure 7 presents the potentiostatic polarization plots of SLM 718 and R 718 in a 3.5 wt.% NaCl solution. As seen in Fig. 7(a), the current densities decreased dramatically in the beginning due to the formation and thickening of the passive films (Ref 37). The current densities finally stabilized at 1.21 ± 0.09 ×10−7 and 1.04 ± 0.08 ×10−7 A/cm2 for SLM 718 and R 718, respectively. A higher current density means that the formed passive film is more stable and vice versa. Thus, the passive film of SLM 718 was more easily corroded than that of R 718. The slopes (k) of the curves for SLM 718 and R 718 were −0.88 and −0.91, respectively. Briefly, k = −1 suggests that the passive film is compact and highly protective, whereas k = −0.5 indicates that the film is porous (Ref 12, 13, 38, 39). Compared with R 718, the k value of SLM 718 decreased significantly, which meant that the compactness of the passive film on SLM 718 decreased. A passive film with high compactness can effectively restrain the damage caused by corrosive substances, thus reducing the dissolution rate of the passive film. (Ref 40) Hence, the protective property of the passive film on SLM 718 was of inferior quality.

Fig. 7
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i-t (a) and logi-logt (b) plots of the SLM 718 alloy and R 718 alloy in a 3.5 wt.% NaCl solution

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Mott–Schottky Analysis

It is well known that the semiconductor properties of passive films are closely related to their corrosion resistance. Mott–Schottky analysis was performed to investigate the semiconductor properties of the passive film. According to Mott–Schottky theory, the space-charge capacitance of a semiconductor is expressed as (Ref 41, 42)

$$ \frac{1}{{C_{{{\text{SC}}}}^{2} }} = \frac{2}{{\varepsilon \varepsilon_{0} eN_{{\text{D}}} A^{2} }}\left( {V - V_{{{\text{FB}}}} - \frac{{k_{{\text{B}}} T}}{e}} \right)\quad {\text{for}}\;{\text{n - type}} $$
(1)
$$ \frac{1}{{C_{{{\text{SC}}}}^{2} }} = \frac{2}{{\varepsilon \varepsilon_{0} eN_{{\text{A}}} A^{2} }}\left( {V - V_{{{\text{FB}}}} - \frac{{k_{{\text{B}}} T}}{e}} \right)\quad {\text{for}}\;{\text{p - type}} $$
(2)

runwhere ε is the dielectric constant of the passive film (12 for Cr2O3 (Ref 13, 43)), ε0 is the vacuum permittivity constant (8.854×10−14 F/cm), e is the elementary charge (1.602×10−19 C), A is the electrode surface area, V is the applied potential, VFB is the flat band potential, kB is the Boltzmann constant (1.38×10−23 J/K), T is the absolute temperature, ND is the donor density, and NA is the donor density. Thus, a positive slope corresponds to n-type semiconductors, and a negative slope corresponds to p-type semiconductors.

The Mott–Schottky plots of the passive film on SLM 718 and R 718 are shown in Fig. 8, and the passive films of SLM 718 and R 718 show p-type and n-type semiconducting characteristics (Ref 44). The passive films acted as a p-type semiconductor below 0.1 VSCE due to the presence of some compounds, such as Cr2O3 and NiO (Ref 45). However, they exhibited n-type semiconductor behavior above 0.1 VSCE due to the presence of Fe2O3 (Ref 46). The acceptor and/or donor densities (Na/Nd) calculated from Fig. 10 are listed in Table 4. SLM 718 had higher values of Na and Nd than R 718. In addition, when the metals were covered with a passive film layer, Na/Nd was the key factor dominating the corrosion current density of the passive film (Ref 38). Therefore, the larger passive current density of SLM 718 could be attributed to the higher Na or Nd (Ref 47, 48). The Mott–Schottky analysis suggested that the passive film on SLM 718 was defective. This finding was in good agreement with previous work showing that passive films of additively manufactured alloys were usually defective (Ref 17). The results of the Mott–Schottky analysis were consistent with the results of the polarization curves and EIS test.

Fig. 8
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Mott–Schottky plots of the SLM 718 alloy and R 718 alloy

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Table 4 Na and Nd values of the passive films on SLM 718 and R 718
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XPS Analysis of the Passive Film

Figure 9 shows the XPS spectra of the passive films formed on SLM 718 and R 718, and the passive films contain C 1s, O 1s, Ni 2p, Fe 2p, Cr 2p, Mo 3d, and Nb 3d peaks; the C came from contamination when the samples were exposed to air. No significant differences were found in the chemical composition of the passive films formed on SLM 718 and R 718. Figure 10 shows the high-resolution Ni 2p, Fe 2p, Cr 2p, Mo 3d, and Nb 3d XPS spectra of the passive films formed on the SLM 718 and R 718 alloys. The Ni 2p XPS spectrum could be divided into Ni2+ (NiO) and metallic Ni. The existence of satellite peaks is a well-known feature associated with certain elements (such as Ni) (Ref 49). The Fe 2p XPS spectrum could be divided into Fe3+ (Fe2O3) and metallic Fe. The Cr 2p XPS spectrum could be divided into Cr3+ (Cr2O3) and metallic Cr (Ref 50). The Mo 3d XPS spectrum could be divided into Mo6+ (MoO3) and metallic Mo. The Nb 3d XPS spectrum could be divided into Nb5+ (Nb2O5), Nb2+ (NbO), and metallic Nb. The passive films on SLM 718 and R 718 were composed of Cr2O3, Fe2O3, NiO, MoO3, Nb2O5 and NbO. As seen in Table 5, Cr2O3, NiO, and Fe2O3 were the dominant components in the passive film. This finding was consistent with published XPS results on Ni-Cr-Fe alloys, in which their passive films consisted of Cr2O3 and NiO (Ref 51). In comparison with R 718, the passive film formed on SLM 718 contained more NiO, which was porous and less protective than Cr2O3. A high Cr2O3 percentage in a passive film is an indication of a compact surface oxide. The Cr2O3 film had a lower point defect density and higher thermodynamic stability than the NiO layer (Ref 12), which led to an increase in the passive current density and dissolution rate of SLM 718. Therefore, the corrosion resistance of SLM 718 was inferior to that of R 718.

Fig. 9
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XPS survey of the passive films formed on the SLM 718 alloy and R 718 alloy after potentiostatic polarization

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Fig. 10
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High-resolution Ni 2p, Fe 2p, Cr 2p, Mo3d and Nb3d XPS spectra of the passive films formed on SLM 718 (a) (c) (e) (g) (i) and R 718 (b) (d) (f) (h) (j)

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Table 5 Chemical composition (at.%) of the passive films formed on the SLM 718 alloy and R 718 alloy
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AFM

The surface corrosion morphology was further examined using AFM, and the 3D morphology of the surface corrosion of SLM 718 and R 718 is shown in Fig. 11. As seen in Fig. 11, the surface roughness of SLM 718 (20.02 ± 0.12 nm) was slightly larger than that of R 718 (19.79 ± 0.11 nm). The presence of an undulating pattern in Fig. 11(a), which revealed that uneven corrosion was present in SLM 718, was attributed to the layered structure of the formed melting traces (Fig. 2a). The surface profile of R 718 suggested uniform corrosion.

Fig. 11
figure 11

AFM micrographs of SLM 718 (a) and R 718 (b)

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Conclusion

In this study, the corrosion behavior and passive films of SLM 718 and R 718 alloys were investigated. The main conclusions can be summarized as follows:

  1. (1)

    Both the SLM 718 and R 718 alloys were composed of γ, γ′ and γ′′ phases. SLM 718 had obvious molten pool boundaries, while R 718 had equiaxed grains.

  2. (2)

    In the electrochemical test, SLM 718 exhibited lower corrosion resistance than the rolled alloy in a 3.5 wt.% NaCl solution at room temperature.

  3. (3)

    The passive films of the SLM 718 and R 718 alloys were composed of Cr2O3, Fe2O3, NiO, MoO3, Nb2O5 and NbO, and the porous passive film formed on SLM 718 had less Cr2O3 and more NiO, thereby exhibiting inferior corrosion resistance compared with the film formed on the rolled alloy.