Corrosion mechanism of amorphous alloy strengthened stainless steel composite fabricated by selective laser melting

Corrosion Science xxx (xxxx) xxxx

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Corrosion mechanism of amorphous alloy strengthened stainless steel composite fabricated by selective laser melting ⁎

Yuanjie Zhanga, Bo Songa, , Jun Mingb, Qian Yana, Min Wanga, Chao Caia, Cheng Zhanga, Yusheng Shia a School of Material Science and Engineering, State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China b State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Selective laser melting Amorphous alloys Stainless steel Pitting corrosion

The anti-corrosion behaviors of SLM-processed 316L stainless steel (SLMed-SS) with amorphous alloy additive (SLMed-composite) have been investigated, where the pitting of SLMed-composite is easy to be suppressed compared to that of SLMed-SS particularly once the pitting occurred. In addition, the pitting incubation time of SLMed-composite is confirmed to be longer than that of SLMed-SS by potentiostatic polarization under the same potential. More importantly, we confirm that the improved anti-corrosion properties are ascribed to the grain refinement and particularly the different passive film on SLMed-composite, which contains the yttrium oxide and solid solution of additional elements such as Cr and Mo.

1. Introduction Commercial 316L stainless steel has a widespread application, where the selective laser melting (SLM) has become a promising additive manufacturing technique for the improved mechanical properties and corrosion resistance. To date, the influence of SLM process parameters such as the laser power, scanning speed, layer thickness, hatch spacing, and scanning strategy have been intensively investigated [1–7]. A correlation between the process parameters and the resultant microstructures and mechanical properties of SLMed-SS was reported, where the effect of laser power on density is confirmed to be significant but the effect of hatch spacing is not obvious over the tested range [2]. In addition, SLM remelted SLMed-SS can decrease the surface roughness and achieve a surface texture (Ra) less than 0.5 μm [4]. Besides, SLMed-SS bars have been also built assisted by SLM along two different orientations, where the influence of these orientations on microstructures and mechanical properties has been investigated [5]. However, the research related to the corrosion resistance after the SLM has been rarely reported, because most studies focused on the mechanical properties [8–12]. Up to now, a similar corrosion behavior was found between SLMedSS and standard bulk 316L SS in a chloride-containing solution, where the breakdown potential was reduced in SLMed-SS due to the porosity [8]. The SLMed-SS with a low porosity shows a better anti-corrosion

⁎

behavior than that of wrought 316L SS, where the pitting potential (Epit) values were ˜300 mV higher than that of wrought 316L SS [10]. The superior pitting resistance of SLMed-SS was attributed to inhibited formation of MnS and Cr-depletion zones, which is caused by the high solidification rates inherent to SLM [11]. Furthermore, the influence of thermal oxidation (TO) process on SLMed-SS was also reported, where Cr2O3 and Fe2O3 formed on SLMed-SS substrate improved corrosion behavior compared with that of the original substrate [12]. Briefly, the SLMed-SS always exhibits the higher corrosion resistance and toughness properties than those of wrought 316L SS, but the poor performances such as low hardness and low frictional property are difficult to be improved. Our previous researches demonstrate that in situ fabrication of hard particles-reinforced alloy composites by SLM is an effective method to obtain high performance materials [13–15]. The mechanical properties of SLMed-SS (e.g., hardness, wear resistance) can be enhanced by particle reinforcements [16–18], particularly the Fe-based amorphous alloy reinforcement, which presents a higher strength, improved mechanical properties and better wettability than those of ceramics [19]. But, the mechanism of corrosion resistances in these cases has not been fully understood yet. Herein, the corrosion and metastable pitting characteristics of the SLMed-composite (vs. SLMed-SS) were studied in detail.

Corresponding author. E-mail address: [email protected] (B. Song).

https://doi.org/10.1016/j.corsci.2019.108241 Received 15 March 2019; Received in revised form 15 September 2019; Accepted 20 September 2019 0010-938X/ © 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Yuanjie Zhang, et al., Corrosion Science, https://doi.org/10.1016/j.corsci.2019.108241

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2.3. Characterizations

Table 1 Chemical compositions of 316L SS and amorphous alloy powders. Element (Wt%)

Fe

Mo

C

Ni

Co

B

Y

Cr

Other

316L SS Amorphous alloy

Balance 46.5

2 23

< 0.03 3.5

12 –

– 8.2

– 0.9

– 3.2

18 14.6

< 1.0 –

The microstructure and composition of specimens were characterized by the X-ray diffractometer (XRD-7000S) equipped with Cu Kα radiation and the wavelength dispersive X ray fluorescence instrument (XRF-1800), respectively. The surface morphology and polished top views were observed by electron probe microanalyzer (EPMA-8050 G) equipped with an energy dispersive X-ray analysis (EDAX) detectors. The EBSD experiments were performed to obtain the grain size distribution, using sirion field emission scanning electron microscope (Sirion 200) equipped with an orientation imaging microscopy/electron back-scattering patterns (OMI/EBSP) and the scanning step was 1 μm. The cyclic potentiodynamic polarization (CPP) was conducted in the 3.5 wt.% NaCl solution at 25 °C. The specimens were reduced potentiostatically at −1.0 V for 3 min initially to remove the air-formed oxides on the surface before the polarization. Then, the specimens were stabilized in the electrolyte, where the open-circuit potentials (OCP) were measured. The CPP test was performed at 0.5 mV/s from −0.3 VAg/AgCl to a current density of 0.1 mA/cm2, then the scan direction was reversed. After that, the Ecorr and icorr were calculated by extrapolating the Tafel line [20]. Each type of electrochemical measurement was repeated at least three times to ensure the reproducibility and consistency of the data. Later, the electrochemical impedance spectroscopy (EIS) measurements were performed at a potential of 0 V, and a perturbing sinusoidal potential was superimposed with an amplitude of 10 mV. The anodic potential was approximately 0.6 V for the SLMed specimens, which is near the pitting potential (Epit). The analysis of the potentiostatic polarization data was performed on the basis of stochastic theory. Then, the morphologies of the corroded SLMed-SS and SLMed-composite specimens were presented. The Mott-Schottky plots were analyzed to obtain information on the formation of the passive film after the specimens were potentiostatically polarized at 0.3 V for 60 min. The chemical compositions of the passive films were determined by the X-ray photoelectron spectroscopy (XPS) (Axis-ultra DLD-600W) with an Al Kα excitation. The specimen surfaces were sputtered by Ar+ for 30 s to remove the contaminants on surface

2. Materials and methods 2.1. Powder feedstock The commercial-purity gas-atomized AISI 316L stainless steel and Fe43.7Co7.3Cr14.7Mo12.6C15.5B4.3Y1.9 (at.%) amorphous alloy powders were used as the matrix and reinforcement, respectively. The chemical compositions of 316L SS and amorphous alloy are listed in Table 1. The 316L SS and amorphous alloy powders with the mass ratio of 90:10 were blended, in which the particle size distribution of mixed powder was about 27–55 μm and the flowability of mixed powder was suitable for SLM processing.

2.2. SLM process Cuboid specimens with the dimensions of 10 mm × 10 mm × 5 mm were fabricated by an EOS M280 SLM machine that was equipped with a Yb: YAG fiber laser (maximum power Pmax = 400 W, wavelength λ = 1060 nm, and focus diameter d = 70 μm) and a F-theta lens system. The forming chamber was placed under vacuum and protected by argon gas with an oxygen content below 100 ppm. In this study, the laser power (P, 285 W), scanning speed (V, 960 mm/s), powder bed layer thickness (h, 40 μm), and scan line hatch spacing (t, 110 μm), and scanning direction (i.e., 67° between layers) were finely controlled accordingly.

Fig. 1. (a)-(b) Top surface view of SLMed-SS and SLMed-composite, (c)-(d) top view of the polished SLMed-SS and SLMed-composite by EPMA. (The inset images of figures are XRD results). 2

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before the XPS measurements. The binding energies of electrons were adjusted by the peak of C 1s (284.8 eV). XPS spectra were analyzed by a least-squares fit with XPSPEAK analytical software.

Table 2 Elements content of the SLMed specimens.

3. Results and discussion

Element (Wt%)

Fe

Mo

Cr

Co

Ni

Other

SLMed-SS SLMed-composite

67.95 65.40

2.46 5.23

17.11 16.89

0 1.08

11.19 9.79

1.29 1.61

3.1. Microstructure analysis Fe in SLMed-composite is accompanied by the yttrium oxidation and other elements solid solution. Then the grains are easy to be interrupted by the other reaction, leading to the smaller grains than those of SLMedSS. As a result, the average grain size of SLMed-composite is calculated to be 6.47 μm, which is less than 12.72 μm of SLMed-SS, fully demonstrating the decrease of average grain size in the specimens. The varied properties of specimens are discussed later.

The first difference of SLMed-composite and SLMed-SS can be observed by EPMA images and XRD patterns. We find that only one phase (γ-Fe) presents on the surface and polished area in the top view of SLMed-SS (Fig. 1a, c). However, the surface of SLMed-composite has another phase of Y2O3 (Fig. 1b), which disappears when the SLMedcomposite was polished (Fig. 1d), indicating that the Y2O3 only distributes on the composite’s surface. The formation of yttrium oxide during the SLM processing is reasonable because the yttrium is a strong deoxygenation chemical. This kind of low-gravity and dystectic oxide (5.01 g/cm3, 2415 °C) can be precipitated and distributes along the molten pools. Then, the yttrium oxide can move towards the surface when the next layer was melted, leading to an accumulation of yttrium oxide in the original top surface. Finally, all the oxide floats to the uppermost surface of the SLMed-composite after repeating the process for each layer. The polished SLMed-composite was further investigated by the XRD in Fig. 2. We find that the (111) peak of the SLMed-composite shifts to the lower angle, indicating a larger interplanar spacing of the SLMed according to the Bragg equation (2dhkl sinθ = nλ ). This is because the interstitial solid solutions, such as B and C, can easily enter the gaps in the matrix as the oxidation of yttrium during the SLM processing, thereby giving rise to the increased lattice plane distance. In addition, the introduction of Co and Mo, and the variation of contents of Fe, Mo, Cr, Co, Ni in SLMed-composite after the SLM process are the other reasons for the increased lattice plane distance. This is confirmed by the increased amount of Mo and Co elements when the amorphous alloy was added (Table 2), where the varied amount of Ni, Fe, Cr in SLMedcomposite can be also found. The solid solution of the elements from amorphous alloy causes a local lattice distortion that increases the obstruction of dislocation movement, resulting in a hard sliding motion. In addition to the interplanar space, the average grain size was also estimated by Scherrer’s equation [21] (Dhkl = kλ / Bcosθ , wherein Dhkl, k, λ , B and θ is the grain size, Scherrer constant (0.89), wavelength, halfpeak width and angle, respectively). We find that the Dhkl of SLMedcomposite (227kλ ) is smaller than that of the SLMed-SS (247.25kλ ), demonstrating the grain refinement after adding the amorphous alloy. The viewpoint of different grain refinement is confirmed directly by the inverse pole figure (IPF) in Fig. 3. We find that the large grains in SLMed-composite are only ˜50 μm compared to the large (˜100 μm) and irregular grains in SLMed-SS (Fig. 3a–b). This is reasonable because the solidification of SLMed-composite powder is more complex than that of pure SLMed-SS powder during the SLM process. The solidification of γ-

3.2. Electrochemical analyses and passive film behaviors The difference of SLMed-composite and SLMed-SS can be demonstrated by the cyclic potentiodynamic polarization (CCP) measurement in 3.5% NaCl solution (Fig. 4), in which a passive film can be formed and characterized (Table 3). We find that the SLMed-composite presents the widest passive region (1.185 VAg/AgCl) and the greatest pitting potential (Epit) value (1.013 VAg/AgCl) over those of the SLMed-SS (0.918 VAg/AgCl, 0.918 VAg/AgCl) and 316L (0.723 VAg/AgCl, 0.591 VAg/ AgCl). In addition, the repassivation potentials (Erep) values of SLMedcomposite are greater than the self-corrosion potential (Ecorr) values. Therefore, the passive film can be regenerated when the potential decreased even the passive film broke down at Epit. However, the Erep values of SLMed-SS are less than the Ecorr values, demonstrating the formed passive film cannot be regenerated during the entire test once it was broken down at Epit. This observation reveals that the pitting of SLMed-composite is easy to be suppressed compared to that of the SLMed-SS once the pitting occurred. Note that the reason of the higher Epit for SLMed specimens can be ascribed to the homogenous microstructure by the rapid solidification rates in SLM (typically ∼107 K/s). The fast solidification speed can decrease the diffusion time for Mn/S and then reduce the amount of formed MnS, thus inhibiting the pitting corrosion events initiated in the vicinity of second-phase particles [11,22], as shown in Table 3. The anti-corrosion mechanism of the SLMed specimens was analyzed first by the EIS in Fig. 5. The Nyquist curves of the SLMed specimens show only one capacitive loop in the entire frequency range, which was fitted by the simplified equivalent circuit (inset of Fig. 5a). In the equivalent circuit, Rs and Rp represents solution resistance and charge-transfer resistance, respectively. CPE (constant-phase element) was used to represent the capacitance in the EIS analysis, and the impedance of CPE is defined as: ZCPE=[Y0(jw)n]−1

Fig. 2. (a) XRD patterns of polished SLMed specimens and (b) enlargements of the (1 1 1) diffraction peak. 3

(1)

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Fig. 3. Inverse pole figures of (a) SLMed-SS and (b) SLMed-composite, the grain size distribution of (c) SLMed-SS and (d) SLMed-composite obtained through EBSD.

Cdl= Y01/n(Rs−1+ Rp−1)(n-1)/n

The values of Rs, Rp, Y0, n and Cdl were calculated by the ZView software, and the results are listed in Table 4. Rp value relates to the passive film, indicating the corrosion resistance of the specimens. A lower Rp value means a higher electrochemical reaction rate. We find that the value of Rp in the SLMed-composite (4.38 ± 1.24 MΩ cm2) is greater than that of the SLMed-SS (1.35 ± 0.21 MΩ cm2). Besides, the value of Cdl in the SLMed-composite (2.15 ± 0.43 μF cm−2) is smaller than that of the SLMed-SS (6.17 ± 2.27 μF cm−2). These results demonstrate that a passive film was formed on the SLMed-composite and it has a greater corrosion resistance, which is consistent with the CPP measurements. The passive film formation, pit nucleation and growth were further analyzed in Fig. 6. The initial rapid decrease in the current density implies the formation of a passive film on the working electrode [24]. The passive film formation rate of SLMed-composite (K2=-0.62) is lower than SLMed-SS (K1=-0.56) in Fig. 6, showing the passive film of SLMed-composite formed faster than that of SLMed-SS. Besides, SLMedcomposite also presents a stable passive film, where the pitting incubation time of SLMed-composite (T2 = 1668s) is much longer than SLMed-SS (T1 = 488 s). The pitting corrosion morphologies of the specimens after potentiostatic experiments (E = 0.6 V) were shown in Fig. 7. We find that the SLMed-SS was corroded after the duration of 1000s, where a pit with 200 μm in width and 70 μm in depth can be observed (Fig. 7a). By contrast, the SLMed-composite was not corroded and there is no pit observed (Fig. 7b). These phenomena reveal that the passive film on the SLMed-composite could protect matrix for a longer time than SLMed-SS in a 3.5% NaCl solution.

Fig. 4. CPP curves of SLMed-SS and SLMed-composite. Table 3 Summary of electrochemical parameters of pristine 316L, SLMed-SS and SLMed-composite from the potentiodynamic polarization curves. Specimens

Ecorr(V)

Epit(V)

Erep(V)

As cast 316L SLMed-SS SLMed-composite

−0.132 ± 0.006 0 ± 0.053 −0.172 ± 0.027

0.591 ± 0.076 0.918 ± 0.035 1.013 ± 0.010

−0.125 ± 0.015 −0.129 ± 0.033 −0.147 ± 0.018

(2)

where Y0 is the magnitude of CPE, j is the imaginary unit, ω is the angular frequency, n is the experience index. The value of n in SLMedSS (0.90 ± 0.03) and SLMed-composite (0.86 ± 0.02) indicated that the CPE is close to capacitance, so the double layer capacitance (Cdl) can be calculated by the Brug’s equation [23]: 4

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Fig. 5. (a) Nyquist and (b) bode curves of SLMed-SS and SLMed-composite. Table 4 Equivalent circuit parameters of EIS results. Specimens

Rs (Ω cm2)

Rp (MΩ cm2)

Y0 (μΩ−1 cm-2 sn)

n

Cdl (μF cm−2)

SLMed-SS SLMed-composite

14.68 ± 3.05 15.95 ± 2.99

1.35 ± 0.21 4.38 ± 1.24

15.70 ± 2.2 9.26 ± 1.28

0.90 ± 0.03 0.86 ± 0.02

6.17 ± 2.27 2.15 ± 0.43

Fig. 6. (a) Potentiostatic polarization curves and (d) distribution of pitting incubation time of SLMed-SS and SLMed-composite.

Fig. 7. The pitting corrosion images of (a) SLMed-SS and (b) SLMed-composite by EPMA.

3.3. Passive film characterizations The difference of passive film can be analyzed by the Mott-Schottky relation. As we know, the passive film on stainless steel has the semiconductor properties. The charge transfer occurs between the semiconductor and the solution when the semiconductor contacts with the NaCl solution. Thus, three kinds of space-charge layers of passive film can be found in different potential ranges when the electrons move in or out of the semiconductor surface: depletion layers, enrichment layers and inversion layers. According to the Mott-Schottky relation in the depletion layer, p-type and n-type semiconductors can be defined by Eqs. 3 and 4, respectively [25,26], as shown hereafter:

1 2 ⎛ KT ⎞ =− ⎜E − EFB − ⎟ C2 εε0 qNA ⎝ q ⎠

(3)

1 2 ⎛ KT ⎞ = ⎜E − EFB − ⎟ C2 εε0 qND ⎝ q ⎠

(4)

where C and ε are the semiconductor capacitance and the dielectric constant (15.6 for stainless steel) of the passive film, ε0 is the vacuum permittivity constant (8.854 × 10−14 F/cm), q is the elementary charge (1.602 × 10-19 C), NA and ND are the acceptor and donor density, respectively, E is the applied potential, EFB is the flat band potential, K is Boltzmann’s constant and T is the absolute temperature. Then, the 5

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inside of SLMed specimens was further analyzed by the XPS. We find that the passive film formed on SLMed-composite shows a different compositional tendency compared with that on the SLMed-SS (Fig. 9ab). In detail, a sudden increase of Fe content and decrease of O content, together with an obvious increase of Cr content at 390 s were observed for the sample of SLMed-composite. By contrast, the gradual increase of Fe content and the decrease of O to 84.85 at.%, together with the slight increase of Cr content demonstrate that the composition of passive film on SLMed-SS is gradient. This result demonstrates that there is a uniform passive film on the SLMed-composite. Meanwhile, the higher concentration of O for the SLMed-composite than that of SLMed-SS is consistent with the results of forming yttrium oxide. In addition, the XPS spectra of the passive films formed at 0.3 V are further shown in Fig. 9c–e. We find that the passive film on the SLMed-composite has a higher content of FeO, MoO3, MoO2 and Cr(OH)3, while the passive film formed on the SLMed-SS has a higher content of Fe3O4 and Cr2O3. These results all confirm that the chemical composition of passive film on SLMed specimens is totally different, which is the main cause of different electrochemical behaviors for the SLMed-composite and SLMed-SS.

Fig. 8. Mott-Schottky curves of SLMed-SS and SLMed-composite in 3.5% NaCl solution.

Mott-Schottky plots of the passive film formed on the SLMed-composite and SLMed-SS are shown in Fig. 8. These plots show that the SLMed specimens present n-type semiconducting behavior between -0.55–0.2 V in the 3.5% NaCl solution, and the flat band potential is approximately -0.55 V. However, the slope of the SLMed-composite greatly differs from that of the SLMed-SS, which means that the surface charge of the passive film changed. The donor densities of the passive film formed on the SLMed-composite and the SLMed-SS are (6.92 ± 1.39)×1020 and (9.77 ± 3.45)×1020 cm-3, respectively. These results indicate that the ion conductivity of passive film on the SLMed-composite is lower, thereby leading to the higher corrosion resistance. The compositional difference from the formed passive film to the

3.4. Microstructure and passive film analysis In addition to the benefit from the formed passive films, the observed yttrium oxide and the dissolved B, C, Co, Cr and Mo elements in SLMed-composite are also responsible for the better anti-corrosion properties. In detail, the addition of Co in SLMed-composite could increase the hardness and wear resistance. While the Cr and Mo in SLMed-composite can enhance the corrosion resistance because they can accelerate the formation of steady passive films and then avoid the corrosion against the Cl− anions. This is because the standard electrode

Fig. 9. Comparison of element content within (a) SLMed-SS and (b) SLMed-composite being sputtered, (c) comparison of the composition of two passive films, XPS spectra of Fe, Co and Cr for the passive film of (d) SLMed-SS and (e) SLMed-composite. 6

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potential (EH0 ) of Cr or Mo is less than that of Fe, which means that the Cr and Mo can form passive films more easily than Fe. Thus, the passive film can be formed within a shorter time with a higher rate. Besides, the detected MoO3 is a steady passive film, which can exist as solid solution in the passive film of stainless steel, thereby enhancing the distribution and stability of the original passive film. In addition, Mo can also decrease the destructiveness of Cl−, because the addition of Mo promotes the reaction between Fe and O, which keeps the Cl− away. Finally, the observed smaller average grain in SLMed-composite is another reason responsible for the enhanced corrosion resistance [27,28]. This is because the activity of electrons at the grain boundaries can be enhanced by grain refinement [29], resulting in the rapid formation of a mechanically strong and stable passive film [28], as shown in Fig. 6a. Thus, the SLMed-composite demonstrates an enhanced corrosion resistance.

[3] [4]

[5]

[6]

[7]

[8]

[9]

4. Conclusions [10]

In summary, the mechanism of anti-corrosion behaviors for SLMedcomposite (vs. SLMed-SS) is studied in detail. We find that the SLMedcomposite has the greater Epit value (1.013 V) and the wider passive region (1.185 V) compared to those of SLMed-SS. Besides, the pitting incubation time of SLMed-composite is confirmed to be longer while the passive film formation on SLMed-composite is faster than that of SLMed-SS. The XPS results all confirm that the chemical composition of passive film formed on SLMed specimens is totally different, which is the root cause of different electrochemical behaviors for the SLMedcomposite and SLMed-SS. In addition, the formed yttrium oxide and solid solution of additional elements together with the smaller grains in SLMed-composite are also responsible for the passive film formation and the improved anti-corrosion properties.

[11]

[12]

[13]

[14]

[15]

Data availability

[16]

The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.

[17]

Declaration of Competing Interest

[18]

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[19]

[20]

Acknowledgements

[21]

This work was sponsored by the Natural and Science Foundation of China (Grant nos.51531003, 51775208), the Hubei Science Fund for Distinguished Young Scholars (No. 0216110085), the National Key Research and Development Program “Additive Manufacturing and Laser Manufacturing” (No. 2016YFB1100101), Wuhan Morning Light Plan of Youth Science and Technology (No. 0216110066), the academic frontier youth team (2017QYTD06, 2018QYTD04) at Huazhong University of Science and Technology (HUST). The authors thank the Analytical and Testing Center of HUST for EPMA examination and the State Key Laboratory of Materials Processing and Die & Mould Technology for XRD examination.

[22]

[23]

[24] [25] [26] [27]

[28]

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