Enhancing the corrosion resistance of selective laser melted 15-5PH martensite stainless steel via heat treatment

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Enhancing the corrosion resistance of selective laser melted 15-5PH martensite stainless steel via heat treatment Li Wanga, Chaofang Donga,*, Cheng Manb, Decheng Konga, Kui Xiaoa, Xiaogang Lia a

Beijing Advanced Innovation Center for Materials Genome Engineering, Key Laboratory for Corrosion and Protection (MOE), Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing, 100083, China b School of Materials Science and Engineering, Ocean University of China, Qingdao, 266100, China

ARTICLE INFO

ABSTRACT

Keywords: Stainless steel Polarization AFM Passive film

The microstructural optimization for selective laser melted 15-5PH stainless steel via heat treatment to enhance corrosion resistance was investigated. The results showed that after aging treatment, Cu-rich nanoparticles (about 10 nm) diffusely precipitated, and approximately 18∼25 % austenite was distributed near the molten pool boundary. The surface potential of the austenite was approximately 15 mV higher than that of martensite by scanning Kelvin probe force microscopy. However, the austenite phase disappeared and the new NbC-(Mn, Si)O duplex particles precipitated after solution treatment and aging treatment, which decreased the pitting corrosion resistance and passive film stability of the SLM 15-5PH stainless steel.

1. Introduction Selective laser melted (SLM), also known as 3D printing, is an additive manufacturing technique for metal composites based on a 3D CAD volumetric model that uses a layer-by-layer deposition method [1–3]. 15-5PH martensite stainless steel is one of the metallic materials that could be well established using this technique. Generally, apart from the high strength, the excellent corrosion resistance is also one of the critical advantages for the extensive application of the 15-5PH stainless steel. The corrosion resistance of the metallic material is mainly dependent on their compositions and microstructures [4–6], and the microstructures of martensitic stainless steels could be significantly changed though the heat treatment [7,8]. Therefore, the heat treatment should be one of the effective methods that could enhance the corrosion resistance of the 15-5PH martensite stainless steel produced by SLM (hereafter SLM 15-5PH stainless steel) via microstructural optimization. Recently, some researchers focused their interests on revealing the relationship between the heat treatments, the microstructures and the mechanical properties [9,10]. Yuchao Bai et al. investigated the mechanical properties of a 3D printed 300 M maraging steel by different aging treatments, and they revealed that Ni3Mo, Fe2Mo and Ni3Ti precipitates gradually precipitated after the solution and aging treatments, and the strength of the 300 M steel was obviously improved [11]. Somayeh Pasebani et al. studied the effect of atomization and

⁎

solution treatment temperatures on the mechanical properties of a 3D printed 17-4PH steel and their results revealed that the microstructure contained an increased amount of martensite after the high-temperature solution treatment, which was harmful to plasticity [12]. Even so, the influence of the heat treatments on the corrosion behavior of the martensitic stainless steels is rarely considered. In the previous work of our group, it was found that the SLM 316 L austenite stainless steel exhibited more excellent pitting corrosion resistance than the wrought 316 L stainless steel in the simulated body fluid [13,14], and the recrystallization heat treatment could improve the durability of SLM 316 L stainless steel in a 0.5 M H2SO4 solution due to the uniform structure [15]. However, the conclusion obtained in the studies of the SLM 316 L stainless steel was difficult to be directly applied in this work, because the variation in the microstructure of the martensitic stainless steel induced by the heat treatments is much more complex than that of the austenitic stainless steel. Most of the variation in the microstructures of the martensitic stainless steels could significantly influence their corrosion behavior, especially the formation of precipitations and the generation of the reversed austenite [16,17]. M23C6 precipitates were always regarded as the nucleation site of the pitting/ intergranular corrosion [18,19]. At the same time, the generation of the reversed austenite was beneficial to the corrosion resistance of the martensitic stainless steel [20]. Therefore, it is necessary for enhancing the corrosion resistance of SLM 15-5PH stainless steel via heat treatment to reveal the effect of the heat treatment process on its

Corresponding author. E-mail address: [email protected] (C. Dong).

https://doi.org/10.1016/j.corsci.2019.108427 Received 1 July 2019; Received in revised form 3 December 2019; Accepted 30 December 2019 0010-938X/ © 2020 Elsevier Ltd. All rights reserved.

Please cite this article as: Li Wang, et al., Corrosion Science, https://doi.org/10.1016/j.corsci.2019.108427

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Table 1 The alloy composition of the SLM 15-5PH stainless steel (Wt. %). Element

C

Si

Mn

P

S

Cr

Ni

Cu

Nb

SLM 15-5PH Standard

0.037 ≤0.07

0.58 ≤1.00

0.458 ≤1.00

0.021 ≤0.04

0.011 ≤0.03

14.96 14-15.5

5.08 3.5-5.5

4.01 2.5-4.5

0.385 0.15-0.45

microstructure and the corrosion resistance. In this work, we optimized the microstructure through different heat treatment processes (aging treatment for 1−10 h, solution treatment + aging treatment for 1−10 h) of SLM 15-5PH stainless steel. The effects of the different heat treatment processes on the microstructure were studied by X-ray diffraction (XRD), electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). Electrochemical testing and scanning Kelvin probe force microscopy (SKPFM) were applied to characterize the corrosion resistance and passive film stability, and the composition of the passive film was measured with X-ray photoelectron spectroscopy (XPS). Finally, the most reasonable microstructure via heat treatment for enhancing the corrosion resistance of SLM 15-5PH stainless steel was proposed and it guided for post-heat treatment of SLM martensite high strength stainless steel. 2. Experimental methods 2.1. Materials

Fig. 1. XRD spectra of SLM 15-5PH stainless steel after different heat treatments.

SLM 15-5PH high-strength steel was produced by an EP-M250 metal 3D printer. The sample size was Ф20 mm × 100 mm. Table 1 shows the

Fig. 2. Microstructures of SLM 15-5PH stainless steel after different heat treatments: (a) xz-SLM; (b) yz-SLM; (c) xy-SLM; (d) xz-AT-10 h; (e) yz-AT-10 h; (f) xy-AT10 h; (g) xz-ST + AT-10 h; (h) yz-ST + AT-10 h; (i) xy-ST + AT-10 h.

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Fig. 3. EBSD images of SLM 15-5PH stainless steel after different heat treatments: (a,b)AT-1 h; (c,d)AT-10 h; (e,f)ST + AT-10 h.

composition of the samples in this study. The 3D printed powders used to fabricate 15-5PH stainless steel were mainly spherical and the average size was approximately 20 μm. The laser power and scanning speed were 195 W and 850 mm/s, respectively. The density of 15-5PH stainless steel was higher than 99.5 % and no obvious manufacturing defects were observed by 400× optical microscope. There were three heat treatment processes for the SLM 15-5PH stainless steel: (1) ST: solution treated at 1050 °C without aging treatment; (2) ST + AT: solution treated at 1050 °C and then aging treated at 500 °C for 1−10 h, which was similar with the traditional heat treatment; (3) AT: immediately aging treated at 500 °C for 1−10 h after SLM.

288 K and 30 V and analysed by TEM (FEI Tecnai G20) and energy dispersive spectroscopy (EDS) on the TEM. The phase distribution was identified by EBSD and XRD (RigakuUltima IV) with Cu Kα radiation. The microstructure and corrosion behavior were studied on the surface of XY plane (parallel to the powder bed). The Vickers hardness of the samples was measured by the indentation method. A diamond quadrilateral head with the opposite angle of 136° was used to apply pressure. The pressure was 200 N and the pressure was maintained for 15 s. At least five valid results were selected for each test parameter during measurement. 2.3. Electrochemical measurements

2.2. Microstructure analysis

The corrosion behavior of the SLM 15-5PH stainless steel was evaluated by the electrochemical tests. The steel electrodes of 10 mm × 10 mm × 2 mm were used for electrochemical tests and were inlaid in epoxy resin, exposing electrodes area of 1 cm2. The sample surfaces were ground using 2000# SiC paper later, rinsed using distilled water and absolute ethanol. The Modulab XM electrical workstation

The confocal microscope was used to observe the microstructure by samples with dimensions of 10 mm × 10 mm. The samples were polished and eroded with the solution (5 g FeCl3 + 50 ml HCl + 50 ml H2O). The samples of Ф3 mm were polished to 30 μm with SiC paper and jet thinned with 5 vol.% perchloric acid +95 vol.% acetic acid at 3

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Fig. 4. TEM images of SLM 15-5PH stainless steel after different heat treatments: (a) AT-1 h; (b) AT-10 h; (c) ST + AT-10 h; (d) EDS mapping results of Cu-rich nanoparticle; (e) EDS mapping results of NbC-(Mn, Si)O duplex particle.

potentiodynamic polarization was 0.1667 mV·s−1. The square-wave potentiostatic pulse test (PPT) was conducted to investigate the impact of microstructure on the passivation film stability of the SLM 15-5PH stainless steel [21,22]. In a current density response curve, the potential was held at a lower potential (E1) for a period of time (t1) and the passivation film formed on the surface of the sample, after which the adjustment was continued at a higher potential (E2) for a period of time (t2), where the passive film was gradually broken down and pitting gradually formed. The potential was cycled 20

was used to conduct the electrochemical measurements on a conventional three-electrode electrochemical cell at room temperature in 0.1 M NaCl solution. The counter-electrode was a platinum plate, and the reference electrode was a saturated calomel electrode (SCE). Prior to the electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization, the open circuit potential (OCP) was tested for 30 min to ensure system stability. The frequency range of EIS was from 100 kHz to 10 mHz with an excitation voltage of 10 mV, and the results were analyzed with ZSimpWin software. The potential scanning rate of 4

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to the multi-pass aging heat treatment [2,23]. When the sample was reheated to the austenite phase during the solution treatment, the solute elements diffused rapidly, and the austenite phase was transformed into the martensite after water cooling. Therefore, the austenite content of the sample after solution treatment was reduced [2,24]. Fig. 2 shows the microstructures of the SLM 15-5PH stainless steel after the different heat treatments. There were obvious molten pool lines on the XZ and YZ planes, and the weld pool lines of XY plane were relatively inconspicuous. There were some molten pool structures with diameters of 100 μm and layer thickness of 20 μm. The strip-like tissue distribution along the selective laser melted direction was observed. At the same time, bulk austenite formed near the weld pool line. After 10 h of aging treatment, the tissue distribution in different printing directions did not change significantly, and there were still weld pool lines and strip-like structures. The reason was that the cooling rate during SLM was about 105-107 K/s, so that martensite was preferentially formed, then the austenite forming element (Ni) was enriched at the bottom of the molten pool; thus, the molten line was eventually formed [25,26]. Moreover, as the cooling rate increased, the weld line was more pronounced [16,27]. After solution treatment at 1050 °C then aging at 500 °C for 10 h, the molten pool line disappeared, and martensite structure with a fine grain size was observed because the hightemperature solution treatment caused the solute elements to diffuse rapidly, resulting in microstructure homogenization. Therefore, the content of austenite reduced, and it was replaced by martensite [28]. Fig. 3 shows the EBSD results of the samples with different heat treatments. The red area represents martensite relative to austenite by the green area. There were 18.1 % austenite distributed at the martensite laths after aging treatment for 1 h. As the aging time increased to 10 h, the austenite content increased to 25.4 % due to the formation of reversed austenite. There were no obvious martensite laths in the samples due to the extremely fast cooling during SLM. When the sample was ST + AT for 10 h, the microstructure was almost free of austenite, and martensite laths were obvious and coarse, which was similar to 155PH stainless steel by the traditional manufacturing process [29]. Related studies have shown that for martensitic high-strength steel, austenite has a beneficial effect on the corrosion of the sample and has no obvious harmful effect on the strength [30,31]. Therefore, the SLM 155PH stainless steel had a large amount of austenite phase after aging treatment, which could increase corrosion resistance and passive film stability. The TEM results of the SLM 15-5PH stainless steel after the different heat treatments are displayed in Fig. 4. It showed that there were a lot of nanoscale precipitates and high density dislocations distributed at the martensite laths. Fig. 4(a) reveals that the precipitates with the size of approximately 1 nm were contained in the sample after aging treatment for 1 h. As the aging time increased to 10 h, the precipitates

Fig. 5. The microhardness of SLM 15-5PH stainless steel after different heat treatments.

times between E1 and E2. In the paper, E1 was 0 mVSCE, E2 was 250 mVSCE, t1 = t2 = 5 s. 2.4. Surface characterization The sample of SKPFM analysis was treated by 0.5 μm polishing paste. The atomic force microscope was MultiMode 8 of Bruker Instruments Inc. The nanoprobe conductive nitride cantilever silicon tips were used to characterize the surface potential of different phases. The force constant was 0.8 N/m and resonant frequency was 300 kHz. The passivation films composition formed on the SLM 15-5PH stainless steel with different heat treatments was measured by XPS. The samples were potentiostatically polarized at 0.05 VSCE for 3 h in 0.1 M NaCl solution and researched by a hemispherical electron analyzer, which conducted at a pass energy of 55 eV. The XPS PEAK software was used to fit all peaks by corrected with the standard peak (C1s, 285.0 eV). 3. Results 3.1. Microstructures Fig.1 displays the XRD patterns of SLM 15-5PH stainless steel after produced by SLM (As-built) after different heat treatments. The samples mainly comprised the martensite phase and the (110) martensite peak was the strongest after the different heat treatments. The original sample and the aged sample contained more austenite, which was due

Fig. 6. Potentiodynamic polarization of SLM 15-5PH stainless steel after different heat treatments: (a) potentiodynamic polarization curves; (b) pitting potential.

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Table 2 Fitting data of the potentiodynamic polarization. Heat treatment Ecorr/mVSCE Epit/ mVSCE icorr/×10−9A·cm-2 ip/×10−8A·cm-2

As-built −181 244 2.398 1.122

ST

AT-1 h

AT-10 h

ST + AT-1 h

ST + AT-10 h

−175 266 26.915 5.082

−166 216 4.955 1.309

−177 153 8.337 1.626

−184 174 14.388 3.707

−182 97 35.075 9.183

mechanical properties due to precipitation of Cu-rich precipitates. 3.3. Corrosion behaviors Fig. 6 shows the potentiodynamic polarization curves of the SLM 15-5PH stainless steel after different heat treatments. Before polarization tests, it took enough time to achieve a relatively stable OCP. There was an obvious passivation interval from -100 mVSCE to 200 mVSCE, which showed that the passive film could spontaneously form on the surface of samples [34–36]. As the potential was higher than 200 mVSCE, the current density increased rapidly indicating the occurrence of the pitting corrosion. There was no significant difference in the cathodic processes of SLM 15-5PH stainless steel with different heat treatments. The oxygen reduction occurred as the cathodic reaction for stainless steel in the NaCl solutions. The electrochemical parameters (corrosion potential (Ecorr), corrosion current density (icorr), pitting potential (Epit), and passive current density (ip)) acquired from the potentiodynamic polarization curves are listed in Table 2. It can be seen that there were less difference in corrosion potential, while Epit, icorr and ip were much different after different heat treatments. The sample with ST at 1050 ℃ had the highest pitting potential (about 266 mVSCE) which was more positive than that of the SLM sample. The pitting potentials of the samples with ST + AT were lower than that of samples with AT. At the same time, the icorr and ip of the samples with ST + AT were also larger than that of the AT samples. This indicated that the chloride ions were more likely to penetrate the passive film and the stability of the passive film was weaker for the sample after ST + AT. The changes in corrosion behavior illustrated that the bulk/thin austenite distribution at the bottom of the molten pool/martensite lath significantly improved the pitting corrosion resistance of the stainless steel. With increasing aging times from 1 h to 10 h, the pitting potential decreased and icorr/ip increased, which was due to the coarsening of Cu-rich precipitates [28]. So that it was concluded that the sample after being AT had a higher pitting potential and more stable passive film than the samples of ST + AT. Fig. 7 shows the current density responses of the PPT experiments. At a lower potential, the passivation film grew steadily, while the passivation film was broken at the high potential [37]. When the potential increased from E1 to E2, the current density of the sample after solution treatment was still relatively small, approximately 8.5 × 10−5 A·cm-2, which was due to the passivation state of the surface film. When the potential was E2, the current density of the AT samples for 10 h was relatively large, approximately 5.5 × 10-4 A·cm-2. As the time increased, the pitting on the sample surface gradually expanded, so that the current density gradually increased [22]. When the sample was ST + AT for 10 h, the current density increased to 1.5 × 10-3 A·cm-2, which was attributed to the breakdown of the passive film at 250 mVSCE. Corrosion gradually occurred on the local areas of the samples, and corrosion products gradually accumulated on the surface of the pits, which resulted in a decrease of the current density with the extension of test time. Thus, it was concluded that the passive film stability of the sample with ST + AT for 10 h was poorer as compared to that of the AT sample for 10 h, which was consistent with the results of polarization curves. The EIS of SLM 15-5PH stainless steel was measured in chlorinecontaining solution, as shown in Fig. 8. The Nyquist plots showed

Fig. 7. Potentiostatic pulse curves of SLM 15-5PH after different heat treatments: (a) potential of PPT experiment; (b) current density responses of different samples.

gradually coarsened to 2 nm. Fig. 4(d) shows the EDS analysis of the precipitates, which proved that these were Cu-rich particles, which was consistent with the precipitates in traditional 15-5PH stainless steel [28]. For the samples underwent the heat treatment of ST + AT for 10 h, the 2−3 nm Cu-rich precipitates diffusely distributed in the martensite matrix as shown in Fig.4(c). At the same time, the TEM-EDS analysis displayed that these were also some NbC-(Mn, Si)O duplex particles (approximately 50−80 nm) in the samples after ST + AT as shown in Fig. 4(e). The presence of oxide particles might reduce the corrosion resistance of the sample [32]. 3.2. Microhardness test The microhardness of the samples with different heat treatments are presented in Fig. 5. The results showed that the microhardness of the SLM 15-5PH stainless steel was 363.78 HV, which was higher than that of the ST sample. This may be due to the presence of many dislocations at the grain boundaries of the SLM 15-5PH stainless steel without solution treatment [3,33]. The microhardness of the AT samples for 1−10 h was approximately 450 HV, which was nearly equal with that of the ST + AT samples for 1−10 h. Therefore, the aging treatment and solution treatment + aging treatment had a similar effect on the 6

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Fig. 8. EIS of SLM 15-5PH stainless steel after different heat treatments: (a) Nyquist plot; (b) bode plot; (c,d) Complex-capacitance plot.

attributed to the microstructure without precipitates. After aging treatment, the impedance value gradually decreased with increasing aging treatment time. After the solution treatment + aging treatment for 10 h, the austenite content was reduced, and NbC-(Mn, Si)O duplex particles gradually precipitated, so that the impedance value decreased significantly. Fig. 9 is the equivalent circuit to fit the EIS data. In the equivalent circuit, Rs is the solution resistance, CPE1 is related to the electrochemical potential of the electric double layer, and R1 is the corresponding charge transfer resistance, CPE2 reflects the electrochemical response of the passive film, R2 reflects the hindrance of passive film to ion migration. The CPE (Q, n) was the constant phase element. The impedance of CPE (ZCPE) could be calculated based on [40,41]:

Fig. 9. The equivalent circuit diagram used to fit the EIS experimental data of SLM 15-5PH stainless steel.

ZCPE =

similar depressed capacitive semicircles, which indicated uniform passivation behavior after different heat treatments [38,39]. The sample after solution treatment had the maximum impedance value, which was

1 Q ( . i )n

(1)

In this equation, Q and n are the admittance value and fitted exponential of CPE, respectively; ω is the angular frequency; and i is an

Table 3 Fitting date of the equivalent circuit used in the experiments. Heat treatment Rs/Ω·cm2 Q1/×10−5 Ω-1·cm-2sn n1 R1/×104 Ω·cm2 Q2/×10−5 Ω-1·cm-2sn n2 R2/×105 Ω·cm2 C∞/×10−6 F·cm-2 dδ/nm

As-built 42.23 5.125 0.892 2.568 5.021 0.912 1.894 6.302 2.18

ST

AT-1 h

AT-10 h

ST + AT-1 h

ST + AT-10 h

42.73 2.487 0.912 4.587 3.125 0.889 2.125 5.459 2.53

48.44 5.698 0.923 3.215 4.012 0.921 1.684 5.638 2.45

43.31 2.568 0.905 2.157 4.725 0.915 1.235 6.738 2.05

47.27 3.259 0.895 2.568 3.897 0.923 1.458 6.609 2.09

46.46 4.258 0.921 1.897 3.598 0.908 1.568 6.336 2.18

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Fig. 10. XPS spectra of the surface film formed on SLM 15-5PH stainless steel after immersion experiment for 24 h in 3.5 % NaCl solution at room temperature.

relationship

Table 4 Binding energy, compounds and compounds content extracted from XPS analysis. Element

Compounds

Binding energy (eV)

ST

Fe

Fe2O3 FeOOH FeO Fe Cr Cr2O3 Cr(OH)3

708.6 709.83 707.4 706.2 573.51 575.6 577.01

19.41 % 44.16 % 13.81 % 22.62 % 6.29 % 20.85 % 72.86 % 3.03

Cr Fe/Cr

AT-10 h

34.83 % 30.67 % 34.5 % 9.65 % 74.42 % 15.93 % 3.29

C =

ST + AT-10 h

% % % %

imaginary number. When n = 1, the CPE is considered an ideal doublelayer capacitor, while CPE is a non-ideal capacitor for 0.5 < n < 1. The CPE behavior was always analysed by the power-law model [42–45]. However, the use of the power-law model introduces a large number of parameters. The capacitance of the oxide film can be extracted from the impedance data by using the complex-capacitance representation [46–48]. The film capacitance was graphically determined from the EIS response using:

C( ) =

1 j [ZF ( )

Rs ]

(3)

where ε and ε0 are the relative permittivity of the passive film and the vacuum permittivity (ε0 = 8.8542 × 10−14 F·cm-1), respectively. All fitting data and passive film thickness from the EIS analysis are listed in Table 3. The value of R2 can reflect the passive film stability of the SLM 15-5PH stainless steel. The R2 of ST samples was maximum (about 2.125 × 105 Ω·cm2) compared to 1.568 × 105 Ω·cm2 for the ST + AT samples for 10 h. As the aging time increased, the R2 and passive film stability decreased, which was owing to the coarsening of Cu-rich precipitates. When the sample was solution treated at 1050 °C and then aged at 500 °C, the passive film stability obviously decreased, which was attributed to the disappearance of the austenite [51]. At the same time, the passive film thickness has been calculated by Eq. (3). It showed that the passive film thickness was about 2∼2.5 nm, which was in good accordance with the literature [28]. The composition of the surface passive film of SLM 15-5PH stainless steel after different heat treatment is shown in Fig. 10. The passive film mainly contained Fe2O3 (708.6 eV), FeOOH (709.83 eV), FeO (707.4 eV), Cr2O3 (575.6 eV), and Cr(OH)3 (577.01 eV) (Table 4). The passive film of the SLM 15-5PH stainless steel did not contain CrO3. There was very little Ni and Nb in the passivation film after the different heat treatments. When the sample was aged at 500 °C for 10 h, the passive film contained 34.83 % FeOOH, which was less than that of the ST samples. The presence of FeOOH tended to form the dense oxide film, which improved the corrosion resistance and passivation film stability of SLM 15-5PH stainless steel. Simultaneously, the ratio of Fe/ Cr was 3.296, which was more than that of the ST samples. When the samples were treated by ST + AT, the passive film contained 43.49 % Fe2O3 without FeOOH, and the ratio of Fe/Cr reached a maximum of approximately 6.3847. The decreased Cr content is mainly related to the oxidative dissolution of Cr in the passive film. The development of

43.49 % 56.51 54.29 22.88 22.83 6.38

0

d

(2)

where ZF is the impedance of the steel electrode and Rs is the electrolyte resistance. In the case of dielectric systems, the complex capacitance representation (Cole-Cole format) has been proved suitable for the determination of the capacitance of the film [49,50]. The results presented in Fig. 8(c) and (d) shows an example of the complex-capacitance plot obtained from the impedance response. From the high frequency limit of the impedance, C∞, the dielectric capacitance of the passive film formed at the electrode surface can be determined. The passive film thickness, dδ, was afterwards calculated from the usual 8

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Fig. 11. The EDS and SKPFM analysis of SLM 15-5PH stainless steel: (a) microstructure of AT samples; (b) local area of (a); (c) the SKPFM result of (b); (d) the surface potential of line.

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reversed austenite at the interface [2,27]. At the same time, there were many dislocations and defects during SLM, which promoted the elemental diffusion and led to the accumulation of austenite-forming alloying elements, such as Ni at the interface, as shown in Fig. 11. Therefore, austenite was distributed along the boundaries in the molten pool [8,61]. On the other hand, a solid-state phase transformation occurred during SLM of the 15-5PH martensitic steel. The solid-state phase transformation caused tensile residual stress to decreasing and an increasing in the compressive residual stresses, which hindered the phase transformation process of martensite and thus an increased amount of austenite was presented at the interface (Fig. 2a) [62]. For the samples with solution treatment at 1050 °C for 0.5 h and aging treating at 500 °C, as the temperature gradually increased to 1050 °C, martensite gradually transformed into austenite at a high temperature. At the same time, the molten pool structure disappeared, which was due to the rapid diffusion of alloy elements. With the cooling from 1050 °C to 100 °C, obvious martensite laths gradually formed, and prior austenite gradually disappeared. During the aging treatment process, there were almost none reversed austenite formed at the martensite lath at 500 °C due to the little nickel of the 15-5PH stainless steel. Some studies showed that the austenite can significantly improve pitting corrosion resistance of martensitic steel, which was also discussed in the previous study [51,60]. Therefore, the corrosion resistance of SLM 15-5PH steel after ST + AT was significantly reduced. Based on all experimental results above, the better corrosion behavior and passive film stability after aging treatment were attributed to the microstructure, especially the formation of austenite.

Fig. 12. Correlation between the heat treatment and microstructure after different heat treatments.

the chromium oxide can form the dense barrier that blocks the contact of sensitive ions with the substrate, ultimately improving the corrosion resistance of the samples [52,53]. In summary, the presence of austenite can improve the pitting corrosion resistance and passive film stability of SLM 15-5PH stainless steel. Fig.11 displayed the alloying element distribution and surface potential of the molten pool zone by EDS and SKPFM. The results showed that austenite formed at the bottom of the molten pool, which was rich in nickel. Marcus et al. [54] showed that nickel enriched at the bottom of the passivation film, which ultimately affects the crystallinity of the passivation film and the content of Cr2O3 in the inner layer of the passivation film. The SKPFM result showed that there was an improvement of 15 mV at the surface potential of the austenite comparing with the martensite. Therefore, the corrosion resistance of SLM 15-5PH stainless steel after aging treatment was relatively better, which was attributed to the presence of austenite [55,56].

5. Conclusions In this paper, we proposed a reasonable heat treatment process for SLM 15-5PH stainless steel to optimize the microstructure and improve the corrosion resistance. The following conclusions could be drawn: (1) The nano-sized Cu-rich precipitates gradually precipitated and coarsened to 2 nm after the aging treatment process. The content of austenite was about 25 %, which distributed at the bottom of the molten pool/ boundary of martensite laths. (2) After solution treatment + aging treatment for 1−10 h, the austenite phase and molten pool line disappeared. The martensite laths were markedly coarsened, and new NbC-(Mn, Si)O duplex particles (approximately 50−80 nm) precipitated. (3) The surface potential of austenite was 15 mV higher than that of martensite by SKPFM. So that the samples after aging treatment obtained a higher pitting potential and more stable passive film than the other samples after solution treatment + aging treatment.

4. Discussion In conclusion, nanoscale particles able to precipitate in the martensite matrix during the aging treatment in the field of high-strength martensitic steel, thereby laeding to precipitation enhancement [57,58]. Therefore, the SLM 15-5PH stainless steel needs aging treatment to improve the strength. Relevant literature showed that there were significant differences in microstructure, mechanical properties and corrosion behaviour after different heat treatment processes [27,59]. The microstructure of SLM 15-5PH stainless steel consisted of martensite, austenite, and nanoscale Cu-rich particles after different heat treatments. The interrelation between microstructure and heat treatment is summarized in Fig. 12. When the samples were aged at 500 °C for 10 h, the bulk/thin austenite still existed at the bottom of the molten pool/ boundary of the martensite lath. The SKPFM result (Fig. 11) displayed that the surface potential of the austenite distributed at the bottom of the molten pool was 15 mV higher than that of the martensite matrix. As indicated by the potentiodynamic polarization curves and EIS results, the austenite led to an increase on the corrosion resistance of martensite steel, which was consistent with the beneficial effects of austenite on corrosion resistance in the literature [51,60]. Several reasons for the presence of the austenite on the microstructure were concluded as described below. On the one hand, the multi-pass heat treatment processes occurred during the multi-track multi-layer selective laser melted process for 155PH stainless steel [2]. Due to the poor phase stability at the interface of the molten pool line, multi-pass heat treatment played a significant role in solid-state phase transformation, which led to the formation of

Author statement We deeply appreciate your consideration of our manuscript. No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously. CRediT authorship contribution statement Li Wang: Conceptualization, Methodology, Writing - original draft. Chaofang Dong: Validation, Supervision. Cheng Man: Data curation, Writing - review & editing. Decheng Kong: Software, Investigation. Kui Xiao: Visualization, Data curation. Xiaogang Li: Project administration. Declaration of Competing Interest All the Authors (Li Wang, Chaofang Dong, Cheng Man, Decheng Kong, Kui Xiao, Xiaogang Li) declare no Competing Financial or NonFinancial Interests. 10

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Acknowledgments [24]

This work was supported by the National Key Research and Development Program of China (No. 2017YFB 0702300), National Natural Science Foundation of China (No. 51871028) and the Fundamental Research Funds for the Central Universities (No. FRF-TP18-002B2).

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