**2. Materials and Methods**

The PREP process of UNS S32707 HDSS powder, the SLM parameters, and the samples size can be found in Shang et al. [15]. The chemical composition of the powder is shown in Table 1, and the median diameter of the prepared metal powder was 45 μm. The samples were water-quenched using an atmosphere-protected quenching furnace (OTF-1500X-80-VTQ, Hefei Kejing Materials Technology Co., Ltd., Hefei, China) following solution annealing at 1050, 1100, 1150, and 1200 ◦C, respectively. The sample numbers of di fferent processing technologies are shown in Table 2.

**Table 1.** Chemical composition of UNS S32707 hyper-duplex stainless steel (HDSS) powder.



**Table 2.** The sample numbers of di fferent processing technologies.

The mechanical properties were measured by a WDW-10E microcomputer controlled electronic universal testing machine. The tensile specimens were built with a nominal gauge section of 25 mm length × 5 mm width × 2 mm thickness. Rectangular samples with the size of 55 × 10 × 10 mm were machined into V-notches for the impact test. The microhardness of the samples was measured by an HV-30 hardness tester. The nitrogen content in the powder and selective laser melted parts was measured by a LECO ONH 836 Oxygen Nitrogen Hydrogen Analyzer (LECO Corporation, St. Joseph, MI, USA).

The microstructure of the selective laser melted sample was observed by a Quanta 450 FEG scanning electron microscope (SEM) (FEI Company, Hillsboro, OR, USA). Prior to the SEM observations, the samples were electro etched in 10% oxalic acid at an operating voltage of 7 V for 25 s. An HKL Channel5 electron backscatter di ffraction (EBSD) analysis system (Oxford Instruments, Oxford, UK) equipped in an FEI Quanta 650F field emission scanning electron microscope (SEM) (FEI Company, Hillsboro, OR, USA) was used to obtain inverse pole figure mappings and phase mappings with a step size of 0.3 μm and 0.5 μm at an acceleration voltage of 20 kV with a working distance of 13 mm. The EBSD samples were polished by Leica EMRES 102 argon ion polishing instrument (Leica Microsystems, Wetzlar, Germany). The samples were first polished at a voltage of 5 kV for 40 min, and then polished at a voltage of 4.5 kV for 50 min. The angle of the ion gun was 15 degrees (2 ion guns). Finally, the voltage was reduced to 3.5 kV, and the polishing time was 60 min at low voltage with the same angle of the ion gun. The microstructure was observed by an FEI Tecnai G2 F20 transmission electron microscope (TEM) (FEI Company, Hillsboro, OR, USA). TEM samples were first mechanically polished to a thickness of about 50 μm, followed by ion-beam milling in a Gatan 691 precision ion polishing system (PIPS) at 5 kV with a final polishing step at 1 kV of ion energies. An FEI Super-energy dispersive spectrometer (EDS) system equipped in an FEI Titan Themis spherical aberration electron microscope (FEI Company, Hillsboro, OR, USA) was used for a high-resolution energy surface scan test. The effect of nitrogen content on the austenite content was simulated by JMatPro software (JMatPro 7.0, Sente Software Ltd., Guildford, UK). The simulated solution annealing temperature was 1150 ◦C.

A Gill AC Bi-STAT electrochemical workstation was used to measure the potentiodynamic polarization curve at room temperature in 3.5% NaCl solution. The classical three-electrode system is as follows: Saturated calomel electrode as the reference electrode, platinum electrode as the auxiliary electrode, and sample as the working electrode. The tests were recorded at a scan rate of 20 mV/min and ranged between −150 and +2000 mV. Prior to the corrosion testing, the samples were electro polished in 10% oxalic acid at an operating voltage of 12 V for 25 s. The composition of the passive film was analyzed by a Thermo Scientific ESCALAB *250Xi* X-ray photoelectron spectroscopy (XPS) (Thermo Fisher Scientific Inc., Waltham, MA, USA), the source of X-ray excitation was Al *K*<sup>α</sup>, and the vacuum of analysis chamber was 10−<sup>9</sup> mbar. The high resolution spectrum of XPS was resolved by PeakFit software (PeakFit 4.12, Seasolve Software Inc., Framingham, MA, USA).

## **3. Results and Discussion**
