**4. Conclusions**

This study has shown that UNS S32707 HDSS prepared by the SLM process can obtain a nearly balanced two-phase structure of ferrite and austenite through the appropriate solution annealing process, which can significantly improve the ductility and toughness. The nitrogen content reduced by 33% in the SLM process, resulting in the imbalance of the two phases and the reduction of the austenite content. The samples with solution treated at 1150 ◦C for one hour had better comprehensive mechanical properties, the measured tensile strength, the yield strength, the elongation, the section shrinkage, the microhardness, and the impact absorption energy were 901 MPa, 658 MPa, 36.4%, 48.4%, 291.5 HV, and 132 J, respectively.

The samples with solution treated at 1100◦C for one hour had better pitting resistance. The pitting potential was 1196 mV. The surface layer of passive film is mainly composed of oxides of Fe and Cr and hydroxide compounds. NH4+ adsorbed on the surface of passive film and protected the passive film. The inner layer of the passive film is mainly composed of metal elements, Cr2N and oxides of Fe, Cr, and Mo.

**Author Contributions:** F.S. is the first author and analyzed the data and wrote the paper. The experiments were performed by X.C., Z.W., Z.J., and F.M., S.R. and X.Q. conceived, designed, and supervised the experiments. In addition, they contributed to the interpretation of data and editing the paper.

**Funding:** This study was financially supported by the National Science and Technology Support Program of the Ministry of Science and Technology of China (No.2015BAE03B00), National Natural Science Foundation of China (No.51874038), Fundamental Research Funds for the Central Universities (FRF-AT-18-014), the Natural Science Foundation of Huaihai Institute of Technology (Z2017001), Lianyungang 521 Project (ZKK201805) and Lianyungang Haiyan Project (2018-QD-013).

**Acknowledgments:** We thank Jiasheng Dong and research fellow Langhong Lou from Shenyang Zhongke Sannai New Materials Co., Ltd. for his help in alloy melting. We thank Shujin Liang from Sino-Euro Materials Technologies of Xi'an Co., Ltd. for his help in PREP. We thank Xiaoming Zhang and Xuehao Zheng from ZKKF (Beijing) Science and Technology Co., Ltd. for TEM and EBSD observations.

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