Selective laser melting (SLM) is a commonly used technology for the additive manufacturing (AM) of metal material. It uses a high-energy laser beam to melt the metal powder layer-by-layer and finally prints the desired parts. During the SLM process, the printed part on the top plane (the XOY plane in Fig. 2 in the vertical printing direction) and the printed part on the side plane (the YOZ plane in Fig. 2 in the parallel printing direction) have different heating histories and temperature gradients. Therefore, the two planes have significantly different microstructures. This anisotropy in the microstructure is bound to introduce anisotropy to the performance. Recently, several studies have been conducted on the effect of microstructural anisotropy on mechanical properties. The unified conclusion is that printed samples have better mechanical properties in the vertical printing direction than in the parallel printing direction. However, few studies have been conducted on the effect of microstructural anisotropy on the corrosion behavior of printed parts, and their conclusions are different. Therefore, it is necessary to further investigate this issue. The aim of this study is to investigate the corrosion behaviors in different directions (the XOY and YOZ planes) in 316L stainless steel (SS) prepared using SLM through electrochemical measurements and propose internal causes of these corrosion behaviors, which have not yet been described.

The 316L SS parts are first prepared using SLM. To obtain samples in different directions, including the XOY and YOZ planes, samples are cut according to the diagram shown in Fig. 3. In this study, the forged 316L SS is used as the counterpart after solution treatment. The body and surface density of 316L SS are measured using the Archimedes drainage and metallographic methods, respectively. The microstructures of the SLMed sample on XOY and YOZ planes are characterized by electron backscattered diffraction (EBSD) and a scanning electron microscope (SEM). The phase structures of all samples are measured by X-ray diffractometry (XRD). The corrosion behaviors are explored by measuring the open-circuit potential (OCP), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). In addition, the internal causes of this effect can be explained by the potentiostatic polarization and characterization of the surface topographies of all parts after corrosion.

The results show that the body density of 316L SS prepared using SLM is 99.38%, which is close to that of its forged counterpart (99.7%). The surface densities of the SLMed sample on XOY and YOZ planes are 99.7% and 99.87%, respectively, indicating that the surface densities in the different directions are almost similar. The XRD results confirm that the additive manufacturing technology does not change the phase structure of the 316L SS (Fig. 5). However, a clear discrepancy is evident in the grain orientation for both planes from the EBSD tests (Fig. 7). On the XOY plane, more (101)-oriented grains are observed, whereas on the YOZ plane, more (111)-oriented grains are observed. According to the literature [26], (111)-oriented grains are more resistant to corrosion. The grain sizes in both planes differ slightly according to the EBSD test results (Fig. 8). The average grain size of the SLMed sample on the YOZ plane (9.51 μm) is slightly larger than that of the SLMed sample on the XOY plane (7.35 μm). However, the grain sizes of SLMed sample on XOY and YOZ planes are significantly smaller than that of the forged counterpart (50-100 μm). The results from the electrochemical tests show that the corrosion resistance of the SLMed sample on the XOY plane is better than that of the SLMed sample on the YOZ plane, and the SLMed sample on both planes are superior to the forged counterpart, as confirmed by the OCP measurements (Fig. 9), potentiodynamic polarization curves (Fig. 10), and EIS measurements (Fig. 11). The improved corrosion resistance of the SLMed sample on the XOY plane is attributed to the fewer (111)-oriented grains on the XOY plane, and consequently, to the more compact passive film formed on the XOY plane based on the results of potentiostatic polarization measurements (Fig. 12). These conclusions are further confirmed by observing the SEM morphologies of the three corroded samples. The sizes of the inclusions on the XOY and YOZ planes of the printed samples are much smaller than those of the forged part (Fig. 13). In addition, the inclusion on the XOY plane remains closely combined with the matrix after corrosion, demonstrating outstanding corrosion resistance. However, for both the SLMed sample on the YOZ plane and its forged counterpart, the case worsens. A clear corrosion gap is present around the inclusions after corrosion, particularly for the forged counterpart, indicating poorer corrosion resistance.

First, compact 316L SS samples are produced using SLM. Their densities are 99.38%, which are considerably close to that of the forged parts (99.7%). There is a notable difference in the microstructure between the XOY and YOZ planes in the printed part. On the XOY plane, more (101)-oriented grains are observed. However, on the YOZ plane, more (111)-oriented grains are observed. This microstructural anisotropy has a significant effect on the corrosion behavior of 316L SS printed using SLM. The corrosion resistance of the SLMed sample on the XOY plane is better than that on the YOZ plane, and the SLMed samples on both planes are superior to their forged counterparts. The better corrosion resistance of the SLMed sample on the XOY plane results from fewer (111)-oriented grains, leading to more compact passive films formed on the surface. The SEM morphologies of inclusions in the three corroded samples show that the size of the inclusions on the XOY plane is smaller than that on the YOZ plane and that of forged counterpart. In addition, the corrosion gap between the inclusion and matrix on the XOY plane is far less than that on the YOZ plane and that of forged counterpart, indicating better corrosion resistance.