Effect of heat treatment on the corrosion resistance of 316L stainless steel manufactured by laser powder bed fusion

Understanding the influence mechanism of microstructures and inclusions during heat treatment on the corrosion resistance of L-PBF 316L stainless steel (SS) is crucial for steel quality control and subsequent industrial application. In this study, the evolution of microstructure, inclusions and passive film in the L-PBF 316L SS during heat treatment at the temperature of 1000 °C and 1200 °C for 2 h, including crystal characteristics, dislocation density, passivation film composition and so on, were characterized by electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The corrosion resistance of L-PBF 316L SS samples was evaluated by Tafel test and electrochemical impedance spectroscopy test. The corrosion mechanisms of L-PBF 316L SS before and after heat treatment were clarified to elucidate the intrinsic effect of microstructure and inclusions on the corrosion resistance of the steel. Results showed that the heat treatment conducted at 1200 °C effectively reduced the number of grain boundaries and induced a substantial number of Σ3 twin grain boundaries in the L-PBF 316L SS, thereby efficiently impeding the precipitation of detrimental phases and reducing corrosion susceptibility at the grain boundaries. Meanwhile, the recrystallization-induced rearrangement of dislocations and the homogenization of grains effectively facilitated the growth of passivation film, thereby increasing the corrosion resistance of HT1200 sample. Additionally, the increase of MoO content compensated for the detrimental impact on the stability of the passivation film resulting from the reduction in chromium oxide content. Transformation from the MnO–SiO–CrO inclusions in the as-built sample to the SiO inclusions in the HT1200 sample would also retard the penetration of corrosive ions into the steel matrix.