Effect of solution treatment on intergranular corrosion behaviour of 316L stainless steel fabricated by selective laser melting

Selective laser melting (SLM) has been widely concerned, due to its high machining precision and broad application scope. 316L stainless steel, which has excellent mechanical properties and corrosion resistance, is one of the early birds in the SLM fields, and it always faces the threat of intergranular corrosion. Additionally, for the 316L stainless steel fabricated by selective laser melting (SLM-316L stainless steel), the solution treatment is one of the post-treatment methods for the optimization of the microstructure and reduction of the residual stress. It has been confirmed that the as-received SLM-316L stainless steel exhibits superior intergranular corrosion resistance than the conventional forged 316L stainless steel, and its intergranular corrosion is mainly caused by the σ phase. It could be speculated that the solution treatment could change the intergranular corrosion behavior of SLM-316L stainless steel, because the intergranular corrosion of austenite stainless steel is one of the properties which are closely dependent on their microstructure. However, little attention is focused on the intergranular corrosion of SLM-316L stainless steel with solution treatments up to now. Based on this, the influence of the solution treatment on the microstructure and intergranular corrosion of the SLM-316L stainless steel. Firstly, the SLM-316L stainless steels were kept at 1150 oC for 0.5 h, 2.0 h, and 5.0 h, respectively. Next, their microstructures and nanoscale oxide particles were detailedly researched using SEM, TEM, EBSD, and EDS. Then the intergranular corrosion was examined through the electrochemical corrosion tests in 10% (NH4)2S2O8 solution, and the depths of the grooves at the grain boundaries were measured with the help of CLSM. The microstructure analysis reveals that the recrystallization takes place on the SLM-316L stainless steel when treated at 1150 oC, where the irregular equiaxed grain is replaced by the regular equiaxed grain and the annealing twins, and the grain size becomes larger with the holding time increasing from 0.5 h to 5.0 h. The oxide particles in the solution-treated SLM-316L stainless steel are a dozen or even dozens of times larger than these in the as-received sample, and the maximum size could reach the micro-scale, especially at the grain boundaries. Meanwhile, the atomic structure of the oxide particle also changes from the Rhombite MnSiO3 to Spinel MnCr2O4, and MnS could be found adjacent to oxides. The intergranular corrosion resistance of SLM-316L stainless steel is deteriorated by the solution treatment, and the intergranular corrosion develop from step-like corrosion to groove-like corrosion. The decrease in the intergranular corrosion resistance should owed to the removal of the inhibiting effect by the cellar structure and element enrichment on M23C6 and Cr-depleted region, and the enhancing activity of grain boundaries by the coarsening of oxide particles.