Triple structuration and enhanced corrosion performance of 316L in laser powder bed fusion

This work focuses on the influence of the feedstock powders on the microstructural properties and corrosion behavior of 316L stainless steel (SS) produced by laser powder bed fusion (L-PBF) from two different suppliers. Microstructural investigations conducted after additive manufacturing reveal many particularities depending on the powders. The first one exhibits a typical microstructure of materials obtained by L-PBF with a heterogeneous and hierarchical structure including the presence of columnar grains, precipitates of Mn and Si oxides, solidification cells (segregation in Cr, Mo and Ni), dislocations. However, the second one presents an atypical microstructure with low columnar grains growth, Mn and Si oxide precipitates which are Cr enriched and the existence of an additional small cell structure, which is organized inside the known large solidification cells. These microstructural differences lead to a different corrosion behavior in boiling nitric acid containing oxidizing ions. The presence of the triple structuration (grains, large cells, small internal cells) shows a positive effect on intergranular and cellular corrosion resistance. The existence of the smaller internal cellular sub-network allows a homogenization of the material composition and a small-scale equilibrium of the anodic and cathodic surfaces favorable to the cellular corrosion resistance. In both cases, L-PBF 316L SS provides better resistance to intergranular corrosion than wrought 316L SS which undergoes fast and severe grain loss over time.