Issues of Localized Corrosion in Additively Manufactured 316L Stainless Steel

Additive manufacturing (AM) is a class of material fabrication methods that has exciting applications in the production of metal alloys such as austenitic stainless steels. In addition to opportunities in material and energy use reduction, the AM process offers opportunities in microstructure control and the refinement of mechanical properties such a strength, ductility, and toughness. AM processes such as laser powder bed fusion (LPBF) and direct energy deposition (DED) create materials with complex microstructures that tend to have spatial variations in composition. As works on AM corrosion are limited, there is a motivation to understand the electrochemical behavior of AM alloys such as 316L. This work is thus divided into three main parts: (1) a holistic assessment of corrosion phenomenology in AM 316L, (2) an investigation of the effect of LPBF processing parameters on the corrosion behavior, and (3) an investigation of intergranular corrosion and sensitization behavior of heat-treated LPBF 316L.

The goal of the first part of this work is to gain a holistic view of understanding electrochemical behavior in AM 316L with particular emphasis on localized/selective corrosion due to the non-equilibrium microstructure of AM alloys. A range of testing environments were utilized from boiling acidic solutions to chloride solutions that induce pitting corrosion. Whether it is through the DED or LPBF process, local differences in elemental composition were observed to be a driving force for preferential corrosion. Distinctions were also able to be established in comparing the phenomenology between active and transpassive dissolution. Overall, it is it shown how local chromium and molybdenum differences drive preferential corrosion whether it is in respect to cellular dendritic structures in the LPBF material or ferrite in the DED material.

While the first part of the work emphasizes the effect of oxidizing behavior on AM corrosion morphology, the second part explores the effect of LPBF processing parameters on susceptibility to surface reactivation. The volumetric energy density (VED) was used as a tool to explore variations in LPBF processing. The double loop potentiokinetic reactivation (DLEPR) testing was used to make this comparison. While it was found that material printed at lower VED favored global reactivation, it was ultimately found that the VED alone cannot be used as a tool in predicting susceptibility to global reactivation. It was ultimately found that conditions that promote rapid solidification lead to materials that have greater susceptibility to reactivation such as materials printed at higher speed at constant VED.

Unlike the first two foci of the dissertation which explore corrosion in as-printed AM material, the third portion focuses on a phenomenon that is relevant to stainless steels exposed to elevated temperatures: sensitization and intergranular corrosion. Like the previous chapter, it was found that VED alone is not adequate to predict susceptibility to sensitization and intergranular corrosion. The deleterious effect of porosity and grain refinement is also established from this study.

The work performed in these studies are of scientific and technological importance. Stainless steels like 316L have diverse applications such as aerospace, naval, and biomedical. This diversity is also reflected in the range of standardized testing environments that are used in screening these alloys. This situation ultimately reflects a need to explore the electrochemical behavior of these materials. In the area of LPBF processing, this work is relevant as complement to other work that highlight the effect of processing on mechanical properties providing potential to gain a more holistic understanding of in-service performances of these alloys. As there is great motivation to implement post-processing techniques that involve exposure at elevated temperatures, this work addresses a knowledge gap involving intergranular corrosion.