Investigation of Localized Corrosion Phenomena in Selective Laser Melted 316L Stainless Steel

Abstract

Selective laser melting (SLM) refers to the additive manufacturing of bulk material from powder on a layer by layer basis through melting via laser and rapid solidification. In recent years, additively manufactured metallic alloys produced by SLM have garnered attention due to opportunities in waste reduction, time and cost efficiency, and fabrication of geometrically complex structures. However, limited work has been performed to investigate the corrosion properties of these alloys. Understanding these mechanisms is crucial in assessing the attendant risks associated with the use of AM alloys in engineering design. Stainless steels such as 316L produced by SLM have presented advantages due to the corrosion resistant properties of these alloys. However, the non-equilibrium microstructure in these SLM materials, significantly different from that of the wrought counterpart, result in heterogeneities that may lead to localized corrosion such as melt pool boundary attack. A point of concern in stainless steels is intergranular corrosion due to the formation of secondary phases such as chromium carbides that deplete chromium from grain boundaries when the material is exposed to temperature ranges of 450 – 800 °C. While low-carbon steels such as 316L have great resistance to deleterious chromium carbide precipitation, this phenomenon has not been investigated in great detail in the case of selective laser melted 316L. However, it has been shown that precipitation of secondary phases is accelerated in SLM alloy systems such as Inconel 625 with respect to the wrought counterpart. Intergranular corrosion can ultimately lead to intergranular stress corrosion cracking, a form of brittle failure. This study investigates localized corrosion phenomena such as melt-pool boundary attack and intergranular corrosion and the associated microstructure-based mechanisms in the corrosion of these SLM 316L alloys. Galvanostatic etching, potentiodynamic scans through the double-loop potentiokinetic reactivation method, and mass-loss testing of wrought and SLM 316L are utilized to reveal the electrochemical behavior of these alloys. Additionally, focused ion beam and transmission electron microscopy are utilized to understand the microstructural origins of the observed corrosion phenomena.