This communication describes observations of unexpected microstructural interface susceptibility to accelerated dissolution in additively manufactured (AM) Type 316L stainless steel prepared by selective laser melting. Observations include accelerated microstructural interface dissolution in the as-built condition, as well as more rapid sensitization of grain boundaries upon exposure to elevated temperature. Electrolytic etching in persulfate solution was used to evaluate the susceptibility of microstructural interfaces to accelerated dissolution in both wrought and AM 316L. Post-test optical microscopy and profilometry on AM 316L revealed that the melt pool boundaries in the as-built condition were susceptible to accelerated attack, although the small grains within the prior melt pools were not. Furthermore, short, elevated temperature exposure (1 h at 675°C) also induced sensitization of the grain boundaries. Identical testing on as-manufactured wrought 316L confirmed that no microstructural interfaces showed susceptibility to accelerated dissolution, and grain boundaries could be sensitized only by extended periods (24 h) at elevated temperature (675°C). Annealing was capable of removing sensitization in wrought 316L, but activated the surface of the AM 316L, leading to widespread, uniform dissolution.
Recent developments in the 3-D printing of austenitic stainless steels have led to the need for standardization of electrochemical techniques used to assess the corrosion performance of these alloys. Currently, ASTM standards for austenitic stainless focus on assessing their resistance to different modes of corrosion such as pitting, crevice, and intergranular corrosion. Due to the complexity of the additive process, selective corrosion occurs in microstructural features such as cellular structures and melt pool boundaries. Standardized corrosion testing needs to incorporate these microstructural features. This study characterizes the corrosion behavior of LPBF stainless steel in a variety of ASTM standards with special attention to melt pool boundary dissolution, cellular structures, and intergranular corrosion.
The recent growth in additive manufacturing (AM) has fueled the motivation to understand the properties and performance of conventional alloys that have been synthesized through these methods. Laser powder bed fusion (LPBF) is a form of AM that uses high energy lasers to construct bulk materials from the melting and rapid solidification of powder on a layer-by-layer basis. While there has been considerable work done to optimize LPBF processing parameters for various metallic alloys and characterize the resulting microstructure and mechanical properties, little work has been performed to characterize the corrosion properties of these 3-D printed alloys. 316L is an austenitic stainless steel that is utilized in various applications due to its excellent mechanical properties and corrosion resistance. Although wrought 316L has a signature microstructure that is nominally single phase and made up of equiaxed grains, the additive process introduces a non-equilibrium microstructure. This leads to the introduction of compositional heterogeneities via melt pool boundaries and cellular structures and provide an origin for selective corrosion, leading to potential issues involving the longevity of these alloys in the as-printed state. Sensitization occurs in stainless steels through the intergranular precipitation of chromium carbides at grain boundaries when the material is exposed to temperature ranges of 450 – 800 °C. This leads to the depletion of chromium at regions adjacent to grain boundaries and may lead to intergranular corrosion and intergranular stress corrosion cracking. The utilization of post-processing methods such as hot isotactic pressing induces temperatures within this range and may initiate this solid-solid phase transformation. The observation of accelerated sensitization in LPBF 316L provides motivation to characterize and understand this sensitization behavior. This study explores corrosion in LPBF 316L with attention to melt pool boundary dissolution and intergranular corrosion. This study also acknowledges the effect of processing parameters on the resulting microstructure and corrosion behavior. Galvanostatic etching and potentiodynamic polarization are performed to quantify and understand the electrochemical behavior of these alloys. Characterization methods such as scanning electron microscopy and white light interferometry are used to probe the microstructure and corrosion morphology of these alloys in comparison to the conventionally wrought counterpart.
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