Additive manufacturing (AM) of alloys such as stainless steels has the potential to be a disruptive technology for design of complex structures which eliminates the potentially damaging effects of mechanical failure and localized corrosion associated with interfaces between separate parts (which can now be printed as part of a single assembly). In addition, increasingly complex geometries can be manufactured with nearly as much ease as simple ones, and new mechanisms for optimization and generative design can allow manufacturers to achieve significant material and weight savings. However, challenges remain, in particular due to the potential for material variability both between parts made with the same nominal build parameters and even within a single build, as the consistency in properties inherent in the commercial-scale forging of alloys to be machined into components is exchanged for the considerable advantages of on-site, distributed custom parts production. Our studies and related work by other groups indicates that this is clearly true in the case of corrosion susceptibility in AM alloys. By studying the relationship between build parameters and electrochemical properties, we propose that it will be possible to tailor alloys for enhanced corrosion properties.AM processes produce materials with hierarchical microstructures containing fusion boundaries at the macroscale, irregular grains and grain boundaries at the microscale, and subgrain dislocation structures at the nanoscale. In laser powder bed fusion (LPBF) formed 316L stainless steel structures, corrosion performance has been discussed in the context of chemical heterogeneities formed in the presence of these hierarchical microstructures. Furthermore, localized corrosion (pitting) is closely related to microstructural defects including dislocation structures, inclusions, and localized elemental segregation which has been found to occur during the very rapid heating and cooling which occurs during laser fusion AM processes.In this presentation, we explore multi-modal synchrotron X-ray techniques for quantifying hierarchical microstructures and their connection to the underlying chemical distribution in 316L stainless steel, as well as localized heterogeneities in the passive film structure. Our results show that the dislocation density depends on the printing conditions with implications for the chemical distribution at the nanoscale, which in turn may play a key role in inconsistent corrosion behavior. Furthermore, we discuss the correlation between this print-formed alloy surface microstructure (dislocation structures, Mn inclusions, and elemental segregation), and local heterogeneities in the passive film formed on LPDF 316L in Cl-containing media. These heterogeneities observed in the passive layer may in turn impact repassivation and other processes critical in localized cession phenomena.Samples formed at varying speeds using pulsed and continuous laser deposition are compared via cyclic polarization in 3.5% NaCl and 0.1M HCl solutions, as well as using Mott-Schottky analysis of passive layers formed in 0.1M HCl. Surface corrosion layers are characterized using laboratory-based X-ray photoelectron spectroscopy to determine the impact on passivity of the aforementioned microsegregation associated with process-induced microstructures. Corrosion rates and pit densities are then discussed to build a connection between printing conditions and corrosion performance vis-à-vis microstructural and chemical information from synchrotron measurements performed at Brookhaven National Laboratory (BNL), including high energy X-ray diffraction using the X-ray Powder Diffraction (XPD) beamline at the National Synchrotron Light Source-II (NSLS-II) which provided data on minor precipitate population(s), atomic structure and dislocation microstructures critical to corrosion behavior. In addition, heterogeneities in elemental composition data is provided by 2D X-ray fluorescence (XRF) and 2D X-ray Absorption Spectroscopy (XAS) nanometer resolution-mapping obtained at the Hard X-ray Nanoprobe (HXN) beamline at the NSLS-II and correlated with process-induced hierarchal structures via use of the microscopy facilities at the Center for Functional Nanomaterials (CFN). X-ray photoemission microspectroscopy/low energy electron microscopy (XPEEM/LEEM) conducted at the Electron Spectro-Microscopy beamline at the NSLS-II provides evidence that the passive layer itself contains inhomogeneities, including areas enriched in oxidized Cr and Mn, as well as microstructural boundaries, reflecting the inhomogeneities in the microstructure of the alloy surface beneath. Mott-Schottky analysis of the passive layer provides further evidence that these inhomogeneities, correlated with alloy microstructure, impact the semiconducting character (and hence protective properties) of the passive layer.By employing a combination of multi-modal synchrotron characterization techniques, electron microscopy, and baseline testing protocol combined with electrochemical polarization experiments, we are able to study the role of microstructure across multiple length scales on electrochemical properties and corrosion performance. Understanding the impact of microstructure and chemical heterogeneities on the susceptibility to pitting and intergranular attack will enable microstructurally-informed process optimization and materials design for enhancing the corrosion resistance of LPBF 316L stainless steel, with implications for AM and subsequent durability of alloys in general.
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Jul 7, 2022
Ahmed A Darwish + 3
Open Access