Oxidation Behavior of Additively Manufactured 316L in a Supercritical CO2 Environment

Abstract

The general properties of additively manufactured (AM) metals have been fairly well characterized to this point with respect to mechanical behavior and even general corrosion response. However, as the fields of application of AM metals broadens, the need for unique/specialized evaluation is required. For instance, there remains a lack in understanding how AM metals behave under atmospheric corrosion exposure conditions where local heterogeneous environments govern the surface reactivity and susceptibility. Similarly, knowledge gaps exist in specialized environment, for example supercritical CO2 (sCO2). Materials and environment parameters such as composition, pressure, and temperature will influence corrosion susceptibility and/or oxidation properties. As AM metals often display vastly different microstructures from their wrought counterparts due to non-equilibrium cooling during processing, an understanding of their behavior in these environments is critical prior to possible application. Additionally, typical engineering design of oxy-combustion gas turbine systems used in these environments, results in atypical conditions, usually localized, that are well known to affect environmental performance of materials (such as crevices, welds, residual stresses, or galvanic coupling).Austenitic stainless steel, type 316L, specimens made by selectively laser melting (SLM) were exposed with their traditional wrought counterparts for comparison at 450°C and 7.6 MPa in pure sCO2 for one week. SLM samples were exposed in the as printed (a contour scan strategy was used) and the ground (ground to 1200 grit) state and compared to wrought samples ground to 1200 grit. Prior to exposure, samples were partially coated with a zirconia film to establish an initial true height to aid ellipsometry. Post exposure, oxide film thickness was measured with respect to the non-reactive coating height. Preliminary analysis of oxide film thickness measurements performed using ellipsometry showed a thinner oxide film grown on the in-process polished SLM samples (0.66 - 0.79 µm) versus the ground SLM (1.1 µm) or wrought samples (0.8 – 1.22 µm). Additional analysis was performed through scanning electron microscopy and focused ion beam combined with transmission electron microscopy to characterize the oxides formed and elucidate possible microstructural and/or morphological dependence of the film growth.SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. SAND2020-4568 A