Corrosion Response of AM350 to Irradiated CFC-11 in the Heat Redistribution System (HRS) for Europa Clipper

Europa Clipper is a NASA-JPL mission to create a radiation tolerant spacecraft that will perform a detailed reconnaissance of Jupiter’s moon, Europa. As part of an initiative to investigate "icy worlds" to find locations in the galaxy that have suitable conditions for life, spacecraft with significant radiation resistance must be designed to withstand the harsh Jovian environment, and therefore require rigorous testing of traditional spacecraft materials under new radiation extremes. Among the critical systems on Europa Clipper, the Heat Redistribution System (HRS) is comprised of multiple pumps, fluid movement devices, and mechanisms to move heat to and from components using a trichlorofluoromethane (CFC11) working fluid. From a materials standpoint, a primary concern is the potential for CFC-11 radiolysis bi-products and adventitious water to exacerbate the corrosion of ferrous alloys. In particular, within the HRS, the bellows accumulator stores and flows the fluid to parts of the spacecraft that require heat transfer, and the internal edge-welded bellows are constructed from AM350 stainless steel. Because this ferrous alloy was selected based on heritage designs from non-Jovian environments at relatively low radiation conditions, the effects of CFC-11 radiolysis products on long-term material stability were unknown.In this work, we aimed to understand the impact of CFC-11 radiolysis bi-products and adventitious water on the corrosion of AM350 employed in the bellows accumulator in the HRS. Configuration-specific samples of the material were completely submerged in either irradiated or non-irradiated CFC-11; samples in irradiated CFC-11 were then further exposed to radiation levels commensurate to those observed on the Europa Clipper mission via a Co60 gamma radiation source, and compared directly with non-irradiated freon and freon-absent controls over specific time intervals (5 days, 4 weeks, 2 months, 4 months). Scanning electron microscopy (SEM), optical microscopy, confocal microscopy, metallurgical analysis of cross-sections, deionized (DI) water rinse analysis for metals and halides, and water content measurements were used to characterize the extent of corrosion of irradiated samples at each time period. Notably, relative to non-irradiated freon and freon-absent controls, AM350 samples clearly exhibited surface-level morphological features (i.e., pitting) in irradiated freon consistent with corrosion mechanisms. Deionized (DI) water rinse analysis for metals and halides corroborated microscopic observation of pitting on AM350 samples, showing an increased iron and chloride content with radiation exposure. If it is deemed necessary to mitigate corrosion of AM350 in the accumulator bellows, future work aims to test a possible getter system in the HRS to facilitate the removal of water and chloride from irradiated CFC-11, and ultimately improve HRS designs for extreme radiation environments for both fly-by and landed missions to Europa (Europa Clipper, Europa Lander), Enceladus (Enceladus Orbilander), and beyond.