Early Results from a Broad Compatibility Study of Various Materials with Ionic Silver Biocide
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Ionic silver is baselined for microbial control in spacecraft potable water systems for future exploration missions, but materials compatibility analysis is required to evaluate the passive depletion of ionic silver concentration onto wetted material surfaces over time. Various articles concerning such testing have been published that examine interactions with water containing ionic silver biocide, but most tests have focused on only a couple of materials each, and comparing results of different evaluations to one another has proved challenging. This paper reports the first results from static exposure testing of a large array of material coupons to a 400 parts per billion (ppb) aqueous silver fluoride (AgF) solution, using a surface to volume ratio of approximately 2/cm. The test is designed in two main stages. Stage 1 is a one-week screening solely for silver uptake. Materials that perform modestly to well after that week are promoted to Stage 2, which is a longer test with periodic sampling to examine the silver uptake rates over time; these samples are evaluated for other water quality parameters in addition to the remaining silver concentration. In a tangential investigation, select materials that take up some silver in Stage 1 may be “aged” by repeating the Stage 1 test to determine whether repeated exposure reduces silver uptake rate, and successfully aged materials may then continue to Stage 2 testing. The materials under test include metallic, and polymeric materials with various surface finishes, treatments, and coatings, as well as select other materials historically used in spacecraft water systems. This test began in August 2019, and thus only includes early results; future follow-on papers will include additional results as the test progresses. The ultimate goal builds a broad, easily comparable data set that can be used to guide material selections for silver biocide-compatible spacecraft water system design.