Corrosion of Iron Stainless Steels in Molten Nitrate Salt

Abstract

Energy storage for concentrating solar power (CSP) is a major area of research that seeks to lower the levelized cost of electricity within the aggressive SunShot goals of 6¢/kW-hrth[1–3]. One viable approach is sensible thermal energy storage (TES), which currently utilizes molten nitrate binary salt, stored at 575°C in the hot tank of a two tank system [4,5]. Increasing the temperature limit within the hot tank requires a detailed understanding of materials corrosion behavior, in addition to salt thermal stability properties.High temperature nickel based alloys are the logical choice for strength and corrosion resistance as elevated temperatures will increase corrosion kinetics, however, the cost of nickel based alloys are nearly four times more expensive than iron based steels [6]. For this reason iron based stainless steels, specifically 321SS and 347SS (nominally Fe-17Cr-9Ni), were chosen for investigation at several temperatures in nitrate salt. 316SS, an elementally similar alloy, was susceptible to stress corrosion cracking while tested at Solar Two [4]. It was suggested that alloys with stabilizing additions of niobium (347SS) or titanium (321SS) would mitigate this deleterious behavior.Flat coupon samples were immersed in binary nitrate salts at temperatures of 400, 500, 600, and 680°C, with air sparging on all tests. Samples were nominally removed at intervals of 500, 1000, 2000, and 3000hours to acquire data on time varying weight gain information while simultaneously employing metallography to identify corrosion mechanisms occurring within the melt.Corrosion rates varied dramatically with temperature according to an Arrhenius-type behavior. 347SS and 321SS had very little oxidation for 400 and 500°C, indicative of a protective corrosion scale and low corrosion kinetics. Data at 600°C showed that 321SS tended toward linear oxidation behavior based on oxide spallation which was observed on the samples upon removal.Corrosion products at 500°C had phases of iron oxide, with obvious chromium depletion as observed in energy dispersive spectroscopy (EDS) scans. 600°C corrosion layers were primarily iron oxide with obvious phases of sodium ferrite on the outer surface. 680°C marked an excessive rate of corrosion with metal loss in both alloys.