Role of headspace environment for phase change carbonates on the corrosion of stainless steel 316L: High temperature thermal storage cycling in concentrated solar power plants

The mechanisms leading to corrosion in stainless steel containments for thermal energy storage through phase change materials, such as carbonates and chlorides, are crucial for understanding the degradation of these steel alloys. A comprehensive study of this area will allow for down-selection of materials suitable for solar thermal energy storage (TES) operation at an elevated temperature range.Samples of stainless steel (SS) 316L were subjected to a cyclic heat environment of 550-750 degrees C in air and argon headspace environments, in the presence of a carbonate salt phase change material (PCM). A series of complementary microscopy, spectroscopy and diffractometry analytical techniques were applied to the corroded SS316L. Corrosion rate, interface formation and chemical products with respect to thermal cycling are presented with associated degradation mechanism explained and comparisons are made among different gas environments and varied immersion conditions.In the PCM under ambient air conditions, steel surfaces were mainly corroded by the penetration of oxidants from air, such as H2O or O2, along grain boundaries. SS316L samples demonstrated more severe degradation when exposed to air with a small fraction of PCM vapour present compared to when they were exposed to liquid or solid PCM. Cycling in Ar resulted in less corrosion in samples as opposed to when they were exposed to an air environment. In an Ar environment corrosion is driven via the formation of chromite, while the SS316L showed a less degradation when exposed to Ar with a small fraction of PCM vapour compared to when it was immersed in PCM. The mechanisms for degradation in air and in Ar are common in that the corrosion products of Cr and Ni dissolve in the PCM thus removing any layer that could protect against corrosion.The present study provides insight into corrosion of stainless-steel when exposed to carbonate salts, in air and inert gas environments, and contributes to down-selection of materials for solar thermal energy storage.