Corrosion of iron–nickel–chromium alloys in high temperature carbonate salt under argon atmosphere

Next generation concentrated solar thermal power can produce receiver temperatures up to 800 °C, however current solar salt based thermal storage is limited to 560 °C. Eutectic salt mixtures with high melting temperatures are potential latent heat energy storage media, and store heat in a solid-liquid phase change. This approach reduces the amount of material required, and therefore physical size of the storage system. This work investigates the compatibility of a binary eutectic mixture of 52.81 wt% K2CO3 and 47.19 wt% Na2CO3 salt with three high temperature alloys (316L, 347H and 800H) was assessed at the maximum proposed temperature of 750 °C under high purity argon cover gas. This salt proved to be very aggressive to the alloys tested, with corrosion rates determined to be approximately 1 mm/year for the 316L and 800H alloys, and up to 10 mm/year for 347H. The corrosion product was found to consist of sodium chromite (NaCrO2) and iron and nickel oxides. After testing niobium was detected in the solidified salt in greater amounts than in the corrosion product, indicating that the niobium corrosion product was soluble in the salt. Additionally, the 800H experienced significant grain boundary oxidation, throughout the test sample. A corrosion mechanism is proposed, based on chemical thermodynamics, which suggests that the salt will dissociate when in contact with metal at a lower temperature than previously determined. Likely corrosion reactions are discussed, and the higher corrosion rate for the niobium containing 347H was attributed to an oxygen producing reaction, which accelerates the attack on the metal. •316L, 347H and 800H alloys were corroded in K2CO3:Na2CO3 salt under UHP argon.•SEM-EDS and XRD were used to determine corrosion structure and rate.•347H experienced an order of magnitude higher corrosion rate than 316L or 800H.•Nb in 347H was found to react with the salt producing O2 and increasing corrosivity.•Nb was found in the salt, however Cr, Fe and Ni were confined to corrosion scale.