Corrosion of steel alloys in eutectic NaCl+Na2CO3 at 700 °C and Li2CO3 + K2CO3 + Na2CO3 at 450 °C for thermal energy storage

Abstract

Stainless steel 316, duplex steel 2205 and carbon steel 1008 were examined for compatibility with the eutectic mixtures of NaCl+Na2CO3 at 700°C and Li2CO3 + K2CO3 + Na2CO3 at 450°C in air for thermal energy storage. Electrochemical measurements combined with advanced microscopy and microanalysis techniques were employed. Oxidation was found as the primary attack in both molten salt environments. However, the availability of O2 controlled the degree of oxidation. Alloy 316 showed the lowest corrosion current densities in each molten salt owing to the formation of films on the surface. The attack morphology on the surface of all materials was uniform corrosion with no localized degradation at 450°C. Microscopy observations showed grain boundary oxidative attack as the primary corrosion mechanism for all studied alloys at 700°C with depletion of alloying elements from grain boundaries in alloys 316 and 2205. The protective nature of austenite phase reduced selective oxidation of the underlying ferrite layers of alloy 2205 in chloride carbonate at 700°C.