C-276 nickel alloy corrosion in eutectic Na2CO3/NaCl molten salt under isothermal and thermal cycling conditions

Abstract

Thermal energy storage (TES) is a key component for the practical application of concentrating solar thermal power. Recently, a push for higher temperatures has increased interest in molten salts with melting points above 550 °C as phase change thermal energy storage media. Corrosion of a nickel superalloy (C-276) with a eutectic salt mixture (59.5 %wt Na 2 CO 3 /40.5 %wt NaCl) at 650 °C was analysed under both isothermal conditions and with thermal cycle between 600 and 650 °C. Corrosion rates of 400–500 μm/year under isothermal conditions and 200–300 μm/year under thermal cycling were determined by measuring thickness loss, and oxide growth was primarily responsible for the metal loss. Thermal cycling reduced corrosion thickness, however it remained intact with no cracking or buckling. Post-test analysis of the salt found that molybdenum, manganese and tungsten were most likely to be removed from the alloy. Isothermal conditions did not cause chromium to be removed from the alloy by the salt. However, chromium was present in the salt after thermal cycling, due to its removal from the oxide layer. Whilst the corrosion was above the maximum acceptable corrosion rate for a TES system, this study shows that oxide layers formed on alloy C-276 have protective capabilities in this salt, gaining maximum thickness after about 400 h and preventing the salt from attacking the metal directly. Longer testing with more samples is require to determine if the corrosion of this alloy is halted due to the oxide layer growth on the metal surface. • Alloy C-276's compatibility with Na 2 CO 3 /NaCl molten salt at 650 °C was assessed. • Tests were undertaken under both isothermal and thermal cycling conditions. • Cross sections were analysed using microscopy for corrosion thickness and structure. • Oxide growth was the main corrosion mechanism, forming 40–60 μm thick scale layers. • Analysis of the salt found Mo, Mn and W dissolved into the salt from the alloy.