Comparison of corrosion resistance between Al2O3 and YSZ coatings against high temperature LiCl-Li2O molten salt

In this study, the high-temperature corrosion resistance of plasma-sprayed ceramic oxide coatings has been evaluated in a LiCl-Li2O molten salt under an oxidizing environment. Al2O3 and YSZ coatings were manufactured by atmospheric plasma spraying onto a Ni alloy substrate. Both the plasma-sprayed Al2O3 and YSZ coatings had a typical splat quenched microstructure which contained various types of defects, including incompletely filled pores, inter-splat pores and intra-splat microcracks. Corrosion resistance was evaluated by the thickness reduction of the coating as a function of the immersion time in the LiCl-Li2O molten salt at a temperature of 650 °C. A linear corrosion kinetic was found for the Al2O3 coating, while no thickness variation with time occurred for the YSZ coating. The ceramic oxide coatings were reacted with LiCl-Li2O molten salt to form a porous reaction layer of LiAl, Li5AlO4 and LiAl5O8 for the Al2O3 coating and a dense reaction layer of non-crystalline phase for the YSZ coating. The reaction products were also formed along the inside coating of the porous channel. The superior corrosion resistance of the YSZ coating was attributed to the formation of a dense protective oxide layer of non-crystalline reaction products on the surface and at the inter-splat pores of the coating.