High temperature molten salt corrosion behavior of aluminide and nickel-aluminide coatings for heat storage in concentrated solar power plants

Sprayed slurry aluminide and nickel-aluminide coatings deposited by means of electrodeposition and slurry application to 9 wt% Cr P91 alloy were studied to mitigate molten salt corrosion in concentrated solar power plants. Both coatings were tested isothermally at 580 °C in contact with the Solar Salt (60% wt% NaNO3, 40 wt% KNO3) under static and dynamic conditions. Uncoated P91 showed considerable mass gains in both conditions and there was evidence of extensive spallation on both cases. Mass loss was therefore also measured after removing the corrosion products by chemical etching so that the corrosion rate could be better estimated. P91 developed a complex, fast growing multilayered oxide scale which included Fe2O3, Fe3O4 and NaFeO2 in molten nitrates whatever the test conditions. All the coated systems and in particular the nickel-aluminide coating in contact with the Solar Salt up to 1000 h performed much better than the uncoated material as they exhibited lower weight variations and no evidence of significant spallation. The aluminide coating developed a thin Na ferrite scale as shown by SEM-EDS and XRD after testing under static conditions. On the tested nickel-aluminide coating NiAl2O4 was detected only by XRD, so it is not possible to establish if it resulted from the oxidation of the residual undiffused material left after heat treatment, or to a thin layer formed on top of the coating after exposure to molten salt. Interdiffusion between the coating and the substrate also occurred in the nickel-aluminide coating whereas the aluminide coating formed at high temperature remained quite stable both in composition and morphology.