Corrosion evaluation of alloys and MCrAlX coatings in molten carbonates for thermal solar applications

Stainless steels (SS) 310, 321, 347, Incoloy 800H (In800H), alumina-forming austenitic (AFA-OC6), Ni superalloy Inconel 625 (IN625), and MCrAIX (M: Ni, and/or Co; X: Y, Hf, Si, and/or Ta) coatings were corroded in molten carbonates in N-2 and bone-dry CO2 atmospheres. Electrochemical tests in molten eutectics K2CO3-Na2CO3 and Na2CO3-K2CO3-Li2CO3 at temperatures higher than 600 degrees C were evaluated using an open-circuit potential followed by a potentiodynamic polarization sweep to determine the corrosion rates. Because the best-performing alloys at 750 degrees C were In800H followed by SS310, these two alloys were selected as the substrate material for the MCrAIX coatings. The coatings were able to mitigate corrosion in molten carbonates environments. The corrosion of substrates SS310 and In800H was reduced from similar to 2500 mu m/year to 34 mu m/year when coated with high-velocity oxyfuel (HVOF) NiCo-CrAlHfSiY and pre-oxidized (air, 900 degrees C, 24 h, 0.5 degrees C/min) before molten carbonate exposure at 700 degrees C in bone-dry CO2 atmosphere. Metallographic characterization of the corroded surfaces showed that the formation of a uniform alumina scale during the pre-oxidation seems to protect the alloy from the molten carbonate attack. (C) 2016 Elsevier B.V. All rights reserved.