High temperature corrosion beneath nitrate melts

Different alloys from ferritic-martensitic, austenitic, and nickel-based were exposed beneath a 60%NaNO3-40%KNO3 eutectic mixture in air for 500h at 600 degrees C in order to estimate the corrosion loss of the different classes of alloys and to evaluate the corrosion mechanism. Investigations were carried out by metallography, SEM with EDX, and microprobe analysis. Ferritic-martensitic materials (P 92) show a significant mass gain compared to austenitic material (304 HCu) and nickel-based alloys. In particular Alloy 617 B suffers from pitting corrosion comparable to sulfate induced hot corrosion. The occurrence of internal nitridation is not clear and has to be evaluated.