The hot corrosion behavior of four Ni-base alloys (Ni-16Cr-2Nb, IN 738, Superalloy 537, and Ni-16Cr) was studied between 900 and 1000 C, and kinetic curves were obtained as weight gains for Na2SO4-coated specimens using a thermobalance. Corrosion products were examined by metallography, scanning electron microscopy (SEM), electron microprobe, x-ray diffraction (XRD), and 35S tracer. It was found that Nb is enriched in the corrosion products of the Ni-16Cr-2Nb alloy. Niobium may decrease the oxide ion activity of fused Na2SO4 by the formation of NaNbO3 such that the reaction between Cr2O3 and the salt film is retarded. For multi-component superalloys, Nb might promote acidic fluxing of the oxide scale. Niobium also promotes continuous segregation of sulfur along the grain boundaries of the Ni-16Cr-2Nb alloy, which accelerates grain boundary sulfidation and oxidation.

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National Association of Corrosion Engineers
1987
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