Hot Corrosion of Some Nickel-Base Superalloys Containing Niobium

The hot corrosion behavior of four Ni-base alloys (Ni-16Cr-2Nb, IN 738,⁽¹⁾ Superalloy 537,⁽¹⁾ 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 multicomponent 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.