Abstract
The corrosion behaviour of three FeCoCrAlYX (where X = V and Nb)-type alloys has been studied in a sulphur-bearing atmosphere ( pS 2 ∼ 10 −1 Pa and pO 2 ∼ 10 −18 Pa) at 750°C. Weight change data revealed protective kinetics and followed a parabolic rate law. The alloys containing higher levels of V and Nb showed better corrosion resistance. Replacement of Co by Fe improved the sulphidation behaviour. Analysis of the exposed specimens by means of SEM, EDAX and XRD demonstrated that the scales formed were duplex in nature. The outer layer of the scale mainly consisted of sulphides of Fe and Cr (Fe 3S 4, Co 9S 8 and Cr 3S 4). The inner layer principally containing sulphides of V, Nb and Cr was compact, adherent and protective, and substantially resisted the outward diffusion of the base alloying elements.
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