SULFIDATION BEHAVIOR OF FE-CO-CR-AL-Y ALLOYS CONTAINING REFRACTORY-METALS

The corrosion behaviour of three Fe-Co-Cr-Al-Y-X (where X = V and Nb)- type alloys has been studied in a sulphur-bearing atmosphere (pS2 appr oximately 10(-1) Pa and pO2 approximately 10(-18) Pa) at 750-degrees-C . Weight change data revealed protective kinetics and followed a parab olic rate law. The alloys containing higher levels of V and Nb showed better corrosion resistance. Replacement of Co by Fe improved the sulp hidation behaviour. Analysis of the exposed specimens by means of SEM, EDAX and XRD demonstrated that the scales formed were duplex in natur e. The outer layer of the scale mainly consisted of sulphides of Fe an d Cr(Fe3S4, Co9S8 and Cr3S4). The inner layer principally containing s ulphides of V, Nb and Cr was compact, adherent and protective, and sub stantially resisted the outward diffusion of the base alloying element s.