High temperature corrosion of nickel‐chromium alloys containing refractory sulphide‐forming elements in sulphur/oxygen environments – theoretical considerations

Despite the fact that combustion atmospheres in gas turbines are normally strongly oxidizing, microexamination of corroded alloys often not only reveals an anticipated external oxide scale and a degree of internal oxidation, but also a significant amount of sulphide phase(s), particularly after turbine operation in marine situations. Thus a degree of selective sulphidation as well as oxidation occurs. While the exact method of the further propagation of the corrosion product(s) is subject to doubt, it is possible that subsequent oxidation of these sulphide particles may be a factor. It is clear, however, that a necessary prerequisite for the reduction of this complex form of attack is the elimination of or control oh the sulphidation which occurs concurrently with oxidation. The theory underlying the possible development of sulphidation resistant alloys via. selective sulphidation of specific alloying element additions is reviewed. On this basis a number of alloying elements, which can form so‐called refractory sulphides of high melting point, and which hitherto have been largely ignored, are put forward in discussion as being potentially useful as additions to Ni‐based alloys for the inhibition of attack in sulphur/oxygen environments. In this assessment thermodynamic data for these refractory sulphides have been calculated and are quoted.