Vanadic and chloride attack of superalloys

Abstract

The high-temperature corrosion of superalloys is associated with contaminants. When comparing contaminant conditions the contaminant flux rate (CFR) should be considered rather than the contaminant level in the fuel or environment. At temperatures above 700° C, vanadates cause fluxing of the protective oxide scales and it is shown that corrosion is determined by the CFR and temperature rather than by material selection. The effects of sulphur level in the fuel on the efficiency of magnesium additives are also considered. Chloride contamination is shown to produce scale rupture and the influence of chloride contamination under gaseous and deposit conditions is examined. In particular the differences between marine gas turbine conditions and laboratory tests to simulate hot corrosion are evaluated. It is suggested that in marine turbines fluxing mechanisms are more appropriate to the alloy than to the protective scale. Finally, the influence of chlorides on low-temperature type II hot corrosion is considered. MST/445