Sulphidation properties of chromium and chromium-containing alloys at high temperatures

Abstract

The sulphidation behaviour of chromium alloys is reviewed in three different sulphur pressure ranges and the effect of chromium on the sulphidation kinetics and scale structures is discussed. It is generally known that chromium is resistant to sulphidation at high sulphur pressures, while it sulphidizes rapidly at low sulphur pressures. This seems to be due to phase changes from Cr 2S 3 or Cr 3S 4 to CrS x. At low sulphur pressures, copious internal sulphidation is formed for NiCr alloys whereas, for FeCr alloys, sulphidation is confined to grain boundaries. The sulphidation properties of chromium and Cr 23C 6 were investigated at 1073 K in the sulphur pressure range from 10 3.5 to 10 −6 Pa. Both chromium and C 23C 6 showed similar kinetics and scale structures, except for the formation of Cr 7C 3 in the inner layer of sulphidized Cr 23C 6. It is concluded that, at the low sulphur pressures which often occur in an internal sulphidation zone, chromium and Cr 23C 6 sulphidize faster than the alloy substrate and also that the grain boundary sulphidation of FeCr alloys is due to the preferential sulphidation of chromium carbides and not of the chromium-depleted alloy matrix.