Internal sulphidation phenomena of heat-resistant alloys at low sulphur pressures

Abstract

Three types of internal sulphidation (general internal, integranular) and a mixture of these occurred beneath relatively thick surface scales at low sulphur pressures where a noble metal sulphide is unstable. This critical sulphur pressure was higher when the surface sulphide scales were highly protective, as observed for FeCrAl and FeMn alloys sulphidized at intermediate sulphur pressures. Copious internal sulphidation was formed by sulphidation of FeMn, FeNi and NiCr alloys. For an Fe26.6 % Cr alloy the internal sulphidation occurred at 1173 K while it was confined along the grain boundaries at 1073 K. With increasing chromium content, a mixture of internal and intergranular sulphidation was formed at both temperatures. Austenitic stainless steels, SUS 316L and Incoloy 800, formed copious internal sulphidation at sulphur pressure of 10 −3.5 and 10 −3 Pa, and with decreasing sulphur pressures this changed rapidly to intergranular sulphidation. The usual internal sulphidation is explained by assuming an enhanced diffusion flux of sulphur due to the sulphur-metal interaction in the alloy phase, which has very low chemical diffusivity. For alloys with relatively high chemical diffusivity intergranular sulphidation takes place, because the metal-scale interface is unstable and the grain boundaries become inward protruding sites.