Effect of alloying additions and preoxidation on high temperature sulphidation resistance of iron–chromium alloys

Abstract

High temperature corrosion of structural alloys in sulphur bearing environments is many orders of magnitude higher than in oxidising environments. Efforts to increase sulphidation resistance of these alloys include addition of alloying elements. This paper reports the effect of yttrium, aluminium and yttrium plus aluminium addition on the sulphidation behaviour of an Fe–20Cr alloy in H2–2%H2S at 700 and 800°C. The effect of preoxidation of these alloys on sulphidation behaviour was also investigated. The initial stages of sulphidation of these alloys were observed and overall sulphidation kinetics was determined. In the presence of yttrium, the sulphidation rate of the alloy decreased by an order of magnitude, and the sulphide layer on the FeCrY alloy was significantly thinner than that formed on the FeCr alloy. Aluminium addition increased sulphidation resistance even more. The FeCrAl and FeCrAlY alloys exhibited parabolic sulphidation kinetics at the two temperatures. On both alloys, initial chromium sulphide (Cr2S3) formation at grain boundaries and Al2S3 and FeCr2S4 formation over the grains was followed by formation of an outer Cr2S3 layer over a mixed sulphide layer. Preoxidation of the two alloys resulted in an ‘incubation period’ with practically no sulphidation during subsequent exposure to S bearing environments. After this incubation period, the sulphidation behaviour of the FeCr alloy was similar to that when the alloy was not preoxidised. The incubation period of the FeCrAl alloy was significantly longer, over 45 h, compared with 2 h for the Al free alloy. Based on thermogravimetric as well as SEM, energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) data, sulphidation mechanisms have been proposed for the alloys before and after preoxidation.