Investigation of scales resulted from a high-temperature sulphidation of Fe-Cr alloys

Scales resulted from a sulphidation of a series of Fe100-xCrx alloys (x less than or equal to 45) at T=1073 K in an atmosphere of H2S/H-2 with the partial pressure of sulphur p(s2)=10(-3) Pa were investigated by means of Mossbauer spectroscopy, X-ray diffraction, electron-and light microscopy as well as electron-probe microanalysis (EPMA). The results obtained gave evidence that morphologically all the scales, excepting the one grown on the most concentrated alloy, were macroscopically homogenous (single-layer). The exceptional scale had a two-layer structure. Crystallographically, all the scales were single-phase and had a structure of the FeS except the one grown on Fe90Cr10 alloy which was, in addition, composed of Fe7S8. The structure of the duplex scale was not determined. Mossbauer spectra recorded both in a transmission geometry as well as in a GEMS mode gave clear evidence that the single-layer scales were microscopically not uniform. The spectra characteristic of the scales grown on the alloys with x less than or equal to 10 consisted of two subspectra: one had spectral parameters similar to those of F1+deltaS, while the other subspectrum had a broad distribution of the hyperfine field. Based on the X-ray diffractograms and EPMA-profiles the latter could have been identified as originated from the (Fe,Cr)(1-delta). The relative amount of (Fe,Cr)(1-delta S) was found to increase with x, and for x greater than or equal to 15 the entire scale was composed of it. A relationship between the kinetics of sulphidation, which was determined from the weight gain of test pieces, the morphology and the structure of the scales formed is discussed, too. (C) 1998 Elsevier Science S.A.