INVESTIGATION OF SCALES RESULTED FROM A HIGH-TEMPERATURE SULFIDATION 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 wi th the partial pressure of sulphur p(s2)=10(-3) Pa were investigated b y means of Mossbauer spectroscopy, X-ray diffraction, electron-and lig ht microscopy as well as electron-probe microanalysis (EPMA). The resu lts obtained gave evidence that morphologically all the scales, except ing the one grown on the most concentrated alloy, were macroscopically homogenous (single-layer). The exceptional scale had a two-layer stru cture. Crystallographically, all the scales were single-phase and had a structure of the FeS except the one grown on Fe90Cr10 alloy which wa s, in addition, composed of Fe7S8. The structure of the duplex scale w as not determined. Mossbauer spectra recorded both in a transmission g eometry as well as in a GEMS mode gave clear evidence that the single- layer scales were microscopically not uniform. The spectra characteris tic 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 th ose of F1+deltaS, while the other subspectrum had a broad distribution of the hyperfine field. Based on the X-ray diffractograms and EPMA-pr ofiles the latter could have been identified as originated from the (F e,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 sca le was composed of it. A relationship between the kinetics of sulphida tion, which was determined from the weight gain of test pieces, the mo rphology and the structure of the scales formed is discussed, too. (C) 1998 Elsevier Science S.A.