ADSORPTION AND SURFACE SEGREGATION OF SULFUR ON FE-17CR-12.6NI(100) AND FE-17CR-14.5NI-2.3MO(100) ALLOYS

Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) were used to study the surface compositions and structures of Fe17Cr12.6Ni(100) and Fe17Cr14.5Ni2.3Mo(100) alloys with or without segregated sulphur. Ion etching of Fe17Cr12.6Ni(100) alloy produces a surface composition identical to the matrix composition. On this alloy, segregation of sulphur at high temperature (∼800°C) induces the cosegregation of chromium. The cosegregation of nitrogen and chromium was also observed at 600°C, leading to the multilayer growth of chromium nitride. Ion etching of Fe17Cr14.5Ni2.3Mo(100) alloy leads to surface enrichment of molybdenum. The thermal segregation of sulphur on this alloy surface does not induce any chromium cosegregation. On both alloys, the LEED patterns show that the monolayer of segregated sulphur has a c(2 x 2) structure. The chemisorption of sulphur at 970°C in H2S*-H2 was investigated using the 35S radioisotope. A complete monolayer of sulphur was observed in the investigated range of partial pressures, 1.9 x 10-5 < p(H2S) p(H2) ≤ 1.5 x 10-4. The measured coverages of sulphur were 39(±-3)x10-9 and 37(±-4)x10-9 g/cm2 on Fe17Cr12.6Ni(100) and on Fe17Cr14.5Ni2.3Mo(100), respectively. These measured coverages are consistent with the c(2 x 2) structure observed with segregated sulphur. © 1990.