XPS STUDY OF THE SEGREGATION OF SULFUR ON NI-21CR-8FE(100) ALLOY

The surface composition and structure of a Ni-21Cr-8Fe (100) single crystal were studied by angle-resolved XPS, AES and LEED. A detailed calibration was done by XPS with the individual metals. The composition of the clean surface is identical to the composition of the bulk alloy and a (1 x 1) structure is observed. The surface segregation of sulphur during annealing sulphur-doped alloys in UHV was studied in the temperature range 600 to 850-degrees-C. At saturation a c(2 x 2) structure is observed by LEED. The segregation of sulphur induces a marked cosegregation of chromium: when the surface is covered by a complete monolayer of sulphur, the surface concentration of chromium is approximately 94%. The free enthalpy of surface segregation of sulphur (DELTA-H(seg)S) was derived from the plot of the surface coverage of sulphur versus temperature. A value DELTA-H(seg)S = -105 kJ mol-1 is obtained. The segregation kinetics (sulphur coverage versus time) were investigated at 550, 660, 620 and 750-degrees-C and a diffusion coefficient D = 0.1 exp(-175 x 10(3) [J mol-1]/RT) cm2s-1 is derived from the experimental results. The adsorption of sulphur was studied by exposing the alloy surface to H2S-H2 gas mixtures of well controlled H2S partial pressure, at 970-degrees-C. The alloy surface is saturated by adsorbed sulphur in the range of p(H2S)/p(H-2) = 7 x 10(-5) to 1.8 x 10(-4). The sulphur coverage, measured with radioactive sulphur (S-35) is 48(+/- 3) x 10(-9) g/cm2, which corresponds to a ratio of sulphur atoms to metal atoms on the surface of 0.56. For partial pressure of H2S above 1.8 x 10(-4), the sulphidation of the alloy is observed, which indicates that the alloy does not behave as an ideal solid solution.