EFFECT OF THE REAL IRON CRYSTAL-STRUCTURE ON THE SEGREGATION OF SULFUR

The kinetics of the segregation of sulphur in monocrystalline iron alp ha-Fe(lll) has been studied using the AES method. The segregated sulph ur originated from bulk contamination. Segregation has been studied at temperatures of 730 and 820 K. The results of the studies are discuss ed taking into account the real mosaic structure of the monocrystallin e sample of iron. The adsorption sites for the segregating sulphur cou ld be located on the surface but also in the defects of the monocrysta lline sample. At lower temperatures segregation occurs only from defec ts open to the surface. If the defects are empty, segregation does not occur. At higher temperatures segregation from the bulk to the surfac e occurs via defects. The ratio of the number of adsorption sites in t he defects of the crystalline lattice to the number of adsorption site s on the surface has been determined from kinetic measurements as N-D: N-S congruent to 1:2. The value of -(Delta G(D) - Delta G(S))/RT vari es between 1 and 4 depending on the concentration. The variations are caused by the inhomogeneity of the adsorption sites in defects. (C) 19 98 Elsevier Science B.V. All rights reserved.