Elsevier

Surface Science

Volume 63, March 1977, Pages 377-389
Surface Science

Equilibrium surface segregation of dissolved nonmetal atoms on iron(100) faces

https://doi.org/10.1016/0039-6028(77)90353-3Get rights and content

Abstract

At elevated temperatures equilibria of surface segregation X (dissolved) = X (adsorbed) have been studied for the nonmetal atoms X = C, N and S. Iron single crystals with (100)orientation have been doped with different concentrations of solute atoms (in the range about 10–100 wt ppm). The samples were introduced into the UHV chamber, cleaned and then heated to temperatures in the α-solid solution range. The surface concentration of the segregated nonmetal atoms was observed by AES for different bulk concentrations in dependence of the temperature. The LEED pattern was also observed during segregation equilibrium at temperatures up to about 750° C. The LEED patterns indicate a c(2 × 2) structure for carbon and nitrogen as well as for sulfur. The temperature dependence of the surface concentration for carbon on Fe(100) can be described by a Langmuir-McLean equation, an average segregation enthalpy of −85 kJ/mol°C is obtained. Since N2 desorption occurs the nitrogen segregation is in virtual equilibrium only at temperatures <500°C. The equilibrium surface concentration of sulfur on α-iron is virtually independent of the solute concentration and the temperature: there is always a saturated layer of sulfur on the (100) faces, even at small bulk concentrations. Since the thermodynamic activity of the nonmetal atoms is well defined in the segregation studies (except nitrogen at higher temperatures) , the results can be correlated with studies in gas atmospheres at atmospheric pressure. The relations to the kinetics of the carburization and the nitrogenation of iron are discussed and the influence of sulfur on these reactions.

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    Carbon segregation to iron surfaces has been investigated in several studies in the literature, both theoretically and experimentally. Carbon segregation to bcc iron surfaces was experimentally observed under vacuum conditions in alloys with carbon compositions as low as 90 ppm leading to full carbon coverage of the surface [44,45]. Jiang and Carter employed density functional theory simulations and calculated a net driving force for carbon to segregate to free surfaces and form graphite [46].

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