Surface segregation of sulphur in pure iron and in FeMo alloys: A comparative AES-radiotracer study

In this paper, we report kinetics of sulphur surface segregation obtained by Auger electron spectroscopy (AES) in pure iron and in FeMo (0.5, 2 and 3.5 at%) alloys over the temperature range 500–630°C. Moreover, we investigate sulphur equilibrium surface adsorption at 800°C using radioactive 35S. We show first that the maximum sulphur amount increases with bulk Mo content at 800°C and second, that sulphur strongly segregates in the temperature range [550–600°C] on Fe and FeMo alloys even if the maximum S segregated quantities are always lower than the maximum sulphur amount obtained in adsorption experiments. At high temperature, the S segregation kinetics observed are governed in a first stage by grain boundary and pipe diffusion. A first equilibrium state corresponding to the S and Mo surface cosegregation on FeMo alloys (comparable with S segregation on iron) has to be considered. 2D (Fe, Mo) sulphides in which the most of Mo atoms are located near the surface (and not in the substitutional sites of the first layer) are formed in place of 2D (Fe) sulphide. The second point concerns nitrogen which is a residual impurity in FeMo alloys. At low temperature (520°C), a superficial 2D Mo nitride (second equilibrium state) is formed. However, as sulphur is the most active species in the FeMo alloys, non equilibrium states and site competition are observed in an intermediate temperature range.