Adsorption, segregation and reactions of non-metal atoms on iron surfaces

Abstract

Information on equilibrium adsorption states of non-metal atoms X which are stable at elevated temperatures on iron surfaces can be obtained from the study of segregation equilibria X( dissolved) ⇌ X( adsorbed) by Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). These ultrahigh vacuum techniques were applied to the investigation of the segregation of X (≡ C, N and S) on iron, and the results can be combined with the results from investigations on the kinetics of surface reactions in the carburization of iron in CH 4H 2 and in the nitrogenation of iron in N 2 or NH 3H 2. From the studies of surface reaction kinetics by resistance-relaxation or gravimetric methods, rate laws are obtained which allow the rate-determining steps of the proposed reaction sequence to be identified. Also some information on adsorption or segregation equilibria of participating species is obtained from the kinetic studies and is in agreement with the AES and LEED results. Strongly surface-active elements such as sulphur retard the carburization and nitrogenation reactions by site blocking. By varying the sulphur activity in the gas phase or in the solid bulk phase, its influence on the reaction kinetics can be studied. From these investigations, adsorption isotherms were derived for sulphur on iron and the identification of the rate-determining steps of carburization and nitrogenation could be confirmed by the analysis of site-blocking effects. Some of the effects of sulphur on surface reactivity have found practical applications in steel sheet processing and corrosion protection. In this paper we review information gathered on the state of the iron surface at elevated temperatures in gas atmospheres and in a vacuum and on the kinetics and mechanisms of some metallurgically important gas-metal reactions.