Thermochemical Surface Treatment Of Iron–silicon And Iron–manganese Alloys
Published 2002 · Chemistry
Two binary alloys (Fe–0.5 wt.% Si and Fe–1.5 wt.% Mn) and a high silicon steel were used to investigate the thermochemical phenomena occurring at the sample free surface during simulation annealing cycles aimed to achieve a hot-dip galvanizing-compatible surface condition. The samples were annealed for 60 s at 800–810°C in atmospheres composed of nitrogen and hydrogen (either 0 or 5 vol.% H2), the dew points of which were varied from −60 to 0°C, corresponding to water vapour contents ranging from 10 to 6000 ppm. The surface chemical conditions were characterized by in situ x-ray photoelectron spectroscopy (XPS). Annealing in a nitrogen–hydrogen atmosphere was shown to lead silicon and manganese to diffuse to the surface and become oxidized, the amount of external selective oxidation decreasing with increasing atmosphere vapour content. After annealing in pure and dry nitrogen, the thin native iron oxide films are reduced to metallic iron, while free surface silicon or manganese oxides are definitely less abundant than after annealing in N2–H2 gas mixtures. Although iron oxide is thermodynamically stable in water-vapour-containing nitrogen atmospheres, our results suggest that, provided the dew point is low enough, the oxygen present in the thin native iron oxide film is transferred to more stable oxide forms (silicon or manganese oxides). The transition from iron oxide reduction to iron oxidation takes place at some critical water vapour content threshold that is dependent on the steel composition. The phenomena observed in this study are explained in terms of the balance between the oxygen inward flow and the solute alloying element outward flow. In this context, several possible gas–metal and exchange reactions are discussed. Copyright © 2002 John Wiley & Sons, Ltd.