Water Vapour Effects on FeO Scale Growth: Differences Between Iron and Steel

Abstract

High purity iron and a low carbon, low silicon steel were oxidised at temperatures of 800–1,200 °C, in atmospheres of N2–H2–H2O and N2–O2–H2O. Scales of wustite grew at low oxygen potentials, and of FeO/Fe3O4/Fe2O3 at high oxygen potentials, both according to parabolic kinetics after an initial transient period. The iron and steel behaved similarly in the O2/H2O gases, but not in H2/H2O, where the steel oxidised much more slowly than the iron. The rate for steel increased with $$ p_{{H_{2} O}} $$ at fixed $$ p_{{O_{2} }} , $$ but for iron was almost independent of $$ p_{{H_{2} O}} , $$ whilst rates for both metals increased with $$ p_{{O_{2} }} $$ at fixed $$ p_{{H_{2} O}} $$ . These results are discussed using point defect models involving hydroxyl anions and cation vacancies. Scaling rates in O2/H2O also increased with $$ p_{{H_{2} O}} , $$ a result attributed to gas phase transport within oxide pores which were present in the scales, but absent in wustite grown in H2/H2O.