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

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 wüstite 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 wüstite grown in H₂/H₂O.