Scale growth on steels at 1200°C: Rationale of rate and morphology

High purity zone refined iron and low alloy steels (<0.3%C) were oxidized at 1200°C in O₂–N₂, O₂-H₂O-N₂, and H₂O-N₂ gas mixtures to elucidate the kinetics and morphology of scale growth on steel under conditions which simulated reheating furnaces. A few oxidation experiments were made with pure iron at 700 and 950°C. The present findings shed additional light on the effect of water vapour in the oxidizing furnace atmosphere on the rate of oxidation of pure iron and Fe-C alloys. In the absence of water vapour, scale detachment caused the Fe-C alloys to be oxidized at a lower rate. In oxidizing gas mixtures containing water vapour, the scale remained attached to the metal surface for considerably longer reaction times, and the scale growth progressed in accordance with the parabolic rate for pure iron. These findings substantiate those of earlier studies and imply that the presence of water vapour in the oxidizing gas enhances the rate of creep in the scale. It was found that with water vapour present in the oxidizing gas, the pores migrate away from the metal interface as the oxidation progresses. When high purity iron is oxidized at 950°C (with or without water vapour in the furnace atmosphere), no pores were developed in the scale.