Scaling of Carbon Steel in Simulated Reheat Furnace Atmospheres

A low carbon steel was exposed to a series of laboratory-simulated combustion gases at temperatures of 700--1200°C. The gases corresponded to the products of burning natural gas with amounts of air varying from 95% to 112% of air--fuel stoichiometric equivalence. At all temperatures, fast, parabolic scaling kinetics were observed in the 112% hyperstoichiometric environment, whereas linear kinetics were always found in a 99% air--fuel gas. At 1100°C, linear kinetics were found in 95%, 99% and 101% air--fuel gases, initial linear kinetics followed by a parabolic regime in 108% air--fuel gas, and simple parabolic kinetics in the 112% gas. At equilibrium, Fe3O4 would be stable in the 95%, 99% and 101% air--fuel gases; however, only FeO was formed during reaction. Also at equilibrium, Fe2O3 would be stable in the 112% and 108% gases, and this phase was found at the surface of scales grown in these gases. Mass transfer calculations show that molecular oxygen was not the principal reactant in any of these gases; instead CO2 and/or H2O were the important species. It is concluded that surface reactions control the linear scaling rates.