Reactions at the Interface between Steel and Oxide Scale in Wet CO2 Containing Atmospheres

Abstract The resistance of structural steels against corrosion in CO2-H2O and CO2-H2O-O2 atmospheres at temperatures above 550°C was observed with high magnification methods. The test atmosphere is the basic composition of an oxyfuel coal combustion atmosphere. The analytical information base was applied to develop reaction schemes for the oxidation of steels. It was demonstrated that the formation of phases and the kinetics of oxidation reactions in this atmospheres is affected by the formation of intermediate iron hydroxide and a high carbon activity. In the temperature regime above 550°C the thermal equilibrium between H2O, CO2 and Fe reacted and formed CO. CO triggers the formation of FeO and carbides and increases the reaction kinetics. Porosity at the interface is established by gaseous phases such as Fe(OH)2 or H2O. H2 formation is not limited to the steel - oxide interface but can occur in the whole oxide scale. Examples of the experimental results were compared with a thermodynamic approach. The objective of the work was to derive qualitative rules of the basic corrosion reactions