Thermodynamic Analysis of the Existing Forms of Iron Oxides in Neutral Oxygen-Containing Aqueous Media of the NPP Power-Generating Units

Abstract

Foreign and domestic software products designed for quantitative description of the flow accelerated corrosion (FAC) rate and modeling erosion–corrosion use physicochemical computational models based on the assumption that the oxide film that protects the surface of steel consists of magnetite. Investigations of the phase composition and structural characteristics of oxide films on the inner surfaces of pipelines and equipment of coolant-circulation circuits of the NPP power-generating units with RBMK high-power channel-type reactors or the third circuits of the BN-800 fast reactors in which correction-free or oxygenated water chemistry conditions are implemented have revealed the presence on the steels' surface of not only magnetite but also other iron oxide forms. At present, the presence of the forms of iron oxides and hydroxides other than magnetite is considered in the computational modeling of the FAC by the introduction of empirical dependences. In the work, thermodynamic analysis of the existence forms of solid iron oxide and hydroxide phases in contact with the aqueous coolant in neutral oxygen-containing solutions has been made for temperatures of 25–200°C. To solve the problem set, the method of minimization of free energies of the system was used that allows one to determine which of the solid phases—iron oxides or hydroxides—will be thermodynamically stable in aqueous process media. On the assumption that the free energy of a system is minimal in the equilibrium state, the method allows determining the composition of the solid phase that is thermodynamically stable under the given conditions. The dependence of the phase composition of the oxide film on the temperature and chemical composition of the aqueous medium in contact with the oxide in question is shown. It is suggested to use the calculated results for refining the physicochemical models of the dependence of the FAC on the temperature for estimation of changes in the solubility of the iron oxides on the temperature and the thermodynamic stability regions of the basic phases in which iron oxides exist in the presence of oxygen and hydrogen.