Thermodynamic Analysis and Crystallographic Properties of MFe2O4, MCr2O4 and MAl2O4 (M = Fe,Ni, Zn) Formed on Structural Materials in Pressurized Water Reactor Primary Circuit under Zinc and Zinc-aluminum Water Chemistry

Zinc injection technology (zinc water chemistry, ZWC) was widely applied in pressurized water reactor (PWR) primary circuits to reduce radiation buildup and improve corrosion resistance of structural materials. The simultaneous injection of zinc-aluminium (ZAWC) is a novel implement created to replace part of Zn²⁺ by Al³⁺. It was reported ZAWC can improve further corrosion resistance of carbon steels and stainless steels. However, ZAWC sometimes showed even negative effects on Nickel-alloys. In this study, mechanism of formation of oxide film on metals was investigated. The reactions of Fe²⁺ Ni²⁺ in oxide films replaced by Zn²⁺, or Fe³⁺ replaced by Al³⁺ in ZAWC were analysed. The thermodynamic data and solubility of mixed oxides (ZnFe₂O₄, ZnCr₂O₄, and ZnAl₂O₄), the products of replace reactions, were calculated. According to the Gibbs free energy difference between products and reactants, ΔGθ(T) values of the formation reaction of ZnFe₂O₄, ZnCr₂O₄, and ZnAl₂O₄ are extremely negative. Solubility of ZnAl₂O₄ is the lowest among mixed oxide products, which implies the oxide film composites of ZnAl₂O₄ may show a lower corrosion rate. In addition, the preferential formation of NiAl₂O₄ on Ni-based-alloy, under ZAWC, was discussed based on crystallographic properties of spinel, which was considered as the cause of negative effects of ZAWC on corrosion resistance of Nickel-alloys. This research provides an analytical basis for the study of thermodynamic stability of oxide films under different chemical chemistry and a theoretical basis for improving corrosion resistance of different metals and optimizing the chemical conditions of PWR primary circuit.