Improvement of the corrosion situation during nuclear power plant (NPP) operation is associated with the enhancement of construction steel resistance against general (uniform) corrosion, with the routes of chemical transformations and corrosion product mass transfer in the coolant under different water chemistry conditions. Based on a look-back analysis of the obtained research results and a comparison of these results with those in available publications, the following conclusions were made:
1. The morphology of corrosion products formed on the inside surfaces of NPP systems has a four-layer structure.
a. A layer of solid corrosion deposits tightly bonded with the surface is formed above the oxide film.
b. Tightly bonded deposits are under loosely bonded (“loose” or dissipative) corrosive deposit layers that are dynamically balanced with the corrosion product particles dispersed in the water coolant.
2. By an aggregate state, the coarse/medium fractions (particle size more than 0.45 µm) and the fine fractions (particle size less than 0.45 µm) are conventionally referred to as nonsoluble and conditionally soluble corrosion products, respectively.
3. The chemical composition of all corrosion products depends on the presence of iron compounds, including the alloy element impurities (Cr, Ni, Mn, and Ti).
4. The radionuclide composition of all corrosion products is qualitatively the same and is presented by the activation products of reactor materials, such as 51Cr, 59Fe, 54Mn, 58Co, and 60Co.
5. The phase composition of solid corrosion products depends on the presence of ferrous (II) and ferric (III) iron oxide-hydroxide compounds whose ratio depends on water chemistry conditions:
a. Under reducing water chemistry conditions, the phase composition of all corrosion products is determined by a spinel structure of magnetite (Fe3O4).
b. Under oxidizing water chemistry conditions, the partial oxidation of ferrous (II) iron ions results in the formation of a defect structure of nonstoichiometric magnetite FeА3+ [Fe2+1-хFe3+]BO4-х, where А and B are two nonequivalent positions in the magnetite structure. At х = 1, a nonstoichiometric magnetite structure changes into hematite, a α-Fe2O3 or maghemite γ-Fe2O3 structure.
It is noted that the mathematical models nowadays used for describing the mass exchange and mass transfer of corrosion products in NPP primary systems do not consider physicochemical processes leading to the formation of such complex (phase, disperse, chemical, radionuclide) compositions of corrosion products. A widely known electrochemical mechanism that considers corrosion as a coupled anodic-cathodic process fails to explain actually observed steel dissolution and the contribution of soluble iron forms to oxide film formation on the corroding steel surface at potentials of cathodic polarization and anodic dissolution.
This paper presents a diffusion model for corrosion product mass exchange and mass transfer in the steel-water coolant system as an alternative to the electrochemical model for general corrosion. This model is based on Frank-Kamenetkiy’s [Diffusion and Heat Transfer in Chemical Kinetics, Nauka (1987)] concept of the macroscopic kinetics of heterogeneous processes with simultaneous chemical transformations of the corrosion product ionic forms and the formation of solid-phase products in the water coolant and on the surface of corrosive steel. The diffusion model provided better insight into understanding how the phase, disperse, chemical, and radionuclide compositions of steel corrosion products are formed in the coolant of the NPP primary system.