ELECTROCHEMICAL CONSIDERATIONS REGARDING GENERAL CORROSION OF MATERIALS IN A BWR PRIMARY CIRCUIT

Electrochemical calculations of 3d-transition metal (Fe, Ni, Co and Cr)-water systems at 558 K were performed to evaluated the general corrosion behavior of materials in a BWR primary circuit. Thermodynamics expressed by Pourbaix diagrams, the solubilities of oxides and the equilibrium activities of H-2, O2 and H2O2 in oxide formations, and kinetics determining the corrosion potential and corrosion rate of the materials were evaluated. The results assist recognition of the following corrosion behaviors: (1) Good corrosion resistance of type 304 SS under various water chemistry conditions is due to the properties of NiFe2O4 and (Fe, Ni)Cr2O4; namely wide stable regions and low diffusion coefficients of metal ions and the ability of these oxides to compensate for each others weakness. (2) A typical increase of the corrosion rate of Alloy X750 and Stellite #6 in an oxygenated aqueous environment containing H2O2 is due to the properties of NiO and CoO; namely high solubilities and high diffusion coefficients of metal ions.