Chemistry variations of the secondary circuit aqueous environment reflected in the characteristics of the superficial films formed on the carbon steel components

The aim of this paper is to establish correlations between the characteristics of the superficial films formed on the carbon steel components in operation conditions of the secondary circuit of the Cernavoda Nuclear Power Station (NPS) and the specific parameters of the aqueous environment in which these films were formed. The main parameters studied in this paper, specific to a secondary circuit environment, are: the value of pH and the substances used to control it and the oxygen concentration dissolved in the aqueous environment, respectively. These filmed samples by autoclaving were studied by: 1. X rays diffraction method; 2. metallographic microscopy and 3. electrochemical methods such as: potentiodynamic and electrochemical impedance spectroscopy (E.I.S). to establish the correlations between the characteristics of the superficial films and the chemistry of the aqueous environment from the secondary circuit. To create films on the carbon steel samples similarly with those formed on the components during the operation of the NPS secondary circuit, some samples of three types of carbon steel - SA106, SA508 and SA516 - were filmed by autoclaving at 260 deg. C (51 atm) in aqueous environments with given physical and chemical characteristics. Using descaling and gravimetric method, there were established the corrosion kinetics of the carbon steel samples, which include: 1. the global corrosion rate (mg/dm2/day); 2. the global corrosion products formed (mg/dm2); 3. the adherent corrosion film (mg/dm2); 4. the non-adherent corrosion products (mg/dm2), etc. (authors)