CHEMISTRY OF THE SECONDARY CIRCUIT REFLECTED IN THE PHYSICOCHEMICAL CHARACTERISTICS OF THE FILMED CARBON STEEL COMPONENTS

摘要

The aim of this paper is to establish some correlations between the characteristics of the superficial films formed on the carbon steel components in some operation conditions of a secondary circuit from 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 regulate it and implicitly the oxygen concentration dissolved in the aqueous environment. The physicochemical characteristics of the filmed samples by exposure to hot water were determined by X-ray diffraction, metallographic microscopy and electrochemical methods such as: potentiodynamic and electrochemical impedance spectroscopy (EIS). Correlations between the characteristics of the superficial films and the chemistry of the aqueous environment from the secondary circuit were established. To create films on the carbon steel samples similar to those formed on the components during the operation of the NPS secondary circuit, some samples of those three types of carbon steel-SA106, SA508 and SA516 - were filmed by exposure at 260 deg C(5.1MPa) in aqueous environments having several physical and chemical characteristics. Using descaling and gravimetric methods, there were established the corrosion kinetics of the carbon steel samples, which were correlated with the characteristics of the films formed on the carbon steel components.