Corrosion and deposition of corrosion products on carbon steel and martensitic steel components of CANDU-6 reactor primary circuit

Cernavoda NPP Unit 1 is a heavy water cooled and moderated CANDU 6 reactor, using natural uranium as fuel. To ensure a minimal degradation of structural materials by fluid contact and to achieve or exceed the design lifetime of the plant, a chemical control and corrosion monitoring program has been established. The corrosion of the Primary Heat Transport System (PHTS) components is minimized by carefully selecting and then controlling a set of chemical parameters to reduce the aggressiveness of the coolant to specific metals used in the system. Chemical control of PHTS and surveillance of structural materials corrosion is directed towards keeping chemical parameters within specified limits in order to minimize corrosion of equipment and related piping, to control the corrosion rate, impurities concentration and fission products and to minimize activity transport and heat transfer surfaces fouling. To monitor the effects of operating chemistry on the corrosion of the system's components and the build-up of activity on construction materials and to show up important effects in the activity transport, an Autoclave System is required. To determine corrosion and to characterize the superficial oxide films formed on carbon and martensitic steel, the following methods have been used: gravimetric, metallographic and electronic microscopy, electrochemical impedance spectroscopy (EIS), X-ray diffraction and XPS. The analysis of the samples exposed for different times in autoclaves (197, 371, 568 and 825 days), allowed us to determine the corrosion rate, the deposition and releasing of corrosion products, as well as the characteristics of the corrosive films formed. The results obtained by corrosion analysis of the carbon and martensitic steel coupons exposed in Y1-Y4 autoclaves, assembled in by-pass of CANDU-6 reactor primary circuit from Cernavoda NPP Unit 1 showed the following: - The continuous, adherent and protective oxide films were formed by generalized corrosion of structural materials. By increasing the exposure times better formation of more adherent oxides was evidenced but not any localized corrosion. The corrosion products were mainly magnetite crystalides. Their quantity increased with exposure time and was higher on samples exposed in autoclave circuits in the reactor inlet header, because the magnetite solubility is smaller, the temperature being smaller, and the coolant is probably still supersaturated in iron. The corrosion products release rate and the corrosion rate for carbon steel SA 106 gr.B decreased, while the exposure time increased. The higher values for these rates, especially at the beginning, were obtained on the samples exposed in autoclave circuits from the reactor inlet header