• Tran, Mai T. T.
• Vivier, Vincent
• Euch, Salma El
• Sutter, Eliane M. M.
• Marion, A.
• Engler, N.

Abstract

Alloy 690 TT is widely used in pressurized water reactors (PWR) of nuclear power plants as the material constituting the steam generators (SG) tubes. Concurrently, SG tubes are considered to be the main source of radioactive Co-58, induced from the Ni cation release in the primary circuit. Several studies have mentioned that the Ni cation release is somehow affected by the inner surface state of SG tubes. In this work, the electrochemical behaviour of 690 TT, with different inner surface states (electropolished, mechanically polished, and as received tubes), was studied in borate solution (pH = 9) at room temperature. The passive films were characterized by different electrochemical measurements: the corrosion potential (Ecorr) as a function of the immersion time, the steady-state polarization curves, and electrochemical impedance spectroscopy (EIS). At the open circuit potential, steady state was estimated to be reached within 18 to 21 hours. The capacitive-like behavior of the impedance diagrams obtained at Ecorr, shown the presence of a passive film. Indeed, the analysis of impedance data highlights a CPE (Constant Phase Element) behaviour in a large frequency domain for all the samples. The oxide layer thickness in the range of a few nm was estimated from the Cole-Cole representation of the impedance data, showing that the thickness of the passive layer is related to the initial surface treatment of the tubes.

Topics

• surface
• corrosion
• phase
• electrochemical-induced impedance spectroscopy
• cloud-point extraction