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Electrochemical and photoelectrochemical analysis of passive films formed at room temperature on the 690 TT alloy


690 TT Alloy is widely used in pressurized water reactors (PWR) of nuclear power plants as constituting material of the steam generators (SG) tubes. Currently, 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 [1], and some authors have also reported an influence between the semiconducting properties of the passive film and the alloy corrosion resistance[2] [3]. 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). The chemical composition and the semiconducting properties were further exanimated by Mott-Schottky plots and photo-electrochemical experiments. The steady state was estimated to be reached within 18 to 21 hours and the open circuit potential was around – 0,55 ± 0,2 V/EMS. The capacitive-like behaviour of the impedance diagrams, obtained at Ecorr, shows 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 nanometres, was estimated from the complex-capacitance representation of the impedance data, showing that this thickness is related to the initial surface state of the tubes. Among the numerical simulations that were applied, the power-law model may be adequate for passive films exhibiting semiconducting properties. Mott-Schottky plots for the passive films exhibited negative slope at low potentials and positive slope at higher potentials, confirming that the semiconducting properties of the passive film are controlled by a n-type and a p-type oxide /hydroxide layer. The chemical nature of these layers will be discussed and compared to results from the literature. [1] L. Guinard, O. Kerrec, D. Noel, S. Gardey, et F. Coulet, « Influence of initial surface condition on the release of nickel alloys in the primary circuit of PWRs », Nucl. Energy, vol. 36, no 1, p. 19‑27, 1997. [2] N. Sato, « The potentials of mixed electrodes of corrodible metal and metal oxide », Corros. Sci., vol. 42, no 11, p. 1957‑1973, nov. 2000. [3] D. H. Hur, « Technical Note: A Correlation between Anodic Film Properties and Stress Corrosion Cracking Behavior of Alloy 600 and Alloy 690 in High-Temperature Caustic Solutions », CORROSION, vol. 59, no 3, p. 203‑206, mars 2003.
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hal-04015918 , version 1 (06-03-2023)


  • HAL Id : hal-04015918 , version 1


Salma El Euch, Eliane M M Sutter, Mai T.T. Tran, Vincent Vivier, Nathalie Engler, et al.. Electrochemical and photoelectrochemical analysis of passive films formed at room temperature on the 690 TT alloy. 69th ISE Annual Meeting, Sep 2018, Bologne, Italy. . ⟨hal-04015918⟩
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