Impedance investigation of the surface film formed on aluminum alloy exposed to nuclear reactor emergency core coolant

A method was proposed for in-situ evaluating the thickness and resistivity of the oxide/hydroxide film formed on the surface of aluminum alloy exposed to sump water formed in the containment after a loss-of-coolant accident. The evaluation entailed fitting a model for the film impedance, which has film thickness and other variables describing the resistivity profile of the film along its thickness direction as fitting parameters, to the practically measured electrochemical impedance data. The obtained resistivity profiles implied that the films formed at pHs25.C 7, 8, 9, 10, and 11 all had a duplex structure; compared to the outer layer in contact with the solution, the inner layer of the film had a much higher resistivity and was inferred to be denser and provide most of the protectiveness of the film. Both the thickness and the total resistance of the film decreased with the increasing solution pH25oC, suggesting that the films formed in more alkaline solutions had less protectiveness against corrosion, consistent with the increasing aluminum alloy corrosion rates previously identified.(c) 2023 Korean Nuclear Society, Published by Elsevier Korea LLC. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Automated summary

A method was proposed to evaluate the thickness and resistivity of the oxide/hydroxide film on aluminum alloy surface exposed to sump water after a loss-of-coolant accident. The obtained resistivity profiles revealed a duplex structure of the films formed at different pH levels, with the inner layer providing higher resistivity and protectiveness. The thickness and resistance of the film decreased with increasing pH, indicating decreased protectiveness against corrosion.