Humid Air Corrosion of Carbon Steel and Stainless Steels under Gamma Radiation: The Role of Solution Volume and Radiolysis Products
Abstract
As nuclear power plants age and their lifetimes are extended, it is critical to be able to accurately assess the long-term integrity of the reactor structural materials. A current investigation into a leak in the End Shield Cooling (ESC) System in Ontario Power Generation’s Pickering Unit 6 reactor has raised a potential issue. The corrosion of the supporting structural materials (carbon steel (CS) and stainless steel (SS)) in the presence of ionizing radiation in small stagnant solutions and humid air conditions therefore needs careful evaluation.
This project investigates the effect of water radiolysis and humid air radiolysis products (H2O2 and HNO3) on the corrosion of CS and SS under different conditions. The parameters studied in this thesis include the concentration of HNO3 and H2O2, solution pH, and the presence or absence of γ-radiation. A series of electrochemical techniques and coupon exposure tests were employed to measure the corrosion rates of different steels. Post-test surface and solution analyses were performed to understand the oxides formed on the corroded surfaces and to determine the amounts of metal ions dissolved in the solution.
This study has shown that CS corrosion involves several elementary reactions and transport processes that determine the overall corrosion rate. The presence of H2O2 and nitrate affect the corrosion rate by providing a more oxidizing solution environment that increases the charge transfer rate via increasing the mass transport rate of metal cations. At relatively high concentrations (³10 mM) of H2O2and in neutral pHs, the redox reactions of hydrogen peroxide can couple strongly with metal redox reactions. High H2O2 concentration can provide the conditions for the formation of a protective oxide and increase the corrosion potential to values at which the redox reactions of hydrogen peroxide can be strongly coupled with the metal oxidation. This study has highlighted the strong feedback that can occur between different processes involved in corrosion. Ignoring this feedback and the effect of nitrate and other solution parameters on the mass transfer process of metal cations could result in inaccurate predictions of the long-term integrity of structural alloys.
The results of this study have improved the mechanistic understanding of the effect of these redox-active species on the corrosion pathways of carbon steel and stainless steels, and will contribute to the development of a corrosion model that can be used to assess the long-term integrity of the CANDU reactor structural materials with confidence.