A model of the annulus between the carbon steel overpack and the stainless steel liner of a supercontainer of the type proposed by ONDRAF/NIRAS for the disposal of high level nuclear waste in a boom clay repository is described. The model seeks to calculate the corrosion potentials of the outer surface of the carbon steel overpack and the inner surface of the stainless steel liner as the overpack surface temperature decays exponentially with time (0-300 years). In the initial state, it is assumed that the pore water in the cementitious fill in the annulus is saturated with oxygen and that the concentration of hydrogen is zero. As time increases, the temperature at the overpack external surface decreases exponentially, and oxygen is consumed at the overpack outer and liner inner surfaces, resulting in a fall in the corrosion potential at each surface, as estimated using mixed potential models. Once the corrosion potentials reach sufficiently negative values, hydrogen evolution becomes a viable cathodic reaction, resulting in the gradual build-up of hydrogen in the annulus. Eventually, the corrosion potential versus time data will be used to predict the form of corrosion that occurs on both the overpack outer and the liner inner surfaces as well as the pressure change due to the consumption of oxygen and the build-up of hydrogen. Further development of the model requires the measurement of values for important model parameters, most notably the passive current density of the steel, the kinetic parameters for hydrogen evolution and oxygen reduction and the parameters contained in the point defect model for the passive state. The latter are being obtained by optimising the point defect model on experimental electrochemical impedance spectroscopic data. Eventually, these data will be used in models that are being developed to predict the corrosion of the carbon steel overpack over the entire storage horizon of 10 000 years. Details of the experimental studies are given in an accompanying paper.
Carbon dioxide corrosion of carbon steel in brine has been a recognized problem in oil production and it is becoming more common with the use of CO2-flooding as an oil recovery technique. The presence of acetic acid is systematic in oil fields, however, its role in corrosion has not been well-recognized and it is still debated. The present work was carried out, first, focusing on an understanding of CO2-corrosion of low-carbon steel and, later to ascertain the effect of acetic acid. This understanding is achieved by developing a deterministic model that explains and accounts for the experimental observations from Electrochemical Impedance Spectroscopy (EIS) data. The impedance model, based on the Point Defect Model, provides a good account of the formation and growth of a bi-layer that comprises an oxide (inner) layer adjacent the surface and a precipitated (outer) layer of siderite, FeCO3, forming on the top of the inner layer.
Abstract. The prediction of corrosion damage of canisters to experimentally inaccessible times is vitally important in assessing various concepts for the disposal of High Level Nuclear Waste. Such prediction can only be made using deterministic models, whose predictions are constrained by the time-invariant natural laws. In this paper, we describe the measurement of experimental electrochemical data that will allow the prediction of damage to the carbon steel overpack of the super container in Belgium's proposed Boom Clay repository by using the Point Defect Model (PDM). PDM parameter values are obtained by optimizing the model on experimental, wide-band electrochemical impedance spectroscopy data.
The prediction of corrosion damage to times that are experimentally inaccessible by a large factor (e.g. to over 1000 years) is vitally important in assessing various concepts for the disposal of high level nuclear waste. Such prediction can only be made using deterministic models, whose predictions are constrained to being 'physically real' by the natural laws [conservation of mass, energy, charge and mass charge equivalence (Faraday's law)]. In this paper, the authors describe the measurement of experimental data that will allow the deterministic prediction of damage to the carbon steel overpack of the super container in Belgium's proposed Boom Clay repository using the point defect model to extrapolate damage to future times. In this paper, the authors describe an experimental programme that is designed to generate values for various model parameters that will be required in making the damage predictions. The impact of chloride, sulphide and thiosulphate anions and mixtures of these species, on the electrochemical behaviour of carbon steel in contact with simulated concrete pore water, is examined.
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