Some important issues in electrochemistry of carbon steel in simulated concrete pore water Part 1 – theoretical issues

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.