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

Abstract

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.

Some important issues in electrochemistry of carbon steel in simulated concrete pore water Part 2 – Experimental

“…1 Both the outer surface of the overpack and the inner surface of the liner will be in contact with concrete pore water, which, for our current purposes is defined as saturated Ca(OH) 2 zNaOH to yield a pH at 25uC of 13?5. It is recognised, however, that if (or when) the stainless steel liner is breached, sulphide species formed by oxidation of pyrite (FeS 2 ) in the near field, Boom Clay environment may penetrate into the annulus and cause accelerated corrosion of the carbon steel.…”

“…The supercontainer concept for the disposal of high level nuclear waste in Belgium's proposed Boom Clay repository comprises a carbon steel overpack containing the waste and an outer, stainless steel liner that defines an annulus containing a cementitious material (similar to Portland cement based concrete), as defined in Part I of this series. 1 Both the outer surface of the overpack and the inner surface of the liner will be in contact with concrete pore water, which, for our current purposes is defined as saturated Ca(OH) 2 zNaOH to yield a pH at 25uC of 13?5. It is recognised, however, that if (or when) the stainless steel liner is breached, sulphide species formed by oxidation of pyrite (FeS 2 ) in the near field, Boom Clay environment may penetrate into the annulus and cause accelerated corrosion of the carbon steel.…”

RTM for Waste Repositories

“…Since corrosion models are at the heart of this problematic, significant advances will be achieved in the near future by coupling RTM codes with electrochemical corrosion models (Bataillon et al 2010;Macdonald et al 2011).…”

The electrochemistry of carbon steel in simulated concrete pore water in boom clay repository environments

“…The super container concept for the disposal of high level nuclear waste (HLNW) in Belgium's proposed Boom Clay repository comprises a carbon steel overpack containing the waste and an outer, stainless steel liner that defines an annulus containing a cementitious material (similar to Portland cement-based concrete) [1]. Both the outer surface of the overpack and the inner surface of the liner will be in contact with concrete pore water, which for our current purposes is defined as saturated Ca(OH) 2 + NaOH to yield a pH at 25…”

“…• C. The calculations presented elsewhere [1] predict that oxygen in the annulus will be consumed quickly by corrosion and that the annulus environment will become essentially anoxic after a few years, after which water reduction, rather than oxygen reduction, becomes the principal cathodic reaction. As the oxygen concentration decreases, the hydrogen concentration increases, and the temperature decreases, the corrosion potential is predicted to increase, with the fall in temperature dominating at least over the first 300 years.…”