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

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 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 of 13.5. It is recognized, however, that if (or when) the stainless steel liner is breached; dissolved sulfur species formed by oxidation of pyrite (FeS2) in the near field environment may penetrate into the annulus and cause accelerated corrosion of the carbon steel. Chloride ion is present in both the concrete and Boom clay environments, to an extent that it, too, must be considered a deleterious species that is capable of inducing passivity breakdown on both carbon steel and stainless steel if the potential is sufficiently high (2). Furthermore, during the initial period of waste disposal, depending upon the type of waste contained (reprocessed waste, non-reprocessed waste), the initial temperature will be in excess of 100 ◦ C and will decay over about 300 years to the ambient temperature of about 22 ◦ 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. Once the temperature has decayed to ambient and the annulus is saturated with hydrogen, the corrosion potential is predicted to be about −0.75 Vshe. In the present paper, we report the corrosion behavior of carbon steel during the initial oxic period, when the potential is expected to be between −0.4 V and +0.3 Vshe. We have been particularly concerned with measuring the steady state value of the passive current density and electrochemical impedance spectroscopy data for carbon steel in simulated concrete pore water (Ca (OH)2 + NaOH