An Investigation of the Corrosion of Carbon Steel in Simulated Concrete Pore Water Under Anoxic Conditions

A series of comprehensive short term experiments (STE) have been performed to study the electrochemical and corrosion behavior of carbon steel in simulated concrete pore water under anoxic conditions at room temperature. The prerequisite of employing an anoxic environment follows from the process of continuous oxygen consumption by the corrosion of the carbon steel overpack and stainless steel liner in the supercontainer for HLW storage, which leads to highly negative corrosion potentials and continuous hydrogen evolution. In this study, the applied potential in the STE were selected intentionally in order to duplicate the highly reducing environment. Two subjects are especially highlighted. One is the apparent corrosion irreversibility exhibited by passive carbon steel when the film formation potential shifts in opposite anodic and cathodic directions; The second, compelling issue occurs is concerned with the fact that a negative current density is observed to develop with time after carbon steel is potentiostatted at the open circuit potential of a previously-established steady state. This is attributed to the gradual enoblement of carbon steel when exposed to simulated concrete pore water at a fixed temperature. The objective of the study is to understand the essential microscopic mechanism leading to the persistence of passive film on carbon steel during cathodic reduction, and create an effective approach to extract the anodic partial current density from the negative current density so that the corrosion rate can be obtained to predict the corrosion damage. Potentiostatic polarization was applied to the carbon steel specimen in a particularly designed manner that the applying voltage was stepped anodically and then cathodically in the reverse direction, where a “practical” steady state was reached at each step. The passive film at steady state has been investigated primarily by electrochemical impedance spectroscopy (EIS) and Mott-Schottky (MS) analysis. The formed passive film is postulated to have a two-layer structure, comprising a highly point-defective inner barrier layer and a porous precipitated outer layer. An n-type electronic character of the barrier layer is found by MS analysis, and iron interstitials are indicated as the dominant point defects from optimization of the Point Defect Model (PDM) on the EIS data. Kinetic parameters for the generation and annihilation of point defects at the metal/barrier layer (m/bl) and the barrier layer/solution (bl/s) interface as proposed in the PDM and extracted from the optimization were studied as a function of voltage during both anodic and cathodic potential stepping. Examination of these parameters reveals that the resistance of carbon steel to cathodic reduction is endowed by much slower (nearly by an order) formation and dissolution of the passive film as the potential is displaced cathodically. This feature of irreversibility has to be recognized in determining corrosion damage of carbon steel during the process of hydrogen buildup in the annulus of the supercontainer (i.e. as the potential shifts cathodically). An unprecedented mixed potential model (MPM), combining the point defect model for the partial anodic process, and the Butler-Volmer equation for the polarization characteristics of the cathodic partial process has been developed to successfully deconvolve the anodic partial current density from the negative total current density. The optimization results based on this model show excellent agreement with both impedance and current density experimental data. The validity of the MPM is demonstrated in numerous other aspects, including the extracted cathodic current densities that comply well with the expected Tafel behavior and the consistency of the calculated donor densities with reference to data in the literature.