Corrosion in geological disposal facilities of high level nuclear waste is of great importance to maintain the integrity of the waste container and thus ensure the public safety. Specifically, a nuclear waste “Supercontainer” has been designed by Belgium with carbon steel as being the main metallic barrier material. The corrosion environment in the Supercontainer has been determined to be anoxic for the vast majority of the service period, and the forms of corrosion are expected to be mainly general passive corrosion and pitting corrosion due to passivity breakdown. In order to realize the prediction of accumulated corrosion damage over the exceedingly long service horizon of the Supercontainer (>100,000 years), employment of mechanistically based or analytic models are necessary. Accordingly, the passivity and passivity breakdown of carbon steel were investigated in this dissertation work experimentally using electrochemical techniques and analyzed within the framework of the Point Defect Model (PDM), an analytical model describing the mechanism of passivity.
A mixed potential model has been developed to deconvolve the negative total current density that is observed at potentials more negative than the open circuit potential, Ecorr, under anoxic conditions into its partial anodic and cathodic components as a function of potential across the passive range. Deconvolution was successfully achieved by optimizing a Mixed Potential Model (MPM) comprising the PDM to describe the partial anodic process and the Butler-Volmer equation to describe the partial cathodic process of hydrogen evolution. In this manner, the corrosion rate can be determined across the entire passive range, including the range of potential (E < Ecorr) within which the net observed current density is negative.
The commonly observed irreversibility of the passive state on carbon steel in alkaline solutions was examined in simulated concrete pore solution under anoxic conditions replicated by applying relatively negative film formation potentials, at which the cathodic process dominates the passive system. The fundamental source of this irreversibility was investigated by describing the kinetics of the passive system by the MPM. Electrochemical impedance spectroscopy, Mott-Schottky analysis and model optimization were performed at each potential when the potential was first stepped in the anodic direction and then in the cathodic direction. The experiment and optimization results demonstrate that the irreversibility of the passive state is closely associated with the discrepancy in the defect structure of the passive film upon opposite stepping directions of the film formation potential, and is essentially caused by the slower film formation and slower film dissolution during the cathodic potential stepping than those during the anodic potential stepping.
The theory of the kinetics of nucleation of metastable pits in terms of the PDM has been applied the first time to describing the evolution of the nucleation rate of metastable pits on a variety of metallic substrates. The PDM successfully accounts for the experimental data that have been reported in the literature on stainless steel, carbon steel, iron, aluminum, and Alloy-22, and which are judged to be reliable and reproducible. Important fundamental parameters related to metastable pitting such as total number density of pitting nucleation sites, dissolution time of the cap over the pit, energy related to absorption of the aggressive ions into oxygen vacancies in the surface of the barrier layer, vacancy condensation rate, and the time at which the nucleation rate of metastable pits is maximum were obtained from the optimization of the PDM on the experimental data, as reported in the present paper. The values obtained for those parameters are in good agreement with values and observations reported elsewhere.
Finally, passivity breakdown on carbon steel was studied in various solutions with different pH values, chloride concentrations, and temperatures. The data are interpreted in terms of the PDM, too. An increase in temperature from 25 °C to 85 °C results in a decrease in localized corrosion resistance according to a linear relationship between the critical pitting potential and log (chloride activity) for all chloride concentrations. The linear dependence of the critical pitting potential on the square root of potential scan rate, as predicted by the PDM, yields an estimate of the critical areal concentration of condensed cation vacancies at the metal/ﬁlm interface that leads to the passivity breakdown.