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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 via a previously developed mixed potential model that combines the point defect model accounting for the anodic process and the Butler-Volmer equation accounting for the cathodic process. 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 deep geological repositories for the storage of high level nuclear waste in Belgium have been proposed to adopt the "supercontainer" concept. 1-3 Corrosion of the carbon steel overpack in concrete pore water (pH of 13.5 at 25 • C 4 ) and under anoxic conditions characterized by very negative corrosion potential due to continuous oxygen consumption and following hydrogen buildup 5 has been determined to be the main threat to the integrity of the supercontainer structure. A huge amount of work 6-27 has been done on the corrosion of iron and steels in highly alkaline environment, including concrete pore solution, where the phenomenon of passivity breakdown that is usually induced by chloride, carbonate, and/or sulfide ions, et al. [6][7][8][9][10][11][12][13][14][15] and the kinetics and composition of the formed passive film 16-27 are the subjects being primarily focused on. However, very few studies employed sufficiently negative potentials that are comparable to the corrosion potential inside the supercontainer, which could reach down to −0.8 V SHE . 28 At such negative potentials, the cathodic reaction dominates the corrosion system and thus the information regarding to the partial anodic process which is required to study the passivity electrochemically, such as the anodic corrosion current density and the impedance associated with the anodic reactions, cannot be obtained directly, a difficulty that, however, has been solved previously by using a mixed potential model proposed by the authors. 29 Additionally, the passive carbon steel also exhibits apparent corrosion irreversibility when the film formation potential shifts in opposite anodic and cathodic directions; specifically, a strong resistance to cathodic potential reduction of the passive state is evident if a pre-exiting passive film h...

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This paper summarises recent experimental results from a programme that was carried out to investigate the effects of radiation on the anaerobic corrosion of carbon steel, in relation to the overpack that will be used in the Belgian supercontainer concept for radioactive waste disposal. Anaerobic corrosion rates of carbon steel were measured by monitoring hydrogen evolution using manometric gas cells and an autoclave and the corresponding electrochemical behaviour was investigated by measuring open circuit potential, linear polarisation resistance and alternating current impedance. The test medium was alkaline simulated pore water, at c-irradiation dose rates of 0 and 25 Gy h 21 , temperatures of 25 and 80uC and chloride concentrations of 0 and 100 mg L 21 . The anaerobic corrosion rates exhibited a slow decline due to the formation of magnetite. Good agreement was found with weight loss measurements. A dose rate of 25 Gy h 21 had a negligible effect on gas generation rates.