Simulation of The State of Carbon Steel n Years After Disposal with n Years of Corrosion Product on its Surface in a Ben
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SIMULATION OF THE STATE OF CARBON STEEL n YEARS AFTER DISPOSAL WITH n YEARS OF CORROSION PRODUCT ON ITS SURFACE IN A BENTONITE ENVIRONMENT
YOICHI KOJIMA*,
TOSHINOBU HIOKI*'** and SHIGEO TSUJIKAWA*
* The University of Tokyo, Faculty of Engineering, Department of Metallurgy Hongo 7-3-1, Bunkyo-ku, Tokyo, 113 JAPAN ** Nakayama Steel Works, Ltd., Funamachi 1-1-66, Taisho-ku, Osaka, 551 JAPAN ABSTRACT
The use of bentonite as buffer and carbon steel as overpack material for the geological disposal of nuclear waste is under investigation. To better assess the long term integrity of the carbon steel overpack, a quantitative analysis of the corrosion behavior on the steel surface for time frames beyond that of feasible empirical determination is required. The state n years after disposal, consisting of Carbon Steel / Corrosion Products + Bentonite / Water, was simulated and the corrosion behavior of the carbon steel in this state investigated. The following facts became apparent. Both the corrosion rate and the non-uniformity of it increased with increase in the corrosion product content in the compacted bentonite. When the corrosion product layer was formed between the carbon steel and the bentonite, it ennobled the corrosion potential and increased the corrosion rate.
INTRODUCTION
The combination of bentonite and carbon steel as a design for the geological disposal of high-level nuclear waste is under investigation. In this design, bentonite is used as a buffer material between the host rock and the packages, and carbon steel is used as the overpack material. To better assess the long term integrity of the carbon steel overpack, a quantitative analysis of the corrosion behavior on the steel surface for time frames beyond that of feasible empirical determination is required. The corrosion rate for the carbon steel is affected by the presence of the buffer [1] and also precipitated corrosion products [2]. The state of the overpack n years after disposal, consisting of Carbon Steel / Corrosion Products + Bentonite / Water, was simulated to give n (n =100, 300,- - - ) years of corrosion products artificially. Then, the incremental corrosion behavior of the carbon steel between n and n+An years after the disposal could be investigated by conducting a An year test under the above simulated environment. In the evaluation of corrosion rate, it is important to not only examine its mean value for the wide surface area of overpack but, it is also imperative to consider the non-uniformity of the distribution of the corrosion rate [3]. A two-electrode-cell type corrosion probe was constructed and set under the Mat. Res. Soc. Symp. Proc. Vol. 353 01995 Materials Research Society
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simulated environment. The time variation of the coupling current between the two electrodes and the corrosion potential were measured. When the coupling current is observed, it implies the difference in anodic and cathodic reaction rate between the two electrodes and the difference corresponds to the nonuniformity of the corrosion reaction on the
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