Theoretical Modeling of Crevice and Pitting Corrosion Processes in Relation to Corrosion of Radioactive Waste Containers
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THEORETICAL MODELING OF CREVICE AND PITTING CORROSION PROCESSES IN RELATION TO CORROSION OF RADIOACTIVE WASTE CONTAINERS
JOHN C. WALTON Idaho National Engineering Laboratory, P.O. Box 1625, Idaho Falls, Idaho 83415 ABSTRACT A mathematical and numerical model for evaluation of crevice and pitting corrosion in radioactive waste containers is presented. The model considers mass transport, mass transfer at the metal/solution interface, and chemical speciation in the corrosion cavity. The model is compared against experimental data obtained in artificial crevices. Excellent agreement is found between modeled and experimental values for the potential. The importance of full consideration of complex ion formation in the aqueous solution is emphasized and illustrated. MATHEMATICAL MODEL Safe disposal of radioactive wastes requires that the wastes be isolated for long time periods. One component in many waste isolation schemes is a metallic container. The exact role of the container in the waste isolation system can best be evaluated from a fundamental understanding of corrosion processes which occur and their controlling factors. To assist in this understanding, models describing the electrochemistry in pits and crevices can be developed. Mechanistic mathematical models have as their eventual goal prediction (or at least bounding) of actual service lives of nuclear waste containers. Realization of this ambitious goal will require significant improvements in our fundamental understanding of localized corrosion; for without this knowledge the sophisticated mathematical models represent little more than mathematical exercises. Thus in the short term we take as our major goal an improved understanding of localized corrosion as applied to nuclear waste containers. Mathematical models can play a useful role in this process by rigorously incorporating theoretical constructs in a coupled manner - something unattainable with subjective analysis of data. In this light, the model presented herein places primary emphasis on modelin$ mass transport with chemical reaction at steady state in simple geometries. When this approach has been developed to the point where it can be applied with confidence to a wide variety of experimental systems, the model can be further developed to reflect time dependent corrosion in arbitrary shaped cavities. The equation for transport of dissolved electrolytes in dilute solutions, subject to diffusion and electromigration is [1]: s nzD,F, (1) Jd = -nDivC. where: Ji = flux of species i in solution (moles/dm 2-s) n = porosity in corrosion cavity D = diffusivity of species i (dm 2/s) V = vector gradient operator C, = concentration of species i (moles/din3)
z, = charge of species i F = Faraday constant R = gas law constant T = absolute temperature 1s = potential in solution (volt)
Two types of chemical reactions are of interest in crevices and pits 1) electrochemical reactions at the metal/solution interface and 2) reactions in the crevice solution (e.g., hydrolysis). The current density resulting from an
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