Corrosion of Copper Nuclear Waste Containers in Aqueous Sulphide Solutions
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Corrosion of Copper Nuclear Waste Containers In Aqueous Sulphide Solutions J. Smith, Z. Qin, D.W. Shoesmith, F. King1, L. Werme2 Department of Chemistry, The University of Western Ontario, London, ON, Canada, N6A 5B7. 1 Integrity Corrosion Consulting Ltd., 6732 Silverview Drive NW, Calgary, Alberta, Canada, T3B 3K8. 2 SKB Swedish Nuclear Fuel and Waste Management Co. Stockholm, Sweden, Box 5864, SE-102 40. ABSTRACT Electrochemical and surface analytical techniques are being used to determine the mechanism and kinetics of Cu corrosion in the presence of sulphide, with the primary goal of developing a mixed potential model for Cu corrosion under disposal conditions. Corrosion potential (Ecorr) measurements indicate the formation of copper sulphide films is rapid leading to rate control by the cathodic reduction of water. Impedance spectroscopy suggests the anodic charge transfer process is rapid leading to film growth limited by diffusive transport. Experiments are underway to determine the current-potential relationships necessary in the development of a mixed potential model. INTRODUCTION A proposed method of disposal of Swedish/Finnish/Canadian high-level nuclear waste is to bury it approximately 500-1000m deep in granite environments. It is proposed that the waste canisters will be buried in boreholes excavated in these rocks. The excavated space between the container and the vault walls will be backfilled with compacted bentonite clay. The shafts and tunnels will also be backfilled with a mixture of bentonite clay and crushed granite. Any available O2 will be limited to that trapped in the clay pores of the buffer material and will be consumed relatively early in the lifetime of the container (King, 1996). A primary candidate for the fabrication of these containers is copper. Copper is selected primarily because of its good thermodynamic stability in the aqueous anoxic environments anticipated in the repositories. It is essential that mathematical models be developed in order to assess the long term effects the repository environment will have on the corrosion and possible failure of these containers. These models must be based on a thorough understanding of corrosion mechanisms of the material, making it necessary to measure the essential reaction rate constants required in model calculations. In the anticipated anoxic aqueous environments, copper is thermodynamically stable. However, possible components of the immediate environment as well as the bentonite clay include pyrite (FeS2) and sulphates (SO42-) both of which are potential sources of sulphide. It is well known that sulphate-reducing bacteria (SRB) exist which can convert sulphates to sulphides. Hence, though dense bentonite will inhibit microbial activity in the vicinity of the container (Pederson, 2000), sulphides produced remotely can subsequently be transported to the container surface, Figure 1. This poses a problem since the presence of sulphide films shifts the corrosion
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potential for copper dissolution to more negative valu
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