Potentiodynamic Polarization Studies of Candidate Container Materials in Simulated Tuff Repository Environments
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POTENTIODYNAMIC POLARIZATION STUDIES OF CANDIDATE CONTAINER MATERIALS IN SIMULATED TUFF REPOSITORY ENVIRONMENTS
NEIL G. THOMPSON*, AND WILLIAM V. HARPER** JOHN A. BEAVERS*, *Cortest Columbus Incorporated 2704 Sawbury Boulevard, Columbus, OH 43235 International 281 Enterprise Drive, Westerville, OH 43081
"**Resource ABSTRACT
Cortest Columbus is investigating the long-term performance of container materials used for high-level waste packages as part of the information needed by the Nuclear Regulatory Commission to assess the Department of Energy's application to construct a geologic repository for high-level radioactive waste. In one task of the program, a cyclic potentiodynamic polarization (CPP) technique was used to evaluate the corrosion behavior of the candidate container materials. In order to evaluate the expected range of environmental variables , a statistical experimental design approach was used. A Resolution IV experimental design for 15 variables was selected. The variables included temperature, pH, species present in the groundwater, and those generated by radiolysis. Complete matrices of CPP tests, which consist of tests in 33 environments, were performed on two candidate container materials; Type 304L Stainless Steel and Incoloy Alloy 825. In these tests, both alloys exhibited a wide range of behavior; including passive behavior, pitting and active corrosion. For each alloy, the environmental variables that affected corrosion behavior were identified. INTRODUCTION Much of the previous research performed on corrosion of container materials for the Tuff repository has focused on a reference groundwater, designated J-13 well water, or minor variations of that well water. A review of this work was recently completed by Beavers and Thompson [1]. While much of this research is a necessary component of the material selection process, it is important to recognize that failures of engineered structures by corrosion rarely occur in environments that resemble the nominal environment for which the alloys were selected. In the Tuff repository, processes such as boiling, precipitation and groundwater-radiation interactions can dramatically affect the composition of the electrolyte at the container surface. Accordingly, it is important to understand the effects of environmental variation on corrosion behavior of the container materials. Because of the large number of environmental variables considered, an accelerated test technique, Cyclic Potentiodynamic Polarization (CPP) in conjunction with a statistical experimental design approach was used. The research was not intended to provide long-term predictive capability but to be a screening program to identify regions of the environmental factor space where additional research is needed. The statistical approach used was specifically designed to examine corrosion behavior in complex environments [2]. By using a statistically based experimental design, a mathematical expression can be developed that relates a dependent variable or response, such as corrosion rate, t
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