Evaluation of The Long-Term Stability of Passive Corrosion On The Drip Shield and Waste Package Under Yucca Mountain Con
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Evaluation Of The Long-Term Stability Of Passive Corrosion On The Drip Shield And Waste Package Under Yucca Mountain Conditions D.W. Shoesmith Department of Chemistry University of Western Ontario London, Ontario N6A 5B7 Canada ABSTRACT Possible long term corrosion scenarios for the engineered barriers proposed for the Yucca Mountain (Nevada, USA) repository are reviewed. INTRODUCTION The materials proposed for the engineered barriers in the Yucca Mountain repository (Nevada, USA), Alloy-22 for the waste packages (WP) and titanium Grade-7 (Ti-7) for the drip shield (DS), appear unlikely to suffer localized corrosion (LC) and have very low passive corrosion (PC) rates (1–3). Since environmental conditions will become more benign as temperatures decline and aqueous environments become more dilute (4), this leads to the prediction of exceedingly long waste package lifetimes. In this review, possible corrosion scenarios are discussed in the context of the anticipated evolution in the repository environment. EVOLUTION OF THE REPOSITORY ENVIRONMENT The corrosion of the DS/WP will be controlled by the repository relative humidity/temperature, and groundwater composition and its evolution with evaporative cycles due to the contact of seepage drips with the hot surfaces. Although significant uncertainty remains, laboratory studies (4,5) have bounded the range of environments anticipated, and yield compositions consistent with those observed at a wide range of arid desert locations in Nevada and California (6). The most likely environment is a mixed anion solution with a pH that could exceed 11. Corrosion and electrochemical tests (1,2) show LC to be very unlikely in this environment. However, there is a very small probability of forming a chloride-dominated neutral brine in which, while the probability of LC of the DS should remain unaffected (7), that of the WP could increase (5). Consequently, providing the DS functions as designed, the probability of LC is very low, and should not increase with time as the environment evolves to cooler less concentrated conditions. POSSIBLE CORROSION-ENHANCING SCENARIOS Physical Instabilities A potential source of instability is the condition of the air-grown oxide film on the DS/WP on aqueous immersion. The key question is whether this oxide will maintain its integrity
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and be susceptible only to PC, or whether it will be fractured and potentially susceptible to spalling and accelerated corrosion. At low temperatures, growth laws for air oxidation are logarithmic or inversely logarithmic but transform first to parabolic and then to linear as temperatures increase (8,9). Parabolic kinetics generally involve recrystallization allowing charge carrier transport along grain boundaries (10), while the further transition to linear kinetics (11) leads to the stressinduced film fracture allowing access of O2 to the metal surface (12,13). Since, WP surface temperatures will be ≤ 250 °C, and could be ≤ 100 °C, logarithmic oxide growth to a thin limiting thickness is likely for both the DS
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