Effects of Fluoride and Chloride Ions on Corrosion of Titanium Grade 7 in Concentrated Groundwaters
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Effects of Fluoride and Chloride Ions on Corrosion of Titanium Grade 7 in Concentrated Groundwaters A. L. Pulvirenti, K. M. Needham, M. A. Adel-Hadadi, and A. Barkatt, The Catholic University of America, Washington DC, 20064 C. Marks and J. Gorman, Dominion Engineering, Inc. 6862 Elm Street Suite 460, MacLean, VA, 22101 ABSTRACT Titanium Grade 7 (Ti-7) was tested for both general and local corrosion in environments consisting of fluoride and chloride salts added to simulated groundwaters. Tests were conducted on both U-bends and disks in order to determine the extent of corrosion under stressed and unstressed conditions. The experiments were run over a broad range of temperatures and pH values for periods of up to 164 days. It was found that Ti-7 is susceptible to intergranular attack in some of the test solutions at moderately elevated temperatures, leading to conspicuous localized corrosion. Pitting was detected within 14 days, with pit depths on the order of 0.1 mm. Surface defects appeared to promote the initiation of pitting. The same environment caused stress corrosion cracking failure in a statically stressed U-bend. INTRODUCTION The current engineered barrier system design for the Yucca Mountain nuclear waste repository includes a drip shield. The drip shield is intended as a barrier to prevent groundwater from dripping onto the surface of the Hastelloy® C-22 waste canister.1 The material chosen for the drip shield is Titanium Grade 7 (Ti-7), commercially pure titanium alloyed with 0.15% palladium. Commercially pure titanium and its alloys are stable over a wide range of temperature and pH,2 and resistant to environments that are corrosive toward other materials.3 Ti-7 was chosen specifically for its superior resistance to pitting and crevice corrosion,4 and for its resistance to cracking from hydrogen embrittlement. 4,5 Although considered immune to most environments, most alloys of titanium are susceptible to environments containing the fluoride ion,6 especially in the presence of chloride.7 Brossia and Cragnolino8 observed electrochemically that the relatively low corrosion rates were not significantly affected by chloride concentration or pH. However, a significant lowering of Ecorr and rise in current density was observed in solutions containing 1 M fluoride. The aim of the present study is to confirm by immersion experiments the electrochemical results reported in the literature, and to test the behavior of Ti-7 in simulated possible groundwaters doped with extra halides. Standard specimens of Ti-7 were exposed to these environments under conditions of relatively mild temperature and pH. EXPERIMENTAL METHOD The test environments used here are simulated groundwaters based on characterization and evaporation studies by the Yucca Mountain Project.9,10 Two base environments were used: simulated concentrated J13 well water (1000xJ13), and simulated unconcentrated J13 well water JJ1.9.1
equilibrated with tuff (EJ13). Nominal compositions of these base environments are listed in Table I. In the experiments,
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