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Collaborating Authors
Results
Corrosion Behavior of Alloy 22 Under Conditions of Dynamic Groundwater Composition
Bishop, E.G. (The Catholic University of America) | Barkatt, Aaron (The Catholic University of America) | Marks, Charles R. (Dominion Engineering Inc.) | Gorman, Jeffrey A. (Dominion Engineering Inc.) | Pulvirenti, April L. (The Catholic University of America) | Needham, K.M. (The Catholic University of America) | Adel-Hadadi, Mohammed A. (The Catholic University of America)
ABSTRACT Solutions that simulated the compositions of saturated (J 13) and unsaturated (UZ) waters that can be found in the vicinity of Yucca Mountain were distilled to near dryness. The pH of both the condensates and residual solutions of J 13-based waters rose during the distillations, while the pH of both the condensates and residual solutions of UZ-based water experienced a large drop in pH during distillation, which could be caused by precipitation and slow redissolution of magnesium oxide solids. Samples of Alloy 22 immersed in condensates and residual solutions of UZ distillations exhibited general corrosion at rates ranging from 30-600 gm/year, and one sample exhibited pitting. Samples tested in solutions of 1000xJ 13 in which the pH was altered during testing showed an increase in corrosion rate over solutions of constant pH. INTRODUCTION In a geologic repository, a major criterion for the selection of container materials is their ability to withstand corrosion by groundwater present within the repository throughout the entire service life of the repository. Accordingly, compositions of all nearby groundwater systems are important in testing candidate container materials. In the case of the proposed Yucca Mountain repository, two groundwater systems have been characterized: saturated zone water, 1'2 represented by J 13 well water, and, more recently, unsaturated zone (UZ) pore water collected from the Paintbrush unit above the repository horizon. 2,3 j 13 and UZ water are only two representatives of a much larger set of largely uncharacterized groundwaters, but the reference compositions 2 of even these two are vastly different. In addition to differences in original composition, the compositions of groundwaters are expected to change as the thermal output of the radionuclides in the repository causes evaporation and concentration of the solutions in contact with the container or in the surrounding structures. An investigation of the progressive thermal concentration of both J 13 and UZ groundwaters has been conducted. 2 Residual powders remaining after complete dryout were also analyzed. Thermal concentration of J 13 by a factor of approximately 300 resulted in an increase in pH from about 8 to about 10.5, likely attributable to loss of CO2 upon heating, with precipitation of CaCO3 and SiO2. Thermal concentration of UZ water by a factor of approximately 500 resulted in a decrease in pH from about 7.5 to 5, with precipitation of CaSO4 ? 2H20 and CaMg2C 16 ? 12H20 upon complete dryout. However, these evolved reference compositions are only a small subset of possible groundwater compositions that may contact the container surface. There is no reason to expect that thermal concentration would cease at these concentration factors, as long as additional water can continue to drip on the container surface. The presence of precipitated salts in equilibrium with a highly concentrated solution would prevent complete dryout of the groundwater solutions, causing highly concentrated solutions to linger even at container surface temperatures. Also, the evolved vapor may be transported to another location, condensate may drip back onto the container surface, creating local aggressive environments of a composition very different from the original groundwater compositions.
Fluoride Corrosion of TI-Grade 7: Effects of Other Ions
Pulvirenti, April L. (The Catholic University of America) | Needham, K.M. (The Catholic University of America) | Adel-Hadadi, Mohammed A. (The Catholic University of America) | Wong, D.S. (The Catholic University of America) | Barkatt, Aaron (The Catholic University of America) | Marks, Charles R. (Dominion Engineering Inc.) | Gorman, Jeffrey A. (Dominion Engineering Inc.)
ABSTRACT The effects of chloride, sulfate, and nitrate on the fluoride ion local attack of Ti-Grade 7 (Ti-7: UNS R52400) were investigated. It was observed that the most severe local attack on Ti-7 occurred at approximately 120°C, and at neutral pH. A large chloride: fluoride ratio (10 : 1 by mole) was necessary to produce visible severe attack in immersion tests, but electrochemical studies detected a narrowing of the passive region at chloride: fluoride ratios as small as 1.1 : 1 by mole. The addition of large amounts of sulfate slowed, but did not inhibit pitting and stress corrosion cracking of Ti-7 U-bends, but the addition of nitrate is suspected to act as an effective inhibitor. Polishing of the titanium surface to micron scale also decreased the likelihood of the initiation of local attack. INTRODUCTION Previous immersion 1 and electrochemical 2 studies of Titanium Grade 7 (Ti-7) have shown that Ti-7 is highly resistant to corrosion in aqueous media in the absence of fluoride. However, Ti-7 is subject to severe local attack in solutions containing fluoride, even at near-neutral pH and temperatures near boiling. 1 Groundwaters from the saturated zone (J 13 water) as well as from the unsaturated zone (UZ pore water) at the vicinity of the proposed repository at Yucca Mountain have been found to contain low, but non-zero, concentrations of fluoride. Both in reference J 13 and in reference UZ pore water, fluoride concentrations range from 2.16 and 2.3 mg/L, 3 while fluoride levels in waters collected from other Yucca Mountain wells were measured to be between 1.3 and 4.8 mg/L. 4 Open thermal heating of the J 13 reference water at 85°C to a volumetric concentration of a factor of 950 has been shown to cause a 700-fold increase in the level of both chloride and fluoride. 3 Previous tests solutions 1'2 that produced the most severe local corrosion of Ti-7 by fluoride were 34 those that contained an excess of chloride over fluoride, as is the case in groundwater. ' Preliminary indications from longer-term tests 1 also indicated that in the presence of chloride the fluoride attack is enhanced. This implies that, in the presence of excess chloride, fluoride may be able to initiate an attack on Ti-7 at much lower concentrations of fluoride than previously thought. Because an excess of chloride over fluoride is also present in the representative Yucca Mountain reference waters, a study of the effect of the ratio of chloride: fluoride was conducted. It was observed in one of the preliminary experiments 1 that removal of sulfate (by precipitation) accelerated the stress corrosion cracking of Ti-7 in aqueous solutions containing fluoride and chloride, suggesting that high concentrations of sulfate, known to inhibit corrosion in many industrial cleaning processes ~ may inhibit the local attack on Ti-7 by fluoride and chloride. Accordingly, studies that systematically investigate the effect of chloride" fluoride ratio, and the effect of the presence of excess sulfate, as well as nitrate, which is also found commonly in groundwater, are described in the present paper. Both immersion tests and electrochemical tests were conducted. In addition, the study addressed the problem of initial surface condition, since surface flaws are well known to accelerate the onset of corrosion. 1
- Well Completion > Well Integrity > Subsurface corrosion (tubing, casing, completion equipment, conductor) (1.00)
- Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers > Materials and corrosion (1.00)
- Production and Well Operations > Production Chemistry, Metallurgy and Biology > Corrosion inhibition and management (including H2S and CO2) (0.76)