Effects of Fluoride and other Anions on the Corrosion of Alloy 22
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Effects of Fluoride and other Anions on the Corrosion of Alloy 22 A.L. Pulvirenti, 1 K.M. Needham, 1 M.A. Adel-Hadadi, 1 and A. Barkatt1 C.R. Marks2 and J.A. Gorman2 1
The Catholic University of America, Washington D.C. 20064 Dominion Engineering, Inc., 11730 Plaza America Drive Reston, VA 20190 2
ABSTRACT Samples of Alloy 22 were tested in solutions containing various anions in order to determine their effect on the corrosion of the alloy. It was found that Alloy 22 is relatively corrosion resistant in HCl and HNO3 at pH 1 and 160°C (general corrosion rates on the order of 10 µm/year), but more susceptible to phosphoric acid, especially under reducing conditions. The presence of fluoride raised the corrosion rate of Alloy 22 to the order 1 mm/year at pH 1, and fluoride is still active towards Alloy 22 at pH levels as high as 3.5. Samples tested in solutions of 1000xJ13 in which the pH was altered during testing showed an increase in corrosion rate over solutions of constant pH. Preliminary electrochemical tests suggest that nitrate may be an effective corrosion inhibitor in fluoride containing solutions, while sulfate is not.
INTRODUCTION A major criterion for the selection of container materials for use in a geologic repository is the ability to withstand corrosion by groundwater. Accordingly, compositions of nearby groundwater systems are important in testing candidate container materials. In the case of the proposed Yucca Mountain repository, saturated zone water [1, 2] is represented by J13 well water. J13 water has been found to contain fluoride at levels of approximately 2 ppm, which is expected to increase as the water thermally concentrates [2], which may be aggressive toward container material. The element phosphorous has also been identified in the Yucca Mountain rock as P2O5 at a level of 0.02% [3], and trace amounts (1 ppm) of the phosphate ion have been detected in J13 water. [4] The reference J13 water is not the only water that can be expected to be present under repository conditions. The waters in contact with the container may mix with unsaturated zone pore waters, and undergo pH changes during service life. Recent work [5] has shown that highly acidic solutions can be obtained from the evaporation of concentrated pore waters, and that evaporation and distillation of J-13 waters will result in solutions of high pH. In the present work, the container material candidate Alloy C-22, a high nickel alloy, was tested in solutions of simulated concentrated J13 groundwater with added the anions phosphate and fluoride, which can be expected to be found in the repository environment. Because the container environment may evolve and change more than once over the repository lifetime, the material was also exposed to an environment of changing
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pH to determine if extremes of pH would effect the passive surface of the alloy. The effectiveness of the inhibitors nitrate and sulfate was tested by electrochemical means. EXPERIMENTAL The corrosion behavior of Alloy 22 (UNS NO6022) was investig
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