Effect of Anions on the Fluoride Corrosion of Titanium-Grade 7
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Effect of Anions on the Fluoride Corrosion of Titanium-Grade 7 A.L. Pulvirenti,1 K.M. Needham,1 M.A. Adel-Hadadi,1 D.S. Wong,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
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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 a chloride: fluoride ratio of as high as 10 : 1 by mole was necessary to produce visible severe attack in immersion tests. Localized attack on Ti-7 was most severe at approximately 120°C, and at neutral pH. However, electrochemical studies detected that a narrowing of the passive region can occur at chloride: fluoride ratios as small as 1.1 : 1 by mole. The addition of sulfate did not significantly inhibit pitting or stress corrosion cracking of Ti-7 U-bends. However, the addition of nitrate is suspected to act as an effective inhibitor. INTRODUCTION Previous electrochemical studies [1] on Titanium Grade 7 (Ti-7) have shown that Titanium Grade 7 (Ti7) is highly resistant to corrosion in aqueous media in the absence of fluoride, but subject to accelerated uniform dissolution in solutions containing fluoride. The corrosivity of these solutions were confirmed by immersion testing [2], in which Ti-7 underwent pitting and stress corrosion cracking, even at nearneutral pH and temperatures near boiling. J13 (saturated zone) and UZ (unsaturated zone) groundwaters in the vicinity of the proposed repository at Yucca Mountain have been found to contain low, but nonzero, concentrations of fluoride of approximate 2 ppm. [3,4] The concentration of fluoride is expected to rise upon thermal evaporation to a concentration of 1500 ppm. [3] Previous tests [2] suggest that an excess of chloride in the presence of fluoride can enhance the fluoride attack on Ti-7. 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. In preliminary experiments [1] 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 [5] may inhibit the local attack on Ti-7 by fluoride and chloride. In this paper, the effect of chloride: fluoride ratio and the effect of the anions sulfate and nitrate on the fluoride attack on Ti-7 were investigated by both immersion and electrochemical techniques. In addition, long-term tests on T-7 U-bends in solutions containing progressively lower levels of fluoride were begun. EXPERIMENTAL DETAILS In all immersion tests, disks (1 inch diameter x 1/8 inch thick) or U-bends (1/8 inch thick, ½ inch bend radius) of Ti-7 (Pd: 0.14%; O: 0.1%; Ti: balance) were mechanically polished to the desired surface finish (600 grit for all), marked to introduce surfac
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