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A review has been made of the effects of fluoride ions on the corrosion behavior of zirconium alloys in high-temperature water. Corrosion was found to occur as the result of contamination of the water or the zirconium surface. A major source of fluorides is undue delay in rinsing the HF-HNO3 pickling solution during surface preparation. The oxide on corrosion-resistant material has been found to contain up to 7600 ppm fluoride from this source. The threshold concentration of fluoride in the oxide film which produces poor corrosion resistance ranges from 8500 to 17,000 ppm. Accelerated corrosion from fluorides in water at 300 to 360 C occurs at about 100 ppm, although increased corrosion has been reported at 10 ppm fluoride in water at 300 C. Fluorocarbon plastics degrade and contribute fluorides to the high-temperature water or to the alloy surface when in direct contact with zirconium. Chlorides (1 to 10,000 ppm) and iodides (1270 ppm) do not adversely affect the corrosion behavior of zirconium alloys in water at 360 C. The mechanism of corrosion is not well understood but apparently is related to the formation of insoluble zirconium oxyfluorides during pickling and during exposure to fluoride-contaminated water.
Berry, Warren E.
Assistant Chief, Battelle Memorial Inst., Columbus, Ohio