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Zirconium is a reactive but corrosion resistant metal. However, in a highly oxidizing or an oxygen deficient solution, corrosion of zirconium can be rapid and a pyrophoric film, which ignites easily, may be formed on the surface of zirconium. Although high corrosion rates only infrequently lead to the formation of pyrophoric films, great care should be exercised with testing coupons or equipment where rapid attack is suspected.
Laboratory tests were conducted to find methods to treat pyrophoric films on zirconium. Burnable specimens were prepared by placing zirconium coupons in 77.5% sulfuric acid (H2SO4) with 200-ppm Fe+3 (as ferric sulfate) at 80°C for 10 days without stirring or agitation. Chemical, X-ray diffraction, and metallographic analyses indicate that the pyrophoric film is composed of γ-hydride, zirconium oxide, zirconium sulfate, and metallic particles. This pyrophoric film could not be removed easily using common pickling solutions, that is, nitric acid-hydrofluoric acid (HNO3-HF) mixtures, or other aggressive media, for example, 80% H2SO4.
It has been found that the pyrophoric film can be rendered nonpyrophoric by treating with hot air or steam. A 20 to 30 min treatment with 250°C air or steam is sufficient to completely eliminate the pyrophoric tendency. Lower temperature air or steam can be used but require longer time: a few hours if the temperature is 200°C, and a few days if the temperature is 100°C. Use of this treatment in the field has proven effective.
zirconium, pyrophoric film, hydrides, metallic particles, hot air treatment, steam treatment
Corrosion group head, Metallurgical Research and Development, Teledyne Wah Chang Albany, Albany, Ore.