Zirconia, prepared by calcining high-purity zirconium sulfate, is chlorinated to produce zirconium tetrachloride, which is reduced to metal with calcium in the final steps of the modern extension of the kroll process for preparing zirconium. The critical chlorination step is done in a fluidized bed reactor in the presence of carbon, and the reaction products plus unreacted chlorine and entrained solids are conducted to a separator where the unreacted solids are removed. The ducting and separator, which are constructed from 1/4- inch nickel sheet, are subjected to severely corrosive conditions, which result in embrittlement and reduction in cross section of the nickel ducts. Corrosion testing of nickel in the temperature range 400 deg to 600 deg c in an atmosphere containing he, cl2, h2o, CO, and co2 to simulate industrial conditions, produced specimens exhibiting uniform surface attack similar to the corrosion of nickel in pure chlorine. Since these observations were not consistent with the industrial service failures, a section of corroded nickel sheet was obtained from an industrial zirconia chlorinator and investigated to determine the corrosion mechanism.
Materials Protection and Performance, V. 10, No. 11, November 1971, PP. 22-24