Cathodic Protection in Molten Fluoride Salts at 1200 F

Abstract

Abstract Corrosion is a major problem in the Fluoride Volatility Process for recovery of nuclear fuel. Relatively large scale pilot plant developments are under way by the Chemical Technology Division of the Oak Ridge National Laboratory. In this process fuel is dissolved by hydrogen fluoride in a molten bath consisting of mixtures of such salts as NaF-LiF-ZrF4. Usual operating temperatures are near 1200 F (649 C). Because metals such as stainless steel and zirconium are to be dissolved in this process it is obvious that the salt mixture must be quite corrosive. This is particularly true because HF is sparged through the bath continuously. Experiments were carried out at Battelle on a laboratory scale to determine the feasibility of using cathodic protection to reduce the attack in process vessels. Polarization curves were developed under simulated operating conditions for the materials of interest, i.e., Inor-8, Zircaloy and graphite. Polarization characteristics were obtained by use of a potentiostat using platinum as a reference electrode and graphite as an anode. Experiments using rectangularly shaped coupons were carried out to determine the amount of protection furnished by the impressed current technique in the most corrosive salt composition. Specimens were made cathodic at current densities ranging from 3 to 50 amp/ft2. Excellent protection was furnished to the submerged portions of the specimens. However, severe attack occurred at the interface at reasonable current densities.