The Solubility and Diffusivity of Fluorine in Solid Copper from Electrochemical Measurements
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9.32
x 10-2 exp
r98,9~:/mole).
The results obtained for the dissolution of fluorine as atoms in solid copper showed large scatter. However, the equilibrium dissolution of fluorine follows Sieverts' law. Above the melting point (770°C) of CuF 2 , the mean solubility of fluorine in solid copper, for the equilibrium Cu(s) + CuF 2 (Z) , follows the relationship N
.
F(atom Iraction) = 0.98
,79500J/mole exp \- ' R T ).
INTRODUCTION MANY chemical and extractive metallurgical processes, including the gaseous diffusion isotope separation method, involve halogen gases and volatile metal halide species. In such intermediate temperature halide processes, metals are generally required as the materials for construction of the system hardware. Because of the high reactivity of halogens with metals, scaling and related degradative corrosion processes occur in service. In comparison to the oxidation of metals and alloys, the detailed mechanisms of halogenation of metals are very poorly characterized. A recent review' surveys the characteristics of the high-temperature halogenation of metals. In relatively dilute alloys containing a noble metal solvent and a more reactive solute, the selective internal fluoridation of the reactive component within the matrix would be dependent upon the dissolution and diffusion of fluorine atoms into the matrix. The literature contains no information concerning the magnitude of the solubility or the diffusivity of any halogen in any metal. In this paper, electrochemical experiments to determine the diffusivity and solubility of fluorine in solid copper at temperatures from 757 to 920°C are described. Also, the solid CaF 2 electrolyte is shown to be applicable to kinetic experiments. THEORY A potentiostatic electrochemical experiment involving the CaF 2 solid electrolyte is used in this work for the determination of the diffusivity and solubility of fluorine in solid copper. The method is similar to S. N. S. REDDY is Research Associate and ROBERT A. RAPP is Professor, Department of Metallurgical Engineering, TheOhio State University, Columbus, OH 43210. Manuscript submitted April 10, 1978. METALLURGICAL TRANSACTIONS B
that used by Pastorek and Rapp" for the determination of solubility and diffusivity of oxygen in copper using the oxygen ion-conducting, stabilized-zirconia electrolyte. Solid CaF 2 is essentially an exclusive fluorine ion conductor of electricity.3,4 For a galvanic cell of the form Ni + NiF 2 1CaF 2 1Cu + F (dissolved)
[I]
the cell voltage E can be related to the activity of fluorine atoms (aF) in the copper electrode at the electrode/electrolyte interface according to the expres-
ston:" E - lion Q ion =
1,
F
"RT
(J.1.F- J.1.F) =
F
aic(Cu) In a'F (Ni + NiF 2 ) [lJ
where lion is the ionic current, F is the Faraday constant, Qion is the ionic electrical resistance of the electrolyte, J.1.F is the chemical potential of fluorine atoms, and the prime and double prime refer to the right- and left-hand electrodes, respectively. The standard state for fluorine atoms is defined
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