AN ELECTROCHEMICAL DETERMINATION OF THE CHEMICAL DIFFUSIVITY IN BINARY LIQUID-METAL ALLOYS (LEAD-CADMIUM, AMALGAM, ION D, THERMODYNAMIC PROPERTIES)

THOMAS J BARLO, Purdue University

Abstract

This research was undertaken to develop a new technique to determine the chemical diffusivity in liquid-metal alloys reliably and conveniently. In the technique, diffusate is deposited from the elec- trolyte onto a column of alloy by a constant current as an electrode at a fixed position in the diffusion zone monitors the potential-time history that describes diffusion in the alloy during and after elec- trolysis. This diffusion coefficient is determined from the slope of linear asymptotic solutions to Fick's second law for the boundary conditions. This technique was explored first in a cell that contained Cd-Hg alloy electrodes and a CdCl(,2) + KCl aqueous electrolyte. The diffu- sion coefficients in two Cd-Hg alloys were determined at nominally 25 C as: (UNFORMATTED TABLE FOLLOWS) N(,Cd) = 0.0353, D = 0.16 (+OR-) 0.05 x 10('-5) cm('2)/sec N(,Cd) = 0.0473, D = 0.42 (+OR-) 0.09 x 10('-5) cm('2)/sec (TABLE ENDS) Nonuniform current distribution over the alloy surface during elec- trolysis was believed to have been responsible for lack of better agreement with an accepted value of 1.5 x 10('-5) cm('2)/sec for D. This technique was applied to five molten Pb-Cd alloys from 375 to 500 C in a cell using a fused-salt electrolyte that contained 5 weight percent CdCl(,2) in the eutectic mixture of KCl + LiCl. The experimental results were marked by unexpected curvature in the potential-time histories not observed with the amalgams, and rela- tively poor reproducibility of the data. Impedance of the diffusion flux by the diffusion-monitoring electrode was believed to be the primary reason for the unexpec- ted behavior. Approximate solutions to the boundary conditions assuming radial diffusion from the monitoring electrode gave the following values for the diffusion coefficient: (UNFORMATTED TABLE FOLLOWS) N(,Cd) = 0.2327, D = 1.4 x 10('-4) exp (-1,655/RT) cm('2)/sec N(,Cd) = 0.4025, D = 9.37 x 10('-1) exp (-15,920/RT) cm('2)/sec N(,Cd) = 0.8046, D = 2.21 x 10('-1) exp (-13,430/RT) cm('2)/sec. (TABLE ENDS) The results agree with those from other investigators. Ancillary cell-potential measurements were obtained that permitted determination of the diffusion coefficient of Cd('++) in the fused-salt electrolyte, and the determination of the thermodynamic properties (DELTA)(')G(,Cd), (DELTA)(')H(,Cd), and (DELTA)(')S(,Cd) for five Pb-Cd alloys. The Arrhenius equation for diffusion of Cd('++) in the fused salt was determined as: D(,Cd)('++) = 3.2 exp (-15,800/RT) cm('2)/sec. The Arrhenius parameters were greater than the only other set of values determined for this system. The partial molar properties for cadmium determined from the cell emf were in very good agreement with selected values in the literature.

Degree

Ph.D.

Subject Area

Materials science

COinS