OXYGEN DIFFUSION IN THE OXIDE AND ALPHA PHASES DURING REACTION OF ZIRCALOY-4 WITH STEAM FROM 1000-DEGREE-C TO 1500-DEGREE-C

The reaction of Zircaloy in steam at elevated temperatures involves the growth of discrete layers of oxide and oxygen-rich alpha Zircaloy from the parent beta phase. The multiphase, moving boundary diffusion problem thus described is encountered in a number of important reaction schemes in addition to that of Zircaloy-oxygen, and can be completely characterized through an appropriate ideal model in terms of oxygen diffusion coefficients and equilibrium concentrations for the various phases. Conversely, kinetic data for phase growth and total oxygen consumption rates can be used to compute diffusion coefficients. In this paper, equations are developed which express the oxygen diffusion coefficients in the oxide and alpha phases in terms of the reaction rate constants and equilibrium solubility values. These equations were applied to recent experimental kinetic data on the steam oxidation of Zircaloy-4 to determine the effective oxygen diffusion coefficients in these phases over the temperature range 1000°-1500°C. Least squares treatments of the data for diffusion in the tetragonal oxide phase (ϕ) and in the oxygen-stabilized alpha Zircaloy (α) yielded the expressions: Dϕ = 0.1270 exp [−34510/RT(K)] cm2/sec and Dα = 3.920 exp [−51000/RT(K)] cm2/sec. © 1979, The Electrochemical Society, Inc. All rights reserved.