High Temperature Oxidation and Nitriding Kinetics of Zirconium

Abstract

<p> An investigation is reported on the oxidation properties of alpha-zirconium at 850°C and beta-zirconium at 950°C in oxygen for periods extending to 400 hr. and 100 hr. , respectively. Nitriding kinetics of zirconium in the range of 750° to 1000°C up to 200 hr. were investigated. The kinetics wen determined by volumetric and gravimetric techniques and may be represented by' a parabolic relationship after a period of more rapid oxidation. The uptakes of oxygen or nitrogen were consistent with the mathematical evaluations based on multi-phase diffusion models. Two diffusion models were advanced; one based upon differential and the other upon integral solutions of diffusion equations. It was possible to separate quantatively the oxygen or nitrogen partitions in the scale, alpha and beta phases of zirconium. </p> <p> The diffusivity of nitrogen in alpha-zirconium was determined by using transverse microhardness measurements. The diffusivity is: D = 0.15 exp (-54100/RT)cm/sec^2 for the temperature range of 750°- l000°C. </p> <p> The influence of oxygen-nitrogen atmospheres on the scaling rate of alpha-zirconium at 850°C was investigated. Small additions of either gas to the other increased the sealing rate. A definite breakaway point was observed in the scaling kinetics and the time interval to the transition point varied with the relative amounts of nitrogen to oxygen. </p> <p> Scaling rates of zirconium at 850° and 950°C in the oxygen-water vapor atmospheres initially obeyed to a good approximation a parabolic relationship which was followed by a much faster scaling rate. </p>