High Temperature Oxidation Kinetics of a Zr-21/2 w/o Nb Alloy

Abstract

<p>An investigation is reported on the oxidation kinetics of a Zr-21/2 w/o Nb alloy in the temperature range 650° - 1000°C in pure oxygen atmospheres. Weight gain data is presented for rectangular plate specimens in the range 650° - 991°C. The kinetics were determined continuously using thermogravimetric techniques and may be represented by a parabolic relationship followed by essentially linear kinetics over the range 650° - 885°C, while over the range 937°- 991°C, parabolic kinetics is followed by a breakaway transition to an increasing reaction rate. In the low temperature range the parabolic rate constant has been calculated to be k = 20.8 exp (-38026/RT) and in the high temperature range kp = 0.22 exp (-30054/RT).</p> <p>The latter part of the investigation was focused on the oxidation of Zr-21/2 w/o Nb at 1000°C in pure oxygen for periods extending to 23 hours. The morphological development of the oxide scale and metal substrate during parabolic oxidation was investigated and is supported by kinetic, X-ray, electron micro-probe and metallographic observations. Spherical specimens were used to minimize edge effects and hence extend the period of parabolic behaviour. The observed microstructures are discussed with reference to the Zr-Nb-0 phase diagram.</p> <p>A qualitative discussion pertaining to observations related to the breakaway phenomenon is also presented.</p>