SCALING OF ZIRCONIUM AT ELEVATED TEMPERATURES. Final Report

Abstract

The three phenomena observed in the scaling of zirconium, i.e., the change in scaling rate, the color change of the oxide, and growth of the underlying metal, were investigated in the temperature range of 600 to 1300 C. Three grades of zirconium, varying in impurity content, were studied in atmospheres of air, oxygen nitrogen, and oxygen-nitrogen mixtures. The three phenomena were found to occur in each grade of zirconium investigated and in all atmospheres except nitrogen. The times required for their occurrence are temperature dependent and are generally decreased by increasing impurity level of the metal. Zirconium initially scales at a relatively low rate which, in time, yields to a faster scaling rate (breakaway). Breakaway does not occur above 1000 C in air, however, no such limiting temperature was observed in oxygen. The initial oxide that forms on zirconium is black. This oxide transforms to white upon continued scaling. X-ray analyses revealed no difference between these scales, both are composed of tetragonal and monoclinic ZrO/sub 2/ with the monoclinic form predominating. This color transition of the oxide appears to be due to the absorption of oxygen and/or nitrogen. The growth suffered by zirconium upon scaling appears to be the resultmore » of a deformation process rather than a volume expansion. Growth was not observed above 1000 C in any atmosphere. This lack of high temperature growth in air appears to be due to a unique globular distribution of zirconium nitride in the oxide. The absence of high temperature growth in oxygen is associated with the manner in which cracks form in the scale resulting in the absence of oxide fingers. Experimental results suggest that all three phenomena are interdependent. A constant relation between the three cannot be stated as it appears to vary with purity of the zirconium and scaling temperature. The most general relationship appears to be as follows: scale color change leading to breakaway which, in turn, leads to growth. (auth)« less