The influence of diffused carbon on the structure and oxidation of hot-pressed Zircaloy-2

The influence of diffused carbon on the structure and oxidation behaviour of hot-pressed Zircaloy-2 was studied. The amount of carbon in the hot-pressed compacts increased with hot pressing time. Micrographs revealed a Widmanstaetten type of structure and the number and type of grains varied with carbon content. In compacts that had been hot pressed for more than 60 min zirconium carbides were observed predominantly in regions close to the edges and along grain boundaries. In chemical etching studies the corrosion of Zircaloy-2 was found to depend on the nature of the etchant. In very dilute etchants the carbides were etched preferentially whereas in stronger etchants regions close to the carbides were etched. Thermogravimetric measurements during isothermal oxidation at 350, 400 and 4500C revealed an increase in weight gain per unit area with hot pressing time during the period of oxidation between 40 and 200 min; no appreciable change in oxidation rate with carbon content was observed thereafter in the pretransition region. The high oxidation rate is attributed to the preferential oxidation of grain boundary carbides. (Auth.)