Influence of Heating Modes on Heat-Resistance of Zr and Zr–1% Nb Alloy

Differences in the oxidation kinetics of zirconium and Zr–1% Nb alloy during heating in air depending on the heating rate, temperature and exposure time are revealed. The increase in the heating rate from 2.5 to 6 and 7.5°C/min reduces the activation energy of the Zr oxidation process in the temperature range of 20–1000°C from 70.2 to 67 and 52.7 kJ/mol, respectively. For the Zr–1% Nb zirconium alloy, increase in the heating rate from 5 to 10 and 20°C/min causes an increase in the activation energy of the oxidation process from 65 to 70.1 and 78.5 kJ/mol, respectively. Such an increase in the heating rate (of zirconium from 2.5 to 7.5°C/min, and of the Zr–1% Nb alloy from 5 to 20°C/min) causes a decrease in the thickness of the ZrO₂ oxide film. During isothermal exposure for 5 h at 750°C, Zr–1% Nb alloy and Zr at a temperature of 800°C are oxidized according to the parabolic law. At 800°C Zr–1% Nb alloy oxidizes according to the combined law: first, parabolic, and then quasi-linear.