The oxidation of zircaloy-4 at 900 and 1100°c in high pressure steam

Abstract

Oxidation experiments on zircaloy-4 PWR tube specimens were conducted at 905 and 1101°C in flowing steam at piessures up to 10.34 MPa (1500 psi). Maximum exposure times were about 45 min at 905°C and 10 min at 1101°C. The growth characteristics of the oxide and oxygen-stabilized alpha layers after these exposures were examined and compared to those observed earlier for oxidation in steam at atmospheric pressure. At 1101°C, no differences in the growth rates of the layers were evident. However, the trend of the data at the lower oxidation temperature indicated that an increase in the rate of oxide layer growth does occur with increasing pressure. The magnitude of the observed effect on the oxide layer growth at 905°C was small for the reaction times studied and for pressures up to 6.9 MPa (1000 psi). For example, the maximum increase in oxide thickness observed at 6.9 MPa was less than 50% for times up to 30 min, and the kinetics continued to show negative deviations from ideal parabolic growth behavior. However, the few experiments at the highest pressure yielded oxide layer thicknesses approximately twice as thick as those anticipated for oxidation at atmospheric pressure. While a detailed mechanism is not apparent, metallographic examination of these oxide layers revealed that part of each layer was different structurally compared to that found for oxidation at atmospheric pressure. It is suggested that this difference is responsible for the increased growth rate of the oxide layer.