Steam Corrosion Kinetics of Zircaloy-4

The steam corrosion kinetics of Zircaloy-4 in the temperature range of 1 000°C-1 300°C has been studied. The Zircaloy-4 cladding tube was used as the experimental material. The experimental device is composed of a LGP-C high frequency furnace, a quartz glass reaction chamber, a small stainless steel high-pressure boiler and a WGJ-01 optical pyrometer. The weight gain method is used to investigate the steam corrosion kinetics of Zircaloy-4; it involves heating several specimens for various lengths of time at a fixed temperature in a flowing superheated steam environment. The experimental results show that, during the initial corrosion period, the relation between the weight gain of Zircaloy-4 and time is in keeping with the parabolic equation, and the relation between the parabolic rate constant K and the temperature T(K) is in keeping with the Arrhenius equation: K=2.008× 10⁸· exp(-17 826 / T). With increasing length of the corrosion time, the corrosion kinetic curve gradually departs upwards from the initial parabolic curve, indicating that the corrosion kinetics no longer follows the initial parabolic law. The longer the corrosion time, the greater the deviation, and the corrosion rate is accelerated. Scanning electron microscopy analysis reveals a large number of surface microcracks. The microcracks in the oxide film not only shorten the diffusion distance for oxygen ions, but also increase the surface area for the decomposition of water. As a result the corrosion rate increases greatly, and the accelerated corrosion seen after long times corrosion is due to microcracking of the oxide surface.