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Three zirconium alloys (Zry-4, Zry-2, and NDA) were corrosion tested at 633 K in pure water and in mixed gas of Ar and oxygen. In both environments, their corrosion rate showed cyclic changes. The time and the weight gain at the transition point changed with their corrosion resistance. In the pure water corrosion test, one test cycle was 600 h, and the cumulative total test duration was as much as 7200 h. After each test cycle, the weight changes and the hydrogen contents were analyzed. The hydrogen pick-up ratio was calculated for each test cycle based on the weight change and the increase of the hydrogen content. The calculated hydrogen pick-up ratios were not constant. They showed periodic changes. In the pre-transition region, the corrosion rate showed a gradual decrease; however, the calculated hydrogen pick-up rate gradually increased. After the transition, in contrast to the increase in the corrosion rate, the calculated hydrogen pick-up rate decreased. The changes of the corrosion rate and the calculated hydrogen pick-up rate for the test interval exhibited an inverse relationship. Transmission electron microscopy (TEM), scanning TEM, energy dispersive x-ray (EDX) spectroscopy, and electron diffraction (ED) pattern analysis of the oxide/metal interface showed the existence of an interface layer that consists of distorted ZrO2 and a sub-oxide. Before the transition, the interface layer was clearly observed. On the other hand, the interface layer disappeared after the transition. The cyclic changes in the corrosion rate were closely related to the interface barrier layer at the oxide/metal interface. The interface layer suppresses the corrosion rate, and the degradation of its barrier property increases the corrosion rate. By comparing the morphology of the oxide/metal interface between the water corrosion samples and the mixed gas oxidation samples, it can be seen that the generated hydrogen in the oxide may have two opposite effects. One is to stabilize the interface layer and to suppress the transport of the oxygen ion, and the other is to degrade the interface layer at the transition and accelerate the generation of the uniform oxide. The effect of the chemical composition on the properties of the oxide-metal interface and the mechanism of the corrosion transition will be discussed.
zirconium alloys, corrosion, transition, hydrogen pick-up, sub-oxide, barrier layer, interface layer
Senior researcher, Kobe Special Tube Co., LTD.,
Manager, Zirco Products Co., LTD.,