2016 Volume 80 Issue 4 Pages 231-239
The oxidation behavior of Zircaloy 4 in Ar-H2O and Ar-O2-H2O (saturated water vapor at 323 K) gas mixtures has been examined under several temperature conditions using hydrogen and oxygen sensors to investigate the dependence of oxygen concentration and temperature changes. The results are summarized as follows. The amount of consumed oxygen that contributed to the oxidation could be calculated from the change in the oxygen partial pressure measured by the oxygen sensor. Furthermore, the amount of hydrogen generated through steam oxidation could be calculated from the change in the hydrogen partial pressure measured by the hydrogen sensor, and the amount of consumed oxygen could be concomitantly calculated. The relative contributions of oxygen-consuming and steam oxidation could be calculated with respect to the oxidation of Zircaloy. The oxygen-consuming oxidation proved to precede in the oxidation reaction, and the oxidation by steam oxidation progressed after the oxygen contained in the original gas had been completely consumed. When conducting isothermal oxidation experiments at 1173 K, only the oxygen-consuming oxidation occurred in an atmosphere above 1% oxygen without the progression of steam oxidation. At 1523 K, steam oxidation was observed even in 5% oxygen, because oxygen was intensely consumed in the initial stage of the reaction. When conducting temperature-increased oxidation experiments, oxygen was completely consumed in an atmosphere with 1% oxygen at approximately 1358 K, above which steam oxidation progressed. When performing thermal cycling oxidation experiments, it was clearly demonstrated that the region in which only oxygen-consuming oxidation occurred and the region in which steam oxidation added to the oxygen-consuming process on varying the temperature. As the cycling frequency was increased, the total amount of oxidation decreased and the contribution from the steam oxidation also decreased. The thermodynamics of the processes of oxygen-consuming and steam oxidation has been discussed.
