## 3 Results and Discussions
### Macro- and Micrograph Results
It should be noted that the porous oxides were only detected in light-colored oxides, and the dense oxides were observed in dark-colored oxides. Spotted cracks in the oxides and nearby metal oxide-metal interfaces were observed and were probably generated during the grinding and polishing steps due to the specimen's embrittlement [8]. Figure 5: Macrographs of selected Zircaloy-4 after 2 and 4 h isothermal tests under various oxygen flow rate conditions at 1000 \({}^{\circ}\)C: (**a**) 20 mL/min, (**b**) 40 mL/min, (**c**) 60 mL/min, (**d**) 80 mL/min, (**e**) 100 mL/min, (**f**) 120 mL/min, (**g**) 140 mL/min, (**h**) 160 mL/min, (**i**) 180 mL/min, (**j**) 200 mL/min, (**k**) 180 mL/min, (**l**) 200 mL/min; (**a-j**) 2 h, (**k**,l**)): 4 h.  
In this study, we aimed to identify the effect of initial temperature escalation on oxidation behaviors and oxide characteristics by isothermal oxidation tests at 1000 \({}^{\circ}\)C. An initial sharp temperature escalation was observed, and its degree increased with an increase in the oxygen flow rate. The initial increase in the mass-gain rate is also attributed to the sudden increase in temperature [1; 4; 6]. After the significant increase in the reaction rate, it followed the almost (sub-)parabolic reaction law; however, it was approximately zero for 180 and 200 mL/min. This might be due to the sudden growth of a thick oxide layer, which acts as a strong diffusion barrier. Even though each case showed a different reaction rate and time for each reaction regime, mass gain followed the general oxidation kinetics. One of the most interesting points in this work is the delay of breakaway when the peak temperature exceeded 1050 \({}^{\circ}\)C. This result could be identified by both TG mass-gain data and post-test investigations. Note that the thickness of the dense columnar oxide increased with an increase in the initial peak temperature, as shown in Table 1. The thickness of dense columnar oxide is assumed to be equal to the thickness at the breakaway because these two thickness values showed similar scales reported by [PERSON] et al. [1]. In addition, they already discussed the relationship between the thickness of the oxide at the transition and the dense oxide, and the thickness of dense oxides is strongly dependent on temperature. This also seems to be related to former findings that the formation of  
Figure 6: Micrographs of selected Zircaloy-4 after 2 and 4 h isothermal tests under various oxygen flow rate conditions at 1000 \({}^{\circ}\)C: (**a**) 20 mL/min, (**b**) 40 mL/min, (**c**) 60 mL/min, (**d**) 80 mL/min, (**e**) 100 mL/min, (**f**) 120 mL/min, (**g**) 140 mL/min, (**h**) 160 mL/min, (**i**) 180 mL/min, (**j**) 200 mL/min, (**k**) 180 mL/min, (**l**) 200 mL/min; (**a**–j**): 2 h, (**k**,**l**): 4 h.  
dense oxide is observed for the entire oxide layer at the highest temperature range, with the absence of breakaway [1,4]. Moreover, it should be noted that the abrupt delay of the breakaway began when the peak temperature overlapped with the tetragonal stabilization temperature region. The highest tetragonal phase fraction in the oxide scale is probably anticipated to be 200 mL/min, which shows the longest time to breakaway in this work. It was suggested that lower tetragonal phase fractions in oxides might reduce the internal stress by phase transformations, which is one of the most important factors leading to a breakaway. Hence, both an improved corrosion resistance and delay of breakaway can be expected [10], but the reverse tendency is observed for the breakaway in our results. It is hard to understand this consequence because it is possibly interrelated with many factors, i.e., the oxide phase [10, 11, 12], microstructure and grain boundary characteristics [13, 14, 15], and so on. To investigate these factors, additional analyses using Raman spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), and other apparatuses are necessary.