Zirconium alloy is frequently used in the nuclear industry as a reactor fuel cladding material. Uniform corrosion of zirconium alloys has received much attention because it is one of the material's life-limited properties. To study their long-term uniform corrosion behavior, two types of Zr-Sn-Nb-Fe-V alloy claddings, one with a high niobium content and one with a low niobium content, were exposed to 400^oC and 10.3 MPa of superheated steam for 800 d. Both alloys clearly exhibit periodic oxide densification-transition. Different quantitative methods were used to study the evolution and mechanism of multiple oxide features. The periodic layers of oxide morphologies are still observed even after two times of transitions. Periodically, columnar grains and the defect layer appear. The undulation intensity (related to the “cauliflower” morphologies) and the equivalent thickness of tetragonal zirconia (t-ZrO₂) at the metal/oxide interface increase with corrosion time in pretransition oxides and decrease at transition time. The evolution of both features in the subsequent oxide densification-transition period is the same as initial densificationtransition cycle. The critical value of the interface undulation intensity is approximately 0.25 for both alloys. The critical thickness of t-ZrO₂ is approximately 450 nm for low-niobium alloy and approximately 200 nm for high-niobium alloy. Both features periodically reach critical conditions. The evolution of undulation intensity provides an excellent explanation for the production of isolated and interconnected lateral cracks. Additionally, the transformation of t-ZrO₂ to monoclinic zirconia (m-ZrO₂) results in cracking of the oxide and produces interconnected, tiny equiaxed defects at the interface. Both lateral cracks and equiaxed defects are both important components of the defect layer. It is hypothesized that the synergetic effects of interface undulation and t-ZrO₂ transformation affect the transition. The occurrence of periodic transitions is strongly correlated with the periodic behavior of oxide features reaching critical conditions. The volume of oxygen vacancy in t-ZrO₂ was presumably evaluated by studying the Raman shift of the characteristic t-ZrO₂ peak of 280 cm^-1. As a result, the amount does not change considerably during oxide densification but decreases during the transition period. The Raman shift of t-ZrO₂ in low-niobium alloys is approximately -1.2 cm^-1 less than that in high-niobium alloys, indicating that the low-niobium alloys have a greater volume of oxygen vacancies. It is proposed that the difference in the corrosion behavior of two alloys is derived from the difference in the volume of oxygen vacancies.