Investigations of oxidation behavior and establishment of life-cycle model during the steam oxidation of Cr-coated Zry-4 at 1200 C

The isothermal steam oxidation behaviors of Cr-coated and non-coated Zry-4 cladding at 1200 degrees C have been investigated systematically. The oxidation of Cr coating is divided into three stages based on the different oxidation kinetics. First is protective stage where a thick Cr2O3 layer is gradually formed on the outer surface. Second is transition stage where the coating failure occurs because ZrO2 short circuit paths accelerates the reduction of Cr2O3. The O2- of Cr2O3 is transferred inward and dissolves in the substrate as alpha-Zr(O). The last is the non-protective stage where the Cr coating is oxidized again and substrate is oxidized as well. The oxidation of substrate presents three periods based on the different oxidation rate, which are accelerated period, normal period and abnormal period. Intermetallic precipitates derived from inward-diffusion of Cr make the oxidation kinetics be logarithmic law in the abnormal period by restraining the oxygen diffusion and t-ZrO2 -> m-ZrO2 transformation. In this study, we find that the Cr coatings has important effect in restraining Zry-4 oxidation even after Cr has been completely consumed.