磁控溅射铬涂层锆合金包壳高温水蒸气氧化行为

To improve the high-temperature oxidation resistance of Zr alloy cladding under nuclear accident conditions, Cr coating is proposed to be deposited on the cladding surface, which is one of the concepts of accident tolerant fuel (ATF) cladding. In this work, the oxidation behavior of magnetron-sputtered Cr-coated Zr-1Nb alloy cladding in 1 100-1 300 ℃ steam environment was studied. The cladding samples were 9.5 mm in outer diameter, 0.57 mm in thickness and 2 cm in length. Cr coating was deposited on the outer surface of the cladding tube. A horizontal tube furnace was used to carry out the tests. The test conditions included 1 100 ℃/3 600 s, 1 200 ℃/3 600 s, 1 300 ℃/1 800 s and 1 300 ℃/3 600 s. The samples experienced double-sided oxidation during the tests. Mass of the samples was measured by an analytical balance before and after the tests. Surface and cross-section morphologies of the samples were characterized by scanning electron microscopes (SEM). The element distribution was analyzed by energy dispersive spectroscopy (EDS). The as-deposited Cr coating was dense without obvious defects. After oxidation, stress existed in the Cr₂O₃ layer, which resulted in plastic deformation to form microscopic blisters or folds. If the stress could not be released in time by plastic deformation, micro-cracks appeared on the sample surfaces. Cr₂O₃ could further react with steam (containing small amount of dissolved O₂) to generate volatile products, resulting in the formation of porous surface structure. After oxidation for 3 600 s at 1 100 ℃ and 1 200 ℃, the layered phases of Cr coating from outside to inside were Cr₂O₃, Cr and ZrCr₂, which had a protective effect on the Zr substrate. ZrCr₂ was formed by inter-diffusion between the metallic Cr and the Zr substrate. Zr diffused along the grain boundaries in metallic Cr. At 1 200 ℃/ 3 600 s, Zr reached the Cr₂O₃/Cr interface, and then Cr₂O₃ was reduced, leading to its local thinning. The diffusion resistance of O in Cr₂O₃ was reduced in the thinned region, thus resulting in an increased O flux from the Cr₂O₃/steam interface to the Cr₂O₃/Cr interface. The Zr combined with O to form ZrO₂ precipitates in the grain boundaries of metallic Cr. ZrO₂ precipitates acted as short-circuit paths for O to pass through the metallic Cr. Therefore, the amount of O absorbed by Zr substrate increased. After oxidation at 1 300 ℃ for 1 800 s, the degradation of Cr coating occurred and a thick ZrO₂ layer grown under the coating. Due to the reduction reaction, the Cr₂O₃ layer was very thin. Zr was oxidized preferentially to Cr due to its larger oxygen affinity, hence a metallic Cr layer (containing ZrO₂ precipitates) was retained. After oxidation for 3 600 s, with the thickening of ZrO₂ layer, the metallic Cr was also completely oxidized. During the oxygen saturation of β-Zr and α-Zr(O), the parabolic rate constant for ZrO₂ growth (k_p) increased. Therefore, the experimental measurement of the thickness of inner ZrO₂ layer deviated largely from the calculation by Cathcart-Pawel correlation in the case of 1 300 ℃/3 600 s. Due to the inner-sided oxidation of cladding samples, β-Zr substrate reached oxygen saturation rapidly, thus k_p of the outer ZrO₂ layer soon entered the secondary increase stage. Therefore, the outer ZrO₂ layer had a higher growth rate than the inner one. According to the simulation results, the thickness of the inner and outer ZrO₂ layers was reduced compared with the case without coating, indicating an improvement of oxidation resistance by Cr coating. The test conditions in this work are similar to state near the burst regions of cladding tubes during nuclear accidents, thus the results could have a reference value for accident management.