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The Cr-coated Zr alloy materials appear to be one promising short-term accident tolerant fuel cladding concept, due to their outstanding high-temperature oxidation resistance and improved resistance to fretting wear. In addition, there are few barriers or challenges to be applied to nuclear reactors for coated Zr alloy claddings. In this paper, 12-15 μm-thickness coatings have been deposited by magnetron-sputtering process on the outer surface of Zr-1Nb alloy tubes. Two-sided isothermal oxidation tests were performed in flowing steam by synchronous thermogravimetric analyzer, at temperatures ranging from 1000°C to 1200°C and for oxidation times ranging from 300 s to 5000 s, with the object of systematically studying the behavior during accidents. Scanning electron microscopy equipped with energy dispersive X-ray spectroscopy and X-ray diffraction were utilized to characterize the microstructure, element distributions and phase of oxide scale to study the oxidation kinetics and mechanisms of Cr coating. Based on these analyses, dense chromia scale was developed on the outer surface of cladding during steam oxidation, preventing the oxygen atoms from diffusing into the substrate, to improve the high-temperature resistance of the composited claddings. In addition, the oxidation kinetics of the Cr coating was nearly parabolic and the rate constants was at least two orders of magnitude lower than the Zr alloy, enhancing high-temperature steam oxidation resistance of Zr alloy claddings.