Influence of Coating Thickness on High-temperature Steam Oxidation Kinetics and Mechanisms of Cr-coated Zr Alloy Cladding
Metal chromium is widely used as an antioxidant material because it is easy to develop a protective oxide scale in oxidizing environments. As a result, Cr has been extensively studied for use as a coating material for nuclear fuel cladding. The Cr-coated Zr alloy cladding is one of the most promising accident tolerant fuel (ATF) products for full engineering applications,due to its outstanding high-temperature steam oxidation resistance, satisfactory resistance to corrosion and fretting wear, and higher technological maturity and economic efficiency, less difficult engineering application. In this work, a Zr alloy cladding with outer diameter of 9.5 mm and wall thickness of 0.57 mm was used as the substrate to prepare Cr coatings by magnetron sputtering techniques. The target thickness of Cr coatings was set to 10, 15, and 20 μm with well control of thickness variations along the Zr alloy tube axis. High-temperature steam oxidation tests were conducted by synchronous thermogravimetric analyzer at 1200 ℃ for oxidation time ranging from 500 s up to 3000 s. The work aims to systematically study the effect of Cr coating thickness on the oxidation behavior, oxidation kinetics and microscopic mechanisms under hypothetical loss of coolant accident conditions for nuclear reactor. The microstructural characteristics including oxide thickness, element distribution, and phase composition were analyzed by energy dispersive X-ray spectroscopy (EDS), field emission scanning electron microscopy (SEM), and X-ray diffraction (XRD). The oxidation kinetics of Cr coatings based on Cr2O3 oxide thickness were constructed, and the effects of Cr coating thickness on high-temperature oxidation and failure mechanisms were also explored. A three-layer structure (Cr2O3, Cr, Zr-Cr from outside to inside) was formed. Interdiffusion at the coating and substrate interface resulted in the formation of a continuous Zr-Cr layer on the order of micrometer in thickness. It was shown that the protective effect on the Zr substrate was limited at a coating thickness of 10 μm. The protectiveness of Cr2O3 oxide scale and residual Cr coatings were completely lost for 2000 s, which the substrate was oxidized. At a coating thickness of 15 μm, in the first stage, the oxidation kinetic of coatings followed a parabolic law with protectiveness, and the oxidation rate constant was 0.03804 μm2/s. In the second stage, the oxidation kinetics shifted, and the protectiveness of Cr2O3 oxide and residual Cr coatings degraded, owing to the redox reaction between Zr and Cr2O3 oxide, and the development of grain boundaries ZrO2 particles within the residual Cr coating. Nevertheless, the oxide scale and coatings after degradation during 2000 and 3000 s could still hinder the oxidation of the Zr substrate to a certain extent. At a coating thickness of 20 μm, the overall oxidation kinetic of coatings always obeyed the parabolic law without transformation, and the oxide scale and residual coating were protective, in which the oxidation rate constant was 0.04962 μm2/s. The results of this study show that the Cr-coated Zr cladding with the thickness of 15 and 20 μm exhibits the most promising behavior with an improved resistance to high temperature steam oxidation. These experimental results could provide data support and theoretical basis for the optimal design of coatings for ATF cladding.
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