Preparation And Oxidation Behavior Of ZrO₂/FeCrAl Composite Coating Based On Accident-tolerant Fuel

After Fukushima Daiichi nuclear disaster in 2011,there is an urgent need for the more advanced nuclear fuel system that can resist the risk of the loss-of-coolant accident over longer periods while maintaining or improving normal operating conditions and performance in the world.Nuclear fuel cladding is the first safety barrier in nuclear reactors.Zircaloy is the most widely used fuel cladding material in current water-cooled reactors with nuclear power.However,the severe oxidation reaction of zircaloy has become a key factor that restricts the improvement of the safety performance of water-cooled reactors under the conditions of high temperature,high pressure,hydrothermal environment,and vapor.In this study,the technology roadmap of the surface treatment on zircaloy was adopted.ZrO₂ buffer layer was prepared on the surface of Zr-4 alloys by the micro-arc oxidation method,and FeCrAl coating was deposited on the surface of ZrO₂ buffer layer by the magnetron sputtering method.In the end,ZrO₂/FeCrAl composite coating,with the accident-tolerant capability,was obtained.In this study,the preparation technology of the buffer layer and composite coating was explored,and the corresponding oxidation reaction in the supercritical water and high-temperature steam environment was studied.The microstructure,phase composition,electrochemical properties,and bonding strength of the coatings were in contrastive analysis.The forming process of the micro-arc oxidation coating and the mechanism of the ZrO₂ buffer layer to improve the high-temperature oxidation resistance of the FeCrAl coating were explored.In this study,the technological parameters affecting the performance of micro-arc oxidation coating were first studied.By exploring the performance of the micro-arc oxidation coatings prepared in different electrolyte systems?NaAlO2,Na2SiO3,and Na3PO4?,it was found that the coating growth rate of the micro-arc oxidation of Zr-4 alloys was the highest in the NaAlO2 electrolyte.In addition,the obtained coating had the highest tetragonal zirconia content.However,there were many defects in the coating.The coating growth rate in the Na2SiO3 electrolyte was the lowest,but the obtained coating had the higher compactness.The obtained coating performances in Na3PO4 electrolyte were between those in the NaAlO2 and Na2SiO3 electrolyte.The effects of different voltage parameters?positive voltage on 350 V,375 V,and 400 V;negative voltage on 0 V,25 V,50 V,75 V,and 100 V?on the performances of microstructure,phase composition,and electrochemical corrosion resistance of ZrO₂ coatings were in further study.The results showed that the negative voltage had a significant effect on the regulation of micro-arc oxidation performance.When the positive voltage was 375 V and the negative voltage was increased from 0 V to 100 V at intervals of 25 V,the tetragonal zirconia content was increased and then decreased,reaching the maximum value at 50 V.At the same time,the highest compactness and the best electrochemical corrosion resistance of the coating were obtained at the 50 V of negative voltage.In addition,the alternating-current impedance data showed that the more compact inner layer of the double layer structure in the micro-arc oxidation was the main reason for the improvement of the electrochemical corrosion performance.By the comparative study on the supercritical water oxidation action of micro-arc oxidation coating and hydrothermal oxidation coating on the surface of zircaloy,the oxidation mechanism of micro-arc oxidation coating and hydrothermal oxidation coating on zircaloy was analyzed.After the supercritical water oxidation at 550?/25 MPa,the oxidative weight gain in micro-arc oxidation coating was lower and the film-based bonding strength was higher than those in the hydrothermal oxidation coating.Therefore,the corrosion resistance performance of micro-arc oxidation coating was much better than that in the hydrothermal oxidation coating.The microstructure of the coating showed that the hydrothermal oxidation coating produced numerous cracks on the surface and inside of the coating after the supercritical water oxidation,resulting in severe oxidative cracking.However,the micro-arc oxidation coating had a more compact microstructure before and after oxidation.The analysis of the phase composition and elemental composition showed that the interior of the micro-arc oxidation coating produced outer layers with multiple t-ZrO₂ and inner layers with little t-ZrO₂ because of the effect of different elements in the electrolyte.After supercritical water oxidation.The inner structure with little t-ZrO₂ was still compacted.The outer layer,due to the phase change from t-ZrO₂ to m-ZrO₂,produced volume expansion and internal cracks in the coating.Although the outer coating produced numerous cracks after oxidation,the compact inner coating provided an important guarantee for the oxidation resistance.In this study,we further explored the oxidation reaction of micro-arc oxidation coatings obtained from different electrolytes?NaAlO2,Na2SiO3,and Na3PO4?in supercritical water and high temperature steam environment.The results showed that the coating obtained from Na2SiO3 electrolyte had the better compactness of microstructure than that in Na2SiO3 and Na3PO4 electrolytes after supercritical water oxidation at 374 ?/22.1 MPa and steam oxidation at 1000 ? and 1200 ?.The surface cracks of the micro-arc oxidation coating obtained in the Na2SiO3 electrolyte under high temperature steam oxidation were in the minimum number,followed by Na2AlO2.The re-nucleation of ZrO₂ on the surface of the coating obtained from the Na3PO4 electrolyte would grow again,causing excessively intergranular defects and loose structure on the surface.In addition,the high temperature steam oxidation needs curves to indicate that the micro-arc oxidation coating cannot accelerate the oxidation of the substrates although it cannot prevent the oxidation of the substrates at 1000 ?and 1200 ?.ZrO₂/FeCrAl composite coatings were prepared on the basis of the study of micro-arc oxidation buffer layers.By comparing the high temperature steam oxidation action of Zr-4 alloy,single FeCrAl coating sample and ZrO₂/FeCrAl composite coating sample,we found that a single FeCrAl coating can improve the oxidation resistance performance of Zr-4 alloy in a steam environment at 800?,but the oxidation of the matrix alloy will be promoted under 1000 ? and 1200 ? steam oxidation conditions,resulting in the higher oxidation rate and oxide thickness than those in Zr-4 alloy.The ZrO₂/FeCrAl composite coatings can effectively protect Zr-4 alloys at 800 ? and 1000 ?.The curves of oxidation kinetics show that the oxidation rate constants of the composite coating samples at 800 ? and 1000 ? are reduced 2 orders of magnitude and 1 order of magnitude than those of Zr-4,respectively.In addition,there is a turning point of oxidation rate of the ZrO₂/FeCrAl composite coating at 1200 ? in the steam environment.Before the turning point,the oxidation rate of ZrO₂/FeCrAl composite coating is approximately 1/2 than that of Zr-4 alloy.After the turning point,the cracks on the surface of the coating and partial spalling of the FeCrAl layer result in an increase of the oxidation rate.The ZrO₂ buffer layer avoids interdiffusion between the Zr-4 alloy and the FeCrAl coating,thus effectively avoiding the formation of brittle and low-melting intermetallic compounds between Zr-Fe and Zr-Cr.Therefore,the stable FeCrAl coating is ensured at high temperatures,eventually achieving significant improvement in oxidation resistance performance.