Study On Preparation And Oxidation/Corrosion Resistance Of Protective Coating For Zr Alloys

Zirconium alloys are often used as nuclear fuel cladding materials due to low neutron cross section and good toughness under neutron irradiation.To achieve higher safety margins of nuclear plants,especially during the loss of coolant accidents whereupon Zr alloy fuel cladding enduring a heavy dose of irradiation can still keep a better integrity for a few time,it is one of the viable approaches to develop a protective coating on fuel cladding.Currently,the studies for Zr alloy coating are mainly focused on the materials with good corrosion and oxidation resistances,such as Cr and FeCrAl alloys.However,there are few researches for the coating fabrication on Zr alloy cladding,and the studies on the failure mechanisms of the coating spalled from the Zr alloy substrate is fewer.For the development of the protective coating on Zr alloy,it is of great importance to understand the failure mechanisms of the coating in depth.This work is focused on the preliminary investigations of the materials,fabrications,and failure mechanisms of the protective coatings for Zr alloys.Through the characteristic analyses of the coating,e.g.Cr₃C₂-NiCr and FeCrAl-based alloys manufactured by thermal spraying and spark plasma sintering methods on Zr alloy,the failure mechanisms of the coating have been investigated,and the reasonable structures and compositions of the coating have also been suggested.The potential of FeCrZr alloy has also been investigated for the application of Zr alloys.According to the results,some conclusions have been drawn as following:1)From the experiments of the coating preparations?e.g.Al,Cr,and Ni-based alloy?and performance tests,it was found that the Zr alloy substratecracked by hydriding after the processing of pack cementation,and the thermal spraying and spark plasma sintering methods could fabricate the required coating on Zr alloy substrate.2)During the oxidation experiments of the Cr₃C₂-NiCr coating prepared by thermal spraying method,it was found that the coating could be well bonded on the Zr alloy and provide protection for the Zr substrate under 1000? air condition.After the corrosion in steam,cracks occurred at the interface between the coating and substrate,and the bimetallic effect could contribute to the oxidation of Zr alloy at interface.3)In the investigation using FeCrAl-based alloys as coatings of Zr alloy under steam condition,the results showed that:i,the corrosion resistances both of FeCrAl and FeCrAl/Si alloys were better than Zr alloy,and could be used as Zr alloy coating;ii,the diffusion layer containing ZrSi₂ between coating and Zr substrate had a better corrosion resistance;iii,the bimetallic effect could accelerate the corrosion rate of the Zr alloy at interface between coating and substrate;iv,the oxide formed at interface between coating and Zr substrate could produce cracks in the diffusion layer,finally causing failure for the coating.4)The FeCrAl alloy coating produced by thermal spraying on Zr alloy showed inner oxidation in high temperature air.During corrosion in steam for the coatings with new structures,the results showed that H₂O could diffuse along the porous structure in the coating to the interface and oxidize the Zr substrate,consequently inducing cracks at interface.5)The arc-melted FeCrZr alloy had better oxidation and corrosion resistances than the Zr alloys,had favorable thermos-physical properties,and was a promising candidate as the protective coating material of Zr alloy or the bond coat between coating and Zr substrate.