Study On Oxidation Mechanism And Kinetics Of Cladding Aluminum Alloy

The purpose of the spent fuel reprocessing process is to separate the spent fuel core and cladding,so as to realize the recycling of spent fuel.As a new post-treatment process,the high-temperature oxidation and volatilization technology can control the simultaneous oxidation and separation of the core and cladding materials and has the advantages of low dissolution difficulty and low processing difficulty.It is an alternative technology to the existing concentrated acid dissolution cladding technology.Therefore,The oxidation performance and oxidation kinetics of aluminum alloy cladding materials in the nitrogen-oxygen mixed gas atmosphere are researched,which provides basic theory for the oxidation of alloy cladding materials.This research is of great significance to the subsequent separation process from spent fuel pellets.In this paper,cladding material 6061 aluminum alloy and the non-cladding material 2324aluminum alloy are selected to comparative study the affects of the oxidation time,oxidation temperature,and the proportion of N₂ in the nitrogen-oxygen mixed atmosphere on oxidation behavior and kinetics of aluminum alloy with XRD,SEM,XPS,TG-DTG-DSC and other test and characterization methods.The first principle calculation method is used to investigate the influence of alloying elements on the oxidation process and the mechanism of O atom adsorption on alloy surface and bulk diffusion.The experimental results show that the surface film layer of aluminum alloy oxidized at580℃in nitrogen-oxygen mixed gas atmosphere is mainly composed of aluminum oxide phase.The oxide film has a dense structure and microcrystalline phase appeared and cracks occurred.Both the oxidation time and temperature affect the morphology of the surface oxide film.With the increase of oxidation time and oxidation temperature,the oxidation weight and film thickness of the alloy increase,the microcrystalline structure of the oxide film grows and the surface of the film layer is rougher.The slightly increase of the N₂ proportion has an impact on the oxidation process.After being adsorbed on the surface of the alloy,N₂ dissolves and diffuses to form Al N and then re-oxidizes with O atoms.The formation of Al N is promoted with the increase of the oxidation time and oxidation temperature.In a nitrogen-rich atmosphere,the more transferable N anion vacancies increase the diffusion rate of O atoms in the film reflecting the promotion of oxidation.6061 cladding aluminum alloy and2324 aluminum alloy have similar oxidation rules,but cuprum content of 2324 aluminum alloy is higher which results in a weaker adsorption capacity of O atoms,while 6061aluminum alloy gains more oxidation weight and faster film thickness increase because of the higher aluminum content.The Flynn-Wall-Ozawa method,the Kissinger method and the Crane method in the equal conversion method are used to calculate the apparent activation energy,pre-exponential factor,reaction order and reaction mechanism function of the oxidation reaction of two aluminum alloys,and establish the weight gain equation,film thickness growth equation and kinetic rate equation of oxidation in oxygen mixed atmosphere.On the basics of first-principles research on the oxidation mechanism of aluminum alloys,it is shown that among four different sites,the adsorption of O atoms at Fcc site is the most stable,then at Hcp site,Bridge site,while the adsorption of O atoms at Top site is the most unstable.The alloying elements have influence on the adsorption of O atoms.The sequence of O atoms adsorptive tendency is Mg atoms,Si atoms and Cu atoms.The adsorption energy of O atoms on the surface of 6061 aluminum alloy is lower than that on the surface of 2324aluminum alloy.This is why 6061 aluminum alloy is easily oxidized.On the duration of O atoms’bulk diffusion in aluminum alloy,O atoms occupied location in tetrahedral interstice is more stable than that in octahedral intersite.Furthermore,the three most probable diffusion paths of O atoms from the alloy surface to interior and interlamination are obtained by transition state simulation calculations are as follow:1.Fcc site→octahedral gap→4-I tetrahedral gap→octahedral gap;2.Fcc site→4-I tetrahedral gap→octahedral gap→4-I tetrahedral gap;3.Fcc site→4-I tetrahedral gap→4-I tetrahedral gap→4-I tetrahedral gap.