Study Of The Aluminized And Anodic Reaction Mechanism Of Uranium In AlCl₃-EMIC Ionic Liquid

Uranium is a key material in the field of energy and national defense security.However.the performance stability and the service life of the uranium workpiece is restricted by its poor corrosion resistance property.The surface modification technologies such as electro-deposition and physical vapor deposition developed in the last century have been applied to the surface of uranium,which have effectively improved the corrosion resistance of uranium workpieces and laid a foundation for the manufacture of long-lasting and reliable weapons.After entering the 21th century,the requirements of the performance stability and service life of uranium workpieces have been improved.and higher standards are needed for the uranium anti-corrosion technologies.Thus.the existing methods cannot meet the requirements for the surface modification of uranium.However,researches on better solutions have basically stagnated at this stage,since neither the innovation of existing methods nor the developments of new methods have been reported.This work proposed a new surface modification method to meet the uranium modification requirements by using ionic liquid.in the light of the essential drawbacks of conventional electrochemical surface modification methods.However,the theoretical basis of the new method is scarce,the confirmatory experiments are missing and many questions are needed to be answered.For the application of this new method,this work conducts researches on several basic,essential and representative problems via the combination of electrochemical methods and various surface characterization techniques.To apply this new method,the compatibility of ionic liquids with uranium is the basic question that needs to be answered.It was found that the chemical reactions between AICl3-EMIC ionic liquids and uranium depend on the Lewis acidic property of the ionic liquid.The Lewis basic and neutral AlCl3-EMIC ionic liquids interacts with uranium oxides to form UCl62-and AlOCl32-;whereas the Lewis acidic ionic liquid interacts with uranium in the form of oxidation of uranium from the substrate while Al2Cl7-is reduced on its surface.driven by the electrochemical potential difference between aluminum and uranium.The clarification of this compatibility not only removes obstacles of applying the surface modification method on uranium by using ionic liquids,but also serves as an important reference for follow-up studies.Beyond that.this part of work also demonstrates a convenient approach to deposit dense Al nano-layer on U by uranium-aluminum displacement,which enriched the surface modification methods on uranium.The nucleation and growth mechanism of the electrodeposited aluminum on the surface of uranium is essential to the question when a high quality aluminum coating could be obtained on the uranium surface.It was found that the deposition process of aluminium on uranium substrates is an overpotential deposition(OPD)process,which mainly consists of two steps:the instantaneous nucleation and diffusion-controlled growth.The formation and growth of nuclei follows a three-dimensional nuclei growth model controlled by instantaneous nucleation and hemispherical diffusion.This research lays the foundation for the new surface modification method on uranium by using ionic liquids,and opens up new a direction for the development of uranium surface modification technologies.The anodic reaction mechanism of uranium in ionic liquids is a representative problem to be solved in order to increase the adhesion of the coating to the uranium substrate.In this work,the anodic reaction mechanism of uranium in AlCl3 66 mol%AlCl3-EMIC ionic liquid was obtained:during the anodic process in AlCI3-EMIC,uranium was oxidized to U3?;the corresponding products accumulated on the metal/ILs interface to form a viscous layer;the formation of the viscous layer led to a current density plateau in the measured polarization curve,which was directly related to the mass transport limit of U3+;the mass transport mechanism of the anodic process followed the salt precipitation model,and the precipitate was proposed to be in the form of U(Al2Cl7)x(AlCl4)3-x.The anodic behavior of uranium was also strongly dependent upon the surface oxide states including thickness and homogeneity of the oxide film,although the oxide layer hardly participates in the anodization reaction.Based on this research,an anodic pretreatment of uranium to improve the adhesion between the aluminum coating and the substrate has been further developed.The adhesion strength was enhanced to 6.2 MPa by electrodepositing aluminum on this oxide-free uranium surface.This series of studies not only provide reliable and effective approach to improve the adhesion property,but also demonstrates re-understanding of the basic scientific problems of the anode behavior of metals in liquid solution.This work verifies that the Al coating electrodeposited in AlCl3-EMIC(66 mol%AlCl3)ionic liquids can effectively enhance the corrosion resistance of U:after electro-deposition of Al in 0.01 mol/L NaCl solution,the self-corrosion current was lower than bare uranium by one order of magnitude,and the polarized resistance was 20 times of bare uranium.Moreover,the amorphous Al-Mn coating obtained in the AlCl3-EMIC-MnCl2 ionic liquids has proven to be a better corrosion protection coating with hardness of 5.2 GPa,and the corrosion rate was about 50%lower than that of pure aluminum coating.This coating first appears in the field of surface modification of uranium.It also provides new ideas to the most fundamental question in this field:which surface modification layer system best fits the actual needs of uranium?This work presents a new exploration in the field of surface modification of uranium,which verifies that the surface modification methods based on ionic liquid can meet the unique material characteristics and engineering requirements of uranium.This work also lays a theoretical foundation for the surface modification of uranium in ionic liquids.In addition,the relevant results in this study may also have certain inspirations for the fields such as precision machining of nuclear materials and spent fuel reprocessing.