Fundamental Aspects of Uranium Electropolishing in AlCl₃-1-ethyl-3-Methylimidazolium Chloride Ionic Liquid

Abstract

A smooth and flat uranium surface with trace of well-defined grain boundaries has been obtained by electropolishing in the Lewis acidic AlCl 3 -1-ethyl-3-methylimidazolium chloride ionic liquid. The surface roughness Ra and Ry of the uranium reduced substantially from 130.1 nm and 899.4 nm to 16.77 nm and 206.89 nm after electropolishing, respectively. The underlying mechanism of electropolishing has been investigated by the combination of electrochemical, spectroscopic and surface characterization methods. A current density plateau in the measured polarization curves of uranium is observed, which is directly related to the mass transport limit of U 3+ . The mass transport mechanism of the process follows the salt precipitation model, and the precipitate is proposed to be in the form of U(Al 2 Cl 7 ) x (AlCl 4 ) 3-x Uranium (U) has been used in military and civilian applications due to its high density and nuclear properties. Mechanical polishing is usually used to reduce the surface roughness of uranium, which could remove the mechanically damaged surface layer and determine the metallographic structure of uranium. However, typical mechanical polishing of uranium is problematic due to its radioactivity and toxicity produced by aerosol.1 Electropolishing is a more suitable method for the polishing of uranium, which could avoid the generation of aerosol. However, there are major limitations associated with the method -most notably that the extensive gas evolution with low current efficiency occurs, along with etch pits and environmental concerns.2 It is proposed that a non-aqueous solution could eliminate the above-mentioned difficulties.Ionic liquids (ILs) served as new non-aqueous solvents have been employed in the field of electropolishing.3-5 The advantage of using ILs is that high current efficiency could be obtained with negligible gas evolution at the anode/solution interface. The liquid used is comparatively benign and non-corrosive compared to the current aqueous acid solutions and bears the potential for repeated usage. Among a large variety of ILs, AlCl 3 -1-ethyl-3-methylimidazolium chloride (AlCl 3 -EMIC) is an appropriate alternative with the merits of high ionic conductivity, low viscosity and wide potential window. 6 We have previously shown that metal uranium can be oxidized to U 3+ and the corresponding products accumulated on the metal/ILs interface form a viscous layer, which could facilitate a uniform dissolution of the substrate surface. Normally, this uniform dissolution could lead to electropolishing under optimum parameters. 8 The study of mass transport mechanisms in electrodissolution systems is critical for the optimization of operating parameters. As reported, mass transport mechanisms of electrodissolution vary from mass transport limiting species. The mass transport limiting species could be the cations of the dissolving metal (salt precipitation mechanism), the anions from the electrolyte (acceptor mechanism), or the water (water mechanism). 9 The salt precipitation mechanism des...

Advantages of Electrochemical Polishing of Metals and Alloys in Ionic Liquids

“…Electropolishing of metallic uranium was carried out in 1-ethyl-3-methylimidazolium chloride ionic liquid in the presence of AlCl 3 [52]. Uranium was found to be electrochemically oxidized with U 3+ formation.…”

“…Photograph of a burgundy film on the surface of uranium after anodization in a 1-ethyl-3methylimidazolium chloride-AlCl 3 mixture (1:2 molar ratio) at 6 mA cm −2 for 2 min (reprinted with permission from ref. [52]; Copyright 2018 the author(s), published by The Electrochemical Society).…”

Electropolishing of Tin in an Amide-Type Ionic Liquid

“…However, the current efficiency of anodic dissolution of metal is low in aqueous solutions due to the oxygen evolution on the anode, leading to the large energy consumption and difficult estimation of a decrease in the thickness of a metal substrate from the passed electric charge. Electropolishing of some metals has been investigated in such nonaqueous electrolytes as deep eutectic solvents (DES) 1,[8][9][10][11][12][13] and chloroaluminate ionic liquids 14,15 to improve the current efficiency of the anodic dissolution of target metal surfaces. The electropolishing of stainless steel was reported to be possible in a choline chloride-ethylene glycol DES system.…”