Redox behavior of yttrium and electrochemical formation of Y-Al alloys in molten chlorides

Abstract

The electrochemical behavior of yttrium and its co-deposition with aluminum were investigated by several transient electrochemical techniques on a tungsten electrode at 973K in NaCl-KCl eutectic melts. The results reveal that the reduction of Y(III) in NaCl-KCl-YCl3 melts is a one-step process with three-electron exchanged and the reaction is a quasi-reversible diffusion-controlled process at low scan rates (0.05∼0.5 V/s). The calculated diffusion coefficient is approximately 2.8 × 10−5cm2/s. After AlCl3 was introduced into the melts, cyclic voltammetry and open circuit chronopotentiometry showed the formation of two Y-Al intermetallic compounds, indicating that under-potential deposition of yttrium occurred on tungsten electrode covered with liquid Al. The electromotive force was measured at 973K to determine the thermodynamic properties of Y-Al intermetallic compounds, such as the activity of Y in the two-phase coexistence state, relative partial molar Gibbs energies, as well as the standard Gibbs energies of Y-Al intermetallic compounds. Finally, potentiostatic electrolysis was conducted to prepare Y-Al alloys from molten NaCl-KCl-YCl3 (1.5 mol%)-AlCl3 (1.5 mol%) by the co-reduction method. The cathodic alloys were characterized using X-ray diffraction (XRD) and scan electron micrograph (SEM)-energy dispersive spectrometry (EDS) and the results indicated that the obtained alloys were mainly composed of YAl2, as well as YAl3 and YAl phases. The Y-rich phase intermetallic compound YAl, formed in the later period of electrolysis just when the concentration of AlCl3 is fairly low.