| Neodymium (Nd) is one of the most important rare earth elements (REE) with electronically special 4f structure and corresponding optical, magnetic, electrical and catalytic properties [1-3]. Because of their unique physical-chemical properties they were widely used in a growing number of applications and had become indispensable for a large varieties of emerging technologies in industrial areas during the past three decades such as permanent magnets, advanced metal alloys and addictives. However, during the engineering applications, it is greatly demanded to improve the corrosion resistance of Nd and its alloys through different surface treatments. The high electrochemical activity of Nd makes it so difficult to perform surface treatments of Nd alloys. It has become an important topic to look for new medium suitable for the surface treatments of reactive metals and to study the related mechanisms in the research area of surface science and technology of reactive metals. Ionic liquid is a kind of new green solvents besides water and organic solvents, which shows its unique advantages in the surface engineering of reactive metals.In the present paper, anodic behavior of Nd in 2:1 acidic AlCl3-l-ethyl-3-methyl-imidazolium chloride (AlCl3-EMIC),0.8:1 basic AlCl3-EMIC and 1-butyl-1-methylpyrrolidinium bis[(trifluoromethyl)sulfonyl] ([BMP]Tf2N) amide ionic liquid was investigated. The potential windows of ionic liquids were tested by cyclic voltammetry. The dissolution behavior of Nd was investigated by conducting linear sweep voltammeter, galvanostatic polarization and chonopotentiometry. A scanning electron microscopy was used to characterize the surface morphologies of Nd and the chemical composition of Nd surface after galvanostatic process was indentified by Raman spectra.The results showed that the dissolved products of Nd anode in 2:1 acidic AlCl3-EMIC were in form of Nd(III). With current densities high than 5 mA/cm2, Nd (III) accumulated at the interface of Nd/ionic liquid resulting in the increase of the viscosity of Nd dissolved ionic liquid. As the anodic process prolonged, a viscous layer formed at the interface due to the high viscosity. Due to the mechanism of "activation-passivation", a flat and homogeneous etching surface was obtained without any pits and the oxide films were removed thoroughly. Otherwise, the Nd electrode kept dissolving at some local areas when the current densities was below 2 mA/cm2, leaving a pitting morphology on the surface.In 0.8:1 basic AlCl3-EMIC and [BMP]Tf2N ionic liquids, the anodic behaviors of Nd electrode were similar. The results showed that the oxide film on the surface of Nd showed great effects on the anodic behavior of Nd. A thick oxide film on the surface inhibited the dissolution of Mg in the ionic liquid and the active dissolution of Nd occurred after the breakdown of oxide films. The oxide films broke down at the potential of 0 V and 1.4 V respectively. After that, Nd dissolved in complex form of [NdCl6]3- and [Nd(Tf2N)x](x-3)--respectively. After galvanostatic process at low current densities (0.2 mA/cm2 and 0.1 mA/cm2 respectively), the surface showed a pitting morphology due to the local breakdown of oxide films; after galvanostatic process at relatively high current densities (2 mA/cm2 and 1 mA/cm2 respectively), the surface of Nd was homogeneous without etching pits due to the complete breakdown of oxide films. However, due to no viscous layer forming during the anodic dissolution, the Nd surface after anodic process was not smooth. |
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