Electrofinishing of metals using eutectic based ionic liquids

Abstract

Ionic liquids are molten salts that have low melting points. This review concentrates upon eutectic mixtures of quaternary ammonium salts and metal salts (usually halides). These so-called eutectic based ionic liquids can be used for the electrodeposition and dissolution of a wide range of metals and alloys. The review highlights the technological challenges involved in the application of ionic liquids to metal finishing.

Anodic Reactions and the Corrosion of Copper in Deep Eutectic Solvents

“…By applying Faraday's law to the anodic current density we obtain: [6] where n' is the apparent valency, F is the Faraday constant, m is the observed mass loss, M is the molar mass, A is the electrode area and t is the dissolution time. Then by substituting equation [6] into [5] and rearranging, the following expression for the apparent dissolution valency is obtained: [7] Clearly, in the case where corrosion is significant equation [7] predicts that n' < n, and only when j corr = 0 is a dissolution valency of n' = n obtained. Moreover, with the condition of cathodic mass transport control we can write: [8] where n c is the number of electrons transferred in the cathodic process,  is the diffusion layer thickness, and D and c are the diffusion coefficient and bulk concentration of the CuCl 4 2-species,…”

“…This suggests that if j lim is raised by increasing c or D (or by reducing ) then j corr is also increased and this results in a lower apparent dissolution valency. Equation [7] can then be used to extract j corr from the apparent dissolution valency data. Figure 12, for example, shows the j corr data extracted in this manner and plotted against  1/2 .…”

“…Typically the eutectic temperature is below 100 o C which makes DES suitable for many plating and surface finishing applications. [5][6][7] A widely used DES is 'ethaline' which is formulated from ethylene glycol (HBD) and choline chloride (HBA) in a 2:1 molar ratio. 5,6 Ethaline has the highest conductivity and the lowest viscosity of all the various DES systems studied.…”

“…6 Additionally, metal chloride salts and metal oxides have a high solubility in ethaline and can therefore be used to formulate practical plating solutions. These 'type-III' DES systems have been used to deposit a wide range of metals including Al, Zn, Fe, Ni, Sn, Cr, Al and Cu [2][3][4][5][6][7] . Studies [8][9][10][11][12][13][14] of copper deposition have been particularly common as it is used extensively in the electronics and microelectronic industries and can be regarded as a model system.…”

Challenges and opportunities in next generation of electropolishing surfaces

“…These electrolytes provide significant advantages such as high solubility of metal salts, high electrical conductivity and high electrochemical stability, that make them ideal for metal processing. Many studies have been carried out demonstrating the feasibility of using ionic liquids as electrolytes in EP for a variety of different stainless steels, Ni/Co alloys [11], Cu [12] or Ti [13], but there is still work to do with the aim of industrializing this process.…”