(Invited) Surface Finishing with Chloroaluminate Ionic Liquids

Abstract

Despite the emergence of many new ionic liquid systems or room-temperature molten salts, chloroaluminate ionic liquids are still one of the most versatile systems for the electrochemical surface finishing of metals, whether it be the electrodeposition of corrosion protective films on steel or the electrochemical polishing of metal surfaces, such as Cu. The most commonly studied chloroaluminate IL is the well-known mixture of AlCl3 and the quaternary ammonium chloride salt, 1-ethyl-3-methylimidazolium chloride (EtMeImCl).1 Among the anhydrous, aprotic ILs prepared to date, this mixture still holds the record for the highest conductivity and lowest viscosity. Save for the need to protect it from exposure to moisture, it is undoubtedly one of the most versatile IL systems for electrochemistry. In most cases, the acidic composition of this IL proves to be the most valuable composition for surface finishing, owing to the presence of the coordinately unsaturated Lewis acidic species, Al2Cl7 -, which is readily reduced to Al metal via the reaction: 4Al2Cl7 -+3e- ⇌Al + 7AlCl4 - (1) The electroplating of Al from chloroaluminate ILs, such as AlCl3-EtMeImCl, is one of the most thoroughly studied processes in the field of ionic liquid electrochemistry, as attested in a recent review.2 However, much less attention has been devoted to the electrodeposition of aluminum alloys from these ILs, many of which have very useful properties. Stable alloys can be obtained during the electrodeposition of Al if certain low valent transition metal ions, Mz+, that are reduced negative of the potential required for the reaction shown in eq 1 are added to the acidic IL, e.g., Ti2+, V2+, Mo2+, and Mn2+, to name a few.2 In this case, it is possible to prepare aluminum-transition metal alloys: xM(AlCl4) p (p-z)- + 4(1-x)Al2Cl7 - + [3-(3-z)x]e- ⇌M x Al1-x + [7(1-x) +px]AlCl4 - (2) where x represents the fraction of the transition metal in the M x Al1-x alloy. For some values of x, these so-called “overpotential alloys” form single-phase metallic glasses. In this case, these amorphous, non-equilibrium alloys exhibit a greatly enhanced resistance to chloride pitting corrosion relative to pure Al. Ternary alloys with metallic glass phases can also be produced by adding a second transition metal to the acidic IL. Articles have appeared in The Journal describing the electrodeposition and properties of such aluminum alloys containing Mo + M, Mo + Ti, and W + Mn. Recently, both the Lewis acidic and basic composition of the AlCl3-EtMeImCl IL system have been explored for the reverse purpose, the electropolishing of Al and Cu by anodic dissolution. The advantages afforded by anodic dissolution in these ILs relative the aqueous acids and salt solutions commonly used for this purpose are quite obvious, i.e., the avoidance of the oxide surface films that invariably complicate the electropolishing process. In this paper, we report recent advances in the electrodeposition of aluminum transition metal alloys for metal surface finishing as well as the thermodynamics and kinetics of the anodic dissolution of metals in the acidic and basic compositions of the AlCl3-EtMeImCl IL system.