Aluminum electrodeposition

Relatively little attention has been devoted to the direct electrodeposition of transition metal-aluminum alloys in spite of the fact that isothermal electrodeposition leads to coatings with very uniform composition and structure and that the deposition current gives a direct measure of the deposition rate. Unfortunately, neither aluminum nor its alloys can be electrodeposited from aqueous solutions because hydrogen is evolved before aluminum is plated. Thus, it is necessary to employ nonaqueous solvents (both molecular and ionic) for this purpose. Among the solvents that have been used successfully to electrodeposit aluminum and its transition metal alloys are the chloroaluminate molten salts, which consist of inorganic or organic chloride salts combined with anhydrous aluminum chloride. An introduction to the chemical, electrochemical, and physical properties of the most commonly used chloroaluminate melts is given below. 

Extensive work has been devoted to aluminum electroplating in nonaqueous systems. Choosing appropriate bath compositions enables aluminum to be deposited at high efficiency and purity from nonaqueous electrolyte solutions. Comprehensive reviews on this matter have appeared recently in the literature [123,455], This work has led to the development of a number of commercial processes for nonaqueous electroplating of aluminum. The quality of the electroplated aluminum is very similar to that of cast metal. For instance, electrodeposited aluminum can be further anodized in order to obtain hard, corrosion resistive, electrically insulating surfaces. It is also possible to electroplate A1 on a wide variety of metal surfaces, including active metals (e.g., Mg, Al), nonactive metals, and steel. 

Because of its very negative standard electrode potential of — 1.7 Kh. aliuninum cannot be deposited from aqueous solutions. Therefore only molten salt and water-free inorganic or organic electrolyte systems are eligible for electrolytic deposition of aluminum. Only through the development of such nonaqueous systems [53, 54, 118, 217, 221] did it become possible to electrodeposit aluminum with the desired quality and properties. 

Acknowledgements. The authors would like to thank Prof. Dr. Dr. h. c. Roland Kammel (Technische Universitat Berlin) and Prof. Dr. Hans-Wilhelm Lieber (Technische Universitat Berlin and Fachhochschule Berlin) for their great interest in and constant encouragement of the development of electrodeposited aluminum from organoaluminum electrolytes. The authors owe special thanks to Dr. Lorraine Aleandri-Hachgenei and to Ms. Helga Wasilewski for the English translation and completion of the manuscript, respectively. 

Hudak NS, Huber DL (2012) Size effects in the electrochemical alloying and cycling of electrodeposited aluminum with lithium. J Electrochem Soc 159 A688-A695... 

Key words magnesium alloy, ionic liquid, electrodeposition, aluminum,... 

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In the case of molten salts, the functional electrolytes are generally oxides or halides. As examples of the use of oxides, mention may be made of the electrowinning processes for aluminum, tantalum, molybdenum, tungsten, and some of the rare earth metals. The appropriate oxides, dissolved in halide melts, act as the sources of the respective metals intended to be deposited cathodically. Halides are used as functional electrolytes for almost all other metals. In principle, all halides can be used, but in practice only fluorides and chlorides are used. Bromides and iodides are thermally unstable and are relatively expensive. Fluorides are ideally suited because of their stability and low volatility, their drawbacks pertain to the difficulty in obtaining them in forms free from oxygenated ions, and to their poor solubility in water. It is a truism that aqueous solubility makes the post-electrolysis separation of the electrodeposit from the electrolyte easy because the electrolyte can be leached away. The drawback associated with fluorides due to their poor solubility can, to a large extent, be overcome by using double fluorides instead of simple fluorides. Chlorides are widely used in electrodeposition because they are readily available in a pure form and... 

Several methods and variations have been developed to electrodeposit compounds. Most of the work described in this article concerns the formation of nonoxide compounds such as II-VI and in Vs. Oxides are probably the largest group of electrodeposited compounds (aluminum anodization for example), but will not be discussed here. The electrodeposition of H-VI compounds has been extensively studied and is well reviewed in a number of articles [24-29], The most prominent compound electrodeposition methods include codeposition, precipitation, and various two-stage techniques. 

Electrodeposition of Transition Metal-Aluminum Alloys from Chloroaluminate Molten Salts... 

In many ways, chloroaluminate molten salts are ideal solvents for the electrodeposition of transition metal-aluminum alloys because they constitute a reservoir of reducible aluminum-containing species, they are excellent solvents for many transition metal ions, and they exhibit good intrinsic ionic conductivity. In fact, the first organic salt-based chloroaluminate melt, a mixture of aluminum chloride and 1-ethylpyridinium bromide (EtPyBr), was formulated as a solvent for electroplating aluminum [55, 56] and subsequently used as a bath to electroform aluminum waveguides [57], Since these early articles, numerous reports have been published that describe the electrodeposition of aluminum from this and related chloroaluminate systems for examples, see Liao et al. [58] and articles cited therein. 

The voltammograms in Figure 9 also indicate that it is possible to electrodeposit Ag-Al alloys in a potential range positive of the potential where the bulk deposition of aluminum is normally observed, i.e., 0 V versus A1(III)/A1. The Ag-Al alloy composition, represented as the fraction of Al in the alloy, 1 — x, was estimated from the voltammograms in Figure 9 by using the following expression... 

Bulk Ag-Al alloys, containing up to 12 a/o Al, were electrodeposited from melt containing benzene as a co-solvent. Examination by x-ray diffraction (XRD) indicated that the low-Al deposits were single-phase fee Ag solid solutions whereas those approaching 12 a/o were two-phase, fee Ag and hep i>-Ag2Al. The composition at which ti-Ag2Al first nucleates was not determined. The maximum solubility of aluminum in fee silver is about 20.4 a/o at 450 °C [20] and is reduced to about 7 a/o at room temperature. One would expect the lattice parameter of the fee phase to decrease only slightly when aluminum alloys substitutionally with silver because the... 

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