Electrodeposition aluminum deposition

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. 

Quing Liao et al. [465] investigated aluminum deposition of high quality on copper substrates in A1C13/MEIC = 60/40 melts. The authors found that the quality of the electrodeposit was greatly enhanced by the addition of benzene as a cosolvent. This improved the properties of the electrolyte by an increase in electrical conductivity and a decrease in viscosity. 

The first attempt to electrolytically deposit an aluminum layer was carried out more than 100 years ago. Since then, other methods of electrolytic aluminum deposition were continued to be published. However, none stood up to careful scrutiny. The wish to electrodeposit a newly-to-be-erected statue of William Penn with aluminum led the city council of Philadelphia to be swindled. A charlatan claimed to be able to complete the electroplating process by using a secret recipe. The aluminum was to protect the statue from corrosion in the sea climate. The contractor had the city finance the construction of the world s largest eletroplating plant. Only subsequently would the defraud be publicized, when it became clear that zinc had been elec-trodeposited instead of eiluminum [203]. 

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. 

The first electrodeposition of aluminum from an ionic liquid was reported in 1994 by Carlin etal. [157], Two years later, Zhao et al. [158] smdied the aluminum deposition processes on tungsten electrodes in trimethylphenylanunonium chlo-ride/aluminum chloride with mole ratio 1 2. It was shown that the deposition of aluminum was instantaneous as a result of three-dimensional nucleation with hemispherical diffusion-controlled growth, underpotential deposition of aluminum, corresponding to several monolayers. Liao et al. investigated the constant current electrodeposition of bulk aluminum on copper substrates was in 1-methyl-... 

Jiang et al. studied the electrodeposition and surface morphology of aluminum on tungsten (W) and aluminum (Al) electrodes from 1 2 M ratio of [Emim]CI/AlCl3 ionic liquids [165,166]. They found that the deposition process of aluminum on W substrates was controlled by instantaneous nucleation with diffusion-controlled growth. It was shown that the electrodeposits obtained on both W and Al electrodes between -0.10 and -0.40 V (vs. AI(III)/A1) are dense, continuous, and well adherent. Dense aluminum deposits were also obtained on Al substrates using constant current deposition between 10 and 70 mA/cm. The current efficiency was found to be dependent on the current density varying from 85% to 100%. Liu et al. showed in similar work that the 20-pm-thick dense smooth aluminum deposition was obtained with current density 200 A/m for 2 h electrolysis [167],... 

Fig. 5.6 The aluminum deposit on the different metal substrates with different electrolysis time (a) electrodeposition on Cu substrate for 4 h, (b) electrodeposition on nuld steel substrate for 4 h, (c) electrodeposition on mild steel substrate for 6 h, and (d) electrodeposition on stainless steel substrate for 6 h...

The electrodeposition of aluminum from ionic liquids has been intensively investigated. The principal rule for ionic liquids is the special design of the anion-cation combination for the metal to be deposited, hr Figure 7.20 the usual ionic liquid for aluminum deposition is shown. 

The equilibrium constant for Equation (1) is reported to be 1.06 x 10 at 175 °C (23). In the 52 48 AlCl3 NaCl melt the AlC and Al2Cl7 concentrations are about 7.68 mol/L and 0.662 mol/L, respectively. Aluminum can be electrodeposited from either one of these electroactive species. The reduction of AlCV occurs at potentials more negative than that required for AljClv reduction and becomes prominent as the acidity (AICI3 content) of the melt is reduced. The aluminum deposition reaction, involving Al2Cl7 as the electroactive species, is normally expressed by equation (2). One can see that aluminum deposition results in a localized acidity decrease near the electrode surface since AljCV is consumed and AlCV is produced. 

Several electrodeposits were formed at potentials ranging from 0 to -O.IV. EDS examination of the as-deposited surface indicated the presence of aluminum and niobium in all of the electrodeposits examined. Chlorine was not detected in any of the samples indicating that the deposits contained no entrained electrolyte. Figure 7 is a plot of alloy composition as a function of deposition potential. The highest niobium concentration detected was 13.5% (atomic fraction). This was observed at a deposition potential of O.OV. As the deposition potential is made more negative, the niobium concentration is dramatically reduced. This implies that the kinetics for aluminum deposition are much faster than that of niobium, or that the niobium reduction is simply mass transport limited in the potential range examined. The fact that pure niobium deposits are apparently not achievable at potentials more positive of the aluminum deposition potential may be an indication that the codeposition of niobium and aluminum at negative potentials follows an mduced codeposition mechanism i.e., niobium deposition is only possible when aluminum is codeposited. 

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... 

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... 

Moffat [80] reported the electrodeposition of Ni-Al alloy from solutions of Ni(II) in the 66.7 m/o AlCl3-NaCl melt at 150 °C. The results obtained in this melt system are very similar to those found in the AlCh-EtMcImCI melt. For example, Ni deposits at the mass-transport-limited rate during the co-deposition of Al, and the co-deposition of Al commences several hundred millivolts positive of the thermodynamic potential for the A1(III)/A1 couple. A significant difference between the voltammetric-derived compositions from the AlCl3-NaCl melt and AlCl3-EtMeImCl melt is that alloy composition is independent of Ni(II) concentration at the elevated temperature. Similar to what has been observed for room-temperature Cu-Al, the rate of the aluminum partial reaction is first order in the Ni(II) concentration. Moffat s... 

Cr-Al, Mn-Al, and Ti-Al alloys can be obtained from acidic melt solutions containing Cr(II), Mn(II), or Ti(II), respectively, only if the deposition potential is held very close to or slightly negative of the thermodynamic potential for the electrodeposition of aluminum, i.e., 0 V. From these observations it can be concluded that the formal potentials of the Cr(II)/Cr, Mn(II)/Mn, and Ti(II)/Ti couples may be equal to or less than E0 for the A1(III)/A1 couple. Unlike the Ag-Al, Co-Al, Cu-Al, Fe-Al, and Ni-Al alloys discussed above, bulk electrodeposits of Cr-Al, Mn-Al, and Ti-Al that contain substantial amounts of A1 can often be prepared because problems associated with the thermodynamic instability of these alloys in the plating solution are absent. The details of each of the alloy systems are discussed below. 

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