Electrolytic aluminum deposition

The Principles and Techniques of Electrolytic Aluminum Deposition and Dissolution in Organoaluminum Electrolytes... 

Electrolytic Aluminum Deposition from Nonaqueous Organic Electrolytes. 175... 

Processing Techniques for Electrolytic Aluminum Deposition from Electrolytes Containing Aluminum Alkyls. 211... 

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]. 

This section provides a survey of the electrolytic deposition of aluminum out of organoaluminum electrolytes, from its discovery to its technical applications. First, the deposition of metals from nonaqueous organic electrolytes is generally discussed, and the corresponding problems and possibilities are pointed out. In detail, concrete examples of electrolytic aluminum deposition from organoaluminum electrolytes and their fundamental complex chemistry and electrochemistry are treated. In a further section, the properties of such deposited aluminum are described, and finally an overall view is given of the development in instrumentation from the first laboratory cell to a coating plant unit with a capacity of 90 mVh. 

Consequently, in early 1953, research on these complex compounds was initiated to determine whether they were suitable for electrolytic aluminum deposition. The first trials ended in disappointment, because the electrolytes, employed as melts, yielded useless aluminum coatings containing large portions of alkali metal. Besides, the electrolytes showed a very low conductivity compared to aqueous systems. Attempts to improve the quality of the aluminum deposits by adding excess triethylaluminum led to unexpected observations. Hence, a detailed investigation of alkali metal fluoride-aluminum trialkyl systems was necessary. 

Fig. 1 shows one of the first electrolytically deposited alumimun coatings to be obtained from this type of electrolyte. Since electrolytic aluminum deposition from this system has no true smoothening effect, thick layers become even rougher, as illustrated by the thickly coated cathode plate shown in Fig. 1. The cathodic deposition and the anodic dissolution of aluminum corresponded to almost 100 <7o of the amount expected according to the Faraday rule, which is an important prerequisite for even considering using this electrolysis technique for technical applications. Independently of the layer thickness, the deposited aluminum layers are found to be ectraordinarily pure. Spectroscopic investigations have revealed purities of up to 99.999%. Even when relatively impure raw aluminum with purities of 99.7% functions as the anode, very pure aluminum can be deposited. Therefore, obviously not only a technique of electroplating aluminum was discovered, but also a method of...

Already in the first year after the discovery of electrolytic aluminum deposition from solutions containing aluminum alkyls, an apparatus for the continuous plating of wire was tested [118], see Fig. 17. 

Outside of these seemingly niche markets the main driving force for using non-aqueous electrolytes has been the desire to deposit refractory metals such as Ti, Al and W. These metals have numerous applications, especially in the aerospace industry, and at present they are deposited primarily by PVD and CVD techniques. The difficulty with using these metals is the affinity of the metals to form oxides. All of the metal chlorides hydrolyze rapidly with traces of moisture to yield HC1 gas and hence any potential process will have to be carried out in strict anhydrous conditions. Therefore the factor most seriously limiting the commercialization of aluminum deposition is the engineering of a practical plating cell. 

The ionic species present in the electrolyte appear to be Na+, A1F63 , A1F4 , F and certain A1—O—F complexes (see above). The fact that sodium is present as free ions, whereas aluminum is bound in complexes, and the fact that the sodium ion is the carrier of current, have led many authors to the assumption that sodium is the primary discharge product at the cathode. However, the thermodynamic data favor primary aluminum deposition on aluminum in cryolite melts. 

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. 

Most metals can be electrolytically deposited from water-free melts of the corresponding metal salts. It is well known that aluminum, lithium, sodium, magnesium, and potassium are mass produced by electrolytic deposition from melts. Industrial processes for the melt-electrolytic production of beryllium, rare earth metals, titanium, zirconium, and thorium are also already in use. Pertinent publications [74, 137, 163] describe the electrolytic deposition of chromium, silicon, and titanium from melts. Cyanidic melts are used for the deposition of thick layers of platinum group metals. It is with this technique that, for instance, adhesion of platinum layers on titanium materials is obtained. Reports concerning the deposition of electrolytic aluminum layers [17, 71-73, 94, 96, 102, 164, 179] and aluminum refinement from fused salts [161] have been published. For these processes, fused salt... 

The final performance limitation of aluminum alkyl electrolyte systems has not, by far, been reached. Substantially higher deposition rates than those realized today by eletrodeposition technology with organoaluminum electrolytes, i.e., 1-1.5 A/dm, are probably attainable [130]. As a result, the industrial efficiency of both galvanic aluminum deposition and eletrolytic aluminum refinement may be improved. 

When aluminum is utilized as an anode, the metal dissolves during electrolysis. Based on the amount of cathodically deposited aluminum and anodically dissolved aluminum, the current yields, not withstanding a few exceptions, are quantitative [118,186,221]. Consequently, during electrolysis only aluminum metal is transported from the anode to the cathode. Apparent deviations from the Faraday rule with regard to aluminum deposition and dissolution actually result from alkali metal codeposition on the cathodic side with the particular electrolytes mentioned above, or in some cases from side reactions on the anodic side, which can be recognized by gas evolution at the anode. 

Table 7. Tafel coefficients as a function of the temperature of aluminum deposition from alkylalumi-num electrolyte (EL II/T).

Fig. 11. Log of the exchange current density for aluminum deposition from alkylaluminum electrolytes as a function of /T (EL II/7) [186],...

An alternative mechanism with primary aluminum deposition claims formation of [R2AI] , [RAl] , or AP cations by electrolytic dissociation of M[R3Al-F-AlR3l complexes [63, 118, 221). Formation of such cations by dissociation of organoalumi-num compounds was unknown at the time. Shortly afterwards, Bonitz [39, 40] observed that aluminum trialkyl dimers possess a certain amount of intrinsic conductivity, and postulated the dissociation Eq. (14). 

The same investigation showed that copper-precoated ceramic plates coated with aluminum can be etched like copper circuit boards. A comparative study of galvanoaluminum layers and other electrolytically precipitated deposits was recently published [140]. 

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