Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Utilizing nonaqueous bath
Reexamination Certificate
2001-05-30
2003-11-25
King, Roy (Department: 1742)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Utilizing nonaqueous bath
C205S237000
Reexamination Certificate
active
06652730
ABSTRACT:
The present invention relates to organoaluminum electrolytes suitable for the electrolytic deposition of aluminum or aluminum-magnesium alloys on electrically conductive materials, and a method for this using soluble aluminum anodes or soluble aluminum and magnesium anodes or an anode made of an aluminum-magnesium alloy.
Organoaluminum complex compounds have been used for a long time for the electrolytic deposition of aluminum (dissertation H. Lehmkuhl, TH Aachen 1954, German Patent 1047450; K. Ziegler, H. Lehmkuhl, Z. anorg. allg. Chemie 283 (1956) 414; German Patent 1056377; H. Lehmkuhl, Chem. Ing. Tech. 36 (1964) 616; EP-A-0084816; H. Lehmkuhl, K. Mehler and U. Landau in Adv. in electro-chem. Science and Engineering (Eds. H. Gerischer, C. W. Tobias, Vol. 3, Weinheim 1994). As suitable electrolytes, there have been proposed complex compounds of general type MX2AlR
3
, which are employed either as molten salts or in the form of their solutions in liquid aromatic hydrocarbons. MX may be either alkali metal (Na, K, Rb, Cs) or onium halides, preferably fluorides. R represent alkyl residues with preferably one, two or four carbon atoms.
The interest in electrolytic coatings of metal workpieces with aluminum has greatly increased due to the excellent corrosion protection by the aluminum layers and their ecological safety. Therefore, the galvanic coating with organoaluminum electrolytes which work at moderately elevated temperatures of between 60 and 150° C. and in closed systems is of great technical importance.
Since it has been sought, in recent years, to develop motor vehicles optimized in terms of consumption and weight, a consequent light-weight construction more and more requires the use of aluminum or magnesium or their mutual alloys. However, the light metal materials have a drawback in that both aluminum and magnesium have a high solution pressure in aqueous medium. Mainly upon contact with steels or conventionally galvanized steels, there is contact corrosion. For this reason, it is required to coat fixing members on magnesium applications in such a way that contact corrosion on the magnesium is avoided, on the one hand, and a long-term stability of the coating is obtained, on the other hand. The galvanic coating of the connecting screws with aluminum alone serves this function only partially since the corrosion products of the construction material magnesium are alkaline and attack the aluminum surfaces of the coating (B. Reinhold, S. G. Klose, J. Kopp, Mat.-wiss. u. Werkstofftech. 29, 1-8 (1998).
Methods for the galvanic deposition of aluminum-magnesium alloys on electrically conducting materials are known: J. H. Connor, W. E. Reed and G. B. Wood, J. Elektrochem. Sc. 104, 38-41 (1957), describe only briefly that they obtained a metal layer with 93% Al and 7% Mg having a good appearance upon electrolysis of AlBr
3
, Li[AlH
4
], MgBr
2
(Mg/Al=0.8) in diethyl ether. J. Eckert and K. Gneupel obtained metal depositions with up to 13% Mg from a similar electrolyte of AlCl
3
, Li[AlH
4
], MgBr
2
in a mixture of THF, diethyl ether and benzene (Mg/Al=0.6) (GDR Patent Specification 244573 A1). The conductivity of the electrolyte was on the order of 1.10
−3
to 7.10
−3
S.cm
−1
. In the GDR Patent Specification 243723 A, the same authors describe an electrolyte solution consisting of ethylmagnesium bromide and triethylaluminum in THF/toluene 1:1 from which metal layers with a maximum of 10% Al were obtained.
Typical electrolytes, which have also proven technically useful for the deposition of aluminum, based on organoaluminum complex compounds of the type M[R
3
Al—X—AlR
3
] (R=Et, iso-Bu; X=F, Cl; M=K, Cs, N(CH
3
)
4
) have been used for the electrochemical deposition of aluminum-magnesium alloys and magnesium by A. Mayer, J. Electrochem. Sci. 137 (1990), and in the U.S. Pat. No. 4,778,575 (priority of Oct. 18, 1988) after the addition of trialkylaluminum (R=Et, i-Bu) and dimethyl- or diethylmagnesium.
However, in a technical application of this method, the following problems arise, which render a continuously operating coating process impossible.
In contrast to aluminum anodes, magnesium anodes cannot be dissolved in the coating process with the proposed electrolytes. Continuous replenishing of the Mg content by dissolving the magnesium anode is not possible using organoaluminum complexes containing fluoride or generally halide as electrolytes.
According to the description in the U.S. Pat. No. 4,778,575, dialkyl magnesium in ethereal solution is employed for preparing the electrolyte. In a continuously working coating method, the dialkyl magnesium would have to be fed constantly in ethereal solution. However, diethyl ether is known to cleave some complexes, e.g., Na[Et
3
Al—F—AlEt
3
] into Na[Et3AlF]+Et
3
Al.OEt
2
(K. Ziegler, R. Köster, H. Lehmkuhl, K. Reinert, Liebigs Ann. Chem. 629, 33-49 (1960)). If the use of ether as the solvent for dialkyl magnesium was to be avoided, dialkyl magnesium would first have to be rendered ether-free, which requires considerable expenditure and costs, or it would have to be prepared in an ether-free form by the reaction of magnesium metal with di-alkylmercury, a very toxic compound.
For the reasons already described, it has been the object of the present invention to provide halide-free organoaluminum electrolytes which combine in themselves optimally the properties required for a technical application for the deposition of aluminum and aluminum-magnesium alloys, such as solubility of both aluminum and, in the case of alloy layers, magnesium anodes by electrolysis, as high as possible a conductivity, homogeneous solubility in aromatic solvents, such as toluene at between 20 and 105° C., cathodic deposition of dense layers of aluminum-magnesium alloys with selectable proportions of the two components of from Al:Mg=95:5 to 5:95.
The object has been achieved by the use of organoaluminum electrolytes which are characterized by containing either (in the case of electrolyte type I) alkali tetraal-kylaluminate M[AlR
4
] or (in the case of electrolyte type II) alkali hexaalkylhydrido-dialuminate and additionally M[AlR
4
] as well as trialkylaluminum AlR
3
(R=CH
3
, C
2
H
5
, C
3
H
7
or n- or iso-C
4
H
9
; M=Li, Na, K, Rb, Cs), while electrolytes of composition M[R
3
Al—H—AlR
3
] have proven particularly useful for the preparation of pure aluminum layers.
For reasons of optimizing solubility, specific conductivity and good accessibility, the ethyl compounds (R=C
2
H
5
=Et) are preferred. An electrolyte according to the invention of type I is dissolved in 2.5 to 6 mol per mole of complex compound of an aromatic hydrocarbon liquid at 20° C., preferably in toluene or a liquid xylene. The trialkylaluminum is preferably triethylaluminum (AlEt
3
), and alkali tetraalkyl-aluminate is preferably a mixture of potassium and sodium tetraethylaluminates. The quantitative ratio of complex : AlEt
3
is from 1:0.5 to 1:3, preferably 1:2. The proportion of Na[AlEt
4
] is between 0 and 25 mole percent, based on the total amount of K[AlEt
4
] and Na[AlEt
4
], but preferably between 5 and 20 mole percent. The addition of low amounts of Na[AlEt
4
] is preferred because, when this component is lacking, the aluminum anodes are dissolved only with moderate to poor current efficiencies, e.g., only about 22% in K[AlEt
4
]/3AlEt
3
/6 toluene, which would lead to a loss of triethylaluminum for extended durations of the electrolysis. The electrolysis is performed at temperatures of between 80 and 105° C., preferably between 90 and 100° C.
An illustrative electrolyte I is 0.8 mol of K[AlEt
4
]/0.2 mol of Na[AlEt
4
]/2.0 mol of AlEt
3
/3.3 mol of toluene. From this electrolyte solution, there is no crystallization even upon extended standing at room temperature, and the specific conductivity at 95° C. is 13.8 mS.cm
−1
.
The a
Lehmkuhl Herbert
Mehler Klaus-Dieter
Reinhold Bertram
King Roy
Leader William T.
Studiengesellschaft Kohle mbH
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