Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Utilizing nonaqueous bath
Reexamination Certificate
1999-11-01
2001-03-27
Gorgos, Kathryn (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Utilizing nonaqueous bath
Reexamination Certificate
active
06207036
ABSTRACT:
The invention relates to an electrolyte for the electrolytic high-speed deposition of aluminum on continuous products, which electrolyte contains an organometallic aluminum complex. The invention is also directed to the use of said electrolyte in the production of corrosion-resistant and decorative coatings on continuous products in a continuous process.
By aluminizing base metals, it is possible to make them corrosion-resistant and provide them with a decorative coating. Optionally, such a coating may also be colored. The aluminum is predominantly deposited by electroplating from electrolytes enabling such an electrodeposition. Amongst the electrolytes are fused-salt electrolytes as well as electrolytes containing aluminum halides or alkyl aluminum complexes. Electrolyte systems based on alkyl aluminum complexes have gained general acceptance in the art. In general, such alkyl aluminum complexes also contain alkali complex compounds or ammonium complex compounds.
Initially, electrolyte solutions containing the NaF.2AlEt
3
complex dissolved in aromatic hydrocarbons such as toluene or xylene have been used almost exclusively in the electrodeposition of aluminum. However, one drawback of these electrolytes has been their very poor throwing power which, in particular, has disadvantageous effects when coating parts of complicated shape as rack products or drum products. With large parts of complicated shape having angles and corners, the poor throwing power results in incomplete and non-uniform coating.
In the course of time, therefore, electrolyte systems have been employed containing potassium halides instead of sodium halides. Potassium halides exhibit superior throwing power and have compositions such as KF.2AlEt
3
. Furthermore, the complexes have superior electrical conductivity compared to the corresponding sodium salt complexes.
One major drawback, however, is the poor solubility of these complexes in aromatic hydrocarbons generally used as solvents, so that the common 3-4 M toluene solutions of these complexes already undergo crystallization at 60-65° C., posing a serious problem when aluminizing rack products. Further dilution of these solutions results in a massive decrease in conductivity and current density resistance, rendering the coating process uneconomic.
The use of potassium fluoride complexes containing triisobutyl aluminum as complex component has neither provided a substantial solution to these problems. Complexes of the composition KF.2Al(iBu)
3
have a substantially lower melting point of from 51 to 53° C., which is lower than that of the corresponding ethyl or methyl aluminum complexes. Even at room temperature and a dilution of 3-4 M in toluene, the isobutyl complexes do not crystallize. One major disadvantage of this compound, however, is to be seen in its poor current density resistance. Even at low current densities, gray coatings are formed on the objects to be coated, and there is undesirable co-deposition of potassium.
EP-A 0,402,761 and U.S. Pat. No. 4,417,954 describe prior art methods intended to solve these problems. To this end, the potassium-containing triethyl aluminum complexes used to date are to be mixed with other alkyl aluminum complexes. Such mixtures have lower melting points compared to pure triethyl aluminum complexes. In addition, they have a higher solubility in aromatic hydrocarbons. Triisobutyl aluminum and trimethyl aluminum are exemplified as admixtures. The compositions obtained in this way are acceptable for rack product aluminizing with respect to electrical conductivity, solubility and throwing power and are used on an industrial scale today.
Likewise, the EP-A 0,084,816 describes electrolytes for the electrodeposition of aluminum, wherein mixtures of aluminum alkyl complexes are used. According to the examples of this document, mixtures of triethyl aluminum and isobutyl aluminum are used, in particular.
However, such electrolytes are disadvantageous as they are not suitable for the continuous coating of continuous products such as wires, tapes, long-profiles, or pipes. Such a process and a corresponding device for the electrodeposition of aluminum on continuous products are described in the German patent application by the present applicant filed simultaneously with the present application.
The electrolytes for the electrodeposition of aluminum available up to now have a low current density resistance of only from 0.2 to 2.0 A/dm
2
at maximum. When exceeding the maximum limiting current density for a specific composition, the result will be burns, rough coatings and undesirable co-deposition of potassium. In particular, this is the case when adding larger amounts of triisobutyl aluminum as is the conception in EP-A 0,084,816 or EP-A 0,402,761, for example.
To date, continuous products such as wire are generally coated continuously for corrosion protection by applying a zinc coating, wherein the galvanizing technique is used. However, this is no high-quality corrosion protection because the protective coating undergoes changes even after a short period of time, forming voluminous white corrosion products on the surface as a result of oxidation of the coated zinc layer. For many applications, there is a demand for a higher quality corrosion protection which can be achieved by using electrodeposition of aluminum. Such a coating remains substantially unchanged and therefore provides a higher quality corrosion protection compared to zinc coating used so far. However, the preconditions for an economic production are that the electrolytes employed can be operated at high current density and quantitative yield, have a long service life, are cheap in production and easy to maintain.
The previously known electrolytes for the electrodeposition of aluminum are not suitable for use in such a process, as the requirements for an electrolyte in continuous coating are essentially different from those in the previously known rack product aluminizing. In the continuous coating of continuous products such as wires, tapes, long-profiles, or pipes, the parts to be coated are simple in geometry. The electrode gaps are equal in most of the cases, so that the macro throwing power of the electrolyte plays a minor role. In contrast to rack product aluminizing, the main requirement in using the electrolyte is a deposition rate as high as possible, where sufficient purity and a compact structure of the deposited layer must be achieved so that, in addition, an electrolyte having a high limiting current density is required.
It was therefore the technical object of the invention to provide an electrolyte which has the properties required for the electrolytic high-speed deposition of aluminum on continuous products, particularly a high deposition rate, a high limiting current density, permits operation with quantitative yield, has a long service life, is cheap in production and easy to maintain.
Said object is achieved by using an electrolyte containing an organometallic aluminum complex of formula (I)
MF.2Al(C
3
H
7
)
3
.nAlR
3
(I),
wherein
M=K, Rb, Cs,
R=a C
3
alkyl group or a mixture of a C
3
and a C
1
-C
2
alkyl group,
n=from 0.1 to 1,
in an aromatic or aliphatic hydrocarbon as solvent.
To date, such an electrolyte compound has not been used in the electrodeposition of aluminum and, in particular, has not been usable in rack product aluminizing. In principle, tri-n-propyl aluminum or triisopropyl aluminum may be used as tripropyl aluminum complex. Particularly preferred, however, is the use tri-n-propyl aluminum.
Furthermore, it can be inferred from formula I that the electrolyte according to the invention also comprises alkyl aluminum admixtures which are possible in addition to the 1:2 complex. Surprisingly, it has been found that this results in higher values for the applicable limiting current density and in a reduction of the macro throwing power which, however, is of minor importance in the high speed deposition on continuous products.
It is preferred that MF in formula I be KF or CsF. In accordance with fo
Aluminal Oberflachentechnik GmbH
Gorgos Kathryn
Leader William T.
Stockton Kilpatrick
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