EP0084816A2 - Electrolyte for galvanic deposition of aluminium - Google Patents

Abstract

An organometallic electrolyte for the electrodeposition of aluminum is described which exhibits high throwing power as well as high conductivity and good solubility and is commercially readily accessible. For this purpose, the invention provides an electrolyte of a formula based upon an organometallic complex of potassium, rubidium or cesium fluoride in combination with a series of organometallic aluminum compounds.

Description

The invention relates to an organometallic electrolyte for the electrodeposition of aluminum and the use of this electrolyte.

Organometallic electrolytes, ie organo-aluminum complex compounds (DE-PS 1 047 450), can be used for the galvanic deposition of aluminum. In the past, a large number of compounds have been described which can be used for galvanic aluminizing, for example onium and alkali complex compounds. In practice, however, only the complex salt NaF described as optimal has been used. 2 Al (C ₂ H ₅ ) ₃ used ("Journal for inorganic and general chemistry", Vol. 283, 1956, pp. 414 to 424).

Electroplating baths with NaF. However, 2 Al (C2H5) 3 as an electrolyte salt have a decisive disadvantage for a technically broad and economical application: the spreadability is too low. It is comparable to that of aqueous chrome baths ("Galvanotechnik", Vol. 73, 1982, pp. 2 to 8). Due to the low spreadability during galvanic aluminizing, strongly profiled parts can only be coated as frame goods where the geometry of the parts allows, using auxiliary anodes. However, this is a technically very complex and therefore expensive process. Due to the low spreadability, the drum aluminizing of small parts is not practical either Can gain importance because the aluminized parts have too large fluctuations in layer thickness or are not coated at all at critical points.

The object of the invention is to provide an organometallic electrolyte for the electrodeposition of aluminum, which has a high scatterability but also a high conductivity and good solubility and is easily accessible commercially.

wobei folgendes gilt:

• Me = K, Rb oder Cs;
• ^{R = H} oder C_xH_2x+1 mit x = 1 und 3 bis 8, wobei wenigstens zwei Gruppen R Alkylreste sind;
• m = 1,3 bis 2,4 und n = 0,1 bis 1,1 , wobei m > 2n sein muß.

In der vorstehenden Formel (1) bedeutet "Me" Metall, "Et" steht für einen Äthylrest, d.h. für C₂H₅; im übrigen können auch verschiedene Metalle nebeneinander vorliegen.According to the invention, this is achieved by an electrolyte which has the following composition:

the following applies:

• Me = K, Rb or Cs;
• ^{R = H} or C _x H _{2x + 1} with x = 1 and 3 to 8, where at least two groups R are alkyl radicals;
• m = 1.3 to 2.4 and n = 0.1 to 1.1, where m must be> 2n.

In the above formula (1), "Me" means metal, "Et" stands for an ethyl radical, ie for C ₂ H ₅ ; otherwise, different metals can also be present side by side.

dabei gilt folgendes:

• m' = 1,8 bis 2,2 (insbesondere 2,0),
• n' = 0,2 bis 0,5 (insbesondere 0,4) und
• R' = CH₃ oder C₄Hg, wobei die Reste R' n- oder iso-Butylreste sein können.

Advantageous embodiments of the electrolyte according to the invention are the subject of subclaims, an electrolyte of the following composition being particularly preferred:

the following applies:

• m '= 1.8 to 2.2 (in particular 2.0),
• n '= 0.2 to 0.5 (in particular 0.4) and
• R '= CH ₃ or C ₄ Hg, where the radicals R' can be n- or iso-butyl radicals.

The organoaluminum electrolyte according to formula (1) according to the invention proves to be extremely progressive in terms of electroplating, i.e. it fulfills the demands placed on electrolytes for a technically widely applicable and economical aluminizing process to a far greater extent than was previously possible. The electrolyte according to the invention namely has a high scattering capacity and, at the same time, electrical conductivity and solubility required for economical aluminizing, as well as good commercial accessibility. For the first time, it combines the electrotechnical properties relevant to electroplating. Another advantage is also that this electrolyte compared to NaF. 2 Al (C2H5) 3 has a significantly lower sensitivity to oxygen and moisture.

The electrolyte according to the invention is based on knowledge which has been obtained on the one hand with regard to the relationships between the composition of organo-aluminum complex compounds and the electroplating requirements, such as scatterability, conductivity and solubility (in low-viscosity aromatic hydrocarbons which are liquid at room temperature and have low water absorption). These relationships were previously unknown.

It has now been shown that the metal ion is decisive for the scatterability, whereas the conductivity is influenced both by the metal ion and by the halogen ion and by the length of the alkyl radicals. The alkyl residues and the metal ion turn out to be particularly relevant for solubility.

The following applies in detail. Scattering, conductivity and technical handling of the electrolyte are improved with increasing ionic radius of the alkali metal, while the halogen ion has an opposite effect. For a high conductivity, the alkyl residues should not take up much space and are short-chain. Small metal ions are more suitable than large ones in order to achieve good solubility.

With the electrolyte according to the invention, a technically useful product was created for the first time. This applies in particular to the technical manageability, i.e. this electrolyte is soluble at room temperature and can be transported in a simple manner in the electrolyte concentration range which is of interest in terms of electrotechnology.

The electrolyte according to the invention is comparable to cadmium electrolytes in terms of scatterability in the field of electro-technical interest. For the first time there is the possibility to aluminize the same product range as for cadming. This creates the electro-technical prerequisite for replacing cadmium with aluminum as a corrosion protection coating.

The electrolyte according to the invention is preferably used in the form of a solution. Aromatic hydrocarbons which are liquid at room temperature, such as toluene, are advantageously used as solvents, advantageously in the following composition: 1 mol of electrolyte salt per 1 to 10 mol, preferably 1 to 5 mol, of solvent.

The invention will be explained in more detail using examples.

1. Production of the electrolyte

Approx. 1140 ml of toluene are placed in a Witt'schen stirring pot (capacity: 3 1), which is equipped with a mechanical stirrer, a dropping funnel, a thermometer and an inert gas transfer system, and has 183.5 g of potassium fluoride slurried in it. 577 g of aluminum triethyl and 250 g of aluminum triisobutyl are gradually added to this slurry with stirring. The electrolyte KF · [1.6 Al (C ₂ H ₅ ) ₃ · 0.4 Al (iC ₄ H ₉ ) ₃ ] · 3.4 mol of toluene forms as clear, with increasing specific conductivity and with heating colorless liquid. After the reaction, this electrolyte composition - at 100 ° C - a specific conductivity of _{2, 25} S cm ^-1.

The same method can also be used to produce electrolytes with a different composition. The solvent-free electrolytes can also be produced in this way. To do this, however, it is necessary to carry out the reaction above the melting temperature of the respective electrolyte.

In the table below is the electrical conductivity (in S 10- ^{2. Cm-1)} at 100 ° C for more electrolytes in the general shape of KF · [(2-n) AlEt ₃ n AlR _3] · 3.4 moles toluene stated .

2. Electroplating attempts

The good scatterability of the electrolyte according to the invention is to be demonstrated on the basis of electroplating tests. A galvanizing cell in the form of a rectangular glass vessel (20 cm x 8 cm x 20 cm) was used to carry out the electroplating tests, and an Al anode plate was attached to each end. Since the aluminum electrolytes are sensitive to air and moisture, the electroplating cell was provided with a special cover that has several openings: for a thermometer, for a conductivity cell, for a gas transfer tube (for flooding the cell with nitrogen), for two stirrers (diagonally opposite in the corners of the cell in front of the anodes) and for charging the test specimens to be aluminized. Served as test specimens. Rectangular angle plates made of steel of a certain size. To determine the scatterability, the thickness of the aluminum layer deposited on the angle plates was determined by means of a layer thickness measuring device.

Before the aluminizing, the individual test specimens were pretreated, as is customary in electroplating, ie pickled and degreased. For this purpose, the one on a Ka The test specimen attached to the test rod was first degreased using an organic solvent and pickled by immersion in dilute hydrochloric acid. Subsequently, the specimen was cathodically degreased and provided for improving adhesion with an approximately 1 _/ um thick nickel layer. After rinsing with water and subsequent removal of the adhering water film (using a dewatering agent and subsequent immersion in toluene), the toluene-moist test specimen was introduced into the electroplating cell, ie into the electrolyte, and - as a cathode - arranged between the two anodes (cathode area: 2 dm ² ; distance between anode and cathode: each about 10 cm).

The electroplating was carried out at an electrolyte temperature of 100 ° C by means of a so-called pulse current (deposition voltage: + 10 V). For this purpose, the test specimens were alternately poled cathodically and anodically, the cathodic deposition time in each case 80 ms and the anodic deposition time in each case 20 ms.

In addition to the electrolyte according to the invention, the known electrolyte NaF was examined. 2 Al (C2H5) 3 and, each in a commercially available form, a cadmium electrolyte (cyanidic), a zinc electrolyte (weakly cyanidic) and a nickel electrolyte (weakly acidic), with the last three electrolytes being galvanized using direct current. This resulted in the following: When electroplating in the normal working range (Al electrolytes: 1 A / dm ² ; Cd, Zn and Ni electrolyte: 2 A / dm ² ), under otherwise identical conditions, is in the form of the known electrolyte from NaF. 2 Al (C ₂ H ₅ ) ₃ .3.4 mol of toluene - the scatterability only about 13%, while the elec trolyte - in the form of KF · [1.6 Al (C ₂ H ₅ ) ₃ · 0.4 Al (iC ₄ H ₉ ) ₃ ] · 3.4 mol toluene - has a scattering capacity of approx. 38%, ie almost three times. In comparison, the scattering capacity is approximately 30% for the Zn electrolyte, approximately 33% for the Ni electrolyte and approximately 40% for the Cd electrolyte.

Claims (4)

1. Organometallic electrolyte for the galvanic deposition of aluminum, characterized in that it has the following composition:

the following applies: Me = K, Rb or Cs; R = H or C _x H _{2x + 1} with x = 1 and 3 to 8, where at least two groups R are alkyl radicals; m = 1.3 to 2.4 and n = 0.1 to 1.1, where m must be> 2n. 2. Organometallic electrolyte according to claim 1, characterized in that the following applies: x = 1 or 3 or 4 and / or m = 1.8 to 2.2 and / or n = 0.2 to 0.5. 3. Organometallic electrolyte according to claim 2, characterized by the following composition:

the following applies: m '= 1.8 to 2.2, preferably 2.0; n '= 0.2 to 0.5, preferably 0; 4, and R '= CH ₃ or ^C ₄ ^H ₉ . 4. Use of the organometallic electrolyte according to one of claims 1 to 3 in the form of a solution in 1 to 10 mol, preferably 1 to 5 mol, of an aromatic hydrocarbon which is liquid at room temperature, in particular toluene.