U.S. patent application number 10/618352 was filed with the patent office on 2004-06-03 for alkaline zinc-nickel bath.
Invention is credited to Hillebrand, Ernst-Walter.
Application Number | 20040104123 10/618352 |
Document ID | / |
Family ID | 7875843 |
Filed Date | 2004-06-03 |
United States Patent
Application |
20040104123 |
Kind Code |
A1 |
Hillebrand, Ernst-Walter |
June 3, 2004 |
Alkaline zinc-nickel bath
Abstract
The anode is separated from the alkaline electrode to avoid
undesirable secondary reactions in an alkali zinc nickel
electroplating bath.
Inventors: |
Hillebrand, Ernst-Walter;
(Wickede, DE) |
Correspondence
Address: |
COOK, ALEX, MCFARRON, MANZO, CUMMINGS & MEHLER LTD
SUITE 2850
200 WEST ADAMS STREET
CHICAGO
IL
60606
US
|
Family ID: |
7875843 |
Appl. No.: |
10/618352 |
Filed: |
July 11, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10618352 |
Jul 11, 2003 |
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09744706 |
Jan 30, 2001 |
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6602394 |
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09744706 |
Jan 30, 2001 |
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PCT/EP99/05443 |
Jul 29, 1999 |
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Current U.S.
Class: |
205/246 |
Current CPC
Class: |
C25D 21/12 20130101;
C25D 3/565 20130101 |
Class at
Publication: |
205/246 |
International
Class: |
C25D 003/56 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 30, 1998 |
DE |
198 34 353.1 |
Claims
1. Alkaline electroplating bath for plating zinc-nickel coatings,
having an anode (2) and a cathode (3), characterized in that the
anode is separated from the alkaline electrolyte by an ion exchange
membrane (6).
2. Electroplating bath according to claim 1, characterized in that
the cathode (3) is separated from the alkaline electrolyte (4) by a
perfluorinated cation exchange membrane (6).
3. Electroplating bath according to claim 1 or 2, characterized by
sulfuric acid, phosphoric acid, methanesulfonic acid, amidosulfonic
acid and/or phosphonic acid as anolyte (5).
4. Electroplating bath according to one of claims 1 to 3,
characterized by a platinum-coated titanium anode.
Description
[0001] The invention relates to an electroplating bath for plating
zinc-nickel coatings, having an anode, a cathode and an alkaline
electrolyte.
[0002] It is known to coat electrically conductive materials with
zinc-nickel alloys in order to improve their resistance to
corrosion. To do this, it is customary to use an acidic electrolyte
bath, for example with a sulfate, chloride, fluoropromate [sic] or
sulfamate electrolyte. In these processes, it is very difficult
and, in practice, generally impossible, in terms of control
technology, to achieve a uniform thickness of the zinc-nickel
coating on the material to be coated.
[0003] For this reason, the alkaline zinc-nickel electroplating
baths which are disclosed in German patent 37 12 511 have recently
been used, having, for example, the following composition:
1 11.3 g/l ZnO 4.1 g/l NiSO.sub.4 * 6H.sub.2O 120 g/l NaOH 5.1 g/l
polyethyleneimine.
[0004] The amines contained in the electroplating bath serve as
complex formers for the nickel ions, which are otherwise insoluble
in the alkaline medium. The composition of the baths varies
depending on the manufacturer.
[0005] The electroplating baths are usually operated with insoluble
nickel anodes. The zinc concentration is kept constant by the
addition of zinc and the nickel concentration is kept constant by
the addition of a nickel solution, for example a nickel sulfate
solution.
[0006] However, after they have been operating for a few hours, the
color of these baths changes from what was originally blue-violet
to brown. After a few days or weeks, this discoloration becomes
more intense and it is possible to detect a separation of the bath
into two phases, the upper phase being dark brown. This phase
causes considerable disruption to the coating of the workpieces,
such as for example nonuniform layer thicknesses or blistering. It
is therefore imperative for the bath to be continuously cleaned,
i.e. for this layer to be skimmed off continuously. However, this
is time-consuming and expensive.
[0007] Furthermore, after a few weeks of operation it is possible
to detect cyanide in the baths. Cyanide pollution requires regular
cleaning of the bath and special wastewater treatment, which has a
considerable effect on the operating costs of the bath. This
applies all the more so if the wastewater has a very high
concentration of organics and, with a COD value of approx. 15 000
to 20 000 mg/l, makes cyanide detoxification more difficult. It is
then only possible to adhere to statutory wastewater parameters
(nickel 0.5 ppm and zinc 2 ppm) by the extensive addition of
chemicals.
[0008] The formation of the second phase is attributable to a
reaction of the amines, which in alkaline solution are converted at
the nickel anodes to form nitrites (including to form cyanide).
Moreover, on account of the amines being broken down, fresh complex
former has to be continuously added to the bath, which increases
the costs of the process.
[0009] Anodes other than nickel anodes cannot be used, since they
dissolve in the alkaline electrolyte, which also has adverse
effects on the quality of the coating.
[0010] In view of this background, the invention is based on the
problem of providing an alkaline zinc-nickel electroplating bath
which provides high-quality zinc-nickel coatings at low cost.
[0011] To solve this problem, the invention proposes separating the
anode from the alkaline electrolyte by an ion exchange
membrane.
[0012] This separation prevents the amines from reacting at the
nickel anode, with the result that there are no undesirable
secondary reactions which cause waste disposal problems or lead to
a second phase of reaction products being deposited on the bath and
adversely affect the quality of the zinc-nickel coating. The
invention obviates the need for this layer to be skimmed off at
high cost and to renew the bath. Furthermore, there is a
considerable improvement in the quality of the coating.
[0013] The use of a cation exchange membrane made from a
perfluorinated polymer has proven particularly advantageous, since
such membranes have a negligible electrical resistance but a high
chemical and mechanical resistance.
[0014] Furthermore, the cyanide poisoning of the wastewater no
longer takes place, thus considerably simplifying the entire
wastewater treatment. Furthermore, there is no need to top up the
complex former in the electrolyte, since it is no longer broken
down and its concentration in the bath remains approximately
constant. As a result, the cost of the process becomes considerably
less expensive.
[0015] In the solution according to the invention, the zinc-nickel
bath functions as catholyte. The anolyte used may, for example, be
sulfuric acid or phosphoric acid. In the electroplating cell
according to the invention, customary anodes, such as for example
platinum-coated titanium anodes, are suitable as anode material,
since they are no longer exposed to the basic zinc-nickel bath.
[0016] The present invention is explained in more detail with
reference to the exemplary embodiment illustrated in the drawing,
in which:
[0017] FIG. 1 shows the diagrammatic structure of an electroplating
bath according to the invention.
[0018] FIG. 1 shows an electroplating cell 1 which has an anode 2
and a cathode 3, which is the workpiece to be coated. The catholyte
4 surrounding the anode is alkaline and consists of a zinc-nickel
electroplating bath of known composition, in which amines are added
as complex formers for the nickel ions. The anolyte 5 surrounding
the anode 2 may, for example, consist of sulfuric acid or
phosphoric acid. Anolyte 5 and catholyte 4 are separated from one
another by a perfluorinated cation exchange membrane 6. This
membrane 6 allows unimpeded flux of current through the bath but
prevents the catholyte 4, in particular the amines contained
therein, from coming into contact with the anode 2, thus preventing
the reactions which were extensively described in the introduction
to the description, including the adverse effects of these
reactions.
* * * * *