ELECTROPLATING METALS

PROCEDE D'ELECTRODEPOSITION

English Abstract

ABSTRACT OF THE DISCLOSURE
This specification relates to the electroplating
of metals and in particular to an electroplating bath having
an acolyte separated from the catholyte by a cation exchange
membrane. This specification provides an electroplating bath
having an anolyte separated from a catholyte by a perfluorinated
cation exchange membrane. One use of the present invention
is in trivalent chromium plating solutions wherein oxidation
of Cr(III) ions to Cr(VI) occurs at the anode and accumulation
of Cr(VI) ions in the plating solution ultimately results in
the cessation of plating. The use of perfluorinated cation
exchange membranes prevents this anode reaction, with negligible
increase in the plating voltage. The anolyte preferably has
a depolarising species therein capable of reducing the
electrode potential of the anode when the bath is in use. In
addition the pH of the catholyte can be stabilised by arranging
the pH of the anolyte to allow hydrogen transport through the
membrane to compensate for hydrogen evolution at the cathode.

Claims

The embodiments of the invention in which an exclusive
property or privilege is claimed are defined as follows:
1. An electroplating bath for depositing chromium having
an anolyte separated from a catholyte by a perfluorinated
cation exchange membrane, in which the anolyte includes
sulphate ions, and in which the catholyte includes chromium (III)
sulphate as the source of chromium and anions for reducing the
plating voltage; and said bath having an anode immersed in the
anolyte, the material of the anode and the anolyte being
determined by their chemical reaction.
2. A bath as claimed in claim 1, in which the anolyte
has a depolarising species therein capable of reducing the
electrode potential of the anode when the bath is in use.
3. A bath as claimed in claim 2, in which the depolarising
species are ferrocyanide anions, hydrazine, quinhydrone,
potassium iodide ions in sulphuric acid solution or ferrous
ions.
4. A bath as claimed in claim 1, 2 or 3, in which the pH
of the anolyte is arranged to allow hydrogen ion transport
through the membrane to compensate for hydrogen evolution at the
cathode thereby stabilising the pH of the catholyte.
5. A bath as claimed in claim 1, 2 or 3, in which the
anolyte is contained within a compartment immersed in the
catholyte, the compartment having at least a part of its
surface provided by the membrane.

6. A bath as claimed in claim 1, 2 or 3, in which the
anolyte is contained within a compartment immersed in the
catholyte, the compartment having at least a part of its
surface provided by the membrane, in which the anolyte is a
gel.
7. A bath as claimed in claim 1, 2 ox 3, in which the
anolyte is contained within a compartment immersed in the
catholyte, the compartment having at least a part of its
surface provided by the membrane, in which the anolyte is a
gel, having a colloidal dispersion of a metal or carbon therein.
8. A bath as claimed in claim 1, 2 or 3 r in which the
anolyte is contained within a compartment immersed in the
catholyte, the compartment having at least a part of its
surface provided by the membrane, in which the compartment
comprises a metal or plastic box-like frame, or in which the
compartment is in the form of a tube.
9. A bath as claimed in claim 1, 2 or 3, in which the
anolyte is contained within a compartment immersed in the
catholyte, the compartment having at least a part of its
surface provided by the membrane, in which the anode is of
carbon, platinised titanium, stainless steel, magnetite or
chromium.
10. A bath as claimed in claim 1, 2 or 3, in which the
anolyte is contained within a compartment immersed in the
catholyte, the compartment having at least a part of its
surface provided by the membrane, in which the compartment
comprises a metal or plastic box-like frame, or in which the

compartment is in the form of a tube, and in which the anode
is of carbon, platinised titanium stainless steel, magnetite
or chromium.

Description

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The present invention relates to the elec~roplating
of metals, and more par~icularly to an electroplating bath
having an anolyte separated from the catholyte by a cation
exchange membrane.
The use of memhranes in electroplating baths has been
suggested but has been inhibited by the fact that formerly
they have exhibited high electrical resistance and consequently
produced unacceptably high plating voltages in electroplating
baths. Also the use of membranes re~uired expensive structural
modification of existing electroplating baths.
The pxesent invention provides an electroplating
bath having an anolyte saparated from a catholyte by a
perfluorinated cation exchange membrane. One use of the
present invention is in trivalent chromium plating solutions
wherein oxidation of Cr(III) ions to Cr(VI) occurs at the anode
and accumulation of Cr(VI) ions in the plating solution
ultimately results in the cessation of plating. The use of
perfluorinated cation exchange membranes prevents this anode
reaction with negligible increase in the plating voltage. The
anolyte preferably has a depolarising species therein capable
of reducing the electrode potential of the anode when the
bath is in use.
Thus the present invention provides an electroplating
bath for depositing chromium having an anolyte separated from
a ~atholyte by a perfluorinated cation exchange membrane, in
which the anolyte includes sulphate ions, and in which the
catholyte includes chromium (III) sulphate as the source of
chromium and anions for reducing the plating voltage; and said
bath having an anode immersed in the anolyte, ~he material of
the anode and the anolyte being determined by their chemical
reaction.
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In addition the pH o the catholyte can be
stabilised by arranging the pH of the anolyte to allow
hydrogen transport through the membrane to compensate for
hydxogen evolution at the cathode.
A preferred perfluorinated cation exchange membrane
is a sheet of NAFION. (N~FION is a trade mark of the
Du Pont Corporation). These per~luorinated polymer membranes
are thin, having negligible electrical resistance and are
mechanically and chemically robust. For trivalent chromium
electroplating baths, and depolarising species may include
ferrocyanide anions, hydrazine or quinhydrone. The effect
of the depolarising species is to significantly reduce the
plating voltage. The plating voltage can be further reduced
by using highly conducting anions in the catholyte which
without the use o~ a membrane could have produced an adverse
- anode reaction. An example of this is chloride
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anions. Chloride anions are desirable as they haye high specific
conductivity and are obtained from the inexpensiYe NaCl salt. Their
- use in trivalent chromium baths is undesirable as chlorine can be
evolved at the anode.
The electroplating baths of the present inventiQn, therefor,
can avoid deleterious anode reactions, and can reduce the plating
voltage by using a perfluorinated cation exchange membrane and by
- using a suitable anolyte. In this way it is possible to separately
optimise both the anolyte and catholyte. In addition, the material
of the anode is not determined by the plating solution and can be
optimised for cost and for its electrochemical reaction with the
anolyte.
The present invention further provides an anode arrangement for
an electroplating bath comprising a compartment having at least a
part of its surface formed by a perfluorinated cation exchange membrane,
and having within the compartment an anode and an anolyte. The
anolyte preferably has a depolarising species thereir capable of reduc-
ing the electrode potential of the anode when the bath is in use.
The anode arrangement is such that it can be substantially the
same shape and size as a conventional anode and can be immersed and
supported in the catholyte without changing the anode supports.
One anode arrangement according to the present invention com-
prises a compartment consisting of a perfluorinated cation exchange
membrane supported on a box-like frame and has an anode supported with-
in the compartment. Alternatively the membrane may be in the form
of a tube. An anolyte is proYided in the compartment or tube,
surrounding the anode, preferably by a gelg such as an Agar gel,
saturated with a depolarising species. In addition the gel may con-
tain a colloidal dispersion of a metal or carbon to increase the
conductivity of the anolyte. The membrane is preferably a NAFION
sheet. The anode arrangement of the present invention provides
a rugged, simple and efficient anode which prevents adverse reactions
UK9-78-015 -2-
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27
and enables the catholyte to be optimised for pla-ting. Also -the
acidi-ty of the anolyte can be arranged so that the pH of the catho-
lyte is stabilised when the bath is in use.
The invention will now be described with reference to the -follow-
ing examples:
EXAMPLE 1
An electroplating bath for chromium comprises an anolyte and
a catholyte have the following constituents.
Anolyte
Potassium Ferrocyanide (K4Fe(CN)6) lM
; Sodium Sulphate (NaS04) 2M
p~ adjusted to 1.6
Catholyte
Chromium (III) Sulphate O.lM
Sodium thiocyana-te 0.2M
Sodium Chloride 2M
Glycine 10 grams/litre
Boric acid 60 grams/litre
Wetting agent (FC98)* 0.1 gram/litre
pH adjusted to 3.5
* FC98 is a product of the 3M Corporation.
The anolyte and catholyte were separated by a sheet of NAFION
(NAFION is a trade mark of the Du Pont Corporation). The bath was
operated at a temperature of 50PC. The effect of using ferrocyanide
as a depolariser in the anolyte was to reduce the plating voltage
by 17 percent using a platinised titanium anode. The ferrocyanide
anions are oxidised to ferricyanide anions during plating so that the
ferricyanide anions must be reduced back to ferrocyanide ions af`ter a
given plating time. The depolarising species may be reduced electro-
chemically in a suitable electrolyte or may be reduced by adding asuitable reducing agent, such as sodium dithionite or zinc, to the
anolyte.
~ UK9-78-015 -3-
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In addition the pH of the catholyte mdy be stabilised by adjust-
ing the pH of the anolyte to allo~ hydroyen ion transpor$ through
the membrane to compensate for -the increase in the p~l o~ the catholyte
by the evolution of hydrogen at the cathode.
Hydrazine and quinhydrone may be substitu-ted for ferrocyanide
anions in the anolyte.
EXAMPLE 2 -
- An anode arrangement for a chromium (III) electroplating bath
having a composition similar to the catholyte given in Example 1
consists of a compartment of box-like shape. The compartment consist-
ing of a NAFION membrane supported by a metal or plastics frame.
Alternatively the membrane may be in the form of a tube. A carbon
anode is supported within the compartment or tube. An anolyte fills
the inside of the compartment or tube surrounding the carbon anode.
The anolyte consists of an Agar gel saturated with a solution of 2M
potassium iodide in O.lM sulphuric acid. The conductivity of the
anolytè can be further increased by including a colloidal dispersion
of carbon in the gel. The sulphuric acid concentration can be
arranged so that the pH of the catholyte is stablised when the bath
is used.
The depolar;sation reaction comprises the oxidation of I ions
to I3 . This reaction is reversible allowing the depolarising species
to be regenerated (reduced) in an aqueous electrolyte or in the
electroplating bath itself by making the anode arrangement a temporary
cathode.
Alternative depolarising species to the iodide anions include
ferrocyanide anions, hydrazine, quinhydrone or ferrous ions. Materials
other than carbon may be used for the anode. Suitable materials are `
stainless steel, platinised titanium, magnetite or chromium depending
on the specific depolarising species.
UK9-78-015 -4

EXAMPLE 3
~n electroplating bath ~or chromium con~prises an anolyte and
a catholyte having the following constituents:
Anolyte
10% by volume sulphuric acid (approximately 2M) and quinhydrone
(to saturation at 18C) t
pH adjusted to 1.0
Catholyte
Chronnium Sulphate O.lM
Sodium thiocyanate 0.2M
Sodium chloride 2M
Glycine 10 grams/litre
Boric acid 60 grams/litre
Wetting agent (FC98) 0.1 gram/litre
pH adjusted to 3.5
The anolyte and catholyte were separated by a sheet of NAFION
membrane. The bath was operated at a temperature of 50C. The use
- of quinhydrone reduced plating voltage by 15% using a platinised
~ titanium anode mesh.
4`
:
.
UK9~78-015 -5-