Note: Descriptions are shown in the official language in which they were submitted.
TRIVALENT CHROMIUM ELECTROPLATING SOLUTIOM AND BATH
The invention relates to chromium electroplating
solutions and baths in which the source of chromium
comprises an equilibrated aqueous solution of chromium ~III)
complexes.
Background Art
The advantages of plating chromium from an equilibrated
aqueous solution of chromium ~III) - thiocyanate complexes
over conventional chromic acid plating are elaborated in our
U.K. Patent 1,431,639. Refinements and modifications of
this basic process have been described in later patents
among which are U.S. Patent 4,141,803 and 4,161,432. The
benefits to the trivalent chromium process of an anolyte and
catholyte separated by a cation exchange membrane are
described in our Canadian Patent 1,120,427, issued March ~3,
1982 to D.J. Barclay et al. Finally our U.K. Patent
2,033,427, issued May 6, 1982 to D .JO Barclay et al and
Canadian Patent 1,150,185, issued July 9, 1983 to D.J.
Barclay et al describe a related solution and process in
which beneficial effects are obtained from a reduction in
the level of chromium and thiocyanate concentration to
levels well below those originally contemplated.
The equilibrated chromium (III) - thiocyanate complexes
from which plating takes place have been prepared from a
variety of starting materials. The originally preferred
starting salts of U.K. Patent 1,431,639 were chromium
perchlorate and sodium thiocyanate. In order to make the
solution sufficiently ~lectrically conductive additional
sodium perchlorate was added as a supporting electrolyte.
U.S. Patent 4,141,803 proposed hexathiocyanatochromium salts
of potassium or sodium (K3Cr~NCS)6 or Na3Cr(NCS)6) to
UK9-80-00~X -1-
which sodium perchlorate or sodium sulphate was a~ded as a
conductivity salt. Po~assium sulphate was also mentioned as
a possible conductivity salt but no example was yiven. In
U.S. Patent 4,161,432 one preferred solution was prepared
from chromium chloride (CrC13) and sodium thiocyanate.
Potassium chloride was added for conductivity. A second
preferred solution was prepared from chromium sulphate
~Cr2(SO4)3) and sodium thiocyanate. In this case sodium
sulphate was added for conductivity.
In Canadian Patent 1,120,427, in which a catholyte and
anolyte are separated by a membrane, the catholyte was
prepared from chromium sulphate ICr2(SO4)3) and sodium
thiocyanate, and sodium chloride was added for conductivity.
The anolyte consisted of an aqueous solution of a
depolarising agent to which sodium sulphate (Na2SO4) was
added for conductivity. The advantage of having sodium
sulphate in the anolyte rather than sodium chloride is that
chlorine evolution from the anode is very much reduced. The
electrolyte employed in Canadian Patent 1,150,185 has
essentially similar constituents to that of Canadian Patent
1,120,427 except that the concentration of chromium is below
0.03 molar and the concentration of thiocyanate is also
proportionally reduced.
It is found that in plating chromium from electrolytes
as described in Canadian Patents 1,120,427 and 1,150,185,
with catholyte and anolyte separated by a cation exchange
membrane, chloride ions from the catholyte are, in practice,
able to penetrate the membrane in sufficient numbers to give
significant chlorine evolution at the anode~ This is not
only environmentally undesirable but prevents the use of
cheap lead anodes because of formation of lead chloride
UK9-80~004X -2-
UK9-8o-oo4E X
l thereon. Ins tead, platinized titanium anodes have had to be
used. A ~urther problem with baths h3ving chloride anions in
the catholyte is that pH stability is poor and needs frequent
adjustment.
Disclosure of the Invention
.
The above stated disadvantages of a chloride supporting
electrolyte point to the use of a sulphate. Several examples
of the use of sodium sulphate as a conductivity sal t for a
supporting electrolyte are given in the above listed prior
art~ This salt is cheap and readily soluble. No noxious
anode gases are liberated and the pH stability of the bath is
improved. However, the efficiency and plating current
density ran~e,of trivalent chromium/thiocyanate plating baths
employing sodium sulphate rather than the chloride are found
to be materially reduced~ It is hypothesized that the reason
for this deterioration in perfonmance may be complexing
between the sulphate ions and the chromium-thiocyanate
cGmplexes which tends to hinder mobility and electrochemical
activity of the complexes in solution.
The present invention stems from the discovery that
potassium sulphate as a conductivity salt for a supporting
electrolyte does not cause such a deterioration in perfor-
mance of the trivalent chromium plating process. Potassium
sulphate had been suggested as a possible conductivity salt
in US patent 4141803 but no examples of its use or
suggestions of this advantage were given. Using potassium
sulphate the efficiency of the ~ath was found to improYe.
, However it was also observed that, although plating was
possible at much higher current densities than with the
sodium sulphate bath, it was not possible at such low current
densities as with the sodium sulphate bathO
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UK9-80-004EX
1 Since there is a direct relationship bet~een current
density and plating voltage for a given electrolyte, this
higher minimum current density requirement dictates a higher
minimum plating voltage.
-
Accordingly, the present invention provides a chromiumelectroplating solution comprising an aqueous
solution of chromium (III) - complexes as the
source of chromium and a supporting electrolyte consisting
essentially of a mixture of sodium and potassium sulphates in
a concentration sufficient to provide electrical conductivity
for the plating process.
By using a mixture of both these salts as the supporting
electrolyte,~;~oth high efficiency and a wide plating range
can be achieved without the need for high plating voltages.
In preEerred examples, efficiencies of up to 9.5~ (at 60
mAcm 2, 60 centigrade and pH 3.51 and a plating range of
10 - 1000 mAcm 2 have been achieved.
One reason for the beneficial effect of the potassium
sulphate on efficiency and plating range is believed ~o be
that the potassium preferentially ion-pairs with the sulphate
in solution thus leaving the mobility of the chromium (III)
complexes largely ~naffec-ted. To maximize the
benefit, it is preferred that ~he potassium sulphate should
be present in saturation concentration.
It is also preferred that the concentration of sodium
sulphate is less than or equal to 1 Molar. Otherwise, with a i
, greater proportion of sodium sulphate than this, efficiency
hegins to fall off again. The optimum concentration of
sodium sulphate appears to be around 0~5 Molar.
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1 It has been found that a mixed sulphate electrolyte
will be beneficial in any chromium (III) plating solution
including, but not limited to, those containing thiocyanate.
In its broadest aspects, this invention comprises a
chromium plating solution comprising an aqueous solution of
chromium (III~ complexes as the source of chromium and a
supporting electrolyte consisting essentially of a mixture
of sodium and potassium sulphates in a concentration
sufficient to provide electrical conductivity for the
plating process.
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UK9-80-004E X
Considering now, in particular, a trivalent
chromium/thiocyanate bath having anolyte and catholyte
separa~ed by a cation exchange membrane, the basic reason for
the use of such a membrane is to prevent anodic oxidation of
bath constituents at the anode. As a result of the blocking
of thiocyanate anions by the membrane, water, instead, is
oxidised at the anode resulting in a steady input of hydrogen
ions to the anolyte. The flux of these hydrogen ions through ,n
the membrane into the catholyte is important in that it
maintains the acidity of the catholyte which would otherwise
decrease because of the steady evolution of hydrogen at the
cathode. Thus the membrane acts to stabilize pH.
The presence of chloride ions in the catholyte but not
the anolyte ~s believed to reduce this pH stabilizing effect
on the catholyte somewhat. The reason for this is not
entirely clear but could be connected with the concentration
differential o chloride across the memhrane. AS noted above
this leads to an inward flux of chloride ions to the
anolyte. It is possible that the flux of chloride ions acts
to reduce the outward flux of hydrogen ions from anolyte to
catholyte. Also the rate of production of hydrogen ions in
the anolyte by electrolysis of water will be reduced because
of the preferential oxidation of the chloride ions.
This additional problem is solved according to another
aspect of the present invention, without greatly affecting
the bath efficiency, by providing a chromium electroplating
bath comprising an anolyte and a catholyt~ separated by a
cation exchange membrane, ~he c~tholyte being chloride free
, and comprising an equilibrated aqueous solution of ~!
chromium (III) - thiocyanate complexes and a supporting elec- ~'
trolyte comprising at least potassium sulphate in a concen
.
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UK9-80-004EX
1 tration sufficient to provide electrical conductivity for the
plating process, and the anolyte also being chloride free and
comprisin~ sulphate ions in aqueous solution.
The plating range of an all potassium sulphate catholyte
may be found inadequate in which case sodium sulphate is
added in an amount sufficient to increase the
range without reducing efficiency to an unacceptable degree.
Sulphate ions in the anolyte are preferably provided as
an aqueous solution of sulphuric acidu
One further important consequence of the chloride free
bath i5 that its anode may be of lead rather than platinized
titanium.
~uantitative results have been obtained from plating
experiments performed in a Hull cell. The electrolyte
employed was one of 0.012M chromium concentration including,
thiocyanate and aspartic acid as complexants, the
conductivity salts, and boric acid as a pH buffer.
In addition to Hull cell experiments, larger baths have
been operated for periods of up to several months. In these
baths both potassium sulphate alone and also a mixture of
potassium and sodium sulphates have been used as conductivity
salts. The larger baths have an anolyte and catholyte
separated by a cation exchange membrane. Topping up of these
baths with "chrometan" (hydrated chromium sulphate3 and
thiocyanate anions replaces depleted chromium without
altering the essential composition of the bath~ Adjustment
of pH, when necessary, can be effected with a mixture of
potassium and sodium hydroxides in the same proportion as the
29 conductivity salt mixtureO
UK9-80-004E X
l Detailed Description
The invention will now be described further with
reference to the following comparative examples and examples.
Comparative Example I
A concentrated chromium plating solution was first
prepared in the following manner:-
a) 60 grams of boric acid (H3BO3) were added to 750 mlof deionised water which was then heated and stirred to
dissolve the boric acid.
b) 33.12 gràms of chromium sulphate
(Cr2(SO4)3.15H2O) and 16.21 grams of sodium thio-
cyanate (NaNCS) were added to the solution which was then
heated and stirred at approximately 70C for about 30
minutes.
c) 16.625 grams of DL aspartic acid (NH2CH2CH(COOH)2)
~ere added to the solution which was then heated and
stirred at approximately 75C for abut 3 hours. During
this time the pH was adjusted from pH 1.5 to pH 3.0 very
slowly with a 10% by weight sodium hydroxide solution.
~ Once the pH of 3.0 was achieved it was maintained at this
value for the whole of the equilibration period.
d) Sufficient sodium chloride ~as added to the solution to
make it approximately lM concentra~ion and 0.1 grams of
FC 98*ta wetting agen~ produced by 3M Corporation) was
also added. The solution was heated and stirred for a
further 30 minutes.
* Trade ~ark
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UK9-80-004EX
1 e) The solution pH was again adjusted to pH 3.0 with sodium
hydroxide s~lution. -'-
f) The solution was made up to 1 litre with deionised water
which had been adjusted to pH 3.0 with a 10% by volume
solution of hydrochloric acid.
The concentrated solution composition may be expressed
as:-
0.1 M chromium sulphate Cr2(SO4)3.15H70
0. 2 M sodium thiocyanate - NaNCS
0~125 M aspartic acid - NH2CH2CH(COOH)2
60 g/l boric acid - H3BO3
60 g/l so~`~ium chloride - NaCl ~;
0.1 g/l FC 98 - Iwetting agent product of 3M Corp)
L
As a result of the equilibration process, the bul~ of the
chromium in the final solution is believed to be in the form
of chromium/thiocyanate/aspartic complexes.
'
120 mls of this solution were made up to 1 litre with a
solution containing 60 grams per litre of boric acid and 60 E~
grams per litre of sodium chloride.
,
The final solution composition (omitting the wetting
agent~ was:-
0.012 M chromium sulphate
O ~ û24 M sodium thiocyanate
0.0'S M aspartic acid
60 g/l boric acid
60 g/l sodium chloride
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UK9-80-004E
l This solution was introduced into a Hull cell having a
standard brass Hull eell panel connected as a cathode and a
platinized titanium anode. At a temperature of 60C and a
solution pH adjusted to 3.5, a total current of 10 amps was
passed through the Hull cell to produce a bright deposit of
chromium on the test plate. To sustain the plating current
required a voltage of 10.6 volts applied to the cell.
Examination of the Hull cell test panel indicated acceptably
bright plating within a current density range of 10-700
mAcm 2. Efficiency measurements were made in a separate
cell, employing an anode bag, and filled with a plating
solution of the above eomposition as catholyte. The anode
bag was a perfluorinated eation exehange membrane separating
the catholyte from a separate anolyte eomprising an aqueous
solution of ~ulphurie acid in 2~ by volume eoncentration. ~,
The plating efficiency of this solution was calculated from
the results of these separate experiments to be 8~ falling to
6% after plating for 4 Ampere hours per litre. The
effieiency was measured at a eurrent density of 75 mAcm 2,
a temperature of 60C and a pH of 3.5. Despite the memhrane
ehloride ions were deteeted in the anolyte in eoneentrations
up to approximately 0.5M, resulting in the evolution of
ehlorine at the anode, furthermore the pH of the bath began
to rise quickly and had to be adjusted frequently.
Comparative Example II
Two plating solutions were made up exactly as for Com-
parative Example I exeept that sodium sulphate ~Na2SO4~ L
replaeed sodium ehloride as the ~onductivity salt2 One
solution had a 1 molar eoneentration of sodium sulphate and
. 30 the other had a 2 molar eoneentration. @
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UK9-80-004E 1~4~
1 The solutions were introduced as electrolytes into a Hull
cell with the same anode as for Comparative Example I. Test
panels were plated at 10 amps total current to produce
bright chromium deposits. In all experiments, the
temperature was 60C and the solution pH was adjusted to 3 5 i-
For the lM sodium sulphate electrolyte, 15.2 volts were
needed across the cell to sustain the current. The current
density plating range in the Hull cell was 20-600 mAcm 2,
For the 2M sodium sulphate electrolyte, 13.2 volts were
needed to sustain the current of 10 amps. The plating range
was reduced as compared with the chloride conductivity salt
to 10-500 mAcm '.
In furthe~ experiments, efficiencies were measured in a
separate cell ~aving an anode membrane and anolyte as or ~-
Comparative Example 1 and employing the lM and 2M sodium
sulphate plating solutions as catholytes. For the lM sodium
sulphate catholyte, the initial efficiency of the solution,
as measured at a current density o 50-55 mAcm 2, a
temperature of 60C and a pH of 3.5 was 7.0%. For the 2M
sodium sulphate catholyte9 the initial efficiency measured
separately under the same conditions as above was 7.5% but
fell rapidly to a sustained efficiency of 4.53.
.
Since no chloride was employed no chlorine could be
evolved at the anode. However, the sustained efficiency and
plating range of the sodium sulphate bath were reduced as
compared with chloride bath. L
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UK9-80-O04EX
l Exam~le I
A plating solution was made up in the manner of Com-
parativ~ Example I except that potassium sulphate (K2504)
replaced sodium chloride as the conductivity salt, potassium
hydroxide was used instead of sodium hydroxide and potassium
thiocyanate replaced sodium thiocyanate. The potassium
sùlphate was present in saturation concentration and was
prepared from potassium hydrogen sulphate.
This plating solution was introduced, as the catholyte,
into a cell having the same anode, anolyte and membrane
arrangement as for the Comparative Examples.
~.
Efficien~ measurements were made at a current density of
50-55 mAcm a, a temperature of 60C and an adjusted pH of
3.5. The initial efficiency of the solution was measured to
be 9~ and fell only to 8.5~ over a long period of time.
Thus, a bath employing potassium sulphate for conductivity
has significantly better current efficiency than one --
employing sodium sulphate (c.f. ComparatiYe Example II~. ~
The pH stabili~y of this bath is also better than the ~i
bath of Comparative Example I. The solution pH only rose
from 3.5 to 4.0 a~ter 40 ampere hours per litre of charge had
passed. It was then adjusted back to 3.5 using sulphuric
acid. It will be recalled that the membrane acts to
stabilize pH by allowing electrolysis of water at the anode
instead of other reactions which would occur preferentially
with catholyte components~ The hydrolysis produces hydrogen
ions which can pass through the membrane to replace those
lost by hydrogen evolution at the cathode. It is believed
29 that since sulphate will not pass through the membrane, the
12 L
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JK9-80-004E X
1 flux of hydrogen ions is greater than it would be with
chloride in the catholyte. Also sulphate, unlike chloride
does not preferentially oxidise at the anode thereby allowing
the maximum number of hydrogen ions to be generated.
In order to determine platïng range and minimum plating
voltage, the plating solution of this example was introduced
as the electrolyte into a Hull cell. Test panels were plated
at a total current of 10 amps to produce bright chromium
deposits. The solution temperature was 60C and its pH was
adjusted to 3.5. A voltage of 11.9 volts was needed to sustain
this plating current. The plating range in th~ Hull cell was
from 25 to approximately 1000 m~cm ~ The upper limit
could not be precisely determined because the test plate was
plated right~to the top edge. As compared with a bath
employing sodium sulpha-te for conductivity, a bath employing
potassium sulphate has an extended upper limit of plating
current density but the lower threshold for plating was
raised.
Thus potassium sulphate has advantages as a conductivity
salt particularly in a bath with a membrane. It does however
have the disadvantage that the lower end of the plating range
is rather high at 25 mAcm ~. As explained earlier this
higher minimum current density requirement implies a high~r
minimum plating voltage than would otherwise be required.
This may be a disadvantage in a working environmen~ where
there is only a limited supply voltage available.
Example II
A plating solution was made up in the manner of Example I
29 but, in addifion to the potassium sulphate in 1 Molar
~:. ~J
UK9-80-004EX
l concentration, sodium sulphate was also added in 0.5 Molar
concentration.
The mixed conductivity salt plating solution was intro-
duced into an electroplating cell as the catholyte with the
same anode, anolyte and membrane arrangement as for the
previous examples. The initial efficiency of plating was
measured, under the same conditions as for Example I, to be
8~.
In separate experiments, the same plating solution was
introduced as the electrolyte into a Hull cell under the same
conditions as for Example Io Test panels were plated at a
total cell current of 10 amps to produce bright chromium
deposits. A~oltage of 11.2 volts was needed to sustain this
current. The plating range in the Hull cell was from 10 to
approximately 100-0 mAcm a. ~his is wider than for Example L
I or Comparative Examples I and II. This implies a
significantly lower minimum voltage for satisfactory plating
in a working bath than would be needed for an all potassium
bath. Thus, a bath employing a mixture of sodium and
potassium sulphate as conductivity salts has both high
efficiency and good plating range while overcoming the
deficiencies of chloride conductivity salts.
Example III
Several plating solutions were made up in the manner of
Example II but having different concentrations of sodium
sulphate. -
le
Plating experiments were conducted in the manner of
28 Example II. In each case, he voltage needed to sustain a
1~ L
current of 10 amps and the current density plating range were
determined in a Hull cell. The initial plating efficiencies
were determined under the same conditions as for Example I,
in a separate cell employing an anode membrane. Sustained
efficiencies were not measured.
The following results were obtained:-
Sodium Hull cell Plating Initial
sulphate voltage Range Efficiency
concentration mAcm 2 %
0.1 M 11.6 20-1000 7-8
0.3 M 11.3 10-1000 7-8
1.0 M 11~2 10-700 6
Example IV
A plating solution was made up in the manner of Example
II but with the differenee that sodium thiocyanate, rather
than potassium thioeyanate was employed. Another difference
was that the concentration of boric acid was increased from
60 to 75 g/l.
Expressed in terms of its initial eonstituents the
composition of the solution was:-
0.012 M ehromium sulphate
0.012 M sodium thioeyanate
0.015 M aspartic acid
75 g/l boric aeid
0.5 M sodium sulphate
1.0 M potassium sulphate
UK9-80-004EX
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~K9~80-004EX
1 ~ull cell experiments were conducted at a temperatUFe of
60C and a solution pH adjusted to 3.5. The plating range
was 10 to approximately 1000 mAcm ~. Since the supporting
electrolyte is the same as for Example II, this implies that
a similar plating voltage as for Example II would be
necessary to sustain an overali current of 10 amps, though
this voltage was not, in fact, measured.
However, the initial efficiency measured separately in
the manner of Example II, improved to 9.5%. The solution
temperature was again 60C and the solution pH was 3.5 but
the current density was 60 mAcm lr
It was also observed that the bright chromium deposits
produced in ~ese experiments were ligh~er in colour than
tho~e produced in Example II.
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L
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l The following comparative e~ample illllstra'_es t.-le
beneficial effects of the mixed sulphate electrolyte ir. -
particular embodiment.
CO~IPARATIVE EX~IPLE III
Comparative examples were carried out on a chromi~m
(III)-thiourea bath employing a sodium sulphate electrol~te
and on the same bath employing mixed sodium and potassium
electrolytes. The benefit of mixed electrolytes was
demonstrated in that the hull cell plating voltage was
reduced from 17 volts to 13.5 volts. Two sulphate baths
were made up, the first of composition 14 gm/L malic acid,
75 gm/L boric acid, 33 gm/L Cr2(SO4)3.15H2O, 50 mg/L
thiourea and 1.5M Na2SO4. The other of the same composition
except that the molarity of the Na2SO~ was reduced to 0.5M,
and the bath was made lM in K2SO4. Both baths had tergitol*
08 as we~ing agent and both were of pH 3.85.
The first bath (just Na2SO4) was warmed to 54C, and a
hull cell was filled with it. lA was passed for two minutes
with a voltage of 4.5Vo With another brass plate, with the
same solution, a lOA test plate was run for two minutes at
50C, this time with a voltage of 17V.
The second plating bath was warmed to 52C, and this
was added to the cleaned hull cell. A lA test plate was run
for 2 minutes 20 seconds with this solution at a voltage of
4V. With the same solution and another brass plate, a lOA
test plate was run at 50C for 2 minutes, with a voltage of
13.5V.
Both solutions plated over a comparable range with the
same quality deposit. Eloweverl the voltage of the mixed
sulphate bath was significantly lower than that of the bath
containing only Na2SO~. Plating current densities ranged
- from 20-800 mA/cm2.
* Trade Mark
UK9-80-004EX 17
.~
1 Additional examples of other particular embodiments of
the invention follow:
In following three groups of examples, a bath
consisting of anolyte separated from a catholyte by a Nafion
cation exchange membrane is used. The anolyte comprises an
aqueous solution of sulphuric acid in 2% by volume
concentration (pH 1.6). The anode is a flat bar of a lead
alloy of the type conventionally used in hexavalent chromium
plating processes.
The catholyte for each Example was prepared by making
up a base electrolyte and adding appropriate amounts of
chromium (III), complexant and the organic compound.
The base electrolyte for each example consisted of the
following constituents dissolved in 1 litre of water:
Potassium sulphate lrl
Sodium sulphate 0.05M
Boric Acid lM
Wetting Agent FC98 0.1 gram
GROUP 1
Example 1
The following constituents were dissolved in the base
electrolyte:
Chromium ¦III) lOmM (from chromium)
DL aspartic acid 10~1
Thiourea lmM
at pH 3.5
Although equilibration will occur quickly in normal
use, initially the electrolyte is preferably equilibrated
until there are no spectroscopic changes which can be
detected. The bath was found to operate over a temperature
range of 25 to 60C. Good bright deposits of chromium were
obtained over a current density range of 5 to 800 mA/cm2.
* Trade Mark
18
.,~....
1 EY~ample 2
The following constituents were dissolved in the base
electrolyte:
Chromium (III) lOmM (from chrometan)
Iminodiacetic acid lOmM
Thiourea lm~1
at pH 3.5
The electrolyte is preferably equilibrated until there
are no spectroscopic changes. The bath was found to operate
over a temperature range of 25 to 60C. Good bright
deposits of chromium were obtained.
Example 3
I
The following constituents were dissolved in the base
15 electrolyte:
Chromium ~III3 lOOmM (from chrometan3
DL Aspartic acid lOOmM
Mercaptoacetic acid lmM
at pH 3.5
The electrolyte is preferably equilibrated until there
are no spectroscopic changes. The bath was found to operate
over a temperature range of 25 to 60C. Good bright
deposits were obtained.
19
:,
Example 4
The following constituents were dissolved in the base
electrolyte:
Chromium (III) lOOmM (from chrometan)
DL Aspartic acid lOOmM
Thiourea lm~l
at p~l 3.5
The electrolyte is preferably equilibrated until there
are no spectroscopic changes. The bath was found to operate
over a temperature range of 25 to 60C. Good bright deposits
were obtained over a current density range of 10 to 800
mA/cm2.
The above examples illustrate a chromium electroplating
electrolyte containing a source of trivalent chromium ions, a
complexant, a buffer agent and organic compound having a -C=S
group or a -C-S yroup within the molecule for promoting
chromium deposition, the complexant being selected so that
the stability constant Kl of the chromium complex as defined
herein ls in the range 10 ~ K1 ~ 10 M
By way of example, complexant ligands having K1 values
within the range 108~ K1 < 1012 M 1 include aspartic acid,
iminodiacetic acid, nitrilotriacetic acid and 5-sulphosalicylic
acid.
The organic compound haviny -C-S group can be selected
from thiourea, N-monoallyl thiourea, N-mono-p-tolyl thiourea,
thioacetamide, tetramethyl thiuram monosulphide, tetraethyl
thiuram disulphide and diethyldithiocarbonate. The organic
compound having a -C-S~ group can be selected from mercaptoacetic
acid and mercaptopropionic acid.
Since the plating efficiency of the electrolyte is
relatively high, a commercial trivalent chromium electrolyte
ean have as low as 5mM chromium. This removes the need for
expensive rinse water treatment sinee the chromium content of
the 'drag-out' from the plating electrolyte is extremely low.
UK9 80-004EX
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1 In general, the concentration of 'che constituents in
the electrolyte are as follows:
Chromium (III) ions 10 to 1l`i
Organic compound 10 to 10 M
A practical chromium/complexant ligand ratio is
approximately 1:1.
Above a minimum concentration necessary for acceptable
plating rates, it is unnecessary to increase the amount of
the organic compound in proportion to the concentration of
chromium in the electrolyte. Excess of the organic compound
may not be harmful to the plating process but can result in
an increased amount of sulphur being co-deposited with the
chromium metal. This has two effects, firstly to produce a
progressively darker deposit and, secondly, to produce a
more ductile deposit.
GROUP II
~
The following constituents were dissolved in the base
electrolyte:
Chromium (III) lOm~l (from chrometan)
DL aspartic acid 1OmM
Sodium thiosulphate lmM
at pH 3.5
Although equilibration will occur quickly in normal
use, initially the electrolyte is preferably equilibrated
until there are no spectroscopic changes which can be
detected. The bath was found to operate over a temperature
range of 25 to 60C. Good bright deposits of chromium were
obtained over a current density range of 10 to 800 mA/cm2.
1 Example 2
The following constituents were dissolved in the base
electrolyte:
Chromium ~III) lOmM ~from chrometan)
Iminodiacetic acid lOmM
Sodium thionate lmM
at pH 3.5
The electrolyte is preferrably equilibrated until there
are no spectroscopic changes. The bath was found to operate
over a temperature range of 25 to 60C. Good bright
deposits of chromium were obtained.
Example 3
The following constituents were dissolved in the base
electrolyte:
Chromium (III) lOOmM (from chrometan)
DL aspartic acid lOOmM
Sodium thiosulphate ln~l
at pH 3.5
The electrolyte is preferably equilibrated until there
are no spectroscopic changes. The bath was found to operate
over a temperature range of 25 to 60C. Good bright
deposits were obtained.
22
1 Example 4
The following constituents were dissolved in the base
electrolyte:
Chromium (III) lOOmM (from chrometan)
DL aspartic acid lOOmM
Sodium thionate lm~l
at pH 3.5
The electrolyte is preferably equilibrated until there
are no spec-troscopic changes. The bath was found to operate
over a temperature range of 25 to 60C. Good bright
deposits were obtained over a current density range of 10 to
800 mA/cm2.
The above examples illustrate a chromium electroplating
electrolyte containing a source of trivalent chromium ions,
a complexant, a buffer agent and a sulphur species having
S-O or S-S bonds for promoting chromium deposition, -the
complexant being selected so that the stability constant K
of the chromium complex as defined herein is in the range
106 C Kl ~ 1012 M 1 and the sulphur species being selected
from thiosulphates, thionates, polythionates and
sulfoxylates.
By way of example complexant ligands having Kl values
within the range 106 ~ Kl ~ 1012 M 1 include aspartic acid,
iminodiacetic acid, nitrilotr~acetic acid, 5-sulphosalicylic
acid and citric acid.
The sulphur species are provided by dissolving one or
more of the following in the electrolyte: sodium
thiosulphate, potassium thio sulphate, barium thiosulphate,
ammonium thiosulphate, calcium thiosulphate, potassium
polythionate, sodium polythionate, and sodium sulfoxylate.
1 Very low concentrations of the sulphur species are
needed to promote reduction of the trivalent chromium ions.
Also since the plating efficiency of the electrolyte is
rela-tively high a commercial trivalent chromium electrolyte
can have as low as 5m~ chromium. This removes the need for
expensive rinse water treatment since the chromium content
o~ the 'drag~out' from the plating electrolyte is extremely
low.
In general, the concentration of constituents in the
electrolyte are as follows.
Chromium (III) ions 10 to 1~1
Sulphur species 10 to 10 M
A practical chromium/complexant ligand ratio is
approximately 1:1.
Above a minimum concen-tration necessary for acceptable
plating ranges, it is unnecessary to increase the amount of
the sulphur species in proportion to the concentration of
chromium in the electrolyte. Excess of the sulphur species
may not be harmful to the plating process but can result in
an increased amount of sulphur being co-deposited with the
chromium metal. This has two effects, firstly to produce a
progressively darker deposit and, secondly, to produce a
more ductile deposit.
GROUP III
,, _
The following constituents were dissolved in the base
electrolyte:
Chromium (III) 5mM (from chrometan~
DL aspartic acid SmM
Sodium sulphite 5mM
at pH 3.5
Although equilibration wil:L occur quickly in normal
use, initially the electrolyte is preferably equilibrated
until there are no spectroscopic changes which can be
detected. The bath was found to operate over a temperature
range of 25 to 60C. Good bright deposits of chromium were
obtained over a current density range o~ 10 to ~00 n~/cm2.
24
1 Example 2
The following constituents were dissolved in the base
electrolyte:
Chromium tIII~ 5mM (from chrometan)
Iminodiacetic acid 5mM
Sodium dithionite 2mM
at pH 3.5
The electrolyte is preferrably equilibrated until there
are no spectroscopic changes. The bath was found to operate
over a temperature range of 25 to 60C. Good bright
deposits of chromium were obtained.
Exam~le 3
The following constituents were dissolved in the base
electrolyte:
Chromium (III~ 50mM (from chrometanJ
DL Aspartic acid 50mL~
Sodium sulphite lOmM
at pH 3.5
The electrolyte is preferably equilibrated until there
are no spectroscopic changes. The bath was found to operate
over a temperature range of 25 to 60C. Good bright
deposits were obtained.
Example ~
The following constituents were dissolved in the base
electrolyte:
Chromium (IIX) 50mM (from chrometan)
5-sulphosalicylic acid 50mM
Sodium sulphite lmM
at pH 3,5
The electrolyte is preferrably equilibrated until there
are no spectroscopic changes. The bath was found to operate
over a temperature range of 25 to 60C. Good bright
deposits were obtained.
1 The abov~ examples illustrate a chromium electroplating
electrolyte containing a source of trivalent chromium ions,
a complexant, a buffer agent and a sulphur species haviny
selected frorn sulphites and dithionites for promoting
chromium deposition, the complexan-t being selec-ted so that
the stability cons-tan-t K1 of the chromium complex as defined
herein is in the range 106 ~ K1 ~ 1012 M 1 and the chromium
ions having a molar concentration lower than O.OlM.
By way of example, complexant ligands having Kl values
within the range 106 ~ K1 ~ 1012 M 1 include aspar-tic acid,
iminodiacetic acid, nitrilotriacetic acid, 5~sulphosalicylic
acid and citric acid.
The above examples also illustrate a chromium
electrolyte containing trivalent chromium ions, a
complexantt a buffer agent and a sulphur species selected
from sulphites and dithionites, the complexant being
selected from aspartic acid, 5-sulphosalicylic acid and
citric acid.
The above examples further illustrate a chromium
electroplating bath comprising an anolyte separated from a
catholyte by a perfluorinated cation exchange membrane, the
anolyte comprising sulpha~e ions and the catholyte
comprising a source of trivalent chromium ions, a
complexant, a buffer agent and a sulphur species selected
from sulphites and dithionites, and in which the source of
sulphate ions is chromium sulphate. Suitable complexant
ligands are aspartic acid, iminodiacetic acid,
nitrilotriacetic acid, and 5-sulphosalicylic acid and citric
acid.
Sulphites can include blsulphites and metabisulphites.
26
l Low concentrations of sulphite or dithionite are needed
to promote reduction of the trivalent chromium ions. Also
since the plating efficiency of the electrolyte is
relatively high a commercial trivalent chromium electrolyte
can have less than 10mM chromium. This removes the need for
expensive rinse water treatment since the chromium content
of the 'drag-out' from the plating electroly-te is extremely
low.
In general, the concentration of the cons-tituents in
the electroly-te are as follows:
Chromium (III) ions 10 to lM
Sulphur species 10 to 10 M
A practical chromium/complexant ligand ratio is
approximately 1:1.
Above a minimum concentration necessary for acceptable
plating rates, it is unnecessary to increase the amount of
-the sulphur species ln proportion to the concentration of
chromium in the electrolyte. Excess of sulphite or
dithionite may not be harmful to the plating process but can
result in an increased amount of sulphur being co-deposited
with the chromium metal. This has two effects, firstly to
produce a progressively darker deposit and, secondly, to
produce a more ductile deposit.
The preferred source of trivalent chromium is chromium
sulphate which can be in the form of a commercially
available mixture of chromium and sodium sulphates known as
-tanning liquor or chrometan. Other trivalent chromium
sal-ts, which are more expensive than the sulphate~ can be
used, and include chromium chloride, carbonate and
perchlorate.
27
l The preferred buffer agent used to maincain the pH of
the bulk electrolyte comprises boric acid in high
concentrations, iOe., near saturation. Typical pH range for
the electroly-te is in the range 2.5 to 4.5.
The conductivity of the electrolyte should be as high
as possible to minimize both voltage and power consumption.
Voltage is often critical in practical plating environments
since rectifiers are often limited to a low voltage, e.g., 8
volts. In an electrolyte in which chromium sulphate is the
source of the trivalent chromium ions a mixture of sodium
and potassium sulphate is the optimum. Such a mixture is
described in United Kingdom Patent 2,071,151.
A wetting agent is desirable and a suitable wetting
agent is FC98, a product of the 3M Corporation. However,
other wettinc3 agents such as sulphosuccina-tes or alcohol
sulphates may be used.
It is preferred to use a perfluorinated cation exchange
membrane to separate the anode from the plating electrolyte
as described ln United Kingdom Patent 1,602,404. A suitable
perfluorinated cation exchange membrane is Naf iOII tTrade
Mark), a product of the Du Pont Corporation. It is
particularly advantageous to employ an anolyte which has
sulphate ions when the catholyte uses chromium sulphate as
the source of chromium since inexpensive lead or lead alloy
anodes can be used. In a sulphate anolyte, a thin
conducting layer of lead oxide is formed on the anode.
Chloride salts in the catholyte should be avoided since the
chloride anions are small enough to pass through the
membrane ln sufficient amount to cause both the evolution of
chlorine at the anode and the formation of a highly
resistive film of lead chloride on lead or lead alloy
anodes. Cation exchange membranes have the additional
advantage in sulphate electrolytes that the pH of the
catholyte can be stabilized by adjusting the pH of the
anolyte to a:Llow hydrogen ion transport through the membrane
to comp~nsate for the increase in pH of the catholyte by
hydrogen evolution at the cathode. Using the combination of
a membrane, and sulphate based anolyte and ca-tholyte a
plating bath has been operated for over 40 Amphours/litre
without pH adjustment.
