Europaisches Patentamt
European Patent Office
© Publication number:
Office europeen des brevets
EUROPEAN
PATENT
564
APPLICATION
int. ci 5 C25D
© Application number: 92305153.6
0 518
3/04
@ Date of filing: 05.06.92
®
© Applicant: M & T HARSHAW
Two Riverview Drive
Somerset, New Jersey 08875(US)
Priority: 13.06.91 US 714818
@ Date of publication of application:
16.12.92 Bulletin 92/51
©
Designated Contracting States:
AT BE CH DE DK ES FR GB GR IT LI LU NL SE
@ Inventor: Newby, Kenneth Russ
77 Sycamore Avenue
Berkeley Heights, New Jersey 07922(US)
Inventor: Jones, Allen Ray
6 Wells Road
Flemington, New Jersey 08822(US)
Inventor: Meng, John
32265 Corte Chatada
Temecula, California 92390(US)
©
Representative: Stoner, Gerard Patrick et al
Mewburn Ellis 2 Cursitor Street
London EC4A 1BQ(GB)
© Sulphate scavenging in chromium electroplating.
© Barium chromate or dichromate is used as a
sulphate scavenger in a chromium electroplating
process. 0.1 to 5g of the barium chromate are used
per 100g of chromic acid. The barium chromate
scavenges excess sulphate without causing undesirable pH increase in the bath. It may be added
at the beginning of plating, also as part of a replenishment composition e.g. a mixture with chromic
acid.
CM
CO
m
oo
m
Rank Xerox (UK) Business Services
A2
1
EP 0 518 564 A2
This invention relates to chromium electroplating, in particular to the scavenging of sulphate ions
in a chromium electroplating process.
A conventional sulphate-catalysed hexavalent
chromium plating bath comprises chromic acid and
sulphate ion as essential basic ingredients. Sulphate is generally provided by sulphuric acid or
sodium sulphate, although other soluble sulphates
may be possible. The use of further additives, e.g.
to promote plating efficiency, is also usual.
The control of sulphate concentration is a problem in chromium electroplating. Sulphate, or species which may be oxidised to sulphate in a solution, can be created by the decomposition of catalyst components or other additives such as mist
suppressants. Chromic acid itself typically contains
sulphate as an impurity. Sulphate may also be
"dragged" forward into the plating process from
treatments occurring before plating. It is also commonly now desired to operate the plating process
in a "zero discharge" fashion i.e. so as to avoid the
release of sulphate and other ingredients from the
plating bath into the environment through drag-out,
misting or other means.
Consequently the sulphate concentration in the
bath tends to increase. As sulphate concentration
increases the chromium metal deposit tends to
become functionally less useful. Furthermore the
plating rate will become slower and slower unless
the chemical imbalance is corrected.
In the prior art, US-A-204261 1 suggests adding
a sparingly-soluble sulphate such as strontium sulphate to the bath. The complex equilibrium with
chromic acid results in a maintained sulphate concentration of about 1wt% relative to chromic acid.
However the method requires the repeated mixing,
stirring, or other agitation of the insoluble materials
at the bottom of the plating tank, in order to maintain the equilibrium concentrations. In deep or large
tanks this may be impossible, or at least a serious
problem. Conversely, plating may need to be halted to allow these necessarily disturbed solids to
settle again, otherwise nodular deposits may result.
Maintaining the equilibrium may require additional
heating of the plating tank. Again this may halt the
plating process.
Another prior art proposal is the addition of
barium carbonate for controlling the sulphate ion
concentration. This method precipitates barium sulphate and forms carbon dioxide and water. It is by
far the most common sulphate scavenging method
in current use in the industry. It works well. However the addition of each carbonate anion consumes two protons from the solution as it reacts to
give water and carbon dioxide. Over an extended
period this can raise the pH of the bath and render
it inoperative. Consequently the method works well
only when the correction of sulphate concentration
5
io
75
20
25
30
35
40
45
50
55
2
2
is small-scale and infrequent.
Furthermore barium carbonate cannot be packaged with chromic acid. The presence of any moisture could result in a hazardous reaction. Consequently barium carbonate cannot be incorporated
in a chromic-containing replenishment composition,
and the percentage of barium carbonate is therefore not easily controlled.
The problem addressed in the present invention is to provide a novel way of influencing sulphate concentration in a chromium electroplating
bath.
We propose to solve this problem by the use
of a barium chromate compound - either barium
chromate, barium dichromate or a mixture thereof in the plating bath. This can control the concentration of sulphate ion.
Accordingly, in one aspect the invention provides a chromium plating bath comprising chromic
acid and sulphate as basic ingredients, characterised by the presence of a barium chromate
compound as a scavenger for sulphate ion.
In another aspect, the invention provides a
process characterised by the inclusion of a barium
chromate compound acting as a sulphate ion scavenger.
In a further aspect, the invention provides a
replenishment composition for replenishing a chromium plating bath, comprising chromic acid and a
barium chromate compound.
In a still further aspect, the invention provides
the use of a barium chromate compound as a
sulphate scavenger in a chromium electroplating
process.
The chromium plating bath will usually consist
essentially of chromic acid and sulphate ion. Optional additional
such
as e.g.
ingredients
silicofluoride ion, sulphoacetic acid, alkylpolysulphonic acid, halogenated alkylpolysulphonic acid,
or a salt of any of these acids, may also be used.
The chromic acid concentration will usually be
from about 100 to about 450g/l, preferably from
about 200 to about 300g/l.
The sulphate ion concentration is preferably
from about 0.5 to about 5g/l. Usually it will be at
least 1.0g/l, more preferably at least 1.5g/l, and
preferably less than about 3.5g/l.
The barium chromate compound is used in a
quantity sufficient for the desired degree of scavenging, preferably from about 0.1 to about 5g per
100g of chromic acid.
Additional bath ingredients may be used if desired, consistent with obtaining a satisfactory plating result. For example a fume suppressant may
be added to improve the handling of the bath.
We find furthermore that barium chromate and
dichromate can be relatively safely packaged with
chromic acid, avoiding the safety problem arising
3
EP 0 518 564 A2
with barium carbonate. Furthermore the presence
of the chromate or dichromate anions from the
barium compound can be useful in replenishing
chromium species in the bath. As regards sulphate
ion scavenging, barium ion reacts with the sulphate
to form insoluble barium sulphate, keeping the sulphate ion concentration stably in a range where
good plating can be achieved. The equilibrium involved is believed to be a simple one, not complex
as in the use of a strontium-based scavenger, and
we find that good results can be achieved without
serious practical problems regarding mixing or stirring of insoluble settled material.
The barium chromate or dichromate may be
added as such, or may advantageously be premixed with chromic and added to the bath as a constituent of a replenishment composition. The exact
ratio of the barium chromate compound to chromic
acid may be determined by a skilled person having
regard to the sulphate accumulation conditions existing in the plating bath.
In the description below, please note that the
term "turnover" is used (as conventional in the art)
to denote the plating-out of a given amount of
chromium and the replenishment of that amount,
usually by addition of chromic acid.
The effectiveness of the invention is illustrated
by the following experiments, including embodiments of the invention as well as comparative
examples.
5
Experiment 2
catalyst, is continuously operated for four metal
turnovers (1,000g/l chromic acid). Sulphate increases from 2.5g/l to 13.5g/l, primarily because of
degradation of the catalyst. As in Experiment 1,
covering power and plating efficiency were reduced.
Experiment 4
10
15
A bath the same as that of Experiment 3 was
used. However, 2.2g of barium carbonate were
added with each 100g of chromic acid used in
replenishing the bath. The sulphate concentration
remained constant. However the pH rose from below 0.5 to over 1.0. This resulted in a slowing of
deposition and a nodular chromium deposit.
Experiment 5
20
25
Plating was carried out as described in Experiment 4, except that the barium carbonate was
replaced by an equivalent amount of barium dichromate. This time both the sulphate concentration and the deposit characteristics were maintained.
Experiment 6
30
Experiment 1
Commercial plating-grade chromic acid containing 0.12wt% sulphate was continuously used as
a replenishment material in a "zero discharge"type plating operation. After four metal turnovers,
equivalent to 1,000g of chromic acid per litre of
solution, the sulphate concentration had increased
from 2.5g/l to 3.7g/l. This increase of nearly 50% in
sulphate concentration is sufficient to reduce significantly the ability to plate in low-current-density
areas of parts being plated.
4
35
A pretreatment activation line resulted in the
"dragging in" of 1.34g of sulphuric acid per metal
turnover to a "zero discharge" plating solution.
3.45g of barium chromate were mixed with the
250g of chromic acid used to maintain a proper
chromium concentration in the bath. The sulphate
level remained constant.
Experiment 7
40
45
In a second chromium bath, 3.1 g of barium
chromate were mixed per 1,000g of the same
commercial grade chromic acid. This mixture was
used as a replenishment composition for the bath.
After four metal turnovers, the sulphate concentration remains constant and the covering power of
the bath does not diminish.
50
Experiment 3
55
A chromium plating bath incorporating a shortchain polysulphonic acid catalyst was operated at
an anodic current density of 300A/dm2 under "zero
discharge" conditions. After four metal turnovers
(additions of 1.000g/l of chromic acid) the sulphate
concentration had risen from 2.5g/l to 10.3g/l and
there was a decrease in both covering power and
plating efficiency.
Experiment 8
A plating bath as described in US-A-4472249,
containing 20g/l sodium sulphoacetic acid as a
3
Repeating Experiment 7 with the addition of
20g of barium chromate to each 1,000g chromic
acid, the sulphate concentration and plating characteristics were maintained at a constant level.
It should be understood that the invention is
not limited to the specific embodiments described
above, and a skilled person will be able to introduce variations to those specific teachings on the
basis of the general teaching given herein.
EP 0 518 564 A2
5
claim 12 in which the barium chromate compound is barium chromate, barium dichromate
or a mixture thereof.
Claims
1.
2.
A chromium plating bath comprising chromic
acid and sulphate as basic ingredients, characterised by the presence of a barium chromate
compound as a scavenger for sulphate ion.
A bath according to claim 1 in which the barium chromate compound is barium chromate,
barium dichromate or a mixture thereof.
5
14. A replenishment composition according to
claim 12 or claim 13 containing from about 0.1
to about 5g of the barium chromate compound
to 100g of chromic acid.
io
15. A replenishment composition according to any
one of claims 12 to 14, further comprising at
least one of silicofluoride ion, sulphoacetic acid
and salt thereof, alkylpolysulphonic acid and
salts thereof, halogenated alkylpolysulphonic
acid and salts thereof.
3.
A bath according to claim 1 or claim 2 comprising from 0.1 to 5g of the barium chromate
compound to each 100g of chromic acid.
4.
A bath according to any one of the preceding
claims, further containing at least one of sulphoacetic acid, alkylpolysulphonic acid, halogenated alkylpolysulphonic acid, a salt of any
of these acids, and silicofluoride.
20
A bath
claims
450g/l,
300g/l,
to any one of the preceding
from about 100 to about
from about 200 to about
acid.
25
A bath according to any one of the preceding
claims containing from about 0.5 to about 5g/l,
preferably from about 1.0 to abut 3.5g/l, of
sulphate.
30
5.
6.
according
containing
preferably
of chromic
7.
A chromium electroplating process characterised by the inclusion of a barium chromate compound acting as a sulphate ion scavenger.
8.
A process according to claim 7 in which the
barium chromate compound is barium chromate, barium dichromate or a mixture thereof.
9.
A process according to claim 7 or claim 8 in
which from 0.1 to 5g of the barium chromate
compound is used per 100g of chromic acid.
is
40
10. A process according to any one of claims 7 to
9 using a chromium plating bath in accordance
with any one of claims 4 to 6.
45
11. A process according to any one of claims 7 to
10 in which the barium chromate compound is
added progressively to the plating bath as plating proceeds, and further chromic acid is also
added to the plating bath as plating proceeds.
50
12. A replenishment composition for replenishing a
chromium plating bath, comprising chromic
acid and a barium chromate compound.
55
13. A replenishment
composition
according
6
to
4
16. Use of a barium chromate compound as a
sulphate scavenger in a chromium electroplating process.
17. Use according to claim 16 in which the barium
chromate compound is used in an amount of
from 0.1 to 5g per 100g of chromic acid used
in the process.