Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to chromium electro-
plating. In particular it relates to electroplating from
aqueous trivalent chromium plating baths.
The potential value of solutions containing trivalent
chromium as electrolytes for chromium plating has been
recognized for many years. However, practical difficulties
have, until recently, prevented the commercial introduction
of any decorative chromium electroplating system based on
trivalent chromium. All commercial decorative chromium
plating has been based on hexavalent chromium, which has
some very serious drawbacks. Recently, howeverS certain
significant advances have been made in respect of triva-
lent chromium plating compositions, in particular a com-
position described and claimed in our U.S. Patent Number
3,954,574 which has had substantial commercial success.
In practice, the decorative appearance of deposits
obtained from trivalent chromium plating baths may some-
times be marred by certain faults such as streakiness, haze
or bands at certain current densities. In ~elgian Patent
Number 843,718, there are described certain faults which
it has been discovered are due to the presence in the
electrolyte of traces of metals such as iron, copper, zinc
and nickel~ We have now found that even when these metals
are substantially eliminated from the electrolyte, a slight
greyish discolouration of the deposits at high current
densities is observed. Surprisingly we have now discovered
that an improved deposit may be obtained when the solution
contains very small traces of certain metals, within a
particular range of concentration.
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Our invention provides a trivalent chromium electroplating
bath comprising from 30 to 15~ ppm of metals selected from iron
and nickel. The trivalent chromium plating bath is preferably
a bath containing at least one water soluble complex of tri-
valent chromium capable of electrodepositing chromium metal.
Particularly preferred are complexes comprising carboxylates
and halides. Such complexes may be preformed or formed
in situ in the bath. Particularly preferred are baths con-
taining a trivalent chromium salt, a formate, a bromide, and
ammonium, substantially as described in the specification of
our aforesaid U.S. Patent.
Alternatively and less preferably the baths may be of the
type containing a preformed complex of trivalent chromium, with
glycollic acid or oxalic acid and a halide such as bromide,
fluoride or preferably chloride substantially as described in
the specification of USP 3,706,639, USP 3,706,640, USP 3,706,641
and USP 3,729,392 or a bath of the same type additionally con-
taining ammonium. Preferably baths according to our invention
contain a borate, such as sodium borate or boric acid, chloride
20 and/or sulphate, and alkali metal such as sodium or potassium.
Customarily a wetting agent is also included. Baths according
to our invention are preferably substantially free from hexa-
valent chromium. Typically they have a pH of between 1 and ~.
Where solutions according to our aforesaid U.S. Patent are
25 employed, the solution may contain bromide, formate ~or acetate)
and any borate ion which may be present, as the sole anion species,
but such solutlons are undesirably expensive. Preferably, there-
fore, the solution contains only sufficient bromide to prevent
substantial formation of hexavalent chromium, sufficient formate
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in complexing the chromium and sufficient borate to be
effective as a buffer, the remainder of the anions required
to balance the cation content of the solution comprising
cheaper species such as chloride and/or sulphate.
For example the solution optionally and preferably
contain~ halide ions in addition to bromide, such as
fluoride, or preferably, chloride. The total amount of
halide including the bromide and any iodide which may be
present as well as any fluoride, and/or chloride may
optionally be sufficient, together with the formate and any
borate to provide essentially the total anion content of the
solution. The latter is determined by the number of equiva-
lents of cation (including hydrogen ion) and is typically
from 4 to 6 molar. Alternatively, and preferably, there
may additionally be present some sulphate ion. In one
embodiment, the sulphate is present in a minor proportion
based on the halide, e.g. a minor proportion based on the
chloride and/or fluoride. Alternatively the sulphate may comprise
a major proportion of the inorganic ion and, less preferably,
may be present in place of chloride and fluoride. Preferably
the solution also contains alkali metal ions, usually
provided as the cations of the conductivity salts, and/or
of some or all of the salts used to introduce the anion
species, which alkali metals are preferably sodium or
potassium. The solution may also contain alkaline earth
metals such as calcium or magnesium.
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The solutions of our invention may additionally contain
minor, compatible amounts of additives, such as wetting
agents (e.g. alkali metal alkyl benzene sulphonates) or
antifoams which are commonly used in plating technology.
Our novel solutions may therefore comprise some of the
ollowing species.
A.Trivalent Chromium
This is an essential ingredient of all the solutions of
the invention. Proportions of less than 0.1 molar or more
than 1.2 molar trivalent chromium result in significant
loss of covering power, and the concentration is preferably
maintained within these limits, most preferably between 0.2
and 0.6 molar. Preferably the solution is substantially
free from hexavalent chromium and preferably the chromium
in the solution is substantially all present as trivalent
chromium before plating.
B. Bromide
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This is a strongly preferred ingredient. The concen-
tration of bromide should preferably be maintained above
0.01 molar, to avoid formation of hexavalent chromium, and
lowering of the plating rate. The maximum concentration
is not critical, but is typically less than 4 molar and
preferably less than 1 molar. Economic and effective
operation normally requires a concentration of bromide
between 0.05 and 0.5. The preferred range is from 0.05 and
0.3 molar. Best results are obtained when the concentration
of bromide is greater than 0.1 molar. Iodide functions in
.c a similar fashion to bromide, but suffers from the dis-
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advantage that free iodine, which would be formed during
plating is only soluble to the extent of 0.03% w/w in water
compared with 4~ for bromine. Consequently attempts to use
iodide in place of bromide lead to unacceptable precipitation
of iodine. Iodide, is, moreover, too expensive to use
economically in place of bromide. However, it is possible
in principle, to replace a minor part of the bromide with
iodide, and references herein to bromide do not exclude
bromide containing traces of iodide.
10 C. Carboxylates
This is a strongly preferred ingredient, formate being
most strongly preferred. Typically the proportion of formate
to chromium should not exceed 3 : 1 on a molar basis, to
avoid unacceptably severe precipitation of the corresponding
chromium salt. If the proportion is less than 0.5 : 1 the
covering power is undesirably reduced. Preferably the pro-
portion of formate to chromium is between 2 : 1 and 1 : 1.
Acetate functions similarly to formate but gives a very much
lower plating speed. Acetate alone is not as effective as
formate in preventing ~he accumulation of free halogen. It
is possible, however, to use acetate as a partial replace-
ment for formate up to about a third of the total weight of
carboxylic acid without serious adverse effect. Solutions
containing acetate as moxe than a third of the total car-
boxylic acid are unlikely to be commercially competitivewith solutions based on formate alone, although they are
superior to prior art electrolytes. Other less preferred
carboxylates include glycollic acid, oxalic acid and also
less preferably other mono, di, poly, hydroxy and aldehyde
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carboxylic acids which are soluble in water and have not more
than 10 carbon atoms. Examples include citric, tartaric,
glyoxalic, maleic, succinic and malonic acids.
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D. Inorganic ~nionic Complexants
It is possible though less prefer~ed to replace at
least part o~ the carboxylate complexant by an inorganic
anionic complexant for chromium such as hypophosphite.
5 E. Ammonia
!~ The pressence of ammoniumlis strongly preferred for
our invention. Generally if the concentration of ammonium is
less than 0.1 molar, there is a risk of forming hexavalent
chromium. The upper limit is not critical and ammonium may
10 be present in amounts of up to saturation, i.e. about 4 molar.
Preferably the ammonium is present in a concentration of at
least 0.2 molar, most preferably from 1 to 3 molar. These
higher concentrations are desirable because deposits tend to
be darker at ammonia concentrations near the minimum and also
15 because the presence of ammonium helps to reduce consumption
of formate. Both ammonium and formate contribute to pre-
venting the buildup of free bromine, but at higher ammonium
concentration, the proportion of ammonium oxidised in this
reaction is greater, with consequent economies with the
20 more expensive formate. It is also possible, though not pre-
ferred, within the scope of this invention to include some
substi'tuted ammonium compounds such as hydroxylamine, hydra-
zonium or alkyl ammonium components in the compositions.
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However, in the absence of ammonium itae~ they do not
25 normally provide ade~uate covering power. Preferably aryl-
ammonium or heterocyclic ions such as pyridinium are abs~nt
sence they tend to inhibit deposition of chromium.
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F. Borate
Although it is possible to plate chromium from
solutions of our invention which do not contain borate,
we have not been able to obtain what we consider fully
satisfactory results, commercially, in the absence of
borate. Concentrations below 0.1 molar result in undesira-
bly low covering power. The upper limit is not critical
and is determined only by the solubility of borate in the
system, but generally we prefer to employ from O.S t~ 1
molar borate. The functionof the borate is obscure. Its
beneficial effects may be in part due to its buffering
action. However, other buffer salts, such as phosphates
and citrates appear relatively ineffective.
11 G. _ nductivity Salts
These are optional but generally preferred. The con-
centration is not critical and may vary between zero and
about 6 molar according to solubility. Preferably they are
present in proportions between 0.5 and 5 molar, e.g. 1 to 4
molar. Conductivity salts is a term used in the plating
art to d.~note certain readily ionisable salts which may be
added to plating baths to increase their electrical con-
ductivity and so reduce the amount of power dissipated in
the bath. Typically they are alkali metal or alkaline
earth metal salts of strong acids which are soluble in the
solution. They should have a dissociation constant at least
equal to 10 . Typical examples are the chlorides and sul-
phates of sodium and potassium.
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H. Hydrogen Ion
.
Best results are obtained when the bath is somewhat
acidic. At low pH values (below 2) there is some loss of
covering power which becomes unacceptable below pH 1. If
the pH is above 4 the rate of plating tends to be undesira-
bly slow. Optimu~ pH is between 2 and 3.5.
I. Chloride and/or Fluoride
This is optional, but at least in the case of chloride,
preferred. The amount is not, however, crticial. It may
vary from zero up to the ma~imum permitted by solubility
considerations. Chloride is generally introduced into the
bath as the anion of the conductivity salt (e.g. sodium
chloride), as ammonium chloride, which is a convenient means
of introducing the ammonia requirement of the bath, as
chromic chloride which may optionally be used to supply at
least part of the chromium requirement, and/or as hydro-
chloric acid, which is a convenient means of adjusting the
pH of the bath. Preferably the chloride content is at least
- 1 molar e.g. 1.5 to 5 molar. A particularly convenient
range is 2 to 3.5 molar.
J. Sulphate
This is an optional but preferred ingredient. The
amount of sulphate is not critical and may, like that of
the chloride, vary between zero and maximum amount which is
compatible with the solution. In one type of bath the
amount of sulphate is less than the total chloride. In a
different type of bath, however, the proportion of sulphate
is greater than the proportion of halide, and may be the
predominant anion in the bath. Like
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the chloride, the sulphate may be introduced into the bath as
the anion of the conductivity sal-t, or of the ammonium or
chromium salts or as sulphuric acid. Particularly preferred is
the use of sulphate as the source of chromium in the form of
chrome tanning liquors which are a basic chromi~n sulphate and
which, being a commercial by~product are a particularly con-
venient and cheap source of trivalent chromium. Typical sul-
phate concentratlons may be between 0 and 5 molar preferably
0.5 to 4, e.g. 0.6 to 3, most preferably 0.6 to 1.2 molar.
Preferably the combined chloride and sulphate concentrations are
at least 1 molar, e.g. at least 2 molar most preferably from 2.5
to 4 molar.
K. Co-depositable Metals
These are an essential inyredient of the bath in the case
15 of iron and nickel. The latter are present in the bath in a
concentration of from 30 to 150 ppm t:otal. They are normally
introduced as their soluble chlorides or sulphates. Other co-
depositable metals such as copper, zinc and lead are preferably
pr~sent in proportions of less than 20 ppm each and more pre-
20 ferably less than 30 ppm total.
L Non Co-depositable Metals
These are optionally but preferably present. In partic-
ular it is preferred to include alkali metals and especially
sodium and/or potassium in the bath in a proportion of at least
~5 0.5 molar up to 4 or 5 molar according to solubility. The
presence of sodium and/or pctassium helps the conductivity of
the solution and also improves the throwing power. Typically
the sodium and/or potassium are added in a proportion of about
2 molar initially, but tend to accumulate during use so that
30 the concentration may rise to saturation value. Other alkali
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metals such as lithium,
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alkaline earth metals such as calcium or magnesium or
other metal ions which will not plate out of the solution
with the chromium may also be present. The amount of such
metals may vary within very wide limits provided that they
do not precipitate in the presence of the other components.
They are generally present incidentally, as the cation
species of the conductivity salt, or of the borate, formate
and/or bromide salts which may be used to provide those
anions species in the solution.
10 M. Surface ~ctive ~gents
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These are optionally but preferably present in effective
and compatible amounts. Wetting agents and antifoams are
used throughout plating technology and many suitable
examples are well known to those skilled in the art. Any of
the wetting agents commonly used in hexavalent chromium
plating may be used in the present invention. However,
since the solutions of the present invention are much less
strongly oxidising than hexavalent chromium solutions it is
possible, and preferred to use the cheaper wetting aqents
commonlv employed in t~e less agqressive tYpes of plating
solution. The principal restriction on the effectiveness
of the wetting agents arises from the ~resence of the free
bromine in the solution. Surfactants which are liable to
bromination are therefore not recommended e.g. most non-
ionic surfactants. The surfactants used according to ourinvention are typically cationic such as those described in
B.P. 1 3~8,749 or preferably anionic e.g. sulphosuccinates,
alkyl benzene sul~honates having from 8 to 20 aliphatic
carbon atoms, such as sodium dodecYl benzene sulPhonate,
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alkyl sulphates having from 8 to 20 carbon atoms such as
sodium lauryl sulphate and alkyl ether sulphates such as
sodium lauryl polyethoxy sulphates. If the solution has
undesirable oaming tendencies
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it is also possible, optionally, to include compatible anti-
foams e.g. fatty alcohols such as ~ alcohol. The choice
of surfactants for use in our solution is a routine matter
easily within the ordinary competence o those skilled in the
art. The amount of wetting ayent used is in accordance with
normal practice, e.g. 0.1 to 10 parts per thousand.
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It is preferred that the solutions of our invention-s~ff~d-
consist essentially of the foregoing species. However, we do
not exclude the presence ~ minor amounts of other species which
ara compatible with the solutions and which do not adversely
affect the plating properties to a material extent. Generally
it is preferred that nitrate ion be substantially absent, since
it tends to inhibit deposition of chromium. Sulphite ion also
is preferably absent, since it can cause hazy deposits in more
than very small amounts. Other species, organic or inoryanic,
which do not inhibit plating of the chromium or materially
reduce covering power or create unacceptable problems of
toxicity, may optionally be present. Whether any particular
species can be tolerated in the solution may be routinely
20 determined by simple testing.
According to the present invention ~he baths are pre-
ferably made up substantially as described in any of the afore-
said specifications but including from 30 to 150 ppm of iron
or nickel in the solution. Preferably the additional metal is
25 ferric iron. Conveniently a sufficient quantity of an appro-
priate salt, e.g. ferric chloride, or preferably ferric sul-
phate is added to the bath at any convenient stage in the
preparation thereof. Alternatively the iron may be introduced
in admixture with any of the other components of the bath. For
30 example, it is possible to select a source of one of the other
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bath components, such as chromic sulphate, which contains iron
as an impurity, in sufficient quantity to provide the necessary
concentration in the bath. Preferably, when replenishing the
bath, iron or nickel is included in the replenishing additions
in a quantity sufficient to maintain the concentration within
the specified limits. Preferably the concentration is 40 to
100 ppm e.g. 50 ppm. If the concentration of iron or nickel
should greatly exceed the specified limits, thus resulting in
a plating fault, it may be reduced by addition to the bath of
a hexacyano-ferrate salt, substantially as described in the
specification of the aforesaid Belgian Patent, but ensuring
that after treatment the concentration of the aforesaid metals
is adjusted as necessary to bring it within the concentration
limits characteristic of this invention.
The invention is illustrated by the following examples:-
1. A chromium plating solution was prepared containing 20 gpl
chromium, the chromium being supplied from commercial chromic
sulphate, and 32 gpl formic acid, the other constituents being
potassium chloride (75 gpl), boric acid ~50 gpl), ammonium
bromide (10 gpl) and ammonium chloride (90 gpl) as described in
our aforesaid U.S. Patent. After preparation and plating out
at 0.5 amps per litre for 60 minutes a Hull Cell panel was run
on the solution at 10 amps for 3 minutes. At current densities
in excess of 400 ASF grey bands could be detected. Analysis of
the solution for trace elements showed 15 ppm iron, 10 ppm
nickel and 1 - 2 ppm of copper and zinc, these metals
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having arisen from traces present in the commercial grades
used.
25ppm of iron was added as ferric chloride (FeC136H30)
(i.e. 0.120 g per litre) and the solution re-run on the Hull
Cell. The grey bands had disappeared and a clean non-banded
panel was obtained. The inal analysis of the solution was
40 ppm iron, 10 ppm nickel, 5 ppm (Cu + Zn).
2. A working solution made up as above and used for pro-
duction became contaminated with nickel and iron, leading
to a plating fault. Analysis of the solution confirmed 110
ppm Fe, 150 ppm Ni, 25 ppm Zn, 5 ppm Cu. The solution was
treated with tetrapotassium hexacyanoferrate (K4Fe(CN)6) at
the rate of 1 ml/ litre of a 20~ w/v solution per 50 ppm
metals i.e. 6 ml/l. After allowing time for the reaction
to reach completion the precipitated metals were filtered
off and the solution reanalysed. Results were 20 ppm Fe,
15 ppm Ni, showing virtually complete removal of metals.
A Hull Cell panel showed a trace of grey bands beginning to
develop at high current densities and work plated in the
electrolyte showed a faint greyish appearance at very high
current density points. 25 ppm iron (as FeC136H20) was
- added to the electrolyte, when the grey bands and the grey
marks on work immediately disappeared. The concentration
of (nickel + iron) was maintained in the concentration range
40 to 100 ppm thereafter by suitable additions of iron to
the replenishing solutions.
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