Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention relates to chromium electro-
plating, and in particular to the maintenance of
electroplating baths based on trivalent chromium.
Although the potential advantages of electroplating
from solutions containing chromium in the trivalent state
have been recognized for over fifty years, until recently
a variety of practical difficulties have prevented the
commercial adoption of any such solution. Chromium has
therefore always been electroplated from baths containing
the chromium in the hexavalent state, despite
certain serious disadvantage of such baths~
Recently, however, various proposals have been made
for overcoming at least some of the aforesaid difficulties.
One type of bath in particular, containing a trivalent
chromium salt, a formate, a bromide and ammonium as essential
ingredients is described in our U.S. Patent No. 3,954,574.
A process based on such a bath has recently been introduced
commercially and is already gaining widespread acceptance ;
as a replacement for hexavalent chromium plating baths.
However, it has been discovered that some trivalent
chromium plating baths, which work satisfactorily under
laboratory or test conditions, sometimes develop plating
defects after they have been installed commercially. In
particular at least one, and usually a combination, of
the following faults may occur:
A. A white haze at high current densities, which
may, in severe cases, spread progressively to
lower current densities.
B. A white band at the lower limit of the plating
range, often accompanied by loss of adhesion
at high current densities.
C. A white deposit at current densities of around
50 amps per square foot.
D. Brown or black smudges between about 100 and 200
amps per square foot.
~e have now discovered that the aforesaid faults,
may, in many cases, be reduced or overcome by adding a
small amount of water soluble ferrocyanide to the plating
solution, w~enever the faults are observed.
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Our invention therefore, according to one aspect,
provides a method for the maintenance of an aqueous trivalent
chromium electroplating bath which has begun to exhibit at
least one of the aforesaid faults, which comprises adding
thereto a sufficient amount of a water soluble ferrocyanide
substantially to reduce or prevent said fault.
We have found that the invention is applicable to
the maintenance of trivalent chromium electroplating baths
generally. For example, it may be employed with baths of
the type described in our aforesaid U.S. patent, or with
baths containing glycollic acid such as are described in
USP 3,706,636 to 643. The invention may also be used,
for example, in combination with baths of the type described
in BP 1,144,913, USP 3,021,267, USP 3,006,823, USP 3,069,333
and USP 3,111,464.
Generally speaking the baths contain a trivalent
chromium salt, such as chromium chloride, sulphate or
fluoride and a complexing agent such as a carboxylic acid,
preferably a formate, or alternatively, for example, an
acetate, glycollate or oxalate. Halides especially bromide
are preferably present. The solution preferably contains
alkali metal ions, for example sodium and/or potassium,
and sulphate ions. Aprotic dipolar solvents such as
dimethyl formamide may also be included but are preferably
absent. Typically the pH of the bath is between 1 and 7,
for example 1.5 to 5.
The ferrocyanide may be any ferrocyanide which is
soluble in the bath, for example an alkali metal or
ammonium ferrocyanide, for example sodium or potassium
ferrocyanide.
The ferrocyanide may conveniently be added to the
bath as an aqueous solution. The concentration of the
ferrocyanide solution is not critical, and will normally
be chosen according to the solubility of the particular
ferrocyanide employed. For example using potassium ferro-
cyanide we prefer to employ a solution containing about 20%
by weight ferrocyanide.
We have found that addition of ferrocyanide in
amounts in excess of those required to eliminate the
4Q aforesaid faults, may cause a deterioration in the perfor-
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- mance of the bath. One way of avoiding this problem when
the onset of any of the aforesaid faults is observed, is to
add the ferrocyanide solution in small increments until
the chromium deposit is satisfactory again. If a sufficient
excess of ferrocyanide has accidentally been added to cause
a significant deterioration, it is possible to remove
the excess by adding a small amount of soluble cation such
as copper, nickel, iron or zinc. In most plating shops this
may conveniently be achieved by adding a small amount of
nickel plating solution to the bath. The addition of metal
ion should be made within 15 minutes preferably within l0
minutes of adding the ferrocyanide, in order to be fully
effective, since on standing the excess ferrocyanide
complexes with the chromium and is then difficult or
impossible to precipitate with the added metal.
We believe that the aforesaid faults may be due to
the accidental contamination of the bath by traces of metal
cations, which are capable of codepositing with the chromium.
Our experiments have shown that fault A can be simulated
by adding copper to the bath; similarly fault B appears to
be associated with the presence of zinc, fault C with lead -
and fault D with nickel or a mixture of nickel and iron.
It seems, surprisingly, that the ferrocyanide is capable
of precipitating substantially all of the potentially harm-
ful trace metals which are most commonly encountered in
very low concentrations as contaminants in commercial
practice, but without precipitating the chromium, which is
a principal cationic constituent of the bath.
According to a preferred embodiment, therefore our
invention provides a method of maintaining ~ trivalent
chromium plating bath which exhibits plating defects
associated with the codeposition with the chromium of trace
metal contaminants, which method comprises analyzing the
bath to determine the concentration of said trace metal
contaminants in the bath and adding a water soluble ferro-
cyanide in an amount sufficient to precipitate said
contaminants.
Preferably the bath or any sample used for analysis
should be filtered to remove any previously precipitated
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metal prior to the analysis.
The analysis of the bath may be carried out by any of
the analytical techniques for quantitative determination of
the trace metals which are well known in the art. Typically
the bath may be analyzed by spectographic means, for example
by spark ionization or atomic absorption. Alternatively
polarographic means may be employed.
Usually it is only necessary to test for copper,
zinc, iron and nickel, since these are the only metals
which, in our experience, are likely to cause difficulties
in practice. The invention is however applicable to the
correction of plating faults due to the presence of other
codepositable metals, including lead, cadmium, silver and
gold, although significant contamination by such metals
is unlikely to occur in practice.
The amount of ferrocyanide added is preferably
substantially stoichiometric based on the trace metal
contaminants present, or slightly less. Any substantial
excess of ferrocyanide should be avoided. Addition of
any effective quantity significantly less than the
stoichiometric amount, while beneficial, may not entirely
remove the plating fault. We have discovered that a good
rule of thumb is to add 1 ml. of 20~ potassium ferrocyanide
solution per litre of plating solution for every 50 ppm of
trace metal contamination. In this way the bath can be
relatively easily maintained in the face of at least the
commonly encountered forms of contamination.
It is preferred to reduce any free halogen in the
bath to halide prior to addition of the ferrocyanide, by
addition of a reducing agent capable of converting halogen
to halide, without adversely affecting the performance of
the bath. One particularly suitable reducing agent for
this purpose is ammonium formate, The proportion of
formate is preferably sufficient to reduce all the free
halogen in the solution. Typically 2 to 3 gms per litre
of formate may ~e added, preferably as an aqueous solution.
The formate is preferably added with agitation, about 10
minutes prior to the ferrocyanide addition. Halogen is
usually present in the bath immediately after plating.
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Our invention further provides a rapid method for
detecting the presence of harmful excesses of either the
ferrocyanide or certain of the commoner metallic impurities, -
so that appropriate remedial action can be taken without
5 delay. -
According to one aspect of our invention there is
provided a method of testing an aqueous solution such as
a trivalent chromium plating bath which comprises contacting
the solution with a water permeable medium, causing the
solution to diffuse upwardly through the medium and contact-
ing separate portions of the diffusing solution in the
medium with a water soluble ferrocyanide salt and with a
water soluble iron salt respectively.
Typically the contact between the diffusing solution
and the two salts is ensured by impregnating separate parts
of the medium with the two salts, the impregnated parts
being disposed in such a way as to intercept separate por-
tions of the diffusing solution. Preferably impregnated
parts should be readily visible to an external observer so
2Q as to facilitate the detection of any colour changes.
The permeable medium is preferably a cellulosic
material such as filter paper or chromatography paper.
However, any medium which is capable of causing aqueous
solutions to diffuse upwardly therethrough, when its lower
part is immersed, may in principle be employed. Preferably
the medium is substantially colourless so as to permit
observation of small colour changes.
According to a particular embodiment our invention
provides a means for testing trivalent chromium plating
baths comprising a water permeable medium, separate, exter-
nally visible, parts of which are impregnated with a water
soluble ferrocyanide salt and with an iron salt respectively.
Preferably the medium is a water permeable paper.
A particularly convenient form of test paper according to
our invention comprises a strip of permeable paper such as
filter paper, which can, for example, be rectangular or any
similar convenient shape, a part at or near one end of
which has been impregnated with the ferrocyanide and a part
at or near the other end of which has been impregnated with
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the iron salt, preferably leaving an unimpregnated central
part between the two impregnated parts. In use, such a
paper mày be bent or folded about the unimpregnated part,
so as to permit the latter to be contacted with the solution,
leaving the two impregnated parts unimmersed. The solution
diffused up each immersed arm of the paper, which separates
out suspended solids, so enabling any colour change in
either arm to be more readily detected.
Test papers according to the invention may
conveniently be obtained by preparing rectangular strips of
permeable paper and immersing the two ends of each strip
respectively in solutions of the two salts, for sufficient ~-
time to permit the two solutions to diffuse into separate,
preferably non-overlapping parts of the paper. The paper
may then be dried, for example in an oven.
The tests according to the invention may alternatively
be performed using two separate test papers impregnated
respectively with the two salts. If the medium is non-
coherent or ~rittle in nature a suitable support means may
be provided. For example, it is possible to perform the
tests using a thin layer of silica gel supported on a plate,
or in the case of po~dery or gelatinous media, to support
the medium in a column (preferably of glass or similar
transparent material).
The ferrocyanide salt is preferably an alkali metal
or ammonium ferrocyanide for example tetra potassium
ferrocyanide. The iron salt may be a ferric or preferably
a ferrous salt, preferably of a mineral acid, for example
a chloride, nitrate or sulphate.
3Q If a blue stain forms on contact between the solution
being tested and the ferrocyanide, then the solution contains
an excess of the metallic impurities, whereas a blue stain
forming on contact bet~een the solution and the iron salt
indicates an excess of ferrocyanide. Preferably an aliquot
of trivalent chromium plating solution is taken and ferro-
cyanide is added, stepwise, thereto. The solution is
checked after each addition ~ith the test paper. The end
point, in mls. ferrocyanide per litre of plating solution
represents a maximum. In practice, preferably about 50%
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of this amount of ferrocyanide is added to the bath,
followed by a further addition of e.g. 25%, if required.
The invention will be illustrated by the following
Examples:
Example 1
A trivalent chromium plating solution which had been
working satisfactorily developed a plating fault, giving
dark smudges in the 100-200 ASF region. A sample of the
solution was put into a 300 ml Hull Cell with circulatory
cooling and a 10 amp 3 minute panel run. This panel showed
black streaks between 100-200 ASF and from previous
experience was diagnosed as being due to nickel and
possibly iron contamination of the solution. It was
established that some nickel plated components had been
lost from plating jigs and had been dissolving in the
electrolyte for some time. The electrolyte was analyzed
spectroscopically for trace metals as follows:
Nickel 134
Copper 13
Iron 193
Zinc 26
Total Metals 366
A 20% w/v solution of potassium ferrocyanide
(K4Fe(CN16.3H2O~ was prepared and 1 ml of this solution
added per 50 ppm total metals perliter i.e. 7 ml per
litre were actually added. The mixture was allowed to
stand for 30 minutes and then used for plating components.
The black streaks were completely absent and normal
plating performance was regained.
3Q Subsequent analysis of the electrolyte gave the
following result:
~E~ % removal
Nickel 20 85
Iron 98* (4
Copper 4 69
Zinc 8 69
*this figure will include a contribution from
any excess reagent.
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Example 2
A similar electrolyte contaminated with iron, nickel,
copper and zinc gave plating faults. In this case no trace
metal analysis was available. Halogen was reduced by
addition to the bath of 5 ml/litre of 55% w/w ammonium
formate solution. The bath was agitated for 10 minutes,
and then 20% potassium ferrocyanide was added in a step-
wise fashion, beginning with 1 ml per litre of electrolyte
and increasing the addition in 1 ml per litre steps,
allowing 30 minutes reaction time between additions. Some
improvement was noted after the addition of 3 ml per litre
and fully satisfactory performance was achieved at 5 ml
per litre. The precipitate of insoluble metal salts was
allowed to remain in the bath and did not interfere with
plating in any way, but the solution was filtered at the
next shutdown to remove the precipitated metals.
Example 3
Test papers were each prepared by dipping one end of
a rectangular strip of filter paper in 20% w/v solution of
tetra potassium ferrocyanide and the other end is a 20% w/v
solution of ferrous chloride. The solutions were each
allowed to diffuse part way towards the centre of the strip,
~hich was then dried in an oven.
Example 4
A trivalent chromium plating solution, after working
satisfactorily for several weeks, developed a fault which
comprised the formation of dark smudges at current densities
of between 100 and 200 amps per square foot. A test paper
prepared according to Example 3 was folded across the
unimpregnated central portion, which was dipped in the
bath. The electrolyte diffused towards both ends of the
paper and produced a blue stain near the ferrocyanide
impregnated end, indicating the presence of metallic
impurities.
35~ w/v aqueous solution of tetra potassium ferr~cyanide
was added in 4 ml increments, allowing 30 minutes after
each addition and then repeating the test. After the second
addition, a blue stain was observed at the iron impregnated
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end of the paper. 2 ml of 20% w/v ferrous chloride solution
was added whereafter no stain was observed. Commercial
plating was resumed and the bath functioned satisfactorily.
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