Note: Descriptions are shown in the official language in which they were submitted.
METHOD OF PVRIFYING TRIVALENT
CHROMIUM ELECTROPI~TING BATHS
_
BACKGROUND OF THE II~VENTION
The present invention broadly relates to
trivalent chromium electrolytes, and more particularly
to a method of purifying and restoring the perEormance
of such electrolytes which have been rendered deficient
in producing commercially acceptable chromium electro-
deposits due to the progressive accumulation and in-
crease in concentration of contaminating metal ions
such as nickel, zinc, iron, copper and lead during
normal commercial operation of the bath. When one
or more of such metal ion impurities attain
concentrati~n levels which adversely affect the per-
formance of the electrolytel the resultant chromium
electrodeposit is rendered commercially objectionable
due to the presence of black streaks, clouds and
hazes which is sometimes further accompanied by a
spongy deposit and a 1QSS or reduction in the cover-
ing power of the electrolyte.
In order to overcome the detrimental effect
of such metal ion contamination in trivalent chromium
electroplating baths, it has heretofore been proposed
in accordance with United States Patent ~o. 4,038,160
to add small amounts of water soluble ferrocyanide
compounds to the electrolyte to effect a precipita-tion
~2~ 3
of such contaminating metal ions which can convenient-
ly be removed by filtration. While the purification
technique as disclosed in the aforementioned United
S-tates patent has been found effective in many instan-
ces, the presence of residual amounts of the ferro-
cyanide precipitating agent adversely affects the
performance of the electrolyte rendering the bath
inoperative for producing commercially acceptable
chromium deposits. When an excess of the ferrocyanide
agent is present in the bath, a further treatment is
required to remove such excess which can be performed
by the intentional addition of contaminating metal ions
to effect a precipitation of the residual ferrocyanide
compound. It will be appreciated that the treatment
proposed in the aforementioned United States patent
requires precise analysis of the contaminants in the
bath in order that a precise quantity of the precipi
tating agent can be added which is not only tedious
and time consuming but also requires skilled operating
personnel. The to~ic nature of the ferrocyanide pre-
cipitate requires special waste disposal measures which
also is objectionable.
It has also been proposed to effect an elec-
trolytic purification of contamina-ted trivalent chro-
mium electrolytes employing a cathode on which a
codepositi.on of such contaminating metal ions is
effected by electrolyziny the bath over a prolonged
time period. While such an electrolytic purification
~ ~3~
techni~ue is somewhat effective for reducing copper
ion contamination, i-t has been found relatively in-
effective for removing nic~el and zinc ions and only
partially effective for removiny iron.
It has heretofore been suggested to employ
dithiocarbamates for precipitating heavy metals from
electroplating rinse waters employing a pH generally
on the al~aline side. Suxprisingly, in accordance
with the discovery comprising the present invention,
it has been found tha-t selected dithiocarbama-te com~
pounds can be employed for purifying aqueous acidic
trivalent chromium electrolytes which preferentially
react with and precipitate the detrimental metal ions
found in trivalent chromium electrolytes without sig-
ni~icantly removing the chromium ions which are pres-
ent in concentrations of about 50 times or more greater
than the level of con-taminating metal ions present in
the bath. Moreover, such trivalent chromium electro-
lytes frequently contain metal ions such as vanadium
as a desirable constituen-t for controlling and reduc-
ing the formation of detrimental hexavalent chromium
ions which also surprisingly are not significantly
removed by the use of such selected dithiocarbamate
additives. Moreover, trivalent chromium electrolytes
are on the acid side usually ranging in pH from about
1.5 up to abou-t 5 which is substantially below the
~35i~
alkaline pH range heretofore proposed for treatment
of electroplating rinse waters.
The unexpected results of the present inven-
tion are further evidenced by the fact that certain
dithiocarbamate compounds such as dibutyldithiocarbamate
are unsatisfactoryO Similarly, dimethyl glyoxine, a
compound well known as a precipitating agent for nickel
ions, has been found ineffective for precipitating
appreciable amounts of contaminating nickel ions from
trivalent chromium baths and, moreover, the precipitate
formed is of a gelatinous character which tends to
cling to the tank walls and work piece surfaces causing
unacceptable plate deposits and a fouling of the fil-
tration equipment. Similar sulfide contai.ning com-
pounds such as, for e~ample, sodium thiocarbonate
(Na2CS3) and sodium thiocyanate (NaSCN) when incorpora-
ted in trivalent chromium electrolytes cause serious
defects in the chromium deposit including dark blue to
black discoloxa-tion of low current density recess areas
2~ and also significantly reduce the covering power of the
ba-th.
SUMMARY OF THE INVENTION
The benefits and advantages of the present
invention are achieved by employing dimethyldithio-
carbamate and diethyldithiocarbamate ions as well as
mixtures thereof as a purifying agent for trivalent
chromium electrolytes by ~hich a selective preferential
reaction and precipitation of detrimental contaminating
ions is effected producing a precipitate of a somewhat
crystalline character which can readily be removed and
separated from the treated electrolyte by conventional
filtration equipmentO Due to the instability of
dimethyldithiocarbamic acid and diethyldithiocarbamic
acid, it is preferred to introduce the corresponding
ions into the bath as bath soluble alkali metal and
ammonium salts such as the sodium salt for economic
considerations. The additive agent is preferably
introduced in the form of a concentrated aqueous solu-
tion to facilitate dispersion in the electrolyte. The
specific quantity of the purifying agent added during
the purification treatment will vary depending upon
the magnitude of contaminating metal ions present in
the electrolyte. The quantity employed is therefore
calculated to be sufficient to at least remove a suf-
ficient quantity of such metal ion contaminants to
reduce their concentration to a level at which the
performance of the electrolyte is restored. Typically,
concentrations of the precipitating agent relative to
the concentration oi contaminating metal ions in an
amount of about ~:1 g/l are employed.
Additional benefits and advantages of the
present invention will become apparent upon a reading
of the Description of the Preferred Embodiments taken
in conjunction with the specific examples provided.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The purification method of the present inven-
tion is particularly effective for purifyi~g trivalent
chromium electrolytes which generally contain trivalent
chromium ions in a concentration of about 0.2 to about
0.8 molar, complexing agents for complexing the chro-
mium ions to maintain them in solution such as formate
ions, acetate ions or mixtures thereof present in con-
centrations to provide a molar ratio of complexing
agent to chromium ions usually of about 1:1 to about
3:1, and conductivity salts such as the salts of alkali
metal or alkaline earth metals and strong acids such as
hydrochloric acid and sulfuric acid present in amounts
up to about 300 g/l or higher to achieve the requisite
conductivity. Among such conductivity salts are
20 fluoboric acid as well as the alkali metal, alkaline
earth metal and ammonium bath soluble salts thereof.
Such trivalent chromium electrolytes can optionally
and preferably further contain ammonium ions present
at molar ratios of total ammonium ion to chromium ion
of from about 2:1 up to about 11:1 as well as halide
~ .
~ 6
ions including chloride and bromide ions present at
a molar ratio of about 0.8:1 up to about 10:1 of
halide ions to chromium ions present. Metallic re-
ducing agents of the types as disclosed in
Canadian Patent Application ~o. 389,254, filed
~ovember 2, 1981, comprising vanadium ions are
also desirably included in amounts to provide a
vanadium ion concentration of from about 0.015 up
to about 6.3 g/l.
Such trivalent chromium electrolytes can also
optionally and advantageously contain buffering agents
of which boric acid present in concentrations of about
0.15 molar up ~o bath solubility has been found par-
- ticularly satisfactory. Wetting agents can also be
advantageously employed of the general types used in
nickel and hexavalent chromium electrolytes in concen-
trations of from about 0.05 up to about 1 g/l.
Trivalent chromium electrolytes also contain
hydrogen ions to provide a pH on the acid side general-
ly ranging from about 2.5 up to about 5.5.
The beneficial results of the present inven-
tion are also obtained employing the purification
additive in trivalent chromium electrolytes of the
types as generally and specifically described in
United States Patents No. 3,954,574; 4,107,004;
~ t`;i~
~3~
4,169,022 and 4,196,063.
During conventional commercial operation
employing such trivalent chromium electrolytes, a pro-
gressive contamination of the electrolyte occurs as
a result of drag-in, dissolution in the electrolyte of
unprotected surfaces of the receptacles or tanks con-
taining the electrolyte, dissolution of the metallic
surfaces o~ the work pieces being plated, dissolution
of exposed portions of the work racks durlng immersion
in the electrolyte, as well as contamination from the
water and chemicals employed for replenishing and
makeup of the electrolyte. As a result, a pxogressive
buildup in the concentration of contaminating metal
ions such as ions of nickel, zinc, iron, copper and
lead occurs which, by experimentation, has indicated
that coneentrations of nickel ions in amounts of about
150 ppm or higher are harmful and cause defects in the
chromium electrodeposit. While the presence of iron
ions in amounts up to about 500 ppm are beneficial to
the trivalent chromium electrolyte in that they tend
to promote coverage of the ehromium deposit, concen-
trations of above abou~ 1,000 ppm (above 1 g/l) are
generally harmful to the chromium deposit~ Similarly,
concentrations of copper ions in amounts above about
15 ppm and zine ions above about 10 ppm and higher are
harmful. When combinations of such metal ions are
present in the bath, the harmful effects of the indi-
vidual ions are cumulative and lower concentrations of
the individual metal ions produce defects in the chro-
mium deposit which is evidenced by the presence of
black streaks~ clouds, and hazes. Under severe con-
taminating conditions, the covering power of the elec-
trolyte is also adversely affected.
In accordance with the present invention,
dimethyldithiocarbamate and/or diethyldithiocarbamate
ions preferably in the form of an aqueous concentrate
of the alkali metal and ammonium salts thereof of
which the sodium salt constitutes the preferred mater-
ial,, are introduced into the contaminated trivalent
chromium electrolyte in an amount sufficient to pre-
cipitate at least a portion of such metal ions reducing
their level to an innocuous concentration in which
satisfactory plating performance is again restored.
The non-toxic precipitate can readily be disposed of
~ by conventional disposal techniques in contrast to the
toxic precipitate produced with ferrocyanide compounds
Unlike with the use of ~errocyanide precipitating agents,
excess amounts of the purifying agent of the present
invention do not detrimentally affect the quality
of the chromium deposit and any unreacted quantities
of the purifying agent remaining in the electrolyte
following the treatmentare progressively decomposed
and/or removed during the normal electrolysis of the
electrolyte during conventional plating operations.
. ~ 9
~Z~3~
The purifying agent is preferably introduced into the
electrolyte in the form of a concentrated aqueous
solution to facilitate uniform dispersion thereof in
the presence of agitation as opposed to the addition
in the form of a dry bath soluble powder. The liquid
concentrate will usually contain about 30 percent by
weight of the purification agent and is usually ad-
justed to provide a pH of above about 8, preferably
above about 9 to provide stability during storage.
When commercial dimethyldithiocarbamate and diethyl-
dithiocarbamate products are employed, it is usually
desirable to carbon filter or otherwise purify such
commercial mixtures to remove any undesirable reaction
by-produc~s therein which may adversely affect the
performance of the electrolyte following the purifica-
tion treatment.
The purification agent of the present inven-
tion is useful over a wide pH operating range. Gener-
ally, trivalent chromium electrolytes operate at a
~ pH ranging from a~out 2 to about S.5 and more typically,
from about 3 to about 4. The purifying agent of the
present invention operates well within the foregoing
pH ranges as well as at at pH as low as about 1.5.
In order to further illustrate the present
invention, the following specific examples are pro-
vided. It will be understood that the examples are
, 1 0
~3~
provided for illustrative purposes and are not intended
to be limitlng of the scope of the invention as herein
described and as set forth in the subjoined claims.
EX~MPLE 1
An aqueous acidic trivalen-t chromium test
electrolyte ~as prepared having the following compo-
. sition:
INGREDIENT CONCENTRATION
__
: Cr~3 24.2 g/l
NH4COOH 44.0 g/l
NaBF4 ~,55.0 g/l
NH~Cl 150.0 g/l
H3BO3 57.1 g/1
VOSO4 1.0 g/l
Wetting agent*2.0 cc/l
*Wetting agent comprises equivalent of 0.13'l4 g/l
dihexyl ester of sodium sulfosuccinic acid and 0.244
g/l of sodium sulfate derivative of 2-ethyl-1-hexanol.
Contamination of the electrolyte with iron,
copper and nickel ions was effected by the addition of
the cor.responding sulfate salts to produce an iron ion
concentration of about 0.312 g/l, a copper ion concen-
tration of about 0.032 g/l and a nickel ion concen-tra-
tion of about 0.110 g/l. To 1 liter of the foregoing
electrolyte containing the contaminating metal ions,
2.5 y/l of sodium diethyldithiocarbamate trihydrate
L3~3
was added and the resultant solutlon was stirred for
approximately 10 minutes at 75F at a pH of about 3.2
and then allowed to stand quiescent for a period of 2
hours. The resultant solution was then filtered through
a carbon filter and analyzed for metallic contamination.
Upon analysis, the iron ions concentration was found to
be 0.168 g/l; the copper ion concentration was 0.012
g/l; the nickel ion concentration was found to be 0.0~2
g/l. Accordingly, about ~6.2 percent of the contamina-
ting iron ions were removed; about 62.5 percent of the
contaminating copper ions were removed; about 62 percent
of the contaminating nickel ions were removed.
E~5AMPLE 2
A 400 milliliter sample of the trivalent
chromium test electrolyte as described in Example 1
was adjusted by the addition of contaminating metal
sal-ts to provide an iron ion concentration of about
0.312 g/l, a nickel ion concentration of about 0.120
g/l and a copper ion concentration of about 0.080 g/l.
To the electroly-te containing the dissolved contami-
na-ting metal ions, 3.5 g/l o~ sodium diethyldithio-
carbamate trihydrate were added and the solution
stirred for approximately one-half hour at 120F at
a pH of about 3. The solu-tion was then allowed to
s-tand quiescent for about 2 hours at 75F whereafter
it was filtered through a carbon filter. Analysis of
the filtrate revealed that the iron ion concentration
was reduced to about 0.072 ~/1, the nickel ion concen-
tration was reduced to about 0.010 g/l while the copper
ion concentration was nil. Accordingly, about 77 per-
cent of the iron ion contamina-tion was removed, about
91.7 percent of the nickel ion contamination was removed
while substantially 100 percent of the copper ion con-
centration was removed.
EXAMPLE 3
In order to evaluate the effect of pH on the
effec-tiveness of the purifying agent of the present
invention, the same test electrolvte as described in
Example 2 containing the same level of contaminating
metal ions was raised to a pH of about 4 by the addi-
tion of ammonium hydroxide. The solution was there-
after treated in the same manner as described inExample 2. Analvsis of the filtrate revealed that the
iron ion concentration in the treated electrolyte was
reduced to about 0.012 g/1, and the nickel ion and
copper ion concentrations were nil. Accordingly,
about 96.2 percent of the iron ions were removed
~hile substantially 100 percent of the nickel and
copper ions were removedO The results of Example 3
indicate that the additive agent is of an increased
effectiveness at a pH of 4 in comparison to a pH of
about 3 as employed in Example 2.
135~8
EXAMPLE 4
A trivalent chromium test electrolyte was
prepared of a composition similar to that described
in Example 1 with the exception that the trivalent
chromium ion concentration was 21.7 g/l, the ammonium
formate concentration was about 51.0 g/l and the boric
acid concentration was about S0.8 g/l with the remain-
ing ingredients the same as described in Example 1.
The electrolyte was adjusted for a pH of about 3.5
and the concentration of contaminating ions was as
follows: iron ions about 0.298 g/l; nickel ions about
0.188 g/l and zinc ions about 0.047 g/l.
A test panel which had been plated in a
conventional Watts-type bright nickel bath at a current
density of about 45 ASF at 145F for a period of 10
minutes to provide a bright nickel deposit of about
0.3 mil thickness, after water rinsing was plated in
the foregoing contaminated trivalent chromium electro-
lyte for a period of 3 minutes at 75F and a cathode
current density of about 100 ASF. The chromium de-
posit was iridescent in the intermediate cbrrent den-
sity areas and had black streaks in the high current
density areas. The chromium deposit was considered
unacceptable from a commercial standpoint due to the
high level of contaminating metal ions therein.
The contaminated trivalent chromium electro-
lyte was thereafter treated by the addition of 3.8 g/l
~3~
of sodium diethyldithiocarbamate trihydrate for a
period of 1 hour at 75F with continuous stirring.
The electrolyte after settling, was thereafter ~iltered
through a carbon filter and the filtrate was analyzed
for residual metal ion contaminants present in the
treated electrolyte. Upon analysis, the iron ion con-
centration was found to be about 0.164 g/l, the nickel
ion concentration was nil and the zinc ion concentra-
tion was found to be about 0.0004 g/l. Accordlngly,
about 45 percent of the iron ions were removed, about
100 percent of the nickel ions were removed while about
~ ?
99 percent of the zinc ion contaminant was removed.
~ nickel plated test panel of the type here-
inabove described was then plated in the treated and
filtered solution under the same conditions as employed
for the untreated solution. The chromium deposit was
overall bright and of normal and uniform appearance.
The plating deposit was considered entirely commercial-
ly acceptable, The plating test further indicates
2~ that the presence of excess sodium diethyldithiocar-
bamate in the electrolyte following the treatment does
not adversely affect the performance of the trivalent
chromium plating solution. This example clearly
evidences the effectivenesss of the present invention
in rejuvena-ting a metal ion contaminated trivalent
chromium electrolyte the performance of which has heen
~3~
rendered c~mmexcially unsatisfactory whereby the elec
trolyte is restored to provide satisfactory chromium
depositsO
EXAMPLE 5
An experimental treatment of a commercial
trivalent chromium electrolyte comprising a 4,000 gal-
lon bath was made having a nominal composition corres-
ponding to that described in Example 1. The perfor-
mance of the electrolyte had become impaired due to
the accumulation of iron and nickel contaminating ions
during normal electroplating operations. Analyses of
samples of the electrolyte before treatment and fol-
lowing treatment were made for iron ions, nickel ions
and vanadium ions present as a reducing agent. The
results of the analyses are as follows:
Before Treatment After Treatment
Fe 1.310 g/l 1.000 g/l
Ni 0.501 g/l 0.351 g/l
V 0.238 g/l 0.238 g/l
The purification treatment of the contam-
inated electrolyte was carried out by the addition of
30 gallons of a solution containing a concentration of
about 350 g/l of sodium diethvldithiocarbamate. While
the bath was being continuously filtrated, but wi-thout
undergoing electrolysis, the solution of additive agent was
16
added. In~ediately following the addition of -the
aqueous additive solution, a dark precipitate formed
which, surprisingly, instead of increasing filter
back pressure actually resulted in a reduction in
filter back pressure. As a result, the precipitate
was easily removed and satisfactory operation of the
electrolyte was restored within about 1 hour after
treatment providing commercially satisfactory chromium
deposits~ The precipitate recovered by the filtration
was analy~ed and found to contain about 66.7 mol per-
cent iron die~hyldithiocarbamate and 33.3 mol percent
nickel diethyldithiocarhamaté.f The selectivity of -the
additive agent for extracting contaminating metal ions
is evidenced by the fact that no vanadium and no chro-
mium was extracted rom the bath during treatment.
Slmilar tests employing ferrocyanide precipitating
agents evidences a significant removal of both chro-
mium and vanadium which are desirable constituents in
the trivalent chromium electrolyte.
While it will be apparent that the preEerred
embodiments of the invention disclosed are well calcu-
la-ted to fulfill the objects above stated, it will be
appreciated that the invention is susceptible to modi-
fication, variation and change without departing from
the proper scope or fair meani.ng of the subjoined
claims.
