U.S. patent number 3,706,636 [Application Number 05/116,890] was granted by the patent office on 1972-12-19 for preparing plating bath containing chromic compound.
Invention is credited to John Edwin Bride.
United States Patent |
3,706,636 |
Bride |
December 19, 1972 |
PREPARING PLATING BATH CONTAINING CHROMIC COMPOUND
Abstract
A bright decorative chromium plate of good appearance and
durability is electrodeposited from a bath prepared from a solution
containing trivalent chromic-compound. More particularly the bath
is made from a preformed complex, or such complex is made in situ,
and is a water-soluble trivalent chromic compound containing
carboxylic acid constituents and halogen constituents. The prepared
bath provides for excellent plating speed in the low current
density region thus offering enhanced chromium thickness in
recessed and difficult to plate areas.
Inventors: |
Bride; John Edwin (Mentor,
OH) |
Family
ID: |
22369835 |
Appl.
No.: |
05/116,890 |
Filed: |
February 19, 1971 |
Current U.S.
Class: |
205/286; 205/289;
205/290 |
Current CPC
Class: |
C25D
3/06 (20130101) |
Current International
Class: |
C25D
3/06 (20060101); C25D 3/02 (20060101); C23b
005/06 () |
Field of
Search: |
;204/51 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Edmundson; F. C.
Claims
I claim:
1. The method of preparing an aqueous electrolytic plating bath for
the plating of bright chromium plate from trivalent
chromic-compound which method comprises:
1. establishing an aqueous medium containing a complex,
water-soluble chromic compound containing halogen constituents
selected from the group consisting of chloride, fluoride, mixtures
thereof and mixtures thereof with other halides, and said chromic
compound containing carboxylic acid constituents supplied by acids
selected from the group consisting of glycolate, lactate oxylate
and mixtures thereof, with said compound supplying not
substantially above about 150 grams of chromium per liter of the
medium, said complex having a molar ratio of chromium atoms to
carboxyl constituent within the range of 1:0.7 to 1:3, and a molar
ratio of chromium atoms to halogen atoms within the range of 1:0.1
to 1:3.5;
2. adjusting the pH of said medium within a range of between about
1.8 and about 4.9; while
3. maintaining the temperature of said medium not substantially in
excess of about 50.degree.C.
2. The method of claim 1 wherein said medium is furnished with
between about 10-70 grams per liter of substance selected from the
group consisting of boric acid, a substance supplying boric acid
equivalent in aqueous solution, and mixtures thereof.
3. The method of claim 1 wherein said medium is furnished with
between about 50-200 grams per liter of a salt of a strong acid
having a dissociation constant of at least K=10.sup. .sup.-2.
4. The method of preparing an aqueous electrolytic plating bath for
the plating of bright chromium plate from trivalent-chromic
compound, which method comprises:
1. establishing a liquid, aqueous reaction medium containing
chromium metal, carboxylic acid supplied by acids selected from the
group consisting of glycolate, lactate, oxalate and mixtures
thereof, and halogen providing compound containing halogen
constituents selected from the group consisting of chloride,
fluoride, mixtures thereof and mixtures thereof with other halide,
and with sufficient of said carboxylic acid to provide said medium
with a molar ratio of total chromium atoms to total acido groups
within the range of 1:0.7 to 1:3, and with sufficient
halogen-providing compound to supply said reaction medium with a
molar ratio of total chromium atoms to total halogen atoms within
the range of 1:0.1 to 1:3.5;
2. permitting reaction of the substituents in said medium, thereby
preparing therein a complex, water-soluble trivalent chromic
compound containing carboxylic acid constituents and halogen
constituents;
3. adjusting the concentration of said complex in said medium to
supply not substantially above about 150 grams of chromium per
liter of the medium;
4. adjusting the pH of said medium within a range of between about
1.8 and about 4.9; and
5. maintaining the temperature of said medium below about
50.degree.C.
5. The process of claim 4 wherein said halogen providing compound
is selected from the group consisting of hydrogen fluoride, sodium
fluosilicate, fluorine gas, hydrogen chloride, chlorine gas, and
mixtures thereof.
6. The method of claim 4 wherein said medium is furnished with
between about 10-70 grams per liter of substance selected from the
group consisting of boric acid, a substance supplying boric acid
equivalent in aqueous solution, and mixtures thereof.
7. The method of claim 4 wherein said medium is furnished with
between about 50-200 grams per liter of a salt of a strong acid
having a dissociation constant of at least K= 10.sup. .sup.-2.
8. The process of claim 4 wherein water supplies all of the liquid
of said reaction medium, such medium is heated to boiling during
reaction, and said chromium metal is in pulverulent form.
9. The process of preparing an aqueous electrolytic plating bath
for the plating of bright chromium plate from trivalent-chromic
compound, which process comprises:
1. bringing together in a liquid reaction medium, chromic acid,
carboxylic acid supplied by acids selected from the group
consisting of glycolate, lactate, oxalate and mixtures thereof, and
halogen-providing compound containing halogen constituents selected
from the group consisting of chloride, fluoride, mixtures thereof
and mixtures thereof with other halide, with sufficient of said
carboxylic acid to provide said medium with a molar ratio of total
chromium atoms to total acido groups within the range of 1:0.7 to
1:3, and with a halogen-providing compound sufficient to supply
said medium with a molar ration of total chromium atoms to total
halogen atoms within the range of 1:0.1 to 1:3.5;
2. permitting reaction of the substituents in said medium, thereby
preparing therein a complex water-soluble trivalent chromic
compound containing carboxylic acid constituents and halogen
constituents;
3. adjusting the concentration of said complex in said medium to
supply not substantially above about 150 grams of chromium per
liter of the medium;
4. adjusting the pH of said medium within a range of between about
1.8 and about 4.9; and
5. maintaining the temperature of said medium below about
50.degree.C.
10. The process of claim 9 further characterized by a liquid
reaction medium containing fugitive liquid that is at least
substantially volatized after preparation of said complex.
11. The process of claim 9 wherein said halogen-providing compound
is selected from the group consisting of hydrogen fluoride, sodium
fluosilicate, fluorine gas, hydrogen chloride, chlorine gas, and
mixtures thereof.
12. The method of claim 9 wherein said medium is furnished with
between about 10-70 grams per liter of substance selected from the
group consisting of boric acid, a substance supplying boric acid
equivalent in aqueous solution, and mixtures thereof.
13. The method of claim 9 wherein said medium is furnished with
between about 50-200 grams per liter of a salt of a strong acid
having a dissociation constant of at least K=10.sup. .sup.-2.
14. The process of preparing an aqueous electrolytic plating bath
for the plating of bright chromium plate from trivalent chromic
compound, which process comprises:
1. establishing a liquid, aqueous reaction medium comprising
chromium-providing material selected from the group consisting of
chromic halide, basic chromic halide, and mixtures thereof
supplying halide constituents selected from the group consisting of
chloride, fluoride, mixtures thereof and mixtures thereof with
other halide, said chromium-providing material being in mixture
with carboxylic acid selected from the group consisting of
glycolate, lactate, oxalate, and mixtures thereof, said acid being
in amounts sufficient to provide said medium with a molar ratio of
total chromium atoms to total acido groups within the range of
1:0.7 to 1:3;
2. permitting reaction of the substituents in said medium, thereby
preparing therein a complex, water-soluble trivalent chromic
compound containing carboxylic acid constituents and halogen
constituents;
3. adjusting the concentration of said complex in said medium to
supply not substantially above about 150 grams of chromium per
liter of the medium;
4. adjusting the pH of sad medium within a range of between about
1.8 and about 4.9; and
5. maintaining the temperature of said medium below about
50.degree.C.
15. The process of claim 14 wherein said reaction medium
additionally contains a strong base yielding hydroxyl ions in said
aqueous reaction medium when said chromic halide is present.
16. The method of claim 14 wherein said medium is furnished with
between about 10-70 grams per liter of substance selected from the
group consisting of boric acid, a substance supplying boric acid
equivalent in aqueous solution, and mixtures thereof.
17. The method of claim 14 wherein said medium is furnished with
between about 50-200 grams per liter of a salt of a strong acid
having a dissociation constant of at least K=10.sup.-.sup.2.
Description
BACKGROUND OF THE INVENTION
Decorative chromium plating from baths which contain chromium in
the trivalent state have been extensively investigated. However it
has been found difficult in commercial practice to prepare such a
bath that will compete successfully with the decorative plating
baths, containing chromium in the hexavalent state, which have
received wide commercial acceptance. For example U. S. Pat. No.
3,006,823 describes a recently developed aqueous electrolytic
plating bath containing a chromium complex of chromic ion and
particular carboxylic acid. Although such bath is of interest, the
plating performance, for example in the low current density area,
can be undesirable.
SUMMARY OF THE INVENTION
A decorative chromium plating bath has now been developed which in
addition to providing a highly desirable bright range and a
decorative plate with a desirable bright finish, shows excellent
plating speed in the low current density region.
Broadly the method of the invention is directed to preparing an
aqueous electrolytic plating bath for the plating of bright
chromium plate upon articles contacted therewith, which method
comprises: establishing an aqueous medium containing a complex,
water-soluble chromic compound containing carboxylic acid
constituents and halogen constituents selected from the group
consisting of chlorine, fluorine, bromine, iodine or mixtures
thereof, with such compound supplying not substantially above about
150 grams of chromium per liter of the medium; adjusting the pH of
such medium within a range of between about 1.8 and about 4.9;
while maintaining the temperature of said medium not substantially
in excess of about 50.degree.C.
The invention is further directed to methods of preparing aqueous
electrolytic plating baths for the plating of bright chromium plate
by first preparing a complex, water-soluble chromic compound.
BRIEF DESCRIPTION OF THE DRAWING
The drawing presents in graphic form and for comparative purposes,
thickness of chromium deposit from a bath prepared according to the
present invention and also from a comparative bath containing a
comparative trivalent chromium complex.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
One of the consistent and most noticeable effects achieved by the
plating baths prepared in the present invention is exemplified in
the drawing which is a plot of chromium plating thickness versus
the cathode current density. The plating thickness shown is in
micro-inches for a three minute plating cycle using 10 amperes of
current in a modified Hull cell, such cell being explained more
fully hereinafter. The cathode current density is given in amperes
per square foot (ASF) of cathode.
In the drawing, two plots are shown, the upper graph line showing
the thicker deposit of chromium, most especially in the lower
current density region, achieved for a bath prepared according to
the present invention and containing a complex of chromium with
carboxylic acid constituent from glycolic acid and with chlorine
supplying the halogen. The lower line on the graph depicts the
plate thickness achieved in a comparative bath, such as described
in U. S. Pat. No. 3,006,823, and containing a comparative chromic
carboxylate complex of chromium with glycolic acid. More detailed
description of the plating conditions employed in the tests
represented by the plots in the drawing, are given hereinafter in
connection with the examples.
The chromic compound for preparing the bath according to the
present invention contains halogen constituents that can be
chloride, fluoride, bromide, iodide or mixtures thereof. However,
in typical plating operation, bromine and iodine are often not
used, for economy and to avoid evolution of visible noxious fumes
at the anode. Therefore chlorine, fluorine and their mixtures are
preferred in preparing the complex. When the halogen is not
supplied by a chromic compound, as will be more particularly
discussed hereinbelow, it is preferably, for economy, supplied
simply as the acid, e.g., hydrogen chloride or hydrogen fluoride.
However, other substances may be used and these include gaseous
chlorine, gaseous fluorine, and sodium fluosilicate.
The carboxylic acid constituent for the complex is most typically
supplied by a carboxylic acid having less than about 10 carbon
atoms, and being a saturated acid free from carbon-to-carbon
unsaturation. The acid is a dicarboxylic acid, or a monocarboxylic
acid, or such dicarboxylic and monocarboxylic acids containing at
least one hydroxyl group, or mixtures of these acids.
Representative such acids which can or have been used most usually
have less than about six carbon atoms and include glycolic acid,
lactic acid, oxalic acid, and their mixtures. Preferably for
enhanced plating performance plus economy the carboxylic acid
constituent is supplied at least in major amount by glycolic acid.
A compound of any of the carboxylic acids, such as a salt or an
ester thereof, which will act in any of the reactions whereby the
complex is formed, in the same manner as the free acid, can be
used.
The plating baths where the complex is formed in situ can be
prepared by any of several methods. One method is the
straightforward combination of chromium metal with carboxylic acid
plus halogen-providing compound, for example hydrochloric acid.
When such combination includes particulate chromium metal to reduce
reaction time, the reaction can be highly exothermic, and therefore
caution needs be taken in carrying out same. Typically for enhanced
reaction efficiency, as the reaction proceeds and the evolved heat
starts to diminish, external heating is applied, and such external
heating may involve refluxing of the reaction mixture if such
complex is being prepared in concentrated form for later dilution
with further aqueous medium and concomitant bath formation.
The complex may also be prepared from the carboxylic and
halogen-providing compound in admixture with chromic acid,
typically charged to the reaction medium as a solution of chromic
acid in water. In an aqueous reaction medium, the chromic acid can
be supplied by any of the suitable substances for forming chromic
acid in water, e.g., chromium trioxide. The reaction resulting from
this method is also exothermic and caution in the use of such
method is thus advisable. As a further aspect of this method,
chromic acid, for example, may be first reacted with
halogen-providing compound such as hydrochloric acid to prepare a
pre-formed reaction medium. To this medium carboxylic acid is
added, preferably with heating to hasten completion of the reaction
for final complex formation. In this reaction fugitive liquid can
form a portion of the reaction medium, with water that may be
supplied from the halogen-providing substance, e.g., a 20 to 37
weight percent solution of HCl in water providing the balance of
the liquid medium. For the fugitive liquid, a secondary alcohol
such as isopropanol is preferred, as taught in U. S. Pat. No.
2,524,803, and preferably one of no more than four carbon atoms, as
such alcohol does not coordinate with the chromium and can be
readily removed from the product by processes familiar in such art,
e.g., evaporation with accompanying crystallization followed by
water washing. The subsequent reaction medium for carboxylic acid
addition is then an aqueous medium.
The complex may further be prepared by reaction of chromic halide,
and carboxylic acid, with such halide corresponding to the halide
that is to be present in the complex. When chromic halide is
reacted with the carboxylic acid, this reaction further involves
the addition of strong base, e.g., an alkali metal hydroxide,
yielding hydroxyl ions in the aqueous reaction medium. For example,
CrF.sub.3.sup. . 9H.sub.2 O may be used in this method and will
readily yield a chromium/carboxylic acid/fluoride complex involving
exothermic reaction conditions.
The complex virtually always contains a molar ratio of chromium
atoms to carboxyl constituent within the range of 1:0.7 to 1.3, and
further contains a molar ratio of chromium atoms to halogen atoms
within the range of 1:0.1 to 1:3.5. Especially preferred ratios,
based upon desirable performance and economy, can depend upon the
acid as well as the halogen constituents of the complex. Thus, for
example, for a complex containing a substantial amount of the
carboxyl constituents supplied by glycolic acid, which complex is
thus the preferred complex, and wherein such complex further
contains chloride as the major amount, to all, of the halogen, the
molar ratio of chromium atoms to halogen is preferably within the
range of about 1:0.4 to 1:1. However when the halogen in such a
complex is preponderantly to all fluoride, the molar ratio of
chromium atoms to halogen atoms is preferably within the range of
about 1:2.6 to 1:3.2. Further, the ratio of chromium to halogen can
depend upon the method of complex preparation. For example in the
above mentioned preparation method employing the CrF.sub.3.sup..
9H.sub.2 0, the resulting complex will contain a molar ratio of
chromium to moles of fluorine of 1:3. From this it can be
appreciated that complex formation relying completely upon a
chromic halide or a basic chromic halide, e.g., Cr(OH) Cl.sub.2,
will always yield a complex wherein the molar ratio of moles of
chromium to moles of halogen will be within the broad range of
1:0.7 to 1:3.
The complex is furnished to the plating medium in an amount to
provide from about 25 to about 150 grams of chromium per liter,
that is, the molar concentration of chromium in the plating medium
is within the range from about 0.5 to about 3. The more highly
concentrated media having augmented viscosity are not well suited
for deposition of chromium onto a metal substrate which is immersed
therein. Thus such media having a molar concentration of chromium
above about 1.5 may be employed in portable plating devices used in
spot plating, e.g., for brush plating, but for bath operation
wherein the article is immersed in a bath, such baths most always
contain an amount of complex providing from about 25 to about 75
grams of chromium per liter. Within this concentration range the
bath will be supplied with sufficient complex to avoid frequent
bath replenishment during working. In this bath the complex is
virtually always used in a liquid medium supplied simply by water
although other liquids may be present, e.g., excess carboxylic acid
other than such acid resent in the complex, but these other liquids
will constitute a minor amount of the liquid medium of on the order
of about five volume percent or less.
Before deposition of chromium, the medium is adjusted to a pH
within the range from about 1.8 to 4.9. Such adjustment of pH can
be readily carried out with a base, particularly alkali metal
carbonates or hydroxides, with sodium or potassium hydroxide or
their mixtures being preferred. Before addition to the medium, such
material for pH adjustment can be initially dissolved in water and
the water solution then added to the medium.
The bath can also be furnished with a salt of a strong acid
preferably, for economy, an alkali metal salt. Such salts enhance
the conductivity achieved in the electroplating operation. Most
preferably, for economy, the cation of the salt is sodium,
potassium or their mixtures, and the strong acid anions should be
those of an acid having a dissociation constant of at least
K=10.sup.-.sup. 2, for example, chloride. The plating bath usually
contains between about 50-200 grams per liter of such salts. The
bath can also be provided with boric acid, or an equivalent to
boric acid in an aqueous solution, such as borax, boron oxide, or
sodium oxyfluoborate. Such compounds operate in the bath to augment
the rate of deposition of the chromium and are typically used in an
amount between about 10-70 grams per liter of bath.
The temperature of the medium during plating may range from about
20.degree.C. up to advantageously not substantially above about
50.degree.C. for enhanced plating performance. The most desirable
pH range and temperature range can depend upon the make-up of the
complex present in the plating medium, with, for example, complex
containing a substantial amount of fluorine as the halogen being in
a bath that is preferably maintained at a slightly more elevated
temperature than for a bath where chlorine supplies the major
amount of the halogen.
For the baths prepared as described herein, the object to be plated
is made the cathode, for example immersed in the plating bath, or
the cathode in a brush plating operation where the plating medium
is contained in the brush, and an inert anode is used such as a
carbon, graphite, platinum or platinized titanium anode. In working
of the bath a small discharge of halogen, e.g., gaseous chlorine,
and trivalent chromium fume may be given off at the anode. However,
halide replenishment as well as replenishment of all chromium,
including that plated from the bath or lost by drag-out, is
adequately taken care of by addition of further complex. Likewise
such addition of complex can adequately replenish any acid which
may be used up in the plating operation during working of the
bath.
The substrates which may be employed during operation of the bath
includes metals such as steel, brass, copper, copper alloys,
bronze, zinc diecastings, as for example to prevent rusting or
tarnishing, or nickel surfaces. Additionally plating can be
performed on plastic surfaces which are activated or prepared for
an electroplating operation. The plating bath can be prepared and
the subsequent plating carried out in any vessel useful for
chromium electroplating such as tanks lined with corrosion
resistant material including glass, ceramic material, polyvinyl
chloride, and the like.
The following examples show ways in which the invention has been
practiced but should not be construed as limiting the invention.
Unless otherwise specified, plating tests in the examples are
conducted in a modified Hull cell. The standard Hull cell is a
trapezoidal box of non-conductive material at the opposite ends of
which are positioned anode and cathode plates, as has been more
particularly described in U. S. Pat. No. 2,129,344. For either the
standard or the modified Hull cell, it is possible to easily
determine the effective plating range of a plating composition
under varying conditions. The current density at any point on a
cathode is determined according to the formula A=C(27.7-48.7 log L)
wherein A is the current density in amps per square foot (ASF) at
the selected point, C is the total current in amps applied to the
cell and L is the distance in inches of the selected point from the
high current density end of the plate.
In the modified version of the Hull cell used herein, 1/2-inch
holes are introduced in the parallel sides of the cell adjacent the
anode and cathode whereby, upon immersion of the cell in another
containing plating solution, into which vessel the cell will fit
very closely, improved electrolyte circulation and consequent
improved temperature control is afforded, as more particularly
described in an article appearing in "Plating" Volume 46, Number 3
(1959) Page 257.
EXAMPLE 1
Into a container there is placed 0.8 mole of chromium metal, 1.8
mole of glycolic acid of 70 percent strength, that is, 70 percent
of glycolic acid and a balance of water, and 0.5 mole of 37.3
percent strength hydrochloric acid which is 37.3 percent by weight
HCl in water. The container is covered and good ventilation is
provided. After the ingredients are placed together in the
container, dissolution of the chromium starts slowly but gradually
increases thus supplying heat to the reaction. As the reaction
continues the temperature of the reaction medium reaches
71.degree.C. without external heating and the chromium metal can be
seen by visual inspection to be substantially dissolved. As the
temperature starts to subside from 71.degree.C., external heating
is applied and the temperature of the reaction medium is permitted
to reach 88.degree.C. until all the chromium is dissolved. Total
reaction time i.e., to complete chromium metal dissolution, is
about 4 hours. Thereupon the solution is heated at reflux, reaching
a temperature of 107.degree.C., for about 2 hours, and is
thereafter permitted to cool.
The resulting complex, having a molar ratio of chromium to glycolic
acid of 1:2.25 and of chromium to chloride of 1:0.625, is added to
water to provide a concentration of chromium metal of 40 grams per
liter (g./l.). To this added 150 g./l. of KCl and 63 g./l. of
H.sub.3 BO.sub.3. The pH of the bath is adjusted to 2.94 by the
addition of 40 percent strength sodium hydroxide, that is, 40
weight percent NaOH in a balance of water, and the final volume of
the bat is adjusted to 1,000 milliliters (mls.) by subsequent
addition of water.
The bath is electrolyzed for 10 to 20 amp hrs./gal. and is then
ready for plating in the above-described modified Hull cell. In the
Hull cell graphite anodes are used and the cathode for the test is
a 3 15/16 inches .times. 2 5/8inches brass panel, each panel being
nickel coated prior to use in the cell. The test is carried out
using 10 amperes current for a 3 minute cycle at 12.5 volts and the
bath temperature is maintained at 87.degree.F. In this plating test
the bright range extended from 800 ASF to 6 ASF at the high and low
current density areas respectively. Over the plating range the
plate exhibits a reflective reflectance value of 50 percent, with
the color of the chromium deposit being of a slightly darker cast
than that observed with commercially available decorative chromium
deposits from hexavalent chromium plating baths, the deposit thus
exhibiting a deep and rich appearance. The results from this bath
have been plotted as the upper graph line in the drawing.
EXAMPLE 2
In the manner of Example 1 there is placed sufficient chromium
metal, together with sufficient 88.4 percent strength lactic acid,
that is, 88.4 percent lactic acid and the balance water, together
with an amount of 37.3 percent strength hydrochloric acid, to
provide a mole ratio of chromium to lactic acid of 1:1.625 and a
mole ratio of chromium to chloride of 1:0.625. The contents of the
container are reacted in the manner of Example 1, i.e., are
permitted to react without external heating, followed by external
heating and subsequently by refluxing.
Upon cooling, a portion of the resulting solution in an amount
sufficient to provide 40 grams of chromium per liter, is blended
with about 150 g./l. potassium chloride, and about 60 g./l. H.sub.3
BO.sub.3, and the bath is maintained at a pH of 2.94-3.0 by the
addition of 40 percent strength sodium hydroxide. Plating tests,
conducted in the manner of Example 1, after electrolyzing the bath
as described in Example 1, are carried out using the Hull cell with
graphite anodes and the nickel plated panel cathodes, and using 10
amperes of current for a 3 minute cycle at 13.5-14.0 volts and with
the bath temperature maintained between 87.degree.-96.degree.F.
From these tests, the bright range can be seen to extend completely
across the panels, representing a range of approximately 1,000 to 1
ASF which is unique in the history of decorative chromium plating.
The showing of a definite chromium thickness at the 5 ASF current
density level of as high as 1.7 micro-inches per 3 minute cycle is
regarded as an indication that the bath prepared in this Example
will still deposit chromium at the 1-2 ASF level.
EXAMPLE 3
In the manner of Example 1, sufficient chromium metal, along with a
sufficient amount of oxalic acid plus 37.3 percent strength
hydrochloric acid to provide a molar ratio of chromium to oxalic
acid of 1:1.8 and a molar ratio of chromium to chloride of 1:0.625,
are placed in a reaction vessel. Reaction proceeds in the manner of
Example 1, e.g., initially without external heating and finally
with refluxing. Bath preparation is completed by adding this
complex to water to provide a concentration of 40 grams of chromium
per liter of the bath, and further admixing therewith about 150
g./l. of potassium chloride and about 60 g./l. of H.sub.3 BO.sub.3.
In the manner of Example 1, the pH of the bath is adjusted to 3.0
by the addition of sodium hydroxide, and water is added to bring
the final volume of the bath to 1,000 mls.
Plating tests are carried out in the manner of Example 1 with the
bath temperature being deliberately varied to show effect on
plating results. The bright range for a test at 92.degree.F., is
from 400 ASF down to 7 ASF, and the bright range for a test at
105.degree.F. is measured from 450 ASF down to 9 ASF. Further, the
chromium plating on the nickel plated panel cathodes for each test
is seen by visual observation to have the same finish as that
described for the chromium plating in the tests of Example 1.
EXAMPLE 4
Into a reaction vessel containing 400 mls. of water there is
dissolved 210 grams of chromium fluoride (CrF.sub.3.sup. . 9H.sub.2
O) and to this is added 153 mls. of the 70 percent strength
glycolic acid of Example 1. While this mixture is constantly
agitated there is gradually added thereto 70 grams of potassium
hydroxide. During addition of the potassium hydroxide, the reaction
is exothermic and, as agitation in the mixture is continued,
external heating is applied until the mixture is brought to boiling
and is refluxed for several hours. Following this the volume of the
mixture is adjusted to 750 mls. by the addition of water and to
this there is added 56.6 grams of H.sub.3 BO.sub.3 and 150 grams of
potassium chloride, with agitation. As the mixture is further
permitted to cool to 90.degree.F., the volume is adjusted to 1
liter and the pH is adjusted to 3.4 in the manner of Example 1.
The resulting bath contains sufficient complex to provide 40 grams
of chromium per litter of the bath and the complex has a molar
ratio of chromium to glycolic acid of 1:2.25 and of chromium to
fluoride of 1:3. Thereafter the bath is electrolyzed for 0.5 hour
at a current of 12 amperes. Plating tests are carried out in the
manner of Example 1 and give a full bright range coverage to the
extreme low current density end of the panel and lacked 2 percent
coverage at the extreme high current density area. The bright range
is then deemed to be from about 950 ASF down to 1-2 ASF. The finish
on the panel is comparable to the finish described in the panels
plated in Example 1, i.e., the panel has a reflective reflectance
value of 50 percent, has a desirably rich and deep appearance, and
the color of the chromium deposit is of a slightly darker cast than
that observed with commercially available decorative chromium
deposits from hexavalent chromium plating baths.
EXAMPLE 5
For comparative purposes only, i.e., to compare the methods of
preparation of the present invention with a bath similarly
prepared, a comparative chromium glycolate complex is prepared in
accordance with the precepts of U. S. Pat. No. 3,006,823. For this,
there is added to a reaction vessel containing 3,000 mls. water,
4,398 grams glycolic acid. This mixture is heated to 70.degree.C.
and is then gradually combined with a solution of chromic acid
containing 2,700 grams chromium, expressed as CrO.sub.3, and 1,680
mls. water. During the slow addition of the chromic acid solution,
the temperature in the reaction medium is maintained at
70.degree.C. After addition of the chromic acid solution, the
reaction mixture is heated to 90.degree.C. and held at that
temperature for 1 hour. Subsequently, the mixture is permitted to
cool and is diluted with water to 11,232 mls.
A portion of this resulting solution sufficient to provide a
chromium concentration of 52 g./l. is blended with 150 g./l.
potassium chloride, 62 g./l. of H.sub.3 BO.sub.3, and 86 mls. per
liter of glycolic acid of 70 percent strength. The pH of this bath
is adjusted in the manner of Example 1 to a pH of about 3 and the
bath is electrolyzed to prepare same for evaluation of plating
performance.
Using the Hull cell as above described, a plating test is run on
this comparative bath in the manner of Example 1, i.e., a nickel
plated brass panel cathode is employed and the bath is operated
using 10 amperes current for a 3 minute cycle. The results of such
test are plotted in the drawing and form the lower line appearing
on the graph of the drawing. By reference to such drawing, the
superior performance obtained by the bath of the present invention
compared to this comparative bath of Example 5, is readily
evidenced.
* * * * *