Weight loss abstract
Weight loss of lead and lead alloy anodes is substantially reduced
by including bismuth arsenic or antimony ions in the plating bath.
Weight loss claims
What is claimed is:
1. A chromium electroplating bath in which solubilizing corrosion
of a lead or lead alloy anode used therein is substantially reduced
(a) a source of said chromium,
(b) an alkyl sulfonic acid or salt thereof, wherein the S/C ratio
is .gtoreq.1/3, and
(c) a source of bismuth, arsenic or antimony ions as an additive
therein, to reduce weight loss or pitting of said anode.
2. A chromium plating bath according to claim 1 wherein (c) is
3. A chromium plating bath according to claim 2 wherein said bismuth
ion is present in an amount of about 0.01 g/l to 25 g/l.
4. A chromium plating bath according to claim 3 wherein said bismuth
ion is present in an amount of about 0.1 g/l to 5 g/l.
5. A chromium plating bath according to claim 1 wherein said source
of bismuth ions is a bismuth oxide or an alkali metal bismuthate.
6. A chromium plating bath according to claim 1 wherein said chromium
bath includes chromic oxide and a catalyst.
7. A chromium plating bath according to claim 6 wherein said alkyl
sulfonic acid is selected from methyl sulfonic acid, ethyl sulfonic
acid, propyl sulfonic acid, methane disulfonic acid and 1,2-ethane
8. A chromium plating bath according to claim 7 wherein the chromic
acid concentration is about 100-450 g/l, the alkyl sulfonic acid
concentration is about 1-18 g/l and the catalyst is sulfate in a
concentration of about 1-5 g/l.
Weight loss description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the electrodeposition of chromium using
lead or lead alloy anodes, and, more particularly, to a chromium
plating bath system and process, in which weight loss of such lead
anodes is substantially reduced.
2. Description of the Prior Art
Chromium plating baths using chromic acid solutions as the source
of chromium almost invariably employ lead or lead alloy anode. However,
as described in the book "The Electrochemistry of Lead"
by A. T. Kuhn, Academic Press (1979) pages 405-407, lead and lead
alloy anodes corrode to a soluble species (leading to anode weight
loss) in chromic acid solutions. This anode dissolution proceeds
at a substantial rate, which is proportional to the acidity of the
solution. For this reason, there has been a need to find corrosion
resistant alloys for this medium. For example, alloys containing
10% Sn and 0.5% Co seem to show good resistance. Also the addition
of 1-2 g/l of Co.sup.2+ ions appears to further reduce corrosion.
While this problem is present in the conventional hexavalent chromium
plating baths and in the mixed catalyst chromium baths, it is particularly
acute in high energy efficient baths, e.g. the so-called "HEEF-25"
baths as described in the U.S. Pat. No. 4,588,481. HEEF-25 baths
contain chromic acid, sulfate and an alkyl sulfonic acid, which,
for functional chromium deposits, is preferably a non-substituted
alkyl sulfonic acid, or salt thereof, wherein the ratio of S/C is
.gtoreq.1/3. Typical alkyl sulfonic acids are methyl sulfonic acid,
ethyl sulfonic acid, propyl sulfonic acid, methane disulfonic acid
and 1,2-ethane disulfonic acid.
Other hexavalent chromium plating baths are described in the following
U.S. Pat. Nos.: 2,750,337; 3,310,480; 3,311,548; 3,745,097; 3,654,101;
4,234,396; 4,406,756; 4,450,050 and 4,472,249.
Accordingly, it is an object of the invention to provide a chromium
plating bath in which the rate of weight loss or pitting of a lead
or lead alloy anode is substantially reduced.
Another object of the present invention is to provide an additive
for a hexavalent chromium plating bath containing an alkyl sulfonic
acid, which substantially reduces the corrosion of a lead or lead
alloy anode therein, which corrosion produces a soluble species
leading to anode weight loss.
Still another object of the invention is to provide a method of
preventing such deleterious corrosion of a lead or lead alloy when
used in a hexavalent chromium plating bath, particularly in a high
energy efficient bath.
These and other objects and features of the invention will be made
apparent from the following more particular description of the invention.
SUMMARY OF THE INVENTION
What is provided herein is an improved chromium plating bath which
substantially reduces corrosion of a lead or lead alloy anode used
therein to a soluble species leading to anode weight loss. The bath
of the invention includes a source of chromium metal, and a source
of bismuth, arsenic or antimony ions, which ions perform as an anode
corrosion-inhibiting additive therein.
As a feature of the invention, the anode corrosion-inhibiting additive
herein is effective in various chromium plating baths including
the conventional, mixed catalyst, or alkyl sulfonic acid-containing
In the preferred form of the invention, the corrosion-inhibiting
additive is bismuth ion, which is present in an amount at least
sufficient to effect the desired result, suitable at least about
0.01 g/l to 25 g/l, and, preferably about 0.1 to 5 g/l.
DETAILED DESCRIPTION OF THE INVENTION
In accordance with the invention, a typical chromium plating bath
exhibiting substantially reduced solubilizing corrosion of lead
or lead anodes used therein includes:
(a) a source of chromium metal and catalyst therewith; and
(b) a source of bismuth arsenic or antimony ions as an anode corrosion-inhibiting
A preferred chromium electroplating system comprises:
(a) a lead or lead alloy anode;
(b) a cathode,
(c) a hexavalent chromium electroplating bath solution including:
(i) a source of chromium and catalyst for deposition of chromium
on said cathode, and,
(ii) a source of bismuthions to substantially reduce solubilizing
corrosion of said anode during deposition of chromium metal on said
cathode or while the anode is at open circuit.
In the most advantageous commercial use of the invention, the hexavalent
chromium plating bath is the HEEF-25 type, as described in U.S.
Pat. No. 4,588,481. This bath contains an alkyl sulfonic acid, where
the S/C ratio is .gtoreq.1/3, e.g. methyl sulfonic acid, in an amount
of 1-18 g/l, chromic acid in an amount of 100-450 g/l, and sulfate
as a catalyst in an amount of 1-5 g/l.
The bismuth ion is suitably present in the bath in an amount sufficient
to effect the desired retardation of corrosion of the lead and lead
alloy anodes used in chromium plating. Deleterious corrosion produces
a soluble species of divalent lead which results in anode pitting
or weight loss. On the other hand, corrosion of the alloy to dense
PbO.sub.2, which is insoluble, is not harmful.
Thus, while the mechanism of the action of bismuth in preventing
solubilizing corrosion of lead or lead alloy anodes is not completely
understood, it is believed that a bismuth ion or anion migrates
to the anode leaving a dense and adherent black film of mixed oxides
of bismuth and lead on the anode. The presence of this black film
is believed to prevent further solubilizing corrosion of the anode.
The bismuth ion may be provided in combination with an anion, which,
however, preferably does not accelerate the dissolution of the lead
anode. Bismuth oxide, a bismuthate, or bismuth salts are preferred
because their lead salts are soluble compounds.
The invention now will be illustrated by reference to the following
A. A chroium plating solution was prepared from 250 g/l chromic
oxide, 2.5 g/l sodium sulfate, and 3.5 g/l methane sulfonic acid.
The anode was a Pb-7% Sn alloy. Electroplating of chromium was carried
out at an anode current density of 1.5 a.s.d. at 60.degree. C.,
a total of 1,050 amphere-hours, after which the anode was inspected.
The anode had a dark brown film thereon and was slightly corroded
B. Example A was repeated with the addition of 5 g/l of sodium
bismuthate. A black film formed on the anode, and its surface was
smooth, with no visible solubilizing corrosion.
A. A hexavalent chromium plating bath for accelerated testing was
prepared by mixing 100 g/l chromic oxide into a 10% by volume solution
of methyl sulfonic acid, and adding 2.0 g/l of sodium bismuthate.
A lead-7% tin oxide anode was used for plating, which was carried
out for 4 min. at 15 amps. The Faradaic weight loss of the lead
alloy anode was observed to be only 1.8%, and a dark, dense PbO.sub.2
film formed on the anode.
B. Example A was repeated without sodium bismuthate being present.
The Faradaic weight loss increased substantially to 7.5%.
C. Example A was repeated without chromic oxide being present in
the bath. The anode weight loss indicated 100% efficiency in forming
D. Example B was repeated without chromic oxide being present in
the bath. The anode weight loss indicated 100% efficiency in forming
Examples 1A-C were repeated using 5 g/l of sodium arsenate, sodium
antimonate, and 5% phosphoric acid. Reduction of weight loss of
the anode was observed.
Bismuth oxide was substituted for sodium bismuthate in the above
examples with similar results.
While the invention has been described with reference to certain
embodiments, it will be understood that changes and modifications
may be made which are within the skill of the art. It is intended
to be bound by the appended claims only.