Title:
Self-regulating plating bath and method for electrodepositing chromium
United States Patent 3920527


Abstract:
Lead sulfate when present in a chromic acid electroplating solution together with other activators or catalysts, such as boric acid, hydrofluoric acid, fluosilicic acid, fluotitanic acid or fluoboric acid or their soluble salts maintains a constant effective catalyst concentration during extended chromium electrodeposition runs. The lead bearing electrolytes permit microcracked chromium plates to be formed directly by electrodeposition.



Inventors:
Dettke, Manfred (Berlin, DT)
Roosen, Manfred (Berlin, DT)
Application Number:
04/792970
Publication Date:
11/18/1975
Filing Date:
01/22/1969
Assignee:
SCHERING AG
Primary Class:
International Classes:
C25D3/02; C25D3/04; (IPC1-7): C23B5/06
Field of Search:
204/51,43,105 148
View Patent Images:
US Patent References:
3461048METHOD OF ELECTRODEPOSITING DUPLEX MICROCRACK CHROMIUM1969-08-12Mahlstedt et al.
3408272ELECTRODEPOSITION OF CHROMIUM1968-10-29Such et al.
2952590Process for chromium plating1960-09-13Stareck et al.
2050478Electrolytic bath for depositing chromium1936-08-11Wickenhiser
1590170Process of plating with chromium1926-06-22Hosdowich



Other References:

Paul Morisset et al., Chromium Plating, pp. 47-50 and 96, (1954). .
Paul Morisset et al., Chromium Plating, pp. 27-32, (1954)..
Primary Examiner:
Kaplan G. L.
Attorney, Agent or Firm:
Padlon, Joseph F.
Claims:
What is claimed is

1. A method for forming a microcracked chromium electrodeposit which comprises making an electrically conductive object the cathode in a self-regulating acidic aqueous chromium electroplating electrolyte comprising between 200 and 400 grams per liter of chromium trioxide, lead sulfate in amounts of from 0.5 to 10 percent by weight based on the concentration of chromium trioxide, and an effective amount of a secondary activator selected from the group consisting of the anions of hydrofluoric acid, fluosilicic acid, fluoboric acid, and fluotitanic acid, said effective amount being sufficient to contribute to the production of a microcracked chromium deposit, at a temperature of 25°C. to 60°C. and at a cathode current density of 5 to 70 amps. per square decimeter.

2. A method as set forth in claim 1, wherein the concentration of said chromium trioxide is between 200 and 250 grams per liter, the temperature is approximately 55°C and the current density is between 15 and 30 amps. per square decimeter.

3. A self-regulating acidic aqueous chromium electroplating electrolyte for depositing a amicrocracked layer of chromium, said electrolyte comprising from about 150 to about 550 grams per liter of chromium trioxide, lead sulfate in amounts of from 0.5 to 10 percent by weight based on the concentration of chromium trioxide, and at least 0.05 weight percent based on the weight of chromium trioxide of a secondary activator selected from the group consisting of the anions of hydrofluoric acid, fluosilicic acid, fluoboric acid and fluotitanic acid.

4. The electrolyte of claim 3 wherein the secondary activator is fluosilicic acid in amounts of 1 to 5 percent by weight based on the weight of chromium trioxide.

5. The electrolyte of claim 3 wherein the activator is from 0.1 to 3 weight percent fluoboric acid.

6. The electrolyte of claim 3 wherein the activator is from 0.1 to 3 weight percent fluotitanic acid.

7. A self-regulating acidic aqueous chromium electroplating electrolyte for depositing a microcracked layer of chromium, said electrolyte comprising from about 150 to about 550 grams per liter of chromium trioxide, lead sulfate in amounts of from 0.5 to 10 percent by weight based on the concentration of chromium trioxide, and from 0.2 to 5 weight percent based on the weight of chromium trioxide of boric acid as a secondary activator.

8. The electrolyte of claim 7 wherein the activator is from 0.1 to 2 weight percent boric acid.

Description:
This invention relates to the electrodeposition of chromium from an aqueous solution of hexavalent chromium ions, and particularly to a self-regulating chromium plating bath and to its use.

Bright chromium metal is deposited electrolytically on a cathode from an aqueous solution of chromium trioxide (chromic acid) when the solution contains known catalytic ions of sulfuric or fluosilicic acid in a precise ratio to the chromic acid present. Because of the criticality of the catalyst concentration, such conventional chromium plating baths must be analyzed frequently and adjusted to the required composition to maintain adequate current efficiency, covering power, and throwing power. The temperature of the known solutions also must be held between rather narrow limits if satisfactory results are to be achieved.

More recently, self-regulating chromium plating baths have been proposed in which the effective catalyst concentration is maintained by solid strontium sulfate in contact with the electrolyte. Strontium sulfate alone is effective in chromium plating baths containing up to 200 g/l CrO3. If the conductivity of the bath is to be improved by a higher chromic acid content, up to about 500 g/l, as is usually desirable, a second catalyst such as sodium or potassium fluosilicate is needed.

It has also been proposed to add 1,3-benzenedisulfonic acid to chromium plating electrolytes as a self-regulating catalyst, but the compound is not fully stable under the conditions normally prevailing in chromium plating, and its decomposition products unfavorably affect the chromium plate formed.

It has now been found that a chromium plating bath containing hexavalent chromium ions as a source of chromium can be made self-regulating as to its activator or catalyst content by the use of lead sulfate in conjunction with a secondary activator which may be an acid from the group consisting of boric acid, hydrofluoric acid, fluoboric acid, fluosilicic acid and fluotitanic acid, or a soluble salt of such an acid.

The chromium plating baths of the invention are effective over a wide range of chromic acid concentrations, the preferred range being from 150 g to 550 g CrO3 per liter, and acceptable results are also obtained at values of chromium trioxide concentration slightly, but not substantially, above and below this range.

Best covering power and a most stable catalyst concentration are achieved when the bath further contains ions of dichromic acid, most conveniently derived from sodium or potassium dichromate in an amount of 10 to 40%, based on the chromium trioxide present and expressed as alkali metal dichromate. The optimum ratio of alkali metal dichromate to chromium trioxide is between approximately 4 : 1 and 5 : 1, that is, 20 to 25% alkali metal dichromate, based on the chromium trioxide. The addition of the dichromate brings about a substantial improvement of the electrodeposit at high current densities and improves the covering power.

The lead sulfate addition may amount to approximately 0.5 to 10% of the chromium trioxide dissolved in the plating bath, best result usually being obtained with 0.5 to 5.5% lead sulfate. 2.6% Lead sulfate produces desirable results under most operating conditions.

The chromium electrodeposits of the invention are distinguished by unusually good covering power and by high brightness free from haze over a wide range of cathode current densities. Even when built up to relatively high thickness values, the chromium electrodeposits produced by the method of the invention have at least a satin sheen. Regardless of thickness, the coatings are smooth and free from dendrites, and thereby far superior to otherwise similar coatings produced from the conventional electrolytes using sulfuric aciid as the catalyst or activator.

Lead sulfate in the chromium electroplating baths of the invention is compatible with a wide range of secondary activators or catalysts which are stable under all operating conditions, and do not form decomposition products capable of being codeposited with the chromium metal to interfere with the quality of the deposit. The secondary catalysts of the invention are acids, namely boric acid, hydrofluoric acid, fluosilicic acid, fluoboric acid, or fluotitanic acid, and salts of these acids which are soluble in the electrolyte to furnish anions of the acids. The amounts of the secondary catalysts are too small to make the nature of their cationic moiety (hydrogen or alkali metal ions) relevant to the success of the method.

The secondary activators are effective in minute amounts. As little as 0.05% - based on the CrO3 in the electrolyte - produces practically significant improvements, but the effects of even smaller amounts can be detected. No critical upper limit of secondary activator concentration could be established, but nothing useful is achieved by increasing the amounts of activator above the preferred concentration range which is 1 to 5% for fluosilicic acid, 0.2 to 5% for boric acid, 0.1 to 3% for fluotitanic acid, 0.1 to 2% for hydrofluoric acid, and 0.1 to 3% for fluoboric acid. The soluble salts are effective as the free acids in equimolecular amounts, the sodium and potassium salts being generally most readily available.

The chromium plating baths of the invention are free from organic compounds. The amount of solid phase present in contact with the electrolyte is extremely small, and equilibrium between the solid phase and the supernatant liquid is quickly established. The agitation of the bath resulting from thermal convection and the insertion and removal of objects to be plated is normally sufficient to maintain equilibrium.

The chromium plating baths of the invention are operated effectively at temperatures from approximately 25° to 60°C and at cathode current densities from 5 to 70 amps, per dm2. The cathode current efficiency is affected by the temperature, cathode current density, and electrolyte temperature as shown on the graphs of cathode current efficiency (per cent) v. chromium trioxide concentration (g/l) in the attached drawing.

The four curves of FIG. 1 respectively relate to cathode current densities of 30, 20, 15 and 10 amps./dm2, at an electrolyte temperature of 30°C. FIG. 2 has four curves relating to the same current density values as in FIG. 1, but measured for an electrolyte temperature of 40°C. The four curves of FIG. 3 were determined at 55°C and at current densities of 70, 60, 30 and 15 amps. per dm2 respectively. The curves of FIGS. 1 and 2 reflect typical conditions for decorative chromium plating whereas the curves of FIG. 3 apply mainly to hard chromium plating.

The method of the invention permits a chromium electrodeposit having microcracks to be produced in the bath itself, and the number of cracks per centimeter to be determined in advance, at least approximately, by suitable selection of plating conditions. As is well known, chromium electrodeposits having microcracks have better corrosion resistance than continuous chromium coatings because of the lack of ductility of chromium. Mechanical stresses may cause a continuous electrodeposit to crack through its entire thickness so that local cells may be formed between the exposed base metal and the electrodeposit. The anode current density in such cells can be very high, and the corrosion correspondingly rapid.

The properties of the chromium layer produced by the method of this invention can be controlled by selecting the chromic acid concentration. A grid of microcracks extending over the entire chromium surface is obtained if the chromic acid concentration is between 200 and approximately 400 g/l. Within this concentration range, the number of cracks per centimeter decreases with rising temperature and with rising chromic acid concentration if the temperature is between 30° and 40°C, the current density between 5 and 30 amps. per dm2, and the plating time between 12 and 15 minutes.

When the electrolyte temperature is 55°C, the current density is between 15 and 30 amps. per dm2, and the chromic acid concentration is between approximately 200 and 250 g/l, the chromium plating baths of the invention produce chromium electrodeposits characterized by a much finer grid of microcracks, approximately 1500 cracks per centimeter. Intermediate values of crack density can be obtained by suitable modification of process variables.

The following Examples are further illustrative of the instant invention.

EXAMPLE 1

An electrolyte suitable for producing mirror bright decorative chromium electrodeposits at 30°-50°C, preferably 38°-40°C, at 5-25 amps./dm2, was prepared by dispersing the following materials in water:

Chromium trioxide, CrO3 350 g/l 100.0% Potassium dichromate, K2 Cr2 O7 88.6 g/l 25.3% Lead sulfate, PbSO4 11.1 g/l 3.16% Potassium fluosilicate, K2 SiF6 11.1 g/l 3.16%

EXAMPLE 2

An electrolyte which produces hard chromium plates at high deposition rates with good current efficiencies under the conditions of Example 1, particularly in the higher range of cathode current densities was prepared from the following ingredients: 8n

Chromium trioxide 250 g/l 100.0% Potassium dichromate 67.5 g/l 27.0% Lead sulfate 9.2 g/l 3.68% Potassium fluosilicate 13.6 g/l 5.45%

EXAMPLE 3 ______________________________________ An aqueous electrolyte was prepared from: chromium trioxide 180 g/l 100% potassium dichromate 45 g/l 25.0% lead sulfate 6 g/l 3.34% potassium fluoride, KF 0.5 g/l 0.26% ______________________________________

A continuous chromium deposit was prepared in the electrolyte of Example 1, and was then covered with a chromium layer from the above electrolyte at 40 °C, 15 - 20 amps./dm2 in 4 - 8 minutes. The top chromium layer showed 400 - 600 cracks per centimeter, the width of each crack being uniform and between 0.1 and 0.2 microns.

EXAMPLE 4 ______________________________________ An aqueous electrolyte was prepared from: Chromium trioxide 300 g/l 100% Potassium dichromate 75 g/l 25.0% Lead sulfate 10.3 g/l 3.34% Fluoboric acid, HBF4 0.8 g/l 0.26% ______________________________________

When operated at 48 °C and 15 - 20 amps. per dm2, the electrolyte produced chromium electrodeposits which had 400 to 600 cracks per centimeter at a thickness of at least 2.0 to 2.5 microns.

Similar results were obtained when the chromium trioxide concentration was reduced to 250 g/l while the ratio of the components in the electrolyte was maintained.

EXAMPLE 5

A chromium base layer was deposited from the electrolyte described in Example 1, and was then covered by a surface layer of chromium from an electrolyte of the following composition at 45 °C and 10 - 20 amps./dm2 in 4 - 8 minutes:

Chromium trioxide 200 g/l 100.0% Potassium dichromate 66 g/l 33.0% Lead sulfate 6.3 g/l 3.15% Boric acid, H3 BO3 1.9 g/l 0.95%

The surface layer had approximately 400 - 600 cracks per centimeter, the width of each crack being uniform and 0.1 - 0.2 microns.

EXAMPLE 6 ______________________________________ An electrolyte was prepared from: Chromium trioxide 260 g/l 100.0% Potassium fluosilicate 7.6 g/l 2.6% Lead sulfate 4.7 g/l 1.8% ______________________________________

At a cathode current density of 60 - 80 amps./dm2 and a nominal temperature of 60°C, a chromium deposit one millimeter thick was formed in 12 - 14 hours. It showed several superimposed, uniform grids of cracks.

EXAMPLE 7

An electrolyte suitable for forming thin chromium plates having microcracks was prepared from:

chromium trioxide 190 g/l 100.0% potassium dichromate 24.5 g/l 12.9% potassium fluosilicate 4.9 g/l 2.6% lead sulfate 1.3 g/l 0.7%

After a plating time of only 4 to 7 minutes at 48 °C and 10-20 amps/dm2, there was obtained a chromium electrodeposit covered with a grid of approximately 400 - 600 cracks per centimeter, each crack having a uniform width of 0.1 to 0.2 microns. The width of the cracks was determined in all Examples by means of an electron microscope.

The electrolyte of Example 7 could also be used for producing a surface deposit of cracked chromium on a chromium base formed from the electrolyte of Example 1.

Substantially the same results as indicated above were achieved when potassium fluotitanate was substituted for the fluosilicate at approximately one-half the rate.

The chromium electrodeposits of the invention are metallic in appearance at all cathode current densities not substantially lower than 1.8 amps. per dm2, and this lower limit is raised only insignificantly at the highest deposition temperatures mentioned above. An iridescent coating of chromic chromates, often observed at low current densities in other chromium plating solutions, is not observed in low current density areas of coatings formed by the method of the invention.

When the chromium coatings of the invention are deposited on a nickel base coating in the usual manner, and it becomes necessary to strip the chromium coating, the nickel surface exposed by the stripping operation may be chromium plated anew without the preparatory steps necessary with the usual chromium electrolytes, such as polishing or deposition of a fresh nickel layer.

The chromium plating baths of the invention can be operated with uniform results over extended periods without requiring adjustment of their composition.