US3790455A - Chromium plating bath - Google Patents

Abstract

D R A W I N G
A CHROMIUM PLATING BATH UTILIZED TO FORM A CHROMIUM PLATED LAYER ON THE SURFACE OF AN ATRICLE TO BE PLATED COMPRISES AN AQUEOUS SOLUTION OF CHROMIUM TRIOXIDE, AND FLUOROSILICATE DISSOLVED IN THE AQUEOUS SOLUTION IN AMOUNTS RANGING FROM 20 TO 45 GRAMS PER ONE LITER OF THE SOLUTION. SIRECT CURRENT IS PASSED BETWEEN AN ANODE ELECTRODE MADE OF METAL, SUCH AS LEAD, AND A CATHODE ELECTRODE CONSISTING OF THE ARTICLE TO BE PLATED, WHICH ARE IMMERSED IN THE PLATING BATH.

Description

Feb. 5, 1974 EIG] KANEKO ETAL CHROMIUM PLATING BATH Filed Jan. 22, 1970 SOLID LINE INDICATES SUSPENSION QUANT ITY 'OF POTASSIUM HEXAFLUOROSILI CATE INVENTORS 151.0 I IIMIEI'LU, NASA NAKAJIIIA United States Patent O1 fice Patented Feb. 5, 1974 Ser. No. 4,810

Int. Cl. C23b 5/06 U.S. Cl. 204-51 8 Claims ABSTRACT OF THE DISCLOSURE A chromium plating bath utilized to form a chromium plated layer on the surface of an article to be plated comprises an aqueous solution of chromium trioxide, and fluorosilicate dissolved in the aqueous solution in amounts ranging from 20 to 45 grams per one liter of the solution. Direct current is passed between an anode electrode made of metal, such as lead, and a cathode electrode consisting of the article to be plated, which are immersed in the plating bath.

CROSS REFERENCE TO RELATED APPLICATION This is a continuation-in-part of applicants copending application Ser. No. 626,406, filed Mar. 28, 1967 and now abandoned.

BACKGROUND OF THE INVENTION This invention relates to an improved plating bath utilized in electroplating of chromium.

The most common chromium plating bath comprises an aqueous solution containing chromium trioxide in amounts ranging from 200 to 300 g. per liter of water, and sulfuric acid of the molar ratio of about i based on the amount of chromium oxide. Chromium plating is performed by immersing an anode electrode of lead or lead alloy and a cathode electrode consisting of the article to be plated in the plating bath and then passing direct current between these electrodes.

It has already been known in the art to incorporate a small quantity of hexafluorosilicates less than the solubility at the plating temperature into such chromium plating bath for the purpose of improving the surface luster of the plate layer formed. However, where one of the hexafluorosilicates is incorporated, as the composition of the bath varies with the progress of the plating treatment, it is difficult to perform continuous operation over an extended period of time, thus causing a non-uniform adhering property of the plated layer to the article.

On the other hand, where potassium hexafluorosilicate is incorporated into the plating bath, the plating etficiency of electroplating is very poor, at most 24% under the most favorable condition. The remaining electric current is consumed for reactions other than that required, i.e. essentially for the electrolysis of Water, thus producing a large quantity of mist from the bath. Such a mist containing a great proportion of poisonous chromic acid greatly impairs operational environment.

SUMMARY OF THE INVENTION An object of this invention is to provide a novel plating bath capable of effecting chromium plating at high current efiiciencies.

Another object of this invention is to provide a novel plating bath capable of forming plated layers of dense structure intimately adehring to the surface of articles to be plated.

Still another object of this invention is to provide an improved plating bath wherein control of the bath is facilitated during plating treatment.

A further object of this invention is to provide a method for producing said novel plating bath.

This invention provides an improved chromium plating bath which permits high current efliciencies at the time of plating treatment and results in good adherence of the plated layer to the surface of the article being plated. The novel plating bath comprises an aqueous solution of chromium trioxide containing fluorosilicate dissolved therein in amounts ranging from 20 to 45 grams per liter.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a diagram showing relationship between the specific gravity of the plating bath and potassium.

DETAILED DESCRIPTION OF THE INVENTION Fluorosilicates to be utilized in this invention are metal salts of hexafluorosilicic acid, preferably salts of alkali metals such as potassium, sodium or cesium.

The solubility in water of these hexafluorosilicates is generally small except for lithium salt. For example, it is generally recognized that the solubility of potassium hexafiuorosilicate in 100 g. of water is 0.12 g. at a tem perature of 17.5 C. and 0.95 g. at 100 C. It is also well known that the solubility of potassium hexafiuorosilicate in an aqueous solution containing chromium trioxide at a concentration commonly used as chromium plating bath is about 0.5 g. per 100 g. of the solution at a temperature of 50 C. Consequently, it has previously been thought impossible to prepare a plating bath having dissolved therein potassium hexafiuorosilicate in greater amounts than 0.5 g. per 100 g. of the solution.

We have now discovered that far greater amounts of hexafluorosilicate than has previously been thought possible can be dissolved in chromium plating electrolytes, when an aqueous solution of chromium trioxide is subjected to a preliminary electrolysis (i.e. preliminary to electrolytic use in plating) corresponding to an electric current of at least 30 a.h./l. (ampere hour per liter) at a temperature of 40 to 100 C., preferably 70 to C. More particularly, when an aqueous solution of chromium trioxide is subjected ot the preliminary electrolysis, and the solution is heated to temperatures ranging from 40 to 0., preferably from 70 to 80 C., hexafiuorosilicate in amounts ranging from 20 to 45 g. per liter of the solution, corresponding to about 20 to 40 times of its generally known solubility will completely dissolve in the solution, contrary to ordinary knowledge regarding solubility, and no precipitate or suspension forms even when the solution is cooled to room temperature. The preliminary electrolysis may be conducted after or simultaneously with the addition of 20 to 45 g./l. of hexafluorosilicate, or the electrolysis may be carried out in two steps respectively before and after the addition of hexafluorosilicate. The solution with a large proportion of the hexafluorosilicate dissolved therein was found very effective in various aspects as a chromium plating bath.

The plating bath embodying this invention ensures high current efliciencies at the time of electroplating under ordinary conditions. Under the same conditions of bath temperature, current density, voltage and so forth, the current efficiency of this bath is about two times higher than the conventional plating bath. As a consequence electrolysis efliciency per unit time is higher, thus enabling continuous and high speed plating. Moreover since the proportion of current flowing through the bath that is consumed for other electrochemical side reactions, for

example for the electrolysis of water, is small, generation of harmful mist can be effectively precluded.

Another important effect of the novel plating bath is its ability of direct plating the surface of iron or die cast alloys, which have been impossible or extremely difiicult to plate with the conventional plating bath. This greatly widens the field of application of the chromium plating, and in addition, the steps required for preliminary plating can be completely eliminated.

A possible theory leading to the greatly increased solubility of hexafluorosilicate in chromium trioxide solution is hereinafter explained in reference to the drawing. The experiment shown in the drawing was conducted by adding potassium hexafluorosilicate into an aqueous chromium trioxide solution (250 g./l.) and by applying preliminary electrolysis corresponding to an electric current of 48 a.h./l., at a temperature of 70 C.

As will be evident from the graph, several peaks in the specific gravity occur with an increase of the quantity of potassium hexafluorosilicate. This phenomenon is presumed to be due to the formations of different kinds of complex compounds. When fluorosilicate is added to the chromium trioxide solution, the complete dissolution is obtained by the addition of an amount up to about 5 g., but an amount in excess of this generates a range of suspension up to about 20 g./l. Above this range the dissolution is renewed up to about 45 g./l., thereafter changing again to a state of suspension.

Although the reason for the occurrence of this critical region is not certain, presumably, a complex compound associated with trivalent and hexavalent chromium irons is formed in the region approximately between g./l. and 45 g./1. It is believed that this complex compound serves to increase the solubility of fluorosilicate, resulting in improved current efiiciency, plating speed, and other advantages.

In the known chromium trioxide-fluorosilicate bath, the presence of sulfate radical is required for the formation of a complex compound which serves for the plating of metallic chromium. In the present invention, however, the presence of sulfate radical is not required, and indeed the presence of sulfate radical lowers current efiiciencies at the time of plating treatment. This demonstrates the formation of a preculiar kind of complex compound different from that of the known chromium trioxidefluorosilicate bath.

By the experiment we have confirmed that the formation of such complex compound occurs when hexafluorosilicate is incorporated into the aqueous solution of chromium trioxide within a range of from to 45 grams per liter. With quantities of less than 20 grams per liter, the object of this invention can not be attained, whereas quantities more than 45 grams per liter the hexafluorosilicate would cause oversaturation thus causing an undissolved component to remain in the bath, and in addition, the viscosity of the bath is increased thus requiring higher voltage for electrolysis.

A small quantity of sulfuric acid may be added to the bath of this invention in order to suitably adjust the hydrogen ion concentration thereof, whereby to provide an adequate electric conductivity. However it is preferable to reduce as far as possible the quantity of incorporation of sulfuric acid because it has a tendency to increase the number of chromium ions formed which function to cause non-uniform structure of the plated layer deposited on the surface of the article. The quantity of incorporation of sulfuric acid adequate to increase the electric conductivity without degrading the characteristics of the plated layer was found to be 2 g. per liter or less.

Aqueous solution of chromium trioxide can be prepared in the ordinary manner wherein chromium trioxide is incorporated in water at room temperature and then stirred to obtain solution. When dissolved in water, chromium trioxide forms chromic acid. As is well known in the art,

reaction promoters such as chromium sulfate, chromous sulfate, chromous chloride, and potassium dichromate may also be used together with chromium trioxide.

Similar to the ordinary chromium plating bath, the concentration may range from 20 to 25% preferably from 22 to 23%. Hexafluorosilicate in the portion mentioned hereinabove is added to this aqueous solution of chromic trioxide and is then all dissolved while heating to from to 100 C.

The novel plating bath can be utilized under the same condition as the conventional chromium plating bath. For example, the bath is heated to a temperature of from 55 to C., and in this heated bath an anode electrode of lead or lead lolaaynd a cathode electrode comprised of an article to be plated are immersed and a direct current of 2 to 5 volts is passed between two electrodes at a current density of from 25 to a/dm. It is to be understood that these conditions of plating should be properly selected dependent upon the composition of bath as well as the material and configuration of the article to be plated and that they are not limited to the particular values noted above. However when above described conditions similar to those of the conventional chromium plating are selected the current efficiency of the novel bath is increased to more than 40%, and in some cases to 47% or more. Preferably, during a chromium plating process using an electrolyte of the present invention, the electrode is maintained at a temperature of from 60 to C.

As the plating treatment proceeds, the composition of the bath varies gradually. The most important change is the decrease in chromium trioxide. However this can be compensated for by continuously or intermittently supplementing chromium trioxide into the bath. At the same time, while a very small portion of fluorosilicate and a compound derived therefrom in the bath are consumed, under ordinary operating conditions it would not be required to supplement the fluorosilicate until the life of the bath is exhausted by other causes.

It is preferable to clean the surface of the article to be treated prior to the plating operation by any suitable pretreatment such as polishing and grinding, degreasing or rinsing. However, as the novel plating bath can form a plated layer having excellent adhesion to the surface to be plated, the presence of a small quantity of grease would not cause any trouble, thus greatly decreasing the time and labor required for vigorous pretreatment.

The following examples are given by way of illustration and not limitation.

EXAMPLE 1 200 g. of chromium trioxide was dissolved in one liter of water. Then, the solution was heated up to 70 C., and 35 g. of sodium hexafluorosilicate was incorporated to the solution. After the addition of the hexafluorosilicate, the solution was treated by preliminary electrolysis for 24 hrs. by applying an electric current of 2 a.h./l. at the temperature of 70 C. Consequently all of the sodium hexafluorosilicate was completely dissolved therein. This chromium oxide-fluorosilicate electrolyte was electrolyzed by passing an electric current at a density of 100 a./dm. for 10 minutes and then the current efficiency at the cathode was measured to be 45.0%

For the purpose of comparison, sodium hexafluorosilicate was added one gram by one gram into an aqueous solution of chromium trioxide (200 g./l.) while stirring at 60 C., and no preliminary electrolysis was applied. As a result any amount exceeding 6 g. of potassium hexafluorosilicate remained as a suspension. The current efliciency at cathode measured with the same conditions as above was 21.5%.

EXAMPLE 2 An aqueous solution of chromium trioxide (200 g./l.) was treated with a preliminary electrolysis corresponding to 48 a.h./l. at 40 C. When 40 g. of potassium hexafluorosilicate per one liter of the chromium trioxide solution was added to the solution and then heated to 60 C., a little amount of suspension was recognized. This suspension disappeared leaving a little amount of yellowish precipitate, as the solution was further electrolyzed by applying an electric current corresponding to 100 a.h./l. The average current efliciency of the electrolyte was about 45%.

EXAMPLE 3 250 g. of chromium trioxide .was dissolved in one liter of water. Then, while the solution was treated with a preliminary electrolysis by applying an electric current corresponding to 38 a.h./l. at the temperature of 80 C., 25 g. of potassium hexafiuorosilicate was incorporated little by little to the solution. Consequently all of the potassium hexafiuorosilicate incorporated was completely dissolved therein and the current efiiciency measured was 42%.

EXAMPLE 4 The experiments in the same conditions as in Example 3 were repeated except that the volume of chromium trioxide was varied within the range of 100 g. to 300 g. However, no substantial change in solubility of the hexafiuorosilicate was recognized.

EXAMPLE 5 The bath prepared in Example 1 was contained in an acid proof vessel and an anode electrode of lead plate and a die cast article to be plated and acting as the cathode electrode were immersed in the bath at predetermined positions. Plating treatment was performed by applying a dirct current voltage of 3.5 volts across two electrodes to pass a current at a density of 50:5 a./dm. The rate of deposition of the plated layer upon the surface to be plated was 6 microns per minute with a current efficiency of 47%.

When the plating treatment was performed with the same conditions as above noted except that the bath temperature was lowered to 20 C. no adverse effect was noted except that the current efficiency has decreased to about 37% which is still far higher than ordinary value of about 10% obtainable in the conventional plating bath.

For the sake of comparison, to the aqueous solution of chromium trioxide of the same concentration, was added potassium hexafiuorosilicate in an amout of 60 g. per one liter of the solution and the solution was thoroughly stirred at a temperature of 70 C. whereby a plating bath was prepared. It was noted that non-dissolved crystals of hexafiuorosilicate remained in the bath and that the quantity of these crystals had increased as the temperature of the bath decreased. The bath when heated to 60 C., while it contains precipitate, is very viscous so that it requires higher voltage of 6.5 volts in order to pass the current at a current density of 50 a./dm. in the plating treatment carried out under the same conditions described above. With such a viscous plating bath, lustrous plated surfaces were formed on the peripheral portions of the surface to be plated, but at the central portion the plated layer was thin so that it was impossible to form a uniform plated layer over the entire surface.

Further, for the sake of comparison, to the aqueous solution of chromium trioxide of the same concentration potassium hexafiuorosilicate was added in an amount of g. per liter of the solution and the solution was thoroughly stirred at a temperature of 70 C. to prepare a plating bath. Again this bath contained non-dissolved potassium hexafiuorosilicate so that voltage of 5.5 volts was required to pass current at a current density of 50 a./dm. between anode and cathode electrode immersed in this bath at a temperature of 60 C.

EXAMPLE 6 The plating bath prepared in Example 2 was maintained at a temperature of 60 C. and a voltage of 4.0 volts was applied across two electrodes respectively con- A plating bath of the same composition as in Example 3 was prepared. Articles to be plated were continuously introduced and taken out of the bath and chromium trioxide was supplemented at a rate of 20 grams every 24 hours. The plating treatment was performed continuously over 2000 hours and at the end of this period a slight change in surface luster of the layer plated on the articles was noted. However a current efiiciency of 45% was maintained over the entire treatment. Thereafter, the plating treatment was interrupted, potassium hexafiuorosilicate was incorporated into the bath at a proportion of 20 grams per one liter of the bath and dissolved therein at a temperature of 70 C. After cooling the bath to 60 C., the plating treatment was resumed under the same conditions as above. By this means, the surface luster of the plated surface formed on the articles was recovered to the original value.

What is claimed is:

l. A chromium plating bath comprising an aqueous solution of chromium trioxide, and fiuorosilicate completely dissolved in said aqueous solution in amounts ranging from 20 to 45 grams per liter of said solution.

2. The chromium plating bath according to claim 1 wherein said aqueous solution of chromium trioxide contains a quantity of sulfuric acid to adjust the concentration of hydrogen ions.

3. The chromium plating bath according to claim 1 wherein, as the minor component, said aqueous solution of chromium trioxide contains a reaction promoter selected from a group consisting of chromium sulfate, chromous sulfate, chromous chloride and potassium dichromate.

4. The chromium plating bath according to claim 1 wherein said fluorosilicate is an alkali metal salt of fluorosilicic acid.

5. The chromium plating bath according to claim 4 wherein said fluorosilicate is dissolved in said aqueous solution of chromium trioxide at a temperature ranging from 6 0 to C. under the influence of preliminary electrolysis.

6. A method for making a chromium plating bath comprising dissolving in solution 100 to 300 g. of chromium trioxide into per liter of water, incorporating 20 to 45 .g. of hexafiuorosilicate per liter of the solution, and applying a preliminary electrolysis corresponding to an electric current of at least 30 a.h./l. at a temperature ranging from 40 to 100 C.

7. A method for making a chromium plating bath, comprising dissolving in solution 100 to 300 g. of chromium trioxide into per liter of water, applying preliminary electrolysis at a temperature ranging from 40 to 100 C., incorporating 20 to 45 g. of hexafiuorosilicate per liter of the solution, and applying again a preliminary electrolysis corresponding to an electric current of at least 30 a.h./l. at a temperature ranging from 40 to 100 C.

7 8 8. A method for making a chromium plating bath, com- 3,457,147 7/ 1969 Dettner '204-5l prising dissolving in solution 100 to 300 g. of chromium 3,041,257 6/1962 Cope et a1. 20451 trioxide per liter of water and, incorporating 20 to 45 g. of hexafluorosilicate per liter of the solution, while apply- OTHER REFERENCES ing a preliminary electrolysis corresponding to an electric 5 Modem Electropllanng by Lowenhelm, 1968 current of at least 30 a.h./l. at a tern rature ran in from to pct g 8 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner References Cited UNITED STATES PATENTS 10 -R- 2,437,620 3/1948 Spcer 204-58 106-4 3,502,549 3/1970 Charveriat 204 s1

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