US3419481A - Electrolyte and process for electroplating black chromium and article thereby produced - Google Patents

Description

United States Patent 3,419,481 ELECTROLYTE AND PROCESS FOR ELECTRO- PLATING BLACK CHROMIUM AND ARTICLE THEREBY PRODUCED John B. Nelson, Jr., Chardon, Ohio, assignor to Diamond Shamrock Corporation, a corporation of Delaware No Drawing. Filed Apr. 8, 1966, Ser. No. 541,075 7 Claims. (Cl. 204-51) This invention relates to a composition and method for the electrodeposition of chromium-containing coatings on a conductive metal substrate as the cathode and more particularly relates to the electrodeposition of chromium coatings on such substrates in such physical and chemical form as to produce a black surface having a high degree of absorptivity for both heat and visible light.

Black chromium deposits find use in areas where their heat and light absorbing properties are important, one example of which is the military field where such items as firearms, communications equipment, personnel ornament, etc. are so coated. Such deposits are also valuable for decorative purposes such as on metal furniture, automobile parts, plumbing fixtures, etc., where their corrosion resistance coupled with their appearance make them superior to other black finishes such as paint.

While several processes and plating bath compositions have heretofore been disclosed in the art for producing black chromium electrodeposits, they have not received wide commercial acceptance for a variety of reasons. For example in one process, a grey-black deposit is obtained from a chromic acid bath having a low (less than 0.07%) sulfate content to which has been added a small amount of a carboxylic acid, preferably acetic acid. The chief disadvantages of this bath arise from the conditions for electrodepositing the black chromium whereby very high current densities, i.e., 10,000 to 20,000 amperes per square meter (935 to 1870 amperes per square foot), must be used at relatively low temperatures, i.e., about 20 C. Therefore, in addition to the excessive consumption of current, refrigerating means must be provided to maintain the proper temperature in this process.

It has also been disclosed that dark grey to black electrodeposits may be obtained from a chromic acid bath from which the sulfate has been removed and which contains large amounts of acetic acid. While this bath is said to operate at low current densities, its effective current density range is quite limited, i.e., 40 to 90 amperes per square foot, and this bath also has the further disadvantages that steam coils must be provided to maintain the desired temperatures for plating 90 to 115 F., and that an exhaust fan is required to remove the noxious acetic acid vapors.

In a more recent process, a black chromium deposit is obtained from an aqueous bath consisting of chromic acid and a fluoride catalyst. While the current density and temperature ranges recommended are satisfactory, reproduction of good results is still quite ditficult to attain because of the criticality of the fluoride catalyst concentration and the necessity according to this teaching of eliminating all other catalytic ions from the bath. Thus, in addition to precluding the presence of sulfate in the bath, it is also necessary to use deionized or distilled water for bath make-up and pre-bath rinsing procedures in order that no foreign ions be introduced. Consequently this type of bath is ditficult to control and not suitable for large scale commercial applications.

It is an object of the present invention to provide an improved bath composition and easily operable process whereby uniform, black, chromium-containing electrodeposits may be obtained.

It is another object of the present invention to provide an improved bath composition which is highly effective over a wide range of operating conditions for electrodepositing black, chromium-containing coatings.

These and other objects of this invention will become apparent from the specification and claims that follow.

It has now been found that uniform, corrosion resistant, black, chromium-containing coatings may be electro-deposited on an electrically conductive member by making said member the cathode in an aqueous solution consisting essentially of from at least 60 grams per liter up to saturation of chromic acid, a fluoride-containing catalyst in an amount sufficient to supply from about 0.03 to about 1 gram fluoride in solution per liter and an inorganic nitrogen-containing compound in an amount sufficient to supply the equivalent of from about 0.35 to about 3.5 grams of (N0 radical per liter, said aqueous solution being free of sulfate ions, and passing a direct current between said cathode and an anode immersed in said solution at a current density of from about 30 to 1500 amperes per square foot while maintaining the solution at a temperature of from about 60 to F.

While solutions containing chromic acid and a fluoridecontaining catalyst have previously been used to obtain black, chromium-containing electrodeposits, it has now been found that the inclusion of relatively small amounts, i.e., amounts suificient to provide the equivalent of from about 0.35 to 3.5 grams of (N0 radical per liter of an inorganic nitrogen-containing compound as defined below, affords substantial advantages. With the use of such inorganic nitrogen-containing compounds a uniform, black, chromium-containing electrodeposit is readily obtained over a wide range of current densities. The deposit itself is also improved in appearance in that it is darker in color and has a glossier finish than that obtained without the use of the inorganic nitrogen-containing compounds.

Any commercially available grade of chromic anhydride (CrO may be used in the practice of this inven tion, but since commercial chromic anhydride generally contains significant quantities of sulfates which interfere with the formation of the desired black, chromium-containing deposits, it is necessary that the chromic acid solutions described herein be treated before use to remove these sulfates. This treatment may be easily eifected by the addition to the solution of a source of barium ion such as barium carbonate or barium oxide. The addition of 5 to 20 grams per liter of barium carbonate will generally provide a sufficient excess to insure a sulfate-free plating solution. The amount of chromic anhydride used may be within the range of from at least 60 grams per liter up to saturation, preferably about 300 to 500 grams per liter. Particularly preferred at this time is a concentration of 450 grams per liter.

Any of the fluoride-containing chromium plating catalysts known to the art which supply fluoride in solutions of chromic acid may be used inthe bath compositions of this invention. Examples of these catalysts are hydrofluoric acid, fluoboric acid, fluosilicic acid and water soluble alkali metal, alkaline earth metal, heavy metal and ammonium salts thereof. Preferred fluoride-containing catalyst are those obtained by reacting a hexavalent chromium compound such as chromic acid, an organic reducing agent such as sucrose and a fluoride-silicon compound such as fiuosilicic acid as described in US. Patent No. 2,841,540. The amounts of fluoride-containing catalyst useful in this invention will vary according to the amount of fluoride which the catalyst can supply to the chromic acid solution, and the amount of .fluoride in solu-- tion may be varied from about 0.03 to about 1 gram per liter, preferably from about 0.1 to 0.25 gram per liter.

The inorganic nitrogen-containing compounds effective in the solutions of the present invention are nitric acid and nitrous acid and the alkali metal, alkaline earth metal and ammonium salts thereof, nitric acid and sodium nitrate being preferred at present by reason of their ready availability and comparatively low price. These inorganic nitrogen-containing compounds are effective for the purposes of the present invention in amounts sufficient to supply the equivalent of about 0.35 to about 3.5 grams of (N radical per liter, preferably about 0.7 to 2 grams of (N0 radical per liter. While amounts in excess of 3.5 grams per liter (N0 may be used, they are without apparent additional beneficial effect. Thus, by way of example, if sodium nitrite is used in the practice of this invention amounts within the range of from about 0.52 to about 5.2 grams per liter, preferably 1 to 3 grams per liter will be effective; if sodium nitrate is used amounts within the range of from about 0.65 to about 6.5 grams per liter, preferably 1.3 to 3.7 grams per liter will be effective.

The plating solution disclosed herein for electro-depositing black, chromium-containing coatings are easier to control than prior art solutions in that it is not necessary to use deionized or distilled water in either the makeup of the solutions or in the rinsing steps prior to immersion of the article to be coated in the solution. The solutions may be used effectively in producing electrodeposits for long periods of time without detrimental effects due to decomposition products and employing only the precautions used in the operation of conventional chromium plating solutions to prevent the introduction of foreign ions.

Uniform, black, chromium-containing deposits are readily obtained over a current density range of about 35 to 1500 amperes per square foot. The temperature of the bath may be maintained within the range from about 60 to 130 F., preferably from 70 to 95 F., thereby eliminating the need for either heating or cooling means in most cases.

While the time required to electrodeposit the black, chromium-containing coatings will vary somewhat depending on the solution composition, temperature, and the thickness of coating desired, 1 to 5 minutes at a current density within the above mentioned range will generally give a uniform, black, chromium-containing deposit having good corrosion resistance.

Any of the insoluble anodes used with conventional chromium plating baths may be employed. Especially to be preferred are lead or lead alloy anodes. Corrosion and erosion of these lead or lead alloy anodes when used in the practice of this invention is on a level equivalent with that experienced in conventional chromium plating solutions. Tanks employed for containing the solutions of the present invention may be lined with any suitable corrosion resistant material such as glass, ceramic material, polyvinyl chloride, lead and the like.

While the best results are obtained by subjecting the electrically conductive member to be pated by the process of this invention to a nickel strike treatment, i.e., cathodically plating nickel on said member for a short period at any conventional current density, quite satisfactory coatings may be obtained by plating directly onto copper, zinc, iron, steel, stainless steel, conventional chromium deposits and other conductive bases.

As is the practice in conventional chromium plating, mists suppressants may be added to the solution before plating. Many of the commercially available products for this purpose (which are generally proprietary surfactantcontaining compositions) are acceptable.

Compositions of this invention are ready for the electrodepositing operation immediately upon dissolution of the proper amounts of chromic anhydride, fluoride-containing catalyst and inorganic nitrogen containing compound in tap water, order of addition not being important, without pretreatment or preliminary electrolysis of any kind other than the addition of excess barium to remove the sulfate.

In order that the compositions and process of this invention may be more readily understood by those skilled in the art, the following specific exampes are provided. Unless otherwise, noted, the examples that follow are the result of evaluation of the compositions and process of this invention in a modified 267 milliliter Hull cell using as the cathode brass panels, 2.5 by 4 inches, which have been given a thin, uniform nickel coating by electrodebeen given a thin, uniform nickel coating by electrodepsition from a commercial bright nickel plating bath. The standard 267 milliliter Hull cell is a trapezoidal box of non-conductive material in the opposite ends of which are positioned anode and cathode plates as is more particularly described in US. Patent No. 2,149,344. By the use of this device it is possible to easily determine the effective plating range of a plating composition under varying conditions. The current density at any point on the cathode is determined according to the formula A:C (27.7-48.7 log L) wherein A is the current density at the selected point, C is the total current applied to the cell and L is the distance of the selected point from the high current density end of the plate. In the modified version used herein, holes are provided in the sides of the Hull cell adjacent the anode and cathode whereby, upon immersion of the cell in a container of plating solution, improved electrolyte circulation and consequently improved temperature control is afforded as is more particularly described in an article by J. Branciaroli appearing on page 257, March 1959 issue of Plating, vol. 46, No. 3 (a publication of the America Electroplaters Society, Inc.).

EXAMPLE 1 An aqueous plating solution is made containing 450 grams per liter of CrO 1 gram per liter of fluoride-containing catalyst (the reaction product of a chromium compound, an organic reducing agent and fluosilicic acid as more particularly described in US. Patent No. 2,841,540 and containing by weight chromium, 25 by weight fluoride and 16% by weight silicon) and 7.5 grams of BaCO to precipitate the sulfate. A panel is electroplated for 3 minutes at 10 amperes and a bath temperature of 96 F. A dull black electrodeposit is obtained from the high current density end of the panel to a current density of about 150 amperes per square foot, a value corresponding to 46% coverage.

To the above bath is added 1.5 ml. of concentrated nitric acid (specific gravity 1.42 gm./ml.). A second panel is now electroplated for 3 minutes at 10 amperes and a temperature of 96 F. The black deposit range is now found to extend to about amperes per square foot or 78% coverage. In addition to the substantially improved plating range the deposit itself is more uniform, is darker and has a glossier appearance than the deposit obtained without the addition of the nitric acid.

EXAMPLES 2-9 A series of Hull cell panels is run as indicated in Table 1. In each instance the solution is treated with an excess of BaCO to precipitate the sulfate and the fluoride-containing catalyst is the same as described in Example 1. The column titled Range indicates the effective plating range obtained, i.e., it indicates that a uniform black deposit is obtained from the extreme high current density end of the panel to the area on the panel corresponding to the stated value.

TABLE 1 Ex. CrO a F-Catalyst HNOa Temp. Range (gram/liter) (gram/liter) (ml./liter) F.) (a.s.l.)

1 Specific gravity 1.42 gin/ml. 2 Amperes per square foot.

These examples show the effect of increasing temperature on the effective plating range of the compositions of this invention and indicate that while a loss of coverage in the low current density areas occurs with increasing temperatures, this may be corrected by increasing the amount of nitrogen-containing compound. At the high temperature of Example 8, a grey band appeared in the deposit at the low current density area. This band disappeared in Example 9 wherein more HNO' was added, with only a slight loss in coverage. Thus, the present invention is shown to yield a uniform black electrodeposit over a broad current density range even with widely varying plating solution temperatures.

EXAMPLE 10 A panel is electroplated in a plating solution containing 450 grams per liter Cr 1 gram per liter of the fluoridecontaining catalyst of Example 1, 1.5 ml. of concentrated HNO (specific gravity 1.42 gm./ml.) and BaCO in excess of that required to precipitate the sulfate present. The bath is maintained at 112 F. for 3 minutes While applying 30 amperes direct current. A uniform, black, chromium-containing deposit is obtained over an effective plating range of from greater than 1500' amperes per square foot to about 30 amperes per square foot.

EXAMPLE 11 To illustrate the use in this invention of inorganic nitrogen-containing compounds other than HNO a plating solution is prepared as follows: 450 grams per liter CrO 1 gram per liter fluoride catalyst (as in Example 1); 2.5 grams per liter Ca(NO -4H O and excess BaCO over that required to precipitate the sulfate. A panel is plated from this bath in a Hull cell at 78 F. for 3 minutes with an applied current of amperes. Although there is some evidence of burning at the extreme high current density end of the panel, a uniform black deposit is obtained to a current density of about 70 amperes per square foot.

EXAMPLE 12 To show the eifectiveness of the compositions of this invention over a wide range of CrO concentrations the following solution is used: 300 grams per liter CrO 1 gram per liter fluoride catalyst (as in Example 1), 2.9 grams per liter of Ca(NO -4H O and excess BaCO over that required to precipitate the sulfate present. The panel is plated for 3 minutes at 72 F. and 10 amperes applied current. A black deposit is obtained to the area on the panel corresponding to about 120 amperes per square foot with only a slight burn at the extreme high current density edge.

Now 300 grams per liter of CrO is added (total 600 grams per liter) and another panel is plated at 81 F. for 3 minutes. An excellent black deposit is obtained to 60 amperes per square foot and the burned area is greatlv decreased.

EXAMPLE 13 To show the effectiveness of inorganic nitrogen-containing compounds other than nitric acid and salts thereof the following plating solution is used: 450 grams per liter CrO 1 gram per liter fluoride catalyst (as in Example 1) and excess BaCO to ensure complete sulfate precipitation. A panel is plated for 3 minutes at 78 F. and 10 amperes applied current. A black deposit is obtained only to about 200 amperes per square foot with considerable burning on the high current density end of the panel. This corresponds to a coverage of about 40%.

Now 1 gram per liter of NaNO is added and a panel is plated under the same conditions as above. A uniform black deposit is obtained to a current density of about 60 amperes per square foot with no burn on the high cur- EXAMPLE 14 To show the use of another fluoride-containing catalyst in the practice of this invention an aqueous solution is used as follows: 450 grams per liter CrO 2 ml. of concentrated HNO (specific gravity 1.42 grams per milliliter), 0.3 ml. of HF (49% aqueous solution) and an excess of BaCO over that required to precipitate the sulfate present. A Hull cell panel is plated for 3 minutes at 73 F. and an applied current of 10 amperes. A black deposit is obtained from the high current density end of the panel to about 70 amperes per square foot.

It is to be understood that although the invention has been described with specific reference to particular cmbodiments thereof, it is not to be so limited since changes and alterations therein may be made which are within the full intended scope of this invention as defined by the appended claims.

What is claimed is:

1. A composition of matter for use in the electro-deposition of black, chromium-containing deposits which is an aqueous solution consisting essentially of from about 60 grams per liter to saturation of chromic anhydride, a fluoride-containing catalyst in an amount sufiicient to supply from about 0.03 to about 1 gram of fluoride in solution per liter, an inorganic nitrogen-containing compound in an amount suflicient to supply the equivalent of from about 0.35 to about 3.5 grams of (N0 radical per liter, said aqueous solution being substantially completely free of sulfate ions.

2. A composition as claimed in claim 1 wherein the inorganic nitrogen-containing compound is selected from the group consisting of nitric and nitrous acid and the water soluble alkali metal, alkali earth metal and ammonium salts thereof.

3. A composition as claimed in claim 1 wherein the fluoride-containing catalyst is selected from the group consisting of hydrofluoric acid, fluoboric acid, fluosilicic acid, water soluble alkali metal, alkali earth metal and ammonium salts of hydrofluoric, fluoboric and fluosilicic acid and the reaction product of a hexavalent chromium compound, an organic reducing agent and a fluoride-silicon compound.

4. A process for the electrodeposition of black, chromium-containing deposits which comprises making the electrically conductive member to be plated the cathode in an aqueous solution consisting essentially of from about 60 grams per liter to saturation of chromic anhydride, a fluoride-containing catalyst in an amount sufiicient to supply from about 0.03 to about 1 gram of fluoride in solution per liter and an inorganic nitrogen-containing compound in an amount sufficient to supply the equivalent of from about 0.35 to about 3.5 grams of (N0 radical per liter, said aqueous solution being substantially completely free of sulfate ions, and passing direct current between said cathode and an anode immersed in said solution at a current density of from about 30 to about 1500 amperes per square foot while maintaining said aqueous solution at a temperature of from about 60 F. to about F.

5. A process as claimed in claim 4 wherein said inorganic nitrogen-containing compound is selected from the group consisting of nitric and nitrous acid and the water soluble alkali metal, alkali earth metal and ammonium salts thereof.

6. A process as claimed in claim 4 wherein said fluoride-containing catalyst is selected from the group consisting of hydrofluoric acid, fluoboric acid], fluosilicic acid, water soluble alkali metal, alkali earth metal and ammo nium salts of hydrofluoric, fluoboric and fluosilicic acid and the reaction product of a hexavalent chromium comcompound.

7. An article electroplated by the process of claim 4.

References Cited UNITED STATES PATENTS Ungelenk et al. 204-51 Schlijtter 20451 Keuffel et a1.

Gilbert et a1 204-5l Smith 204-51 Smith 204---51 8 FOREIGN PATENTS 624,887 8/1961 Canada. 881,596 7/1953 Germany.

22,105 10/1964 Japan. 5 135,319 5/1960 U.S.S.R.

OTHER REFERENCES Griffin, John L., Experimental Chromium Electrodeposition as Affected by a Variety of Possible Catalysts, 10 Plating, v01. 53, N0. 2, pp. 196203, February 1966.

JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner.

Claims (1)

1. A COMPOUND OF MATTER FOR USE IN THE ELECTRO-DEPOSITION OF BLACK, CHROMIUM-CONTAINING DEPOSITS WHICH IS AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF FROM ABOUT 60 GRAMS PER LITER TO SATURATION OF CHROMIX ANHYDRIDE, A FLUORIDE-CONTAINING CATALYST IN AN AMOUNT SUFFICIENT TO SUPPLY FROM ABOUT 0.03 TO ABOUT 1 GRAM OF FLUORIDE IN SOLUTION PER LITER, AN INORGANIC NITROGEN-CONTAINING COMPOUND IN AN AMOUNT SUFFICIENT TO SUPPLY THE EQUIVALENT OF FROM ABOUT 0.35 TO ABOUT 3.5 GRAMS OF (NO2) RADICAL PER LITER, SAID AQUEOUS SOLUTION BEING SUBSTANTIALLY COMPLETELY FREE OF SULFATE IONS.